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OCR AS Level Biology A
By
Mo Awe

MODULE 2
...

Maximum magnification = x1,500
Maximum resolution = 200nm
Scanning electron microscope:
We can see 3D structures and surface of the cell as electron are bounced of the surface
Maximum magnification = x100,000
Maximum resolution = 10nm
Transmission electron microscope:
We can see the internal structures as electrons are passes through
Maximum magnification = x500,000
Maximum resolution = 0
...
Therefore, multiple images can be combines to form a 3D structure
...

Making a permanent slide involves:
1
...

3
...


Fix using formadelyde
Dehydrate with ethanol
Impregnate with paraffin wax
Slice using a microtome

The use of staining in light microscopy
The main objective in staining is:
•
•
•

Make the object visible
Increase contrast so object can be seen more clearly
Use of differential staining to identify parts of tissue

Stains used in differential staining are eosin which stains the cytoplasm, sudan red which stains lipids,
iodine which stains cellulose cell wall yellow
...

The interrelationships between organelles in the role of protein production and secretion
1
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3
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The nucleus contains the gene for protein
Ribosomes is the site of protein synthesis
The sent to the Golgi apparatus in vesicles where it is further processed and packaged
Sent to plasma membrane where vesicle fuses in exocytosis

The similarities and differences between prokaryotic and eukaryotic cells
Similarities:
• Presence of plasma membrane, cytoplasm, ribosomes, DNA and RNA
Differences:
• Prokaryotes have no centrioles, nucleus, membrane bound organelles unlike eukaryotes
• Prokaryotes have smaller ribosomes known as 70s and cell wall is made from peptidoglycan
The ultrastructure of eukaryotic cells and the functions of the different cellular components + the
importance of the cytoskeleton
STRUCTURE
Nucleus, nuclear envelope and nucleolus
•

This is surrounded by a double
membrane known as the nuclear
envelope

Rough endoplasmic reticulum
•

Ribosomes are embedded in the
structure

Smooth endoplasmic reticulum

FUNCTION
• The nucleolus is the site where
ribosomes are made
• The pores allow mRNA to leave the
nucleus to be involved in translation
• The job of the nuclear membrane is to
separate the contents of the nucleus
from the rest of the cell
• Synthesis protein
• Cisternae transport substances from one
area of a cell to another area
•

Contain enzymes that are involved in
lipid metabolism

OCR AS Level Biology A
By
Mo Awe
• Lack of ribosomes on structure
Golgi apparatus
•

This is a series of cisternae but unlike
the endoplasmic reticulum, they are not
interconnected
Mitochondrion
•
•

A membrane bound organelle
Inner membrane is folded into cristae

•
•
•
•
•
•

Chloroplasts
•
•

A membrane bound organelle
Contain loops of DNA and starch
granules
• Stacks of thylakoids form grana which
are joined together by lamellae,
surrounded by the stroma
• Only found in plant cells
Vacuole

Lysosomes

•
•
•

•

They are formed from centrioles and
each contain microtubules
Ribosomes
•
•

Produced from ribosomal RNA
Synthesised in the nucleolus as two
subunits
• Present attached to RER or free in the
cytoplasm
Centrioles
•
•

Produced from bundles of microtubules
Microtubules are made of protein called
tubulin

Cytoskeleton
•

This is a series of protein threads made
from micro tubulin and microfilaments

Has own genetic material therefore can
replicate
The site of ATP production in aerobic
respiration
Common in cells that have a high
metabolic rate e
...
liver cells
The site for photosynthesis
Light dependent stage occurs in the
grana and light independent stage takes
place in the stroma
Common in leaf cells e
...
palisade
mesophyll layer

•

Important in maintaining turgidity as it
is filled with water and solutes as
pushes against cell wall

•

Responsible for keeping hydrolytic
enzymes separate from rest of cell

•

Cilia are found in airways to beat and
move mucus
Undulipodium present in sperm to allow
it to move
Ribosomes that are attached to the RER
are predominantly exported out of cell
Ribosomes free in cytoplasm are
usually for cell’s own use

•

Contain powerful hydrolytic enzymes
therefore abundant in neutrophils and
macrophages
• Formed from the Golgi apparatus
Cilia and Undulipodia

Synthesises lipids
Modify lipids and proteins delivered by
the endoplasmic reticulum
Manufacture lysosomes

•
•
•

•

Involved in making spindle required
during metaphase of mitosis and
meiosis
• Also, responsible for producing cilia
and undulipodia
• Centrioles are absent in plants
Extracellular Movement
Cilia and flagella moves cells
Intracellular Movement
Thread from tracks so organelles can move
Strengthening and support

OCR AS Level Biology A
By
Mo Awe
Strengthens and support organelles
• Prevents plant cells from bursting as
provides strength and support

Cellulose cell wall
•

Present in plants and made from
cellulose fibres

MODULE 2
...

Polar substances are attracted to each other by hydrogen
bonding
...

Lower density
Ice is less dense than water therefore floats which is useful as ice forms an insulating layer on
top of water so living things can survive below ice as temperature remains constant
Cohesion
Hydrogen bonds pull water molecule towards each other therefore water flows
...

Adhesion
Hydrogen bonds causes water molecules to be attracted to surfaces so water can move up
tubes with the help of cohesion
Tension
Hydrogen bonds pull water molecules inwards e
...
insects can skate
High specific heat capacity
This is the energy required to raise the temperature of 1g by 1 degree
...
Living things
needs a stable temperature for enzyme controlled reactions
High latent heat vaporisation

OCR AS Level Biology A
By
Mo Awe
A lot of energy is required to break hydrogen bonds so suitable coolant e
...
sweat evaporates
so cools the body
The concept of polymers and monomers and importance of condensation and hydrolysis
reactions
Monomers: small molecules that join together to from polymers
Polymers: large molecule made from smaller molecules known as monomers
A condensation reaction occurs when two molecules are joined together by the removal of
water
...

TYPE OF MOLECULE
Carbohydrates
Proteins
Nucleic acids

MONOMER
Monosaccharides
Amino acids
Nucleotides

POLYMER
Polysaccharides
Polypeptides or protein
DNA or RNA

The chemical elements that make up biological molecules
TYPE OF MOLECULE
Carbohydrates
Lipids
Proteins
Nulcleic acids

ELEMENTS PRESENT
C, H, O
C, H, O (lower proportion of oxygen than
carbohydrates)
C, H, O, N , S
C, H, O, N, P

The ring structure and properties of glucose as an example of a hexose monosaccharide
and the structure of ribose as an example of pentose monosaccharide

Glucose is known as a hexose
sugar as contains 6 carbons
...
The chemical bond
also contains a lot of energy

This is an example of a pentose sugar
as contains 5 carbons known as ribose

OCR AS Level Biology A
By
Mo Awe

The synthesis and breakdown of a disaccharide and a polysaccharide by the formation and
breakage of glyosidic bonds
Disaccharides are made by two
monosaccharides:
Alpha Glucose + Alpha Glucose à Maltose
Alpha Glucose + Fructose à Sucrose
Beta Galactose + Alpha glucose à Lactose
Beta Glucose + Beta Glucose à Cellobiose
When they join together in a condensation
reaction a bond known as a glycosidic bond is
created
...

The hydroxyl group is hidden inside the coil so less soluble, so does not affect water
potential
...

Amylopectin
This is a branched chain of alpha glucose joined by 1,4 and 1,6 glycosidic bonds
...
As the chain is branched, glucose can be
released quickly
...
The hydroxyl groups are exposed so can
form hydrogen bonds with adjacent molecules
...

Glycogen
This is a branched chain of alpha glucose
...

The structure of a triglyceride and a phospholipid as example of macromolecules
Macromolecules: A chain of small molecules formed from different monomer units

OCR AS Level Biology A
By
Mo Awe
Examples of lipids include triglycerides, phospholipids, waxes,
resin and steroids
Triglycerides are formed from glycerol and three fatty acids
called an ester bond, which results in the loss of three waters
...

The properties of triglycerides include:
•
•
•
•
•

Protects major organs
Provides insulating layer
Provides buoyancy as less dense than water
Energy source
When triglycerides are oxidized, they release hydrogen which bonds to oxygen to
from water known as metabolic water, which is useful for desert animals when there
is limited water
...
g
...
g
...
The part
hydrophilic heads and part hydrophobic tails leads to
an effective membrane barrier
Cholesterol
Cholesterol is small and hydrophobic which means it can bind to hydrophobic tails of the
phospholipid bilayer and therefore control fluidity
...

The general structure of an amino acid
Function of a protein:
•
•
•
•
•

enzyme
hormone
antibodies
structural e
...
collagen
functional e
...
amylase

Peptide bond:
When two amino acids join together via condensation reaction

OCR AS Level Biology A
By
Mo Awe
The levels of protein structure
Primary structure
Sequence type and number of amino acids and position of disulphide bond
Secondary structure
Particular shape by coiling or folding as a result of bonds which can lead to an alpha helix or
a beta pleated sheet
Alpha helix - Hydrogen bonds held between the NH and CO groups weak but a number of
them make them stable
Beta pleated sheet - Folds in concertina way with hydrogen bonds connecting adjacent
pleated sheets
Tertiary structure
3D shape of protein when secondary structure undergoes further coiling and folding
...
If protein twists into a long structure then
it is called a fibrous structure
Globular shape allows enzymes to have specifically shape active site
Fibrous protein plays structural roles e
...
keratin
What bonds are present in protein
•
•
•
•

Hydrogen bonds
Ionic bonds
Disulphide bonds formed between the R groups of two cysteine ---> strong covalent
Hydrophobic and hydrophilic interactions

Quaternary structure
When some proteins are made up of more than one polypeptide chain
The structure and function of globular protein
Haemoglobin has quaternary structure with haem group containing inorganic ion
4 polypeptide chains, two alpha chains and two beta chains
Haem is a prosthetic group (non-protein part that is permanently attached)
Haemoglobin is a conjugated protein as has a prostatic group attached to a globular protein
The structure and function of fibrous proteins

OCR AS Level Biology A
By
Mo Awe
Collagen
Provides strength e
...
in artery walls collages prevents vessels from bursting and present in
tendon that connect muscle to bone
...
Keratin is found in
fingernails, hair and horns
...

Elastin
Elastin is found in skin as well as in lungs to help deflate and inflate
...
3: NUCLEIC ACIDS

The structure of a nucleotide as the monomer of nucleic acids
Contains a pentose sugar which can be either ribose or deoxyribose, an organic nitrogenous
base (A,T,G,C) and a phosphate group
...
Purines are larger
than pyrimidines as have 2 carbon-nitrogen rings compare to one in pyrimidines
...

A and T form 2 bonds
G and C form 3 bonds
They synthesis and break down of polynucleotides and breakage of phosphodiester bonds
Nucleotides are joined together by a condensation reaction to form a polynucleotide
...

The structure of DNA

•
•
•

OCR AS Level Biology A
By
Mo Awe
Two polynucleotides are joined together by hydrogen bonds to from a double helix
Strands are anti parallel as run in different directions
Molecules is stable and integrity of coded information is protected

Semi-conservative replication

DNA helicase
unzips DNA
and therfore
breaks
hydrogen
bonds

Two separate
strands act as
a template for
a new strand

Free floating
nucleotides
join to
exposed bases
catalyses by
DNA
polymerase in
a
condensation
reaction

Hydrogen
bonds and
sugar
backbone
reforms

The nature of the genetic code
Degenerate: This means there are different codes for the same amino acids therefore if a
mutatiowhich blocks the apoplast pathway so
water must be diverted to symplast pathway for selective mineral uptake
Symplast pathway
Symplast pathway goes through cytoplasm from cell to cell via plasmodesmata
Adaptions of plant to availability of water
Xerophytes
•
•
•
•
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Reduced stomata
Stomata sunken in pits to shelter from air movement there lower diffusion rate
Stomata surrounded by hairs to trap water vapour lower diffuse rate
Waxy cuticle thicker to reduce water vapour loss
Leaf may be rolled to allow high humidity to build up inside coil

OCR AS Level Biology A
By
Mo Awe
Hydrophytes
•
•
•
•
•

Stomata on the upper surface to allow gas exchange to occur in air rather than water
Leaves have large air spaces which give them buoyancy
Floating leaves are thin + flat
Thin waxy cuticle
Reduced root system

The mechanism of mass flow
This is movement from a source to a sink
Source is leaves as produces sucrose and sink is an area that requires sucrose e
...
growing
points shoot

Active transport
loads solute into
sieve tubes of
phloem at source

Lowers water
potential therefore
water enters via
osmosis from xylem

Creates high
pressure inside sieve
tube (hydrostatic)
force sucrose away
from source and to
sink

The mechanisms of active loading
1
...
Facilitated diffusion of hydrogen ions back to companion cells
3
...
Sucrose diffuses through plasmodesmata from cc to sieve tube

OCR AS Level Biology A
By
Mo Awe
MODULE 4
...
Mucus and saliva will
carry the pathogen
Potato/tomato blight common during wet summers as spores need water and Malaria only
present in warm countries as mosquito needs warm weather
...
g
...
g
...
g
...
g
...
Examples include lupus and arthritis
...


OCR AS Level Biology A
By
Mo Awe
Vaccinations can be:
•
•
•
•

Dead organisms
Weakened organisms
Toxoid
Surface antigens

Herd vaccination – This is the vaccination of all members of a population
Ring vaccination - This is a response to an outbreak, all the people in the area surrounding
the outbreak are vaccinated
HIV/AIDS
1
...
DNA polymerase replicates it
3
...
2: BIODIVERSITY

How sampling is used in measuring biodiversity
Random sampling – This avoids bias by splitting sample areas into a grid and using a
random generator to choose coordinates
Non- random-sampling
Opportunistic – Using prior knowledge to choose sample sites

OCR AS Level Biology A
By
Mo Awe
Stratified – If there are sub habitats, samples can be taken from each
Systematic – This can be achieved by using a transect to carry out sampling at fixed intervals
Collecting random and non-random samples
Plants
Small plants can be counted using quadrats split into 50 by 50 grid so percentage cover can
be estimated
...
This is done by placing the apparatus in the
ground and recording number of species that touch the needles
...

Other ways include:
•
•
•
•

Use of pooter
Shaking a tree and placing white sheet on the ground
Sweep netting
Netting in pond

Measuring specie richness and evenness
Specie richness – The number of species in a habitat
Specie evenness – The abundance of each species in a habitat
The use and interpretation of Simpsons’ Index of Diversity
Simpson’s index measures biodiversity
...

D = 1 – ( ∑(n/N)2)

n is the number of individuals in a species
N is the number of species
High value
•
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•
•

High biodiversity
Stable environment
High specie evenness and richness
Habitat is worth conserving

OCR AS Level Biology A
By
Mo Awe

Low value
•
•
•
•

Low biodiversity
Unstable environment
Low specie evenness and richness
Habitat is possibly manmade

Howe genetic biodiversity may be assessed
High genetic diversity means there are lots of alleles for a given locus, which is known as a
polymorphic gene locus
Proportion of polymorphic gene loci = number of polymorphic gene loci/total number of loci
Factors affecting biodiversity
Human population growth – The rapid growth of population is putting a strain on resources
and available space
Increasing use of intensive agricultural methods – Specifically monoculture which
decreases biodiversity
Climate change – The release of waste pollutants e
...
carbon dioxide
The reasons to maintain biodiversity
Ecological reasons
• Keystone species are
important in the
workings of the
ecosystems
• The environment
provides us with
ecosystem services
e
...
soil fertility and
formation

Aesthetic reasons
• Provides jobs for
locals
• Ecotourism
• Relieve stress and
helps recovery of
injured people

Economic reasons
• We rely on bees to
pollinate crops
• Natural predators
reduce the need for
pesticides
• Source of medicine

In situ and ex situ conservation
In situ- conserving a habitat in its own habitat e
...
wildlife reserves and marine conservation
zones

ADVANTAGES
Organisms will act normally
Generates work for local people
Ecological tourism

DISADVANTAGES
Poaching or hunting cannot be controlled
Disease cannot be treated
There may be food shortages

OCR AS Level Biology A
By
Mo Awe
Cannot monitor health of organisms
Ex situ – conserving a habitat outside its normal environment e
...
zoos and botanic gardens
ADVANTAGES
Health can be monitored
Controlled breeding reduces the likelihood
of inbreeding which reduces genetic
variation

DISADVANTAGES
Organism may not mate
Unnatural habitat

International and local conservation agreements
The Convention on International Trade in Endangered Species
This is an international agreement between governments to monitor trade in wild animals and
specimen so does not affect their survival
The Rio Convention on Biological Diversity
This agreement promoted sustainable development and use of ex situ conservation
The Countryside Stewardship Scheme
This agreement entails looking after habitats and tradition character of the land

MODULE 4
...

The first name is the genus and the second name is the specific species name
...

The advantage is the fact that it avoids confusion all across the world if local names were
used
The features of five kingdoms
Cellular
Nucleus
Membrane
Bound
Organelles

Prokaryotes
Unicellular
Absent
Absent

Protoctista
Unicellular
Some
multicellular
Present
Present

Fungi
Acellular

Plantae
Animalia
Multicellular Multicellular

Present
Present

Present
Present

Present
present

OCR AS Level Biology A
By
Mo Awe
Cell wall
Peptidoglycan Present in
Chitin
many species
Nutrition
Heterotrophic Heterotrophic HeteroParasitic
Parasitic
trophic
Autotrophic
Autotrophic
Locomotion Some have
Some have
None
flagella
Undulipodium

Cellulose

None

Autotrophic

Heterotrophic

None

Muscular
tissue

Evidence that has led to new classification systems
Recent research in the sequence of bases in RNA of ribosomes has revealed that actually the
Prokaryote can be split into Archaea and Bacteria
Phylogeny
The evolutionary relationship between organisms and groups of organisms
Evidence for natural selection and evolution
Fossil evidence – Similarities between fossils can be used to reveal evolutionary
relationships
Molecular evidence – Closely related species have similar DNA and more distantly related
species would have difference in their DNA
Different types of variation
Intraspecific variation – Variation between members of the same species
Interspecific variation – Variation among different species
Continuous variation – There is not distinct group and is causes by genes and the
environment e
...
height and weight
Discontinuous variation – There are distinct groups and is caused by genes e
...
blood group
Anatomical adaption – These are associates with structure e
...
predators have sharp teeth to
kill prey
Physiological adaptions – These are associated with the working of the body system e
...

some C4 plants collect carbon dioxide at night so can keep their stomata close during the hot
day
Behavioral adaptions – These are associated with eating and mating e
...
plants close their
stomata to make their leaves wilt, so smaller surface area exposed to the sun

OCR AS Level Biology A
By
Mo Awe

The mechanism of natural selection
Genetic variation exists and the environment might change which is known as a selection
pressure
...
g
...


PRACTICALS

BIURET TEST
This is a test for protein
The test solution needs to be alkaline therefore sodium hydroxide added and copper II
sulphate added
Positive solution turns purple
Negative stays blue
BENEDICTS TEST FOR REDUCING SUGAR
This is a test for reducing sugar
Dissolve in water the add benedicts reagent and heat at 80 degrees for 3 minutes
A precipitate forms and positive colour change form blue to brick red
BENEDICTS TEST FOR NON-REDUCING SUGAR –
This is a test for non-reducing sugar e
...
sucrose
Hydrolyse the bond to free up the reducing groups
Dissolve in water and add dilute HCl and boil for 2 minutes, then neutralise by adding
sodium hydrogencarbonate solution then add benedicts reagent
A precipitate forms and positive colour change form blue to brick red

OCR AS Level Biology A
By
Mo Awe
IODINE TEST
This is a test for starch
Dissolve in water and add iodine solution
A positive change to deep blue from yellow brown as the triiodide ion slips in middle of
amylose helix causing colour change
EMULSION TEST
This is a test for lipids
Dissolve in alcohol and add water
Positive when water is added the clear filtrate will turn cloudy or milky
BIOSENSORS
Converts a chemical variable to an electrical signal
Glucose diffuses towards immobilised enzymes which catalyses a reaction that releases
hydrogen peroxide which reacts with platinum electrode to generate a current which is
proportional to glucose concentration
COLORIMETER
Use a centrifuge to separate precipitate from excess benedicts and pipette into cuvette then
placed in colorimeter
After each reading colorimeter needed to be reset e
...
water
Create calibration curve and plot glucose concentration on percentage transmission



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