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Biology A-Level
Section 1: Biological Molecules
The similarities in the biochemistry of organisms provides indirect evidence for the theory of
evolution
...
They are formed by
polymerisation reactions
Condensation reaction - The chemical bonding of two smaller molecules to form a more complex
molecule through the elimination of a small molecule, usually water
Hydrolysis - The breaking of a chemical bond between two molecules through the addition of a
molecule of water
Metabolism - The sum of the chemical processes taking place in a living organism collectively
Organic molecule - A molecule containing carbon
...


It also plays a structural role in cell walls and is part of the glycoprotein and glycolipid receptors
found within plasma membranes
...
They are simple sugars and
have the general formula (CH2O)n
...

Glucose is a hexose sugar with the formula C6H12O6
...
When these monomers join the polymers formed show different structural
properties
...


Monosaccharides can be tested for using the Benedict’s test:
1) The solid food sample is ground up and added to water in a test-tube
2) An equal volume of Benedict’s reagent (alkaline copper sulphate) is added to the sample
3) The test-tube is heated in a gently boiling water bath for 5 minutes
4) A positive result results in the solution turning from blue to green, then yellow, then orange,
then brick-red
...
It should then be neutralised with sodium hydrogen-carbonate solution

until alkaline
...

Disaccharides are two monosaccharides chemically joined by a glycosidic bond formed through a
condensation reaction
...

A glucose and fructose unit make fructose
...

Under suitable conditions the addition of water to a disaccharide causes hydrolysis so that the
glycosidic bond is broken releasing the constituent monosaccharides
...

These molecules are larger and more complex, making them insoluble
...
Also, their large size means
that they cannot diffuse out of cells easily, again making them useful for storage
...
g
...
Large amounts are found in seeds and storage organs of plants, so it is a
major energy source in most diets
...


Starch is made up of around 200-100000 alpha
glucose molecules joined by glycosidic bonds
formed by a series of condensation reactions
...
Amylose constitutes for around 20%
of starch; it’s alpha helical structure makes it
compact, so lots can be stored in a small
amount of space
...


Also, starch is large and insoluble, doesn’t
affect water potential and is made up of alpha
glucose which can be easily transported and
readily used in respiration, making it ideal for
energy storage
...
It is never found
in plant cells
...


It is similar to amylopectin starch, however the
polysaccharide chains are bonded to a protein
...
It is also
compact and the many branches can be
readily acted on by enzymes
...

Cellulose is a polysaccharide made up of beta glucose molecules
...
These beta pleated sheets run parallel to each other, allowing hydrogen
bonds to form between adjacent chains
...
Cellulose molecules group together to form
microfibrils, which are arranged to form fibres
...
It also
prevents the cell from bursting by exerting inward pressure on water that enters the cell by
osmosis
...


Lipids Lipids are organic compounds containing carbon, hydrogen and oxygen
...
They are insoluble
in water, but soluble in organic solvents such as alcohols and acetone
...
They also play a role in insulating, as they slow conductors of heat and electricity
(e
...
they help make up the myelin sheath around nerves)
...
g
...
Finally, fat is
used to protect delicate internal structure, such as the kidneys
...
Each fatty acids forms an ester bond
with glycerol in a condensation reaction
...
A fatty acid is a carboxylic acid with a long
aliphatic hydrocarbon chain
...

The high chemical strength of the C-H bond
makes triglyceride molecules useful for energy storage, alongside their low mass to energy ratio
...
Triglycerides are non-polar and therefore
insoluble in water, so they do not affect water potential of cells, again making them useful for
storage
...
g
...

Phospholipids are another type of lipid
consisting of two fatty acids and one
phosphate group attached to one glycerol
molecule
...
This makes the
molecule polar, causing phospholipids to
form micelles and bilayers in aqueous
environments, such as the cell surface
membrane
...


Phospholipids are an essential part of the
cell surface membrane, as they help provide
flexibility and allow glycolipids to form - this
is important for cell recognition
...

1
...
Add 5cm3 ethanol to 2cm3 of sample
3
...
Add 5cm3 of water and shake gently
5
...
This is due to light refracting off oil
droplets emulsified in water
[Using water instead of a sample can be used as a control measure]

Proteins Proteins are complex, large molecules composed of amino acid monomer units
...

There are two basic types of proteins; globular proteins such as enzymes, which carry out
metabolic functions and fibrous proteins such a collagen, which have structural functions
...
Collagen is made up of tightly packed unbranched chains wound around
each other and is found in muscle tendons
...

Amino acid monomers combine through a condensation reaction to form dipeptides and
polypeptides
...

The primary structure of a protein is the polypeptide resulting from the polymerisation of many
amino acid monomers
...
Proteins consisting of one polypeptide
chain are known as simple proteins
...
The hydrogen bonds
between certain amino acids causes the polypeptide chain to twist into a 3D shape, for example an
alpha helix or beta pleated sheet
...
This is maintained by strong disulphide bridges (formed
between the sulphur atoms of two molecules of cysteine), slightly weaker ionic bonds and even
weaker hydrogen bonds
...
For proteins made of one
polypeptide chain this is the final 3D structure
...
One
common example is haemoglobin, which has an iron-containing haem group within its structure
...

1
...
Add a few drops of dilute (0
...
The mixture should turn from blue to purple/lilac if a protein is present
Enzymes Enzymes are globular proteins that act as biological catalysts to increase the rate of chemical
reactions without undergoing permanent changes themselves
...
They do so by holding molecules that need to react close
together, reducing repulsion, and putting a strain on bonds that need to be broken down in a
breakdown reaction
...
The active site forms a small depression within the much larger enzyme,
but it is within this depression that enzyme-substrate complexes form when substrates molecules
with a complementary shape to the active site form temporary bonds with the amino acids of the
region
...
As it changes its shape, the enzyme puts strain on the substrate
molecule, distorting particular bonds within it and thus lowering activation energy
...
g
...

The induced fit mechanism replaces the previous scientific theory behind enzyme action known as
the lock and key model
...
However, this model didn't
explain why enzymes shapes could be altered by molecules binding to areas other than the active
site
...

Enzyme activity can be measured by recording the rate of the enzyme-catalysed reaction (using
product formed over time, or reactant used over time)
...

Factors affecting enzyme activity tend to affect the shape of the enzymes active site and the way
the enzyme and substrate come into physical contact
...
This causes enzyme and substrate molecules to form more
enzyme-substrate complexes in any given time
...
However, past a certain temperature the enzyme is denatured
...
Enzyme-substrate complexes cannot form and this
change is irreversible
...


• pH
pH is a measure of hydrogen ion concentration and alterations in pH beyond a certain range can
denature an enzyme
...
Also, the changes
can disrupt the bonding within the enzyme (primarily the ionic bonding), causing the tertiary
structure to alter
...
However, when the substrate becomes the limiting factor (i
...
at the
saturation point), an increase in enzyme concentration will have no effect on the rate of reaction
...
At this point the rate of reaction has reached its maximum
...
The enzyme is therefore no longer available to bind to the
substrate and the rate of reaction slows
...
The greater the concentration of substrate, the greater the chance a substrate will fill
the enzymes active site before the inhibitor
...
By
doing so they alter the shape of the enzymes active site, making it no longer functional
...

End-product inhibition is usually non-competitive and is a negative feedback metabolic pathway
where the end product inhibits the pacemaker enzyme, resulting in less end product being formed
...
Two important nucleic acids
are RNA and DNA
...
These bases include cytosine, thymine, uracil, adenine and guanine
...

When the pentose sugar of one mono-nucleotide and the phosphate group of another join through
a condensation reaction, a phosphodiester bond is formed
...
Many
joined nucleotides form a polynucleotide
...

DNA Deoxyribonucleic acid (DNA) contains a deoxyribose sugar, a phosphate group and the possible
bases are A, G, C and T
...
DNA is made up of two long polynucleotide chains,
joined by hydrogen bonds between base pairs
...
The base pairing is specific; adenine always pairs with
thymine and guanine always pairs with cytosine
...
The ratio of A to T is always the same, as is the ratio of C to G, but the proportion of the
pairs varies within DNA
...
There is also base stacking, which occurs when there are interactive forces
between the base pairs, helping to hold the DNA molecule together
...

DNA structure serves in aiding its function
...
The variety in the possibility of base sequences
provides genetic diversity within organisms
...
The semi-conservative model of
replication is universally accepted;
1
...
The DNA helix unwinds into two separate strands
3
...
DNA polymerase catalyses the condensation reaction which form phosphodiester bonds
between the newly bonded nucleotides
5
...
The

DNA polymerase works in opposite directions on the two individual new strands of DNA, because
the original DNA strands run antiparallel to each other
...
Energy is initially absorbed from sunlight by producers
...
Consumers can then take in
and oxidise these molecules to make adenosine triphosphate (ATP), which is used as a source of
energy
...
It is the bonds between these phosphate groups that make ATP a good source
of energy
...
When ATP is hydrolysed it forms ADP and an inorganic
phosphate group, as well as releasing energy, which can be coupled with other reactions to initiate
them
...

ATP synthesis is catalysed by the enzyme ATP synthase and is a condensation reaction
...
It
occurs in both plant and animal cells during respiration (oxidative phosphorylation) and substratelevel phosphorylation
...

The instability of these phosphate bonds and ability to rapidly reform makes ATP a good immediate
energy source for a cell
...
It
also requires a single reaction only to be broken down
...
g
...

Cells may require ATP for;
• Metabolic processes - such as building up macromolecules
• Movement - i
...
muscle contraction
• Active transport - ATP provides the energy that causes carrier proteins to change shape
• Secretion - ATP helps form lysosomes needed for the secretion of cell products
• Activation of molecule - e
...
the addition of phosphate to glucose in glycolysis
ATP is a good energy source because;
It stores and releases energy in small, manageable amounts, so less is wasted
It is small and soluble, so is easily transported
It is easily broken down for quick energy release
It is quickly remade through phosphorylation
It can be used for the phosphorylation of other molecules to make them more reaction
It cant pass out of the cell, so gives the cell an immediate supply of energy

-

Water Water has a dipolar charge due to the delta negative charge on the oxygen and the delta positive
charge on the hydrogen atoms
...

These properties include;
• High specific heat capacity, as the hydrogen bonds need more energy to be overcome
• High latent heat of vaporisation, making it effective means of cooling
• Cohesion
• Surface tension
Water is the main constituent of all organisms
...
g
...
g
...
It is determined by the wavelength of the microscope
Cell fractionation Cell fractionation is the process where cells are broken up and the different organelles are
separated out
...
The homogenate is spun in a centrifuge, causing
heavier organelles to form a pellet at the bottom
4) The pellet is removed and the process is repeated using the remaining supernatant
Order of organelles by weight;
1) Nuclei
2) Mitochondria
3) Lysosomes
4) Endoplasmic reticulum
5) Ribosomes
Microscopy The electron microscope has a higher resolution 2000 times than light microscopes because it has
a shorter wavelength of just 0
...
However, air molecules deflect electrons, therefore
samples must be viewed in a vacuum
...
There are two types of
electron microscopes
...
The photomicrograph is 2D and monochromatic
...
The beam is passed back and forth across a portion of the specimen in a
regular pattern
...

The main limitations of the SEM are the same as that of the TEM, except the specimen does not
need to be as thin
...

Cells are measured in light microscopes using an eyepiece graticule; a glass disc with a scale that
is placed into the eyepiece of the microscope
...
This is done using a stage micrometer slide
...
01 mm
...

Eukaryotic cell structure Within the ultrastructure of eukaryotic cells there is a distinct nucleus and membrane-bound
organelles
...
It is also the site of ribosome synthesis
...


It is made up of the nuclear envelope (a double membrane that joins onto ER), nuclear pores,
nucleoplasm, chromosomes and the nucleolus
...



- Mitochondria

Mitochondria are the site of aerobic respiration and therefore responsible for the production of
ATP from glucose
...
The inner
membrane is folded to form cristae, providing a large surface area for the attachment of
enzymes involved in respiration
...



- Chloroplasts

Chloroplasts are organelles found in plant cells, which carry out photosynthesis
...


They are surrounded by the chloroplast envelope; a highly selective double plasma membrane
and the matrix is filled with stroma
...
Grana
are stacks of thylakoids (which contains chlorophyll for photosynthesis) joined by lamella
(thylakoid membrane)
...


Chloroplasts contain enzymes, DNA and ribosomes for the quick manufacture of proteins
needed for photosynthesis
...
The membranes
enclose a network of tubules and flattened sacs called cisternae
...


RER carriers out the synthesis and transport of proteins and glycoproteins throughout the cell
...



- Golgi apparatus

Golgi apparatus have a similar, but more compact structure, to SER
...
The substances stored in the ER are
passed to the Golgi apparatus, where they are modified (e
...
adding carbohydrates to form
glycoproteins)
...
The contents are released
...



- Lysosomes

Lysosomes are vesicles containing lysozymes, proteases and lipases
...



- Ribosomes

Ribosomes are small cytoplasmic granules that are the site of protein synthesis
...
In eukaryotic cells ribosomes are 80S (25 nm
in diameter) and in prokaryotes they are 70S, which is smaller
...
In plant cells the cell wall is
made up of the polysaccharide cellulose, cemented to other cells by a middle lamella made up
of pectins
...
A
plant cell wall also contains plasmodesmata; channels for exchanging substances between
adjacent cells
...



- Vacuoles

Vacuoles are fluid-filled sacs surrounded by a single membrane called the tonoplast
...
It may also contain pigments called
anthocyanin, which attract pollinating insects
...

Cell specialisation Unless they are a stem cell, all cells are specialised; the genes that they express help them to
carry out a certain function
...




Tissues - specialised cells that are aggregated to carry out a specific function
...
In mammals
an example is the stomach; it contains muscle tissues to churn food, epithelium to produce
secretions and connective tissues to hold the organ together
Organ systems - Organs that work together as a single unit (e
...
the digestive system the
respiratory system and the circulatory system)
Prokaryotic cell structure -

Prokaryotes cells are smaller than eukaryotes
...
Prokaryotic cells are be less than 2 micrometers in diameter,
whereas eukaryotic cells tend to be up to 50 times bigger
...
1 to 10 micrometers in length
...
Within the cell wall there is a cell-surface membrane, 70S ribosomes, glycogen
granules and oil droplets (for food reserves)
...
There
are also separate circular strands of DNA called plasmids, which can reproduce independently
...


Some bacteria also have a flagellum for locomotion
...
The circular DNA replicates and both copies
attach to the cell membrane
...
The cell membrane grows between the
two DNA molecules and pinches inwards
...

Viruses are another type of prokaryote
...
A protein capsid with freefloating nucleic acids is contained within a lipid envelope covered in attachment proteins
...


Viruses replicate by attaching to their host cell via the attachment proteins on their surface
...
The components for a new virus are assembled, until
they burst out of the host cell
...
They also have a protein coat and no sites where antibiotics can attach
...
Within its lipid envelope, it contains the enzyme reverse transcriptase, which
can be used to catalyse the production of DNA from RNA
...
CD4 is most commonly
found on helper T cells
...


The reverse transcriptase converts the virus’s RNA into DNA, which enters the nucleus of the
helper T cell and is created into mRNA
...


As the HIV particles burst out of the helper T cells, they use a piece of the cell-surface membrane
to create their lipid envelope
...

Mitosis Mitosis is the process by which eukaryotic cells divide, resulting in two daughter cells that are
genetically identical to the parent cell (except in the case of mutation)
...
Interphase is divided
into three stages; gap phase 1, where cells grow and new organelles and proteins are made,
synthesis, where the cell replicates DNA, and gap phase 2, where the cell keeps growing and the
proteins needed for cell division are made
...

Centrioles move to opposite poles of the cell and develop spindle fibres
...


The nucleolus disappears and the nuclear envelope breaks down
...

2) Metaphase

During metaphase the chromosomes arrange themselves across the equator of the cell, so that
they line up
...

The energy is provided by mitochondria surrounding the spindle fibres
...

4) Telophase and cytokinesis

Once the chromosomes have reached their respective poles they become longer and thinner
until they form only wide spread chromatin
...
The cell divides through cytokinesis
...
The
rate of division is controlled by protooncogenes and tumour suppressor genes
...
However, these treatments are likely to
impact normals cells, in particular cells like hair cells, which divide at a rapid rate
...
It allows for the cell’s internal conditions to differ from those of the
environment and controls the passage of substances
Components of the cell surface membrane:

- Phospholipids 


-

-

-

-

Phospholipids form a bilayer with the hydrophilic heads facing the outside of the cell membrane
and the hydrophobic tails facing the centre of the cell membrane
...
Water-soluble substances cannot pass through the
phospholipid bilayer, only small non-polar molecules
...
Proteins at the surface give mechanical support or may be part of
glycolipids and receptors
...
Proteins can help cells adhere together
...
Protein carriers bind
to ions/molecules and move the molecules across the membrane through a conformational
change in shape
...

Cholesterol

Cholesterol molecules add strength to the membranes
...
They increase rigidity of the
cell membrane by binding to the fatty acid tails of the phospholipids, helping to reduce lateral
movement of molecules
...

Glycolipids

Glycolipids are formed of carbohydrates covalently bonded to lipids
...
Glycolipids also help maintain the stability of the membrane and
help cells attach to form tissues
...
As well as acting as recognition sites for specific chemicals,
glycoproteins also help cells attach together to form tissues
...


Fluid-mosiac model of the cell-surface membrane The model is known as fluid because the individual phospholipid molecules can move relative to
one another, providing flexibility to the structure
...


Most substances do not freely diffuse across the phospholipid bilayer of the cell-surface membrane
because they are not lipid soluble, too large or they are polar molecules
...

At temperatures below 0 degrees Celsius the membrane is rigid, the transport proteins denature,
so the permeability of the membrane increases
...

At temperatures between 0 and 45 degrees Celsius the membrane is partially permeable because
the phospholipids are mobile
...

At temperatures above 45 degrees Celsius the phospholipid bilayer begins to break down,
increasing permeability immensely
...
Transport membranes again denature
...
g
...
g
...
g
...
It is a passive
process that does not require energy from an external source (i
...
ATP)
...
Passive diffusion mainly occurs in small, non-polar
molecules
...

Facilitated diffusion -

Because plasma membranes are not readily permeable to charged/polar molecules they cannot
diffuse through the hydrophobic centre of the phospholipid bilayer
...
Facilitated diffusion is a
passive process and the movement of particles occurs down the concentration gradient
...
The channels are specific and selective and only bind to
certain ions causing a conformational change in shape
...

The rate of facilitated diffusion is affected once again by the steepness of the concentration
gradient and the number of transport proteins within the membrane
...

A solute is any substance that is dissolved in a solvent to form a solution
...
Under
standard conditions pure water has a water potential of zero
...
Only water
move from the area of higher water potential to lower water potential, because the solute
molecules cannot pass through the selectively permeable membrane
...


When a solution is put in hypotonic solution (i
...
the solution has a higher water potential than the
cell), water enter the cells causing it to swell and burst
...

The rate of osmosis is affected by the water potential gradient the thickness of the exchange
surface and the surface area of the exchange surface
...


Unlike in passive transport, active transport uses metabolic energy in the form of ATP to
individually move molecules against the concentration gradient using selective carrier proteins
...
The molecule binds to the receptor sites on the carrier protein

2) On the inside of the membrane ATP binds to the protein, causing it to be hydrolysed into ADP
and a phosphate group
...
This causes the protein to revert to its original shape
4) Several molecules can be moved in the same direction, or in some cases, like in the sodiumpotassium pump, molecules can be moved in opposite directions at the same time
The rate of active transport is affected by the speed of individual carrier proteins
...
s
Absorption of glucose in the ileum The epithelial cells lining the small intestine (ileum) have microvilli; finger-like projection around 0
...
These increase the surface area for the insertion of carrier proteins, so
more diffusion, facilitated diffusion and active transport can take place at one time
...

Diffusion in absorption

When there is a greater concentration of carbohydrates and amino acids in the ileum than in the
blood, the substances move down the concentration gradient into the epithelial cells surrounding
the ileum
...

Active transport in absorption

One the concentration inside the ileum and the blood are equal, diffusion can no longer occur
...
The
sodium-potassium pump is used
...
As they do this they carry a glucose or amino acid molecule into the protein
with them
4) The glucose/amino acid passes into the blood plasma by facilitated diffusion
...
Humans are born with around 10 million different types of
lymphocytes, all of which are specific to a particular protein
...
Instead the lymphocytes collide
with body cells during the development of a foetus
...
Only anti-self lymphocytes should remain
...
g
...
Phagocytes
are a type of white blood cell that engulf and break down pathogens
...

1) The phagocyte is attracted to the pathogen by the pathogen’s chemical products (known as
attractants)
2) The phagocyte uses the receptors on its cell-surface membrane to attach to the chemicals on
the surface of the pathogen
3) The phagocyte engulfs the pathogen, forming a phagosome vesicle
4) Lysosomes within the phagocyte migrate towards the phagosome; they contain lysozymes
which hydrolyse the cell walls of the ingested material
5) The soluble products are absorbed by the phagocytes cytoplasm, and the phagocyte becomes
an antigen presenting cell
Cell-mediated immunity This is a specific type of response that can lead to long term immunity
...
T cells mature in the thymus
...
The antigen must be specific to that T cell
2) Receptors on the specific helper T cell fit exactly onto these antigens
3) The attachment activates the T cell to divide by mitosis to form genetically identical clones
4) The cloned T cells develop into memory cells, more helper cells which stimulate phagocytosis
and B cells, and cytotoxic T cells
...
B cells mature in the thymus
...
The antigen gets presented on the B cells surface, which
then binds to helper T cells
...
Each clone produces one specific antibody, referred to as monoclonal antibodies
...
Memory cells can remain in the blood for up to a
decade, and divide into plasma and more memory cells upon reinfection
...
They react with an antigen on the
surface of non-self materials
Monoclonal antibodies - Antibodies that are isolated and cloned
...


Each antibody has two identical binding sites, which are complementary to a specific antigen and
form an antigen-antibody complex
...
The rest is known as the constant region
...
For example,
monoclonal antibodies can be specific to antigens on cancer cells, so they bind to the cancer
cell
...

Antibiotics cannot be used to treat viruses because they work by interfering with metabolic
reactions and enzymes within bacteria
...



- Medical diagnosis

- Pregnancy testing using antibodies that are complementary to hCG bound to a coloured bead,
which then bind to another immobilised antibody to hCG to give a positive result (i
...
a change in
colour)


- Indirect ELISA test for HIV

The enzyme linked immunosorbent assay is used to detect the quantity of HIV antigen in a
patients blood

1) Apply the sample to a slide to which all the antigens will attach

2) Wash the surface to remove unattached antigens

3) Add the antibody specific to the HIV antigen and leave to bind

4) Wash the surface to remove excess antibody

5) Add a second antibody with an enzyme attached, which binds to the first antibody

6) Add the colourless substrate of the enzyme (causing a reaction and colour change)
...

Passive immunity - The introduction of antibodies from an external source, so that no direct contact
with the antigen is needed to induce immunity
...
Immunity takes time to develop and is
long term
Natural active immunity - Immunity resulting infection
Artificial active immunity - Immunity resulting from vaccination, so an immune response is
generated without symptoms
Vaccine - The introduction of dead, inactivated or parts of pathogens in order to stimulate an
immune response against a particular disease
...
g
...
g
...
The short diffusion pathway and the large surface area to
volume ratio of single celled organisms means diffusion can occur quickly enough for cell function
...
Insects contract their abdominal muscles
resulting in the mass transport of air, which enters the spiracles on their body and goes into their
tracheae
...
These increase the surface area for gas exchange and bring oxygen directly to
respiring tissues
...


The ends of the tracheoles are filled with water
...
The water in the
tracheoles moves by osmosis into the muscle cells, causing it to decrease in volume
...
However, this leads to greater water evaporation
...
Gills are made up of gill filaments with gill lamellae at
right angles to these
...
Water pass over the blood
flow within the gill lamellae in the opposite direction; this is known as the countercurrent exchange
principle
...
If the flows were parallel then the
concentration gradient would level out
...
There is a short, fast
diffusion pathway and flat leaves increase the surface area to volume ratio, so there is no need for
a specific transport system for gases
...
The stomata are pores on the leaves, found mainly on the
underside
...
When the
water potential of the plant decreases, water moves out of these cells by osmosis, causing them to
close and preventing further transpiration
...
The spongy mesophyll layers has gaps so gases can readily come into contact
with cells
...
They are located within the ribcage of the body
as air is not dense enough to protect their structure and they require a constant source of moisture,
or else the would dry out
Trachea - The flexible airway supported by cartilage rings to prevent them from collapsing under
pressure
...
They produce mucus to trap dirt
particles and have cilia to move mucus towards the throat
...
They can constrict to control air flow into the alveoli
Alveoli - Air sacs at the end of bronchioles lined with epithelium, and joined by collagen and elastic
fibres so that they can stretch
...

Air is moved into the lungs by inhalation
...
The ribs are pulled upwards and outwards and the diaphragm
flattens, causing an increase in the volume and decrease in the pressure of the thorax
...



Alveoli are suited for gas exchange because;

- The epithelial cells lining them are only 0
...
The internal intercostal muscles
contract, whilst the external intercostal muscles relax, moving the ribcage downwards and in
...
Pressure is greater within the lungs than within the atmosphere, so air moves out of the
lungs down the pressure gradient
...
The measure of lung function are tidal volume
(volume of each breath), ventilation rate (breaths per minute), forced expiratory rate (maximum
volume than can be breathed out in a second) and forced vital capacity (maximum volume that can
be forcefully breathed out after a deep breath in)
...
this reduced tidal volume
- Fibrosis; formation of scar tissue in the lungs that causes the lung tissue to become less elastic
and thicker than before, which again reduces tidal volume
- Asthma; inflammation of the airways that causes the bronchioles lining to contract and produce
mucus, constricting the airways
...

- Emphysema; foreign particles become trapped in the alveoli and are attacked by phagocytes
...
,
resulting in the destruction of the alveoli walls, reducing their surface area and thus the rate of
gas exchange
...
g
...
g
...
It churns food in order to
break it down
Ileum - A long muscular tube where more enzymes are produced by the walls and glands to further
digest food
...
The villi have thin walls to decrease the diffusion pathway and the
constant movement of food and rich supply of blood vessels ensure a steep concentration gradient
is maintained
Large intestine - Where the absorption of water, mainly from hydrolysis reactions, occurs
Rectum - Where the faeces are stored before being removed via the anus by egestion
Salivary glands - Produces amylase, which hydrolyses starch into maltose
Pancreas - Secretes pancreatic juice, which contains amylase, proteases and lipases
Carbohydrases - Hydrolyse carbohydrates into monosaccharides
Lipases - Hydrolyse lipids into glycerol and fatty acids
Proteases - Hydrolyse proteins into amino acids
Carbohydrate digestion Amylase is produced by the salivary glands (released in the mouth) and the pancreas (released
into the small intestine) and hydrolyses alternate glycosidic bonds of starch to produce the
disaccharide maltose
...
Maltase is membrane
bound in the ileum
...
Mineral
salts in the saliva and alkaline salts produced by the pancreas and intestine walls maintain a
neutral pH so amylase can function
...


Glucose and galactose are absorbed via co-transport proteins, whereas fructose is absorbed
through facilitated diffusion
...
Lipids are first split up in micelles by bile salts in a process known
as emulsification
...

Micelles break down when they come into contact with the epithelial cells lining the villi of the
ileum
...
The monoglycerides and fatty acids inside the
cell are recombined to form triglycerides in the endoplasmic reticulum
...
Chylomicrons transport
dietary lipids from the intestines to other locations in the body
...
Chylomicrons pass through these lymphatic vessels to the blood, where they are hydrolysed
by enzymes in the endothelial cells of blood capillaries
...


Endopeptidases hydrolyse the peptide bond within an amino acid to form smaller peptide
molecules
...

Dipeptidases hydrolyse the bond between dipeptides
...

Amino acids are absorbed via sodium-dependent transporter proteins, which carry the amino acids
into the epithelial cells of the ileum
...
Each
Fe(II) ion can bind with one oxygen molecule, so each heamoglobin can carry 4 oxygen molecules
to form oxyhaemoglobin
Loading/associating - The process by which haemoglobin binds with oxygen
Unloading/dissociating - The process by which haemoglobin releases its oxygen
Affinity for oxygen - The ease at which haemoglobin will bind with oxygen and not release it at a
given partial pressure of oxygen
Partial pressure of oxygen - A measure of oxygen concentration of dissolved oxygen in cells
...
The shape of haemoglobin makes it difficult for oxygen to bind
at first, so at low oxygen concentrations little oxygen binds and more is unloaded from
oxyhaemoglobin
...
This is known as positive cooperativity, and
causes oxygen to bind more easily with a little increase in partial pressure
...

The further to the left the curve, the greater the affinity of haemoglobin for oxygen
...

The partial pressure of carbon dioxide also affects affinity; the higher the partial pressure, the lower
the affinity of haemoglobin for oxygen
...
This is because dissolved carbon dioxide is acidic,
lowers pH and thus changes the shape of haemoglobin
...

Different organisms have different types of haemoglobin depending on the oxygen concentration of
the environment they liv in, their activity levels and their size (i
...
their metabolic rate, as animals
with a higher surface area to volume ratio have a higher metabolic rate)
...
Animals use muscular contractions, where as plants rely on
passive processes such as transpiration
...
Deoxygenated blood is passed to the heart from the body
...
The lungs then pass the oxygenated blood back into the
heart in order to increase its pressure
...


Atrium - Thin-walled and elastic to stretch as it
collects blood
Ventricle - Has thick, muscular walls as it has to
contract more strongly to pump blood a further
distance
Valves - Flexible, fibrous tissue that are cuspshaped to prevent the back flow of blood
Atrioventricular valves - The valves between the
atria and ventricles
Semi-lunar valves - The valves between the
ventricles and the aorta and pulmonary vein
Valve tendons (cords) - Attach the atrioventricular
valves to the ventricles to stop then being forced
up into the atria when the ventricles contract
...
As the
atria fill, the pressure rises
...
Because the
ventricles are relaxed, their pressure is lower than that in the aorta, so the semi-lunar valves
are closed
...
The ventricles remain
relaxed
...
This increases the pressure in the ventricles, so that
is greater than that in the aorta, forcing the atrioventricular valves shut and the semilunar
valves open
...

Cardiac disease Cardiac output can be measured by multiplying the stroke volume (volume of blood pumped in
each heartbeat) by heart rate (beats per minute)
...
g
...
Over
time the white blood cells, lipids and connective tissue build up and harden to form the fibrous
plaque
...
This can burst, causing a
haemorrhage
- Thrombosis; this is the formation of a blood clot due to an atheroma
...
These eventually form a thrombus, which can either block that
artery or become dislodged and block another vessel
- Myocardial infraction; the blockage of the coronary arteries, cutting off supply of blood to the
heart, resulting in irreversible damage
Risk factors for cardiovascular disease:
- High blood pressure increases the chance of atheroma formation
- High blood cholesterol increase fatty deposits in atheromas’
- High salt intake increases blood pressure
- Smoking reduces haemoglobin’s affinity for oxygen, result in reduced oxygen for the heart
...
There are no valves in arteries, as there
tends to be no back-flow
...
This tends to be at a lower pressure
...
This is so there can be greater
constriction of the lumen, controlling the supply of blood into the capillaries surrounding tissues
...

Veins transport blood slowly at low pressures from capillaries to the heart
...
The
thin wall also allows them to be flattened easily by skeletal muscle contractions, aiding the flow of
blood
...

The flow of blood in capillaries is much slower to allow for the exchange of materials between
blood and tissues
...
They are numerous and branched to increase their surface area
...
Networks
of capillaries in tissue are called capillary beds
...
At the same time, waste materials such as
carbon dioxide are collected from tissues
...
The increased pressure forces the tissue
fluid out of the blood by ultrafiltration
...

By the time the blood reaches the venous end of the capillary network its hydrostatic pressure is
lower than that of the tissue fluid, so tissue fluid moves down the pressure gradient and into the
capillaries
...

The rest of the tissue fluid is carried back by the lymphatic system
...


Transport in plants Water and mineral ions in solution move through the xylem vessels of a plant, from the roots to the
leaves
...

The humidity of the air surrounding the stomata is greater than that of the atmosphere, so water
vapour molecules diffuse out into the atmosphere
...

This is known as transpiration
...


This transpiration lowers the water potential of the mesophyll cells, creating a water potential
gradient that pulls water up from the xylem
...
This causes tension in the xylem vessel, giving rise to the name cohesiontension theory
...
The increased tension of the xylem causes the diameter
of tree trunks to shrink
- If a xylem vessel is broken water can no longer be drawn up
...

The movement of organic solutes and some mineral ions occurs in the phloem of a plant by a
process called translocation
...
Each
sieve tube has an associated companion cell
...
The solutes are sometimes referred to as assimilates
...
Hydrogen ions then diffuse through carrier
proteins into the sieve tube elements, co-transporting sucrose molecules
3) Sieve tubes have a lower water potential than the xylem, so water moves from the xylem into
the sieve tubes by osmosis, increasing the hydrostatic pressure
...
This lowers
their water potential, so water moves from the sieve tubes and into the respiring cells by
osmosis
...
Enzymes also maintain a
concentration gradient from the source to the sink by changing the solutes at the sink
Evidence for the mass flow hypothesis:
- Sap is released when sieve tubes are cut, so they are under pressure
- Sucrose concentrations are higher in leaves (sources) than roots (sinks)
- Downward flow in phloem ceases when leaves are shaded/at night
- Increased sucrose concentrations in the leaves are later followed by increased sucrose
concentrations in the phloem
- Metabolic poisons/anaerobic conditions inhibit translocation of sucrose in phloem
- Companion cells possess many mitochondria for ATP production
Evidence against the mass flow hypothesis:

- The function of sieve plates is unclear, as they should hinder mass flow
- Not all solutes move at the same speed, which would occur under mass flow
- Sucrose is delivered at the same rate to all regions, rather than reaching those with the lowest
concentration first

Transport can be investigated in plants using ringing experiments
...
When the protective bark and phloem are removed from the midsection of a tree, the
section above the missing ring begins to swell, where as those below wither and die
...
It is the phloem that transport organic molecules
and respiratory substrates
...

For example, radioactively labelled carbon dioxide using 14-C can be incorporated into the sugars
produced during photosynthesis
...
The blacked areas correspond to phloem tissue in the stem,
suggesting phloem is responsible for the translocation of sugars
...
Its position on a chromosome is known at its locus
Introns - Non-coding sequences of DNA
Exons - Coding sequences of DNA
Codon - The sequence of three bases on mRNA that codes for a single amino acid
Genome - The complete set of genes in a cell
Cellular proteome - All the proteins that can be produced by a given type of cell, at a given time,
under specific conditions
Complete proteome - The entire set of proteins that is/can be expressed by an organism
Allele - A different form of the same gene
...


DNA contains four bases - adenine, guanine, cytosine and thymine
...


The amino acid methionine is used as the start codon of polypeptides
...


Three triplets do not code for amino acids, but are rather stop codons and stop the synthesis of an
amino acid
...
These loops
form chromosomes
...

Homologous pairs of chromosomes are found in sexually reproduced organisms, where each
offspring has a chromosome from each parent
...

During meiosis the daughter cell receives one chromosome from each homologous pair
...

Alleles are alternative forms of genes affecting the phenotype of an organism
...

Ribonucleic acid is a polymer made up of mono-nucleotides
...
Uracil is complementary to adenine
...

Two types of RNA are used in protein synthesis; messenger RNA and transfer RNA
...
It
transports genetic information from the nucleus, through the pores in the nuclear envelope and
enters the cytoplasm
...

Transfer RNA is a small, single stranded, clover shaped molecule made up of around 80
nucleotides
...
The anticodon is a sequence of three organic bases
...

Protein synthesis 1
...
One of these strands is known as the template strand and pairs with complementary free
floating nucleotides (note at this point adenine binds to uracil, not thymine)
3
...
At the same time the
DNA strands reform
4
...
After transcription, the pre mRNA goes through splicing to remove non-coding introns
...
The mRNA leaves the nucleus through a nuclear pore and attaches to a ribosome at its starting
codon
7
...
The tRNA carries a specific amino acid
9
...
The two amino acids are joined by a peptide bond in an enzyme-catalysed reaction requiring
ATP
11
...
At the same time, tRNA detaches from joined amino acids, so is
free to collect more amino acids from the available pool in the cell
12
...
The polypeptide chain may then coil and fold to form a secondary, tertiary or quaternary
structure

Section 9: Genetic Diversity and Adaptation
Mutation - Any change to the quantity or the base sequence of the DNA of an organism
Diploid number (2n) - The number of chromosomes in an autosomal body cell; each cell contains
two of each chromosomes, one inherited from each parent
...
If the base codes for
the same amino acid the mutation is known as a silent mutation and has no phenotypic effect
- Deletion of bases

The deletion of a base from a DNA sequence causes a frameshift, so the DNA sequence is read
differently
- Chromosome mutations

Changes in the structure or number of whole chromosomes is known as a chromosome
mutation
...
If homologous pairs of
chromosomes don't separate during meiosis then the gametes have one more/less
chromosome
...

Genetic diversity During sexual reproduction two gametes fuse to give rise to a diploid zygote
...


Meiosis involves two nuclear divisions that occur immediately one after the other
...
This
causes tensions which break apart the chromatids
...
Each daughter cell of
the first division has one chromosome from each pair
...
Because of the crossing over in meiosis 1 and the independent segregation of
homologous chromosomes meiosis produces genetic variation
...
This
is excluding the possibility of crossing over and only takes independent segregation of homologous
chromosomes into account
...
It is increased through
mutation and gene flow (new genes being introduced to a population) and can be useful in helping
a species adapt to changes in their environment

Population - A group of individuals of the same species that live in the same habitat and can
interbreed
...
Within any population there is a gene pool containing a variety of alleles
...
The individual passes said alleles on to the next generation when they
reproduce; this process continues so that the number of individuals with the ‘advantageous allele’
becomes greater, leading to an adaptation in the species
...
Natural selection results in
adaptations that are either;
- Anatomical (structural)
- Physiological (metabolic processes)
- Behavioural
Selection is the process by which organisms that are better adapted to their environment survive
and breed, whereas those who aren’t don’t
...
g
...
Phenotypes of one
extreme of the population are being selected for, whereas the other extreme is being selected
against
- Stabilising selection

When environmental conditions are stable, individuals closest to the mean are favoured (e
...

human baby weights)
...



Section 10: Species and Taxonomy
Taxonomy Classification - The organisation of living organisms into groups
Phylogeny - The study of evolutionary history of groups of organisms, shown using a phylogenetic
tree
In the Binomial System organisms are identified by two name; the generic name (which starts with
a capital) followed by the specific name (which starts with a lower case letter)
...
This is based on evolutionary
relationships and places organisms in a hierarchy
...
g
...
Shared
features from common ancestors can be identified
...
Domain; which is subdivided into Bacteria, Archaea and Eukarya
...


Archaea are another type of unicellular prokaryote, but unlike bacteria they have no murein in
their cell walls, their DNA and RNA polymerase is more similar to that of eukaryotes and their
membranes contain glycerides
...
They possess
membrane bound organelles, glycerides within their membranes and 80s ribosomes
...

2
...

3
...
Class
5
...
Family
7
...
Species
Ways of classifying organisms -

- Courtship behaviours


Courtship behaviours enable organisms to recognise members of their own species, identify
fertility, form a pair bond, synchronise mating and become able to breed
...
e
...
When the
antibodies for an antigen from one species are inserted into the blood serum of another species
a precipitate is formed
...

Diversity Biodiversity - The number and variety of living organisms in a particular area
Species diversity - The number of different species and number of individuals of each species
living within one community (species richness)
Genetic diversity - The variety of genes possessed by the individuals that make up a population
Ecosystem diversity - The range of different habitats
Species richness - A measure of species diversity; it is the number of different species in a
particular area at a given time (a community)
...

Sampling bias can be reduced by using a larger sample
and statistically analysing the data collected (e
...
using
a normal distribution curve, the mean, the mode, etc
...
It
shows variety from the mean
...

Agriculture and diversity Agricultural ecosystems reduce biodiversity
...
The genetic variety of different alleles
possessed also decreases
...
The index of species diversity is low in
agricultural ecosystems
...
g
...
It is a
metabolic pathway that occurs in a series of small reactions controlled by enzymes
...


Leaves are adapted for photosynthesis because;
They have a large surface area to absorb for sunlight
They are arranged to minimise overlapping and overshadowing
They are thin to decrease the diffusion pathway for gases
They have a transparent cuticle to allow sunlight to penetrate the epidermis
Their photosynthetic upper mesophyll cells contain many chloroplasts
They have numerous stomata and many air spaces within the spongy mesophyll layer for
efficient gas exchange
- Stomata open and close in response to light intensity
- They have specialised mass transport systems; the xylem and phloem

-

Chloroplasts It is within the chloroplasts of the leaves that photosynthesis takes place
...


They are surrounded by a double membrane, and within them contain grana
...
There
are many types of photosynthetic pigments, such as chlorophyll a, chlorophyll b and carotenoids,
each of which collect different wavelengths of light
...
Photosystems collect light energy and pass it inwards to a main reaction
centre
...
Photosystem I absorbs light a 700 nm and photosystem II absorbs light at 680 nm
...


The stroma is the fluid-filled matrix within the chloroplast where the light-independent reaction
occurs
...

Chloroplasts are adapted for photosynthesis because;

- The thylakoid membranes provide a large surface area for the attachment of molecules involved
in the light-dependent reaction

- A network of proteins within the grana hold the chlorophyll in a manner that maximises light
absorption

- The granal membranes contain ATP synthase channels and are selectively permeable, allowing
the establishment of a proton gradient

- Chloroplasts contain DNA and ribosomes, so can easily manufacture proteins involved in the
light-dependent reaction
Coenzymes are molecules that aid the function of an enzyme by transferring a chemical group
from one molecule to another
...


The light-dependent reaction 1) Chlorophyll molecules absorb light energy, exciting a pair of electrons within them
...
This is known as
photoionisation
...
They are passed down a chain of
electron carriers in a series of oxidation-reduction reactions
...
As the electrons pass down the transfer chain from photosystem II to
photosystem I, they release energy
4) This energy is used to pump protons out of the thylakoid membrane and into the stroma
...
This catalyses the
formation of ATP from ADP and phosphate molecules
5) When the electron arrives at photosystem I it replaces another pair of electrons that have been
excited by light
...
This reduces NADP to NADPH, which is used in the light-independent
stages of photosynthesis
...
[NB:
photophosphorylation is the light-driven synthesis of ATP]

The light-independent reaction (The Calvin Cycle) 1) Carbon dioxide from the atmosphere diffuses into the leaf through the stomata and dissolves in
water around the walls of the mesophyll cells
...
This reaction produces two
3-carbon molecules of glycerine 3-phosphate (GP), as the 6-carbon compound is very unstable


3) NADPH is used to reduce GP to triose
phosphate (TP) using energy supplied by ATP
...
It would take 6 cycles of the
Calvin cycle to produce one molecule of glucose
The stroma is adapted for the light-independent
reaction because;
- The fluid contains all the enzymes and substrates need for the the Calvin cycle, and can
maintain these molecules
- The products of the light-dependent reaction can readily diffuse into the stroma
- It contains DNA and ribosomes, so can easily manufacture some of the proteins involved in the
Calvin cycle
Optimum conditions for photosynthesis:

- High light intensity of a certain wavelength
...
4%, otherwise the stomata start to close
- Water
These factors limit the rate of photosynthesis until they have reached their saturation point
...
In respiration the key coenzymes are NAD, coenzyme A and FAD
Stages of aerobic respiration 1) Glycolysis takes place in the cytoplasm
...
The phosphorylated glucose is more
reactive, as it lowers the activation for the splitting of glucose into two TP molecules
...
The two TP molecules are converted to
pyruvate (an acid), each regenerating two ATP molecules
...
Glycolysis can occur without
oxygen, and is the only stage in anaerobic respiration
...


The pyruvate is decarboxylated and oxidised to acetate, resulting in the loss of a carbon
dioxide molecule and two hydrogens
...


The two carbon acetate combines with coenzyme A to form acetyl-coenzyme A
...


3) The Krebs cycle also occurs in the mitochondria
...


This molecule is then decarboxylated and dehydrogenated to form a 5-carbon molecule and
carbon dioxide
...
Citrate has been converted to
oxaloacetate though substrate-level phosphorylation
...
One molecule of glucose therefore produces double these quantities
...


4) Oxidative phosphorylation is the final step of aerobic respiration
...


It occurs within the matrix and cristae of the mitochondria
...
These electrons are transferred
down a series of electron carrier molecules and are involved in a series of oxidation-reduction
reactions
...
After they
accumulate, they diffuse back into the mitochondrial matrix, driving ATP synthase as they do
so
...

If sugars are unavailable, other respiratory substrates, such as lipids and proteins can be used
...
The glycerol can be phosphorylated into TP
...
Because lipids release more hydrogen

atoms when they are broken down, there are more protons available for oxidative phosphorylation,
which is why lipids release double the energy as the same mass of carbohydrates and lipids
...
Following deamination they can be
converted into 3-, 4- and 5-carbon molecules
...
The pyruvate
is reduced to form either lactate or ethanol, as well as oxidised NAD
...

One glucose molecule makes 2 ATP during anaerobic respiration, where as it produces 32 through
aerobic respiration
...


Producers are photosynthetic organisms that manufacture organic substances using light energy,
water, carbon dioxide and mineral ions
...
The biological molecules that plants produce which aren’t respiratory
substrates form the biomass of the plant; this is how energy is transferred between organisms
...
Primary
consumers directly eat producers
...
Consumers may be predators,
scavengers or parasites
...
They break down the complex materials in dead
organisms, releasing minerals and elements in forms that can be absorbed by plants
...
Saprobiotic nutrition involves digesting food externally, which is
known as extracellular digestion
...
Each stage in the food chain is known as a
trophic level
...

Biomass Biomass is the total mass of living material in a specific area at a given time
...
The
chemical energy store within the dry mass is calculated using calorimetry
...

Dry mass is the biomass of an organism with the water removed
...
The mass of carbon is usually 50%
of the dry mass
...

Energy transfer Only 1% of the sunlights energy is absorbed by plants, because 90% is reflected back by clouds,
not all wavelengths of light are absorbed, light may not fall on the chlorophyll and photosynthesis
may be inhibited by a limiting factor
...

Net primary production (NPP) is equal to GPP minus respiratory losses (R)
...
It is also available for other trophic
levels
...

Because the energy passed on at each trophic level decreases, there are usually only 4 or 5
trophic levels
...

Nutrient cycles show cyclical flows of nutrients within an ecosystem
...
The producer passes on the nutrient to consumers
...
The nutrients are released for reuse
...
g
...
g
...


The nitrogen cycle -

Nitrogen is required by organisms to manufacture proteins and nuclei acids
...
Plants use nitrate ions, which they absorb from the soil through active transport
...


Animals obtain nitrogen from the nitrogen-containing compounds within the plants they eat
...
g
...
Saprobiontic microorganisms feed on dead faeces, releasing
ammonia, which then forms ammonium ions in the soil
2) Nitrification is the conversion of ammonium ions into nitrate ions through an oxidation reaction
...
Oxygen is needed for this reaction, which is why
farmers aerate soil through ploughing
3) Nitrogen fixation is the process by which nitrogen gas is converted into nitrogen-containing
compounds
...


Mutualistic nitrogen-fixing bacteria live in the nodules of roots of plants
...
This is carried out by denitrifying bacteria
...

The phosphorus cycle Phosphorus is an important component of ATP, phospholipids and nucleic acids
...

1) Weathering of rocks helps phosphate ions become dissolved
2) Dissolved phosphate ions can be absorbed by plants and are assimilated into the plants
biomass
3) Animals feed on these plants, transferring them into the food chain
4) Excess phosphate ions are excreted out (e
...
guano from sea birds)
5) When plants and animals die, decomposers break them down, releasing phosphate ions found
in shells and bones back into the soil
...
It
returns a significant amount of phosphate ions to soils in coastal areas
Mycorrhizae Mycorrhizae is a type of fungi made up of ling strands called hyphae, that act like extensions of
plant root systems
...
This can help improve the uptake of more scarce ions (e
...

phosphate ions)
...

Agricultural ecosystems Agricultural ecosystems are primarily concerned with improving yield
...
Farmers must use fertilisers to
improve nitrate availability of the soil
...

Organic fertilisers tend to be plant and animal waste, where as inorganic fertilisers are pellets
produced from mined rocks
...
Leaching is the process by which nutrients in
the soil are dissolved in water and are transported to watercourses
...
This is the process where high nitrate concentrations in
watercourses causes overgrowth of plants and algae within the water
...
Plants are
unable to photosynthesise so die, and are decomposed by saprobiontic bacteria
...
As more bacteria are respiring
aerobically, the oxygen concentration of the water decreases
...
The water becomes putrid
...
A motile organism responds to changes by either moving toward a favourable stimulus known as positive taxis - or away from an unfavourable one - known as negative taxis
...
The stimulus does however change
the speed of movement and rate at which an organism changes direction in order to reach more
favourable conditions
...

Tropisms Tropisms are growth of part of a plant in response to a directional stimulus
...

Phototropism - Response to light
Hydrotropism - Response to water
Geotropism - Response to gravity
Plant growth factors are produced in order for plants to respond to external stimuli
...
Unlike hormones, they are not always produced by organs and may
only affect direct tissues, rather than target organs
...
This auction controls plant cell elongation
...
Older plant cells have more rigid cell walls, so
are less able to respond
...

For example, a horizontally-growing root responds to gravity;
- IAA produced in the tip of the plant is transported along the root
- Initially this is done evenly, but gravity causes the movement of IAA to the lower side of the root
...

The peripheral nervous system is made up of the pairs of nerves that originate from with the brain
or spinal cord
...

The motor nervous system can be divided further into two subdivisions; the autonomic nervous
system
...
g
...
The voluntary nervous system carries voluntary impulses
to body muscles
...
The two subdivisions
work antagonistically
...
Cardiac muscle is myogenic rather than
neurogenic; contraction is generated from within the muscle itself and not nervous impulses
...
It
is known as the body’s pacemaker;
- A wave of electrical excitation spreads from the SAN across both atria, causing them to contract
- The non-conductive atrioventricular septum prevents the wave from entering the ventricles
- The wave spreads to the atrioventricular node (AVN) between the two atria
- After a short delay (to let the blood in the atria to drain from the atria into the ventricles), the AVN
sends a wave of electrical excitement through the Purkyne tissues in the Bundle of His
- The Bundle of His conducts the wave through the atrioventricular septum to the base of the
ventricles, causing them to contract from the base upwards
Heart is controlled by the medulla oblongata in the brain, which responds to internal stimuli from
pressure receptors and chemoreceptors
...

Blood pH is lowered by increased carbon dioxide concentration (e
...
during exercise which
increases the rate of respiration)
...
This increases
heart rate, so more carbon dioxide is expelled by the lungs
...

Pressure receptors are found in the walls of the carotid arteries and aorta
...


When blood pressure is too low, pressure receptors (baroreceptors) send signs to the medulla
oblongata to send impulses via the sympathetic nervous system to the SAN
...
A receptor detects a stimulus and generates a nerve impulse
2
...
A coordinator or relay neurone links the sensory neurone to the motor neurone in the spinal
cord
4
...
This can protect the body from harm
...
It transduces
the mechanical energy of the stimulus is into a generator potential
...


A single sensory neurone contains lamella separated by a gel
...


In the neurones resting state the channels are closed, but as pressure is applied to the neurone
they open
...
This increases their permeability to sodium ions, so more influxes into the
neurone, depolarising it
...



- Rod cells and cone cells 

These are the transducers in the eye that convert light energy in electrical impulses
(photorecpetors)
...
This bleaches the pigments, causing a chemical change and changing the
permeability of the membrane to sodium ions
...


Rod cells are found in the periphery part of the retina and are absent from the fovea
...

However, they are more numerous than cone cells and several rod cells are connected to one
bipolar cell
...
But the brain cannot distinguish the single sources of light, so they give
low visual acuity
...


There are three different types of cone cells, each of which responds to a different wavelength of
light
...


There are fewer cone cells, concentrated at the fovea of the retina
...


As iodopsin requires a higher intensity of light to be bleached and broken down before it creates
a generator potential
...
Even thought this means they don’t
respond to low levels of light intensity, it also means that the brain can distinguish where the
source of the stimulus is
...


Section 15: Nervous Coordination and Muscles
Coordination in animals There are two main forms of coordination in animals;
- The nervous system, which uses nerve cells to pass electrical impulses stimulating targets cells
by using neurotransmitters
...

- The hormonal system, which produces hormones that are transported in the blood stream to
their target organs
...


The response is slow, widespread and longer lasting
...

Neurones - Specialised cells adapted to rapidly carrying nerve impulses
Cell body - Contains the nucleus and rough endoplasmic reticulum associated with the production
of proteins and neurotransmitters
Dendrons - Extensions of the cells that that divide into dendrites, which carry nerve impulses
towards the cell body
Axon - A single long fibre that carriers nerve impulses away from the cell body
Schwann cells - Electrically insulating cells that surround the axon
...

Myelin sheath covers the axon
Nodes of Ranvier - Constrictions between adjacent Schwann cells where there is no myeline
sheath
Nerve impulse - A self-propagating wave of electrical activity that travels along the axon
membrane
...
g
...
The inside of the axon has a negative charge relative to the outside, so is therefore
polarised
...


Resting potential is maintained through the sodium-potassium pump;
1) Sodium and potassium ions cannot diffuse through the phospholipid bilayer of the axon plasma
membrane, so much move through voltage gated protein channels
2) Potassium ions are actively transported into the axon
3) Sodium ions are actively transported out of the axon
...
Sodium
diffuses back in, whilst potassium ions diffuse back out
5) Most of the gates that allow potassium out are open, whereas most of the gates that allow
sodium in are closed, thus creating a more negative charge within the axon
Action potential Action potentials occur when the axon is depolarised and the relative charges are reversed
...

1) At resting potential, some potassium voltage-gated channels are open, whereas sodium
voltage-gated channels are closed

2) The stimulus provides energy which causes some sodium voltage-gated channels to open,
increasing the permeability of the membrane to drum and allowing it to diffuse in
3) As more sodium diffuses into the axon, more sodium channels open, having a positive
feedback effect
4) Once an action potential of around +40 mV is established the sodium channels close and
potassium channels open, causing depolarisation of the axon
5) The outward diffusion of potassium ions cases a temporary overshoot of the electrical gradient,
causing the inside of the axon to be hyper-polarised
...

Another action potential cannot be generated during this time, because the sodium voltagegated channels remain closed during the refractory period
An action potential moves rapidly along an axon, maintaining the same size from each end
...
Due to the refractory period where the axon is repolarising, the direction of the nerve
impulse is unilateral and nerve impulses are discreet (i
...
separated)
...
The influx of sodium ions cause the opening of sodium
ions causes the opening of opening of sodium voltage-gated channels within the next region of the
axon
...
It is at the Nodes of Ranvier that action potentials occur
...
Essentially, the
action potential jumps between Nodes of Ranvier
...
The factors that affect speed are;
- The myelin sheath, which increases speed through saltatory conduction by around 3 times
- The diameter of the axon, as a greater diameter leads to less leakage of ions from the axon,
making it easier to maintain a membrane potential
- Temperature affects the rate of diffusion of ions and the enzymes which provide ATP for the
active transport of ions in the sodium-potassium pump
Nerve impulses are described as ‘all-or-nothing’, because a threshold value for a strength of a
stimulus must be reached before an action potential is generated
...

The difference in the size of stimulus is detected through the frequency of impulses and by having
neurones with different threshold values
...
When an action potential reaches the
synaptic knob, these vesicles fuse with the pre-synaptic membrane to release the neurotransmitter
Postsynaptic neurone - The neurone that receives the neurotransmitter and posses specific protein
receptors on its membrane
Synapses transmit information from neurones as chemicals called neurotransmitters
...


Temporal summation occurs when one presynaptic neurone releases neurotransmitter several time
over a given time period in order to make it accumulate to the threshold value
...

1) The presynaptic neurone releases a neurotransmitter that binds to chloride ion protein
channels, causing them to open and allow chloride ions to move in via facilitated diffusion
2) The binding of the neurotransmitter also causes potassium ion protein channels to open,
allowing potassium ions to diffuse out of the neurone
3) The postsynaptic membrane becomes more negative than the outside, causing hyper
polarisation and therefore increasing the number of sodium ions needed to reach the threshold
value and generate an action potential
The transmission of nerve impulses in the cholinergic synapse:
1) The arrival of an action potential in the end of the presynaptic neurone causes calcium ion
protein channels to open and diffuse into the synaptic knob
2) The calcium ions cause the synaptic vesicles to fuse with the presynaptic membrane, releasing
acetylcholine into the synaptic cleft
3) Acetylcholine binds to receptor sites on the sodium ion protein channels in the membrane of
the postsynaptic membrane, causing them to open and allow sodium ions to diffuse into the
postsynaptic neurone
4) This generate a new action potential in the neurone
5) Acetylcholinesterase hydrolyses acetylcholine into choline and ethanoic acid, which diffuse
back into the presynaptic neurone
...
Sodium ion protein channels close in the absence of
acetylcholine
6) ATP released by mitochondria is used to recombine choline and ethanoic acid into
acetylcholine, which is stored in the synaptic vesicles
Muscles Cardiac muscle - The muscle found in the heart
Smooth muscle - The muscle found in the walls of blood vessels
Skeletal muscles - Muscle attached to bone, which acts under voluntary control
Muscle fibres - Aggregations of muscle cells that share nuclei and sarcoplasm, which contains a
large concentration of mitochondria and RER
Slow-twitch muscle fibres - Contract more slowly and less powerfully but work over a longer period,
so are adapted to aerobic respiration by containing a large store of myoglobin, a rich supply of
blood vessels and numerous mitochondria
Fast-twitch muscle fibres - Contract more rapidly and produce powerful contractions for a short
period of time
...
They also contain a store of
phosphocreatine to quickly regenerate ATP from ADP
Myofibrils - Groups of muscle fibres that make up individual muscles
Myofibrils are made up of two types of protein filament; actin and myosin
...
Myosin consists of rod-shaped
tails with bulbous heads that intertwine
...

Tropomyosin is another protein found in muscles and forms a fibrous strand around actin filaments
...
They also run parallel to each other so the force is generated in one
direction
...

I bands - Isotropic bands where thick and thin filaments do not overlap
A bands - Anisotropic bands where thick and thin filaments do overlap

H-zone - This is found in the centre of the A band
Z-line - The centre of the I band where the tails of myosin molecules meet
Sarcomere - The distance between adjacent Z-lines
Muscle contractions A neuromuscular junction occurs where a motor neurone meets a skeletal muscle fibre
...

All muscle fibres that are supplied by a single motor neurone and act as a single unit are known as
a motor unit
...

Muscles work by moving the skeleton they are attached to
...
This is why muscles work in antagonistic pairs
...
This depolarises the muscle cell
3) The action potential generated travels through a system of T-tubules deep within the fibre
...
They then flex in unison,
pulling the actin filament along the myosin filament
...
The energy from the hydrolysis of ATP is
used to detach the myosin heads, which then return to their original angle
...
Myosin molecules
are joined tail to tail, so the sets of heads work in opposite directions, thus pulling the actin
filament in opposite directions
...
e
...
Energy is released through aerobic respiration slowly and the muscles have a
rich supply of blood vessels and mitochondria
...
Slow twitch fibres are rich
in myoglobin, which stores oxygen
...

Energy is quickly released through anaerobic respiration
...
There are few blood vessels, mitochondria
and myoglobin
...
It is monitored by
receptors
Receptors - detects a stimulus that is a deviation from the optimum point and informs the
coordinator
Coordinator - coordinates information from receptors and sends instructions to an appropriate
effector
Effector - brings about the change needed to return the system to the optimum point (e
...
a muscle
or gland)
Negative feedback system - when the change produced by the control system leads to a change in
the stimulus detected by the receptor and turns the system off; the system is returned to the
optimum level after a change to the norm, preventing overshoot (e
...
maintaining glucose
concentration)
...
g
...
Some positive
feedback systems occur when a control system stops functioning, such as with hypothermia
...

The human internal environment is made up of tissue fluids that bathe each cell
...
It has a composition similar to that of blood
plasma, but without the proteins
...

Homeostasis is vital for survival
...
Even if they aren’t denatured, then their rates of reaction may be lowered
...

Organisms that are able to maintain body temperature tend to have a wider geographical range
...
For example, the temperature receptors in skin and receptors in the hypothalamus
need to be coordinated so that body temperature isn’t raised or lowered too much
...

Temperature is maintained differently in different organisms
...
This
includes vasoconstriction/dilation, shivering, raising/lowering hair, sweating and changing
metabolic rate (by generating certain hormones, like thyroid stimulating hormone)
...

Ectotherms on the other hand obtain their heat from external sources, so mainly use behavioural
mechanisms to control body temperature
...


Regulation of blood glucose concentration Glucose is an essential respiratory substrate and provides a
source of energy for almost all organisms
...
Also,
fluctuations in glucose levels affect the osmotic potential of
the blood, which can cause issues such as dehydration
...
These changes are buffered
primarily by the hormones insulin, glucagon and adrenaline
...
There are two types of islet of
Langerhans; alpha cells, which produce glucagon
and beta cells, which produce insulin
The effects of these hormones takes places in the liver
...
The liver can store around 75g-100g of
glycogen, which can maintain blood glucose for around 12 hours at rest
• Glycogenolysis - the breakdown of glycogen to glucose, which is then diffused out of the liver
and into the blood stream
• Gluconeogenesis - the production of glucose from non-carbohydrate sources, primarily amino
acids and glycerol
Insulin is a globular protein made up of 51 amino acids arranged in two chains
...


Cells with glycoprotein receptors that are complementary and specific to insulin (i
...
almost all
cells, except red blood corpuscles) bind to insulin
...
It also causes glucose transporter proteins stored within vesicles in the
cytoplasm of the cell to fuse with the cell surface membrane, increasing the number of available
channel proteins
...

Finally, insulin causes enzymes to convert glucose into glycogen and fat
...
It is a negative feedback system because the lowering of blood
glucose concentration is detected by the beta cells of the islets of Langerhans, causing them
reduce insulin secretions
...
It attaches to specific protein receptors
on the cell surface membrane of liver cells, activating enzymes that catalyse the conversion of
glycogen, amino acids and glycerol into glucose
...

Insulin and glucagon work antagonistically
...
This way blood glucose levels remain within a certain threshold (remembering that there
is always a time lag between effect and response)
...
It is produced by
the medulla of adrenal glands during times of stress (e
...
exercise) and excitement
...

Adrenaline binds to the transmembrane protein receptors of liver cells
...
Within the
membrane, the enzyme adenyl cyclase is activated, which catalyses the conversion of ATP into
cAMP (cyclic AMP)
...
The active protein kinase enzyme catalyses glycogenolysis
...


Diabetes Diabetes is a metabolic disorder characterised by the inability to control blood glucose
concentrations
...
Some of the symptoms include excessive tiredness, thirst, hunger and urination
...

Type 1 - this type of diabetes is known as insulin dependent diabetes, as the body is unable to
produce insulin
...
It develops quickly over a few weeks, and tends to develop
during childhood
...
Carbohydrate intake and exercise levels need to be monitored
...
There may also
be inadequate supply of insulin from the pancreas
...
It develops more slowly, but accounts for 90% of
cases
...
Some drugs can slow glucose absorption
...
The overall gain in
water and ions should balance the overall loss
...
3 litres per day of water and produces around 0
...
To counteract this, around 1
...
4 litres is
expired, 0
...

It is the kidneys that carry out osmoregulation in mammals
...

Fibrous capsule - the outer membrane
that protects the kidney
Cortex - the outer section of the
kidney, which contains renal
Bowman’s capsules, convoluted
tubules and blood vessels
Medulla - the inner region of the
kidney made up of loops of Henle,
collecting ducts and blood vessels
Renal pelvis - funnel-shaped cavity
that collects urine into the ureter
Ureter - carry urine from the kidney to
bladder

• Renal Bowman’s capsule - this the start of the nephron
...
The inner layer 

is made up of podocytes
...
Its walls are made of epithelial cells with
microvilli
• Loop of Henle - a loop that extends from the cortex of the kidney into the medulla
• Distal convoluted tubule - similar to the proximal convoluted tubule, but is surrounded by fewer
capillaries
• Collecting duct - where the distal convoluted tubules converge to
...
It supplies the nephron with blood and forms a
glomerulus
...
The glomerulus recombine to form the efferent arteriole, which leaves the
renal capsule
...
They reabsorb mineral salts,
glucose and water
...

Osmoregulation

The role nephrons play in
osmoregulation occurs in a
series of stages;

1
...

The walls of the glomerular
capillaries are made up of
epithelial cells with pores
between them
...
This
squeezes out smaller water, glucose and mineral ions to form the glomerular filtrate
...
Proteins and red blood corpuscles are too large to be filtered
out of the Bowman’s capsule
...
Movement of filtrate from the glomerulus to the Bowman’s capsule

The movement of the glomerular filtrate into the renal capsule is resisted by connective tissue
and epithelial cells of the blood capillaries, epithelial cells of the renal capsule, hydrostatic
pressure within the renal capsule space and osmotic pressure from the low water potential in
the glomerulus
...
The endothelium of the
glomerular capillaries also has 100 nm spaces between cells
...
Selective reabsorption of glucose and water by the proximal convoluted tubule

Nearly 85% of the filtrate and 85% of water is reabsorbed in the proximal convoluted tubule
...


The tubules have microvilli to increase surface area, as well as infoldings at their bases to
provide a larger surface area to transfer substances into blood capillaries
...


Sodium ions are actively transported out of the lining of the proximal convoluted tubule and

carried away by blood capillaries
...
Consequently, co-transporter carrier proteins to facilitate the diffusion of sodium
and other useful substances (e
...
glucose, amino acids, chloride ions) from the lumen of the
proximal convoluted tubule into its epithelial lining cells
...


4
...


Sodium ions are actively transported out of the ascending limb of the loop of Henle
...
The water potential of the
interstitial fluid is lowered
...
The walls of the descending limb are, however, permeable to water, so
water moves out via osmosis, decreasing the water potential of the filtrate
...
Any water that passes out of the
descending limb and collecting duct are carried away by blood vessels
...


5
...
It absorbs water and salts, as
well as controlling pH of the blood by reabsorbing certain ions
...
The water
potential may lower if too little water is being consumed, many ions are being taken in or sweating
is occurring
...
When these cells then shrink the hypothalamus produces antidiuretic
hormone (ADH)
...

When it reaches the bloodstream within the kidneys, it binds to specific protein receptors on the
cell surface membrane of the distal convoluted tubule and collecting duct
...
Overall ADH increases the permeability of the tubule and
collecting duct to water, so it can pass out via osmosis
...

This encourages further osmosis of water out of the collecting duct
...
At this point nerve impulses are sent to the thirst
centre of the brain and when water potential is increased through more water intake, the pituitary
gland reduces the release of ADH - this is an example of a negative feedback mechanism
...


Section 17: Inherited Change
Genotype - The genetic constitution of an organism, describing all the alleles that that organism
has
Phenotype - The observable/biochemical characteristics of an organism, as a result of the
interaction between genotype expression and environment
Gene - A length of DNA or sequence of nucleotide bases that code for a particular polypeptide
...
Only one allele of a gene can occur at the locus of any
one chromosome
Homologous chromosome - The pairs of chromosomes that have the same gene loci, found in
diploid organisms
Homozygous - When the pair of alleles for a character are the same on each of the chromosomes
of an organism
Heterozygous - When the pair of alleles for a character are different on each of the chromosomes
of an organism
Dominant allele - The allele that expresses itself in the phenotype of a heterozygote
...
A
homozygous organism may be homozygous recessive
Codominant alleles - When both alleles contribute to the organisms phenotype so both features are
represented
Incomplete dominance - When both alleles contribute to the organisms phenotype, but represented
as a blend of the features
Multiple alleles - When a gene has more than two alleles
...

For example, there are three alleles determining blood group - A, B and O
...
Assuming
crossing over, the to genes should not separate during segregation, meaning the number of
possible outcomes for a heterozygous organisms gametes reduces from four to two
Epistasis - When one gene affects the expression of another in the phenotype (e
...
in mice one
gene controls the distribution of melanin pigment, where as another controls production - these
may interact)
...

Genetic diversity The law of segregation states that in diploid organisms, only one of each pair of alleles can be
present in a single gamete
...

In monohybrid inheritance, where heterozygous organisms are interbred, the phenotypic ratio is
1:2:1
...
This is known as the law of independent assortment each member of a pair of alleles may combine randomly with either of another pair
...

Sex-linked chromosomes Any gene carried on the X or Y chromosome is said to be sex-linked
...

This is why characteristics determined by the recessive allele on this non-homologous portion are
more likely to be inherited by males
...
An example of a
common sex-linked disease is haemophilia, where the body produces faulty clotting factor
...
Pedigree charts can be used
to understand the genotypes of families based on who has/had a sex-linked disease
...
The equation makes the assumption that the proportion
of dominant and recessive alleles of any gene in a population remains constant given these
conditions are met;
- No mutations arise
- The population is isolated
- There are no selective
pressures
- The population is large
- Mating occurs randomly

Causes of variation in phenotype - Mutations
- Independent segregation and crossing over in meiosis
- Random fertilisation of gametes in sexual reproduction
- Environmental influences
Natural selection and selective pressures Natural selection occurs when selection pressures (e
...
predication, disease and competition)
determine the frequency of alleles within the gene pool
...
For example, over-production of offspring does not occur
because increased population size leads to more intraspecific competition for limited resources;
the struggle to survive is increased
...
These individuals are more likely to
survive, and pass on their alleles to their offspring
...

Directional selection favours phenotypes that vary in one direction of the mean and is selective
against one extreme phenotype (e
...
the peppered moth)
...

Speciation Speciation is the evolution of a new species from existing ones
...
Members of the
same species are reproductively isolated from other species
...
The allele
frequencies of each population changes
...
They may
adapt to different environments as a result, leading to speciation
...


One of the main causes of speciation is genetic drift
...
The Founder effect occurs when a some of a population
leave a larger population
...
The
increased frequency of certain alleles increases their probability of being passed on
...
A genetic bottleneck has similar effects, but occurs when a large population is
reduced for at least one generation
...
g
...
g
...
g
...
g
...
g
...
g
...
No two species occupy the
same ecological niche due to the competitive exclusion principle
Abiotic factors Temperature
Light
pH
Water and humidity

-

Competition Intraspecific competition is competition within the same species
...
The greater the availability of resources,
the larger the population
...

Interspecific competition is competition between different species
...
If the conditions remain the same there will be complete removal of one species;
this is known as the competitive exclusion principle, which states the species that uses resources
most effectively will eliminate the other
...

A predator is an organism that feeds on another organism, which is known as the prey
...

The predator-prey relationship affects population sizes, which drives evolution as it provides
selective pressures;
1) The predators eat their prey, reducing their population size
2) There is greater competition for food amongst predators
3) The predators population size is reduce as some individuals are unable to obtain prey to
survive
4) Fewer prey are eaten, so can survive and reproduce, increasing their population size
5) More prey are available as food, so the predator population increase
Due to the range of foods that organisms eat in a natural ecosystem, fluctuations in population size
aren’t that severe
...

Investigation populations Populations are investigated using:
- Random sampling with frame/point quadrats

A point quadrat is a horizontal bar with hole at regular intervals into which a pin is dropped
...


A frame quadrat is a square frame divided into equally sized subdivisions
...


The size of the quadrat, the number of sample quadrats used and the position of the quadrants
affect the validity and reliability of the results
...
g
...

- Systematic sampling along a belt transect

This is used more frequently when there is a transition in the abundance of a species within an
area
...


Abundance can be measured by frequency or percentage coverage
...
it is
particularly useful for sampling the abundance of motile organisms, who could not be counted in
quadrats
...
They are released and after a certain
time interval, a sample of the same size are recaptured randomly
...


This method makes the following assumptions;
- The promotion of marked to unmarked individuals in the sample is the same as than in the
whole population
- The organisms from the first sample has enough time to evenly distribute themselves amongst
the remaining population
- The population has a marked boundary with no net migration
- There are few births and deaths within the population during the time of sampling
- The marking is non toxic and doesn’t affect their ability to survive (e
...
by make them more
conspicuous to predators)
- The mark is not rubbed off during the investigation
Succesion Succession describes the changes in species, over time, that occupy a particular area
...
Succession leads to;
- The abiotic factors becoming less hostile
- A greater number and variety of habitats and niches
- Increased biodiversity
- More complex food webs
- Increased biomass
Primary succession:
1) A pioneer species colonise an inhospitable environment
...
This releases nutrients into the soil,
making is less hospitable
3) This repeats until eventually a climax community is reached
...
The process is very slow
Secondary succession occurs when land that has already sustained organisms is suddenly altered
(e
...
due to a forest fire)
...


Conservation Conservation is the management of the Earth’s resources to ensure they are maintained in a
sustainable manner
...
The
reasons for conservation are;
- Personal
- Ethical
- Economic
- Cultural and aesthetic
Conservation may be achieved through managing succession, seed banks, captive breeding,
fishing quotas and maintaining protected areas
...
This can be done by
preventing a change to the next stage
...

The climax community for a particular climate is called its climatic climax
...
g
...
Types of
mutagenic agents include base analogs (act as bases), chemicals that alter/delete bases and
substances that alter DNA structure
Cell differentiation and specialisation - Differentiation of cells results from differential gene
expression of cells
...

Stem cells - Cells that retain the ability to differentiate into other cells
Types of gene mutation -

- Substitution of bases


-

-

-

When a nucleotide in a section of the DNA molecule is replaced by another nucleotide with a
different base
...


It may cause the formation of a different amino acid, which changes the primary and therefore
tertiary structure of the formed protein, again forming a non-functioning protein (e
...
an enzyme
with a no longer complementary active site)
...
The mutation will not be expressed in the organisms phenotype
...
The gene
is read in the wrong three-base group, forming a completely different non-functional polypeptide
...

If three bases are inserted there will not be a frame shift, so the impact on the organisms
phenotype may not be as considerable
...

Inversion of bases

The base order of a section of DNA is reversed, negatively affecting the amino acid sequence
produced
...
This type of mutation causes certain cancers
and reduce infertility
...
g
...
They can still differentiate into any type of body
cell

- Multipotent stem cells

Cells that can differentiate into multiple types of cells, within a range that is more limited than
pluripotent stem cells
...

- Unipotent stem cells

These are derived from multipotent stem cells and have the ability to differentiate into one type
of cell only (e
...
cardiomyocytes and spermatogonial stem cells)
Induced pluripotent stem cells (iPS cells) are a type of pluripotent stem cell produced from
unipotent stem cells
...
These four factors are introduced in the
form of a modified virus containing genes coding for transcriptional factors, and reactivate genes in
the adult cell so that its form and function is similar to that of an embryonic stem cell
...
These iPS cells may be able
to be used to repair damage cells due to accidents (e
...
burns) or disease (e
...
Parkinson’s)
...
These factors bind to a
specific DNA base sequence, causing mRNA to be produced
...

In eukaryotes, transcriptional factors, known as activators or repressors, bind to promoter regions
...
It combines to
the binding site on transcriptional factors, causing confirmation changes in shape
...
As oestrogen is lipid-soluble it diffuses through the cell surface membrane
2
...
This causes a conformational change in shape of the transcription factors DNA binding site, so
it can now bind to DNA, activating the gene
4
...
This stimulates transcription of that portion of DNA
Because oestrogen increases the rate of cell division and growth, it increases the risk of
oncogenes forming, which may lead to the development of breast cancer
...

Epigenetic control of gene expression Epigenetic's is the study of how external factors cause changes in the phenotype of an organism,
without altering the DNA base sequence
...
The epigenome affects the shape of the DNA-histone complex, resulting in changes in
activated genes and thus gene expression
...

Environmental signals stimulate certain proteins to attach to specific base sequences of the DNA
...
It decreases the positive charges on the histones, causing decreased attraction
between the histones and phosphate groups of DNA
...
The gene is thus switched on
...

- Methylation of DNA by attracting enzymes that can add or remove methyl groups
...
This prevents the
binding of transcriptional factors to the DNA, as well as attracting proteins that condense the

DNA-histone complex by inducing deacetylation, making the DNA inaccessible to transcription
factors
...
The decreased methylation in normal
cells causes uncontrolled cell growth, increasing the risk of chromosomal mutations during
replication
...
Methylation of promoter regions, which usually contains genes that
aid in DNA repair, means these genes become inactive and switched off
...

Epigenetic treatments involve reversing the epigenetic changes that cause disease
...

RNA Interference Small interfering RNA (siRNA) is a type of double-stranded RNA that breaks down transcribed
mRNA
...
Double stranded siRNA is broken down by an enzyme
2
...
The enzyme catalyses the break down of the mRNA into smaller sections, so is not translated
into a polypeptide - the gene is not expressed
Gene expression and cancer Benign tumours are abnormal cell growths that are non-cancerous
...
They produced adhesion molecules
between cells so tend to remain stuck together and compact
...
They can grow to a large size, but tend to not be as life-threatening as
malignant tumours
...
Their cells tend to be undifferentiated and DNA
is condensed within the nucleus
...
They have systemic effects on
the whole body and are more life-threatening than benign tumours
...
Cancer cells are
usually derived from a single mutant cell, causing uncontrolled mitosis
...
There are two main types of genes involved in
the development of cancer;
- Oncogenes

Oncogenes are mutations of protooncogenes - the genes that code for proteins that stimulate
DNA replication
...
These can cause the receptor
proteins on the cell-surface membrane to be permanently activated, so that cell division is
switched on despite the absence of growth factors
...
This type of mutation tends be be acquired, rather than
inherited
...
They are also
responsible for apoptosis
...
If BRCA1 is hypermethylated it tends to lead to
breast cancer
...


Section 21: Recombinant DNA
Recombinant DNA Recombinant DNA is when the DNA of two different organisms is combined to form a transgenic/
genetically modified organism
...

DNA fragments can be produced using:

- Reverse transcriptase to make complementary DNA from an mRNA template
- Restriction endonuclease enzymes, which recognise specific palindromic sequences
(recognition sequences) and cut DNA at these places

- A gene machine to create oligonucleotides

Steps of gene transfer and gene cloning 1) Isolation of the DNA fragments with the gene that codes for a specific protein

This can be done using reverse transcriptase to produce cDNA from mRNA, using restriction
endonuclease and creating the gene in a gene machine
...
mRNA
coding for the desired protein acts as a template for a single-stranded complementary copy of
DNA (cDNA) to be formed using reverse transcriptase
...


Restriction endonucleases are enzymes that cut DNA at recognition sequences
...
If the cuts occur at opposite base pairs the straight edge is known
as a blunt end
...
It is these unpaired bases that can be used to combine DNA from
two different organisms, using the enzyme DNA ligase
...
The original sequences must first be checked for biosafety and
biosecurity
...
Using sticky ends, the gene can be inserted into bacterial plasmids, which act as a
vector for the gene
...
These artificial genes can be
produced in as little as 10 days with great accuracy
...


DNA is prepared for insertion the fragment has promoter and terminator regions attached
...


Once the DNA fragment has been isolated and the promoter and terminator regions have been
added, the fragment can be inserted into a vector
...
DNA ligase then binds the plasmids and DNA fragments when they have been
incorporated
...
These calcium ions alongside changes in temperature affect the permeability of
the bacterial cell surface membrane, allowing the plasmids to enter the bacterias cytoplasm
...


4) Identification of successful host cells using gene markers

Marker genes are second, separate genes on the plasmid and they may be resistant to

antibiotics, make fluorescent proteins or produce identifiable enzymes
...
Bacteria that are able
to survive antibiotics in a medium show resistance, and therefore have the incorporated
plasmid
...
If it is, the plasmid containing the desired gene hasn’t
been incorporated
...
It produces the green
fluorescent protein (GFP)
...
Any bacterial cells without the desired gene will fluoresce
...


Enzyme markers are genes that produce enzymes, such as lactase
...
For example, lactase may be used as the gene
marker in a plasmid
...
Those bacterium that haven’t will be unable to change the colour of the
substrate
...
g
...
Tax polymerase is used in
PCR as it is thermostable and does not denature during the process
Primers - Short sequences of nucleotides that have a set of bases complementary to the ends of
the DNA fragments
Thermocycler - A computer-controlled machine that varies temperature precisely over a period of
time
In vitro cloning takes place outside of living organisms
...

1) The DNA fragments are mixed with primers, DNA polymerase and free nucleotides in a vessel
in a thermocycler
...

2) The mixture is cooled to 55 degrees Celsius allowing the primers to anneal to their
complementary bases at the ends of the DNA fragment
...
It also prevents the two strands from rejoining
...

Advantages of in vitro gene cloning - It is extremely rapid and 2n copies are made, where n is the number of cycles of PCR
...
g
...
DNA probes
and DNA hybridisation can be used to locate specific gene sequences
...
This
may be a radioactive label using 32-P in nucleotides or fluorescent labels
...
The DNA probe is made up of base sequences complementary to the
base sequence of the desired allele of a gene
...
Once the DNA probe has
annealed, the site where the DNA probe binds can be easily identified
...
It can also
help doctors to personalise treatments
...
By testing is a patient has these enzymes, a doctor can determine how effective a treatment
will be for someone
...

The comparison of the number of VNTRs at each place in the DNA between different people is
called genetic fingerprinting
...
This is done by by placing the
fragments in gel and covering them in an electrically conducting buffer solution
...
The shorter the DNA, the faster and further the
fragment travels

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