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UNIT 2 BIOLOGY
Size and surface area:
Surface area: volume ratio – Affects how quickly substances are exchanged
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Single celled organisms – Substances can diffuse directly into cells so there is no need for an
exchange system
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Multicellular organisms – Diffusion across the outer membrane is too slow
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Difficult to exchange enough
substances to supply to animal with substantial amount
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Efficient system needed to carry substances to and from individual cells – mass
transport system
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Small organisms lose heat more easily
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Animals with high SA: V lose water more quickly as it evaporates from their surface
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Smaller mammals have thick layers of fur or hibernate when weather gets cold
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They have adaptations e
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elephants have large flat ears
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Each gill is made of lots of
gill filaments which gives it a large surface area for gas exchange
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Lamellae have lots of capillaries and a thin
surface layer of cells to speed up diffusion between water and blood
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Need to maximise rate of diffusion of oxygen from the water
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Only half the
available oxygen enters the blood
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Maintains a steep
concentration gradient
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Insects:

Insects can open and close their spiracles to alter level of ventilation/ prevent water loss
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Trachea supported by rings of chitilin whereas trachioles have thin permeable walls for gas
exchange
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No very large insects as the tracheal system doesn’t work above a certain size
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Spiracles are also sunken or have hair to trap hair
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Haemoglobin:
Human haemoglobin is found in the red blood cells – Transports oxygen around the body
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Each chain has a haem
group containing iron – gives read colour
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Oxygen joins to haemoglobin in the lungs to form oxyhaemoglobin – reversible reaction
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Oxygen joining haemoglobin – association/loading
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Affinity – Tendency for oxygen to bind to a molecule
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As pO2 increases haemoglobins affinity for
oxygen increases
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At high oxygen concentrations in the lungs where the curve levels off haemoglobin is almost fully
saturated with oxygen
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Steep drop due to large change in
percentage saturation
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Happens during respiration as it produces carbon dioxide and uses oxygen
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Happens at high altitudes and low
depths
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Unloads oxygen as
usual in tissue cells
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Walls are thick and muscular and have elastic tissue to cope with high pressures produced by
heartbeat
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All carry oxygenated blood but the pulmonary artery which carries deoxygenated blood to the lungs
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Form a network throughout the body – blood is directed to different areas of demand by muscle
inside arterioles which contract to restrict blood flow or relax to allow full blood flow
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Wider lumen than arteries with little muscle/ elastin tissue
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Blood flow helped by contraction of surrounding body muscles
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Capillaries:
Arterioles branch in capillaries
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They are adapted for efficient diffusion
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Walls are one cell thick – short diffusion pathway
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Tissue fluid:

Right side – hydrostatic pressure
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Forces fluid
out of the blood
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Force generated mainly by proteins in the plasma
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Tissue fluid is formed at the arteriole end (right) as hydrostatic pressure is greater than water
potential force
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Hydrostatic pressure drops
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Water leaves the
blood and the proteins create a low water potential
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Waste materials diffuse back into the blood e
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carbon dioxide
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The fluid passes onto progressively
larger lymph vessels before draining into the blood system in the upper part of the chest cavity
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Vacuole:
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Contains cell sap, a weak solution of sugar and salts
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Surrounded by a single membrane – take over large proportion of cell
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Turgor – swells and pushes against the cell wall making the cell turgid, providing
support
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Thylakoid – houses chlorophyll, the house of light dependent reactions
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Starch grains – contain glucose and starch is insoluble so doesn’t affect water
potential inside chloroplast
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Double membrane – Permeable to oxygen, carbon dioxide, glucose and some ions
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Found on the upper surface – pallaside cells under upper epidermis
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Polymer of B-glucose – straight chained
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Hydrogen bonds give it strength
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Herbivores have bacteria and protocists in their gut which can make cellulase
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Amylose – made from a-glucose
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Amylopectin/glycogen – Made from a-glucose and is branched because of 1-4 and 1-6
linkages
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Cellulose – made from B-glucose and has strong straight chained linkages held together by
hydrogen bonds which form parallel fibres which have great strength
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Thin – Short diffusion pathway
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Upper epidermis transparent – allows light to penetrate the pallaside layer
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Spongy mesophyll cells have large air spaces between them and contain chloroplasts –
allows gas exchange between atmosphere and cells
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Stomata – allow gas to enter and leave air spaces to and from the atmosphere
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Absorption of water:
Soil water has higher water potential than root hair cells
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Apoplast pathway – water moves through gaps in and around the cell walls – takes place
through living tissue
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Takes place through
living tissue
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Found around
endodermal cells in the roots – thin layer which separates xylem cells from root tissue
blocking the Apoplast pathway
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How water gets to leaves:
Root pressure – pushes water up the xylem
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If there is no transpiration or if it is slow ions are not transported up
the stem so there is a build up at the root of the xylem
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Water continuously enters and pushes a column
of water upwards
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Main driving source of transpiration which
pulls water up the plant
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A continuous column of water is pulled up from the roots
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Rate of transpiration:
Light – stomata open when it is light to allow gas exchange
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Temperature – warmer conditions create more kinetic energy which leads to faster
evaporation
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Wind – leads to faster transpiration as diffusion shells around the stomata get blown away
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Potometer – Leaf transpires which creates tension in the xylem
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The amount of water drawn in can be measured by the air bubble
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Reduction of leaves or spine needles – reduces surface area
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Hair on leaves/sunken stomata – reduces concentration gradient and traps moisture
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Rolled leaves – traps a layer of moisture and reduces surface area
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Long and deep or shallow and extensive root system – collect more water during dry
periods
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Mitosis – parent cell divides to produce two genetically identical daughter cells
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It is needed for growth of multicellular organisms and for repairing damaged tissue
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Separate strands of chromosomes are called chromatids
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Interphase:
G1: cells start to grow and new organelles and proteins are made
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G2: Cell keeps growing and proteins needed for cell division are made
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The Centrioles move to
opposite ends of the cell forming a spindle
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Metaphase – The chromosomes move to the centre of the cell and attach to the spindle by
their centromere
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The spindle contracts
pulling the chromatids to opposite poles of the cell
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A nuclear envelope forms around each group of
chromosomes so there are two nuclei
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Cancer:
Cancer is a tumour which invades surrounding cells
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Benign – none cancerous tumour
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Some treatments are designed to disrupt the cell cycle however they don’t distinguish
between normal and tumour cells
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Chemotherapy prevents synthesis of enzymes needed for DNA replication
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Radiation and some drugs damage DNA
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Genes and meiosis:
Genes – sections of DNA
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Exons code for proteins
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Introns are sections of DNA which do not code for proteins
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Introns are removed during protein synthesis
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Normal body cells – Diploid
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During fertilisation gametes fuse making a diploid cell called a zygote
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(SEE DIAGRAM FOR MEIOSIS)
Crossing over – during meiosis homologous pairs of chromosomes come together and pair
up
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The
chromatid now has a different combination of alleles which results in variation
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When the gametes are produced they have
different combinations of maternal and paternal chromosomes in each cell
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Polymers are made up of many monomers
linked together during a series of condensation reactions
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The sugar is pentose as it
contains five carbon atoms
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G pairing with C and A pairing with T
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DNA denatures at temperatures of 90*c
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DNA needs to be stable so that the code is not damaged and it needs to be compact so it
can store a lot of information
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Genes are polypeptides
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Their experiment used two isotopes of nitrogen, N14 (light nitrogen) and N15
(heavy nitrogen)
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As the bacteria reproduced they took up nitrogen
from the broth to help make nucleotides for new DNA
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A sample was taken from each batch and spun in a centrifuge
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Bacteria grown in the
heavy nitrogen broth were taken out and put in a broth containing light nitrogen
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The DNA sample settled in the middle
as it contained one strand of heavy nitrogen (old DNA) and one strand of light nitrogen (new
DNA) meaning semi-conservative replication had taken place
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The DNA
molecule is very long so has to be wound up to fit into the nucleus
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The protein also helps support DNA
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Prokaryotic cells – Molecules are short and circular
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Replication:
The enzyme DNA helicase breaks down hydrogen bonds between two polynucleotide DNA
strands
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Each single strand acts as a template for a new strand
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Nucleotides on the new strand are joined by the enzyme DNA polymerase
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Each new DNA strand contains one strand from the original DNA
molecule and one new strand
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Genotype – the genetic makeup of an organism
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Environment – environmental factors influence characteristics
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Phenotype – expressed characteristics
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Types of variation within species:
Discontinuous – Features that are usually controlled by single genes
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Continuous – features show a large range of values e
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Height, weight, intelligence
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This type is usually polygenic – it is controlled by several genes
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Bias – A particular proportion of the data is favoured
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In random sampling each individual of a population has an equal chance of being
selected as part of the sample
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Standard deviation – measure of the spread of data about the mean
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A gene mutation is a change in the base sequence of DNA
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This results in a different tertiary
structure
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Mutations that occur in body cells will only affect the cell the mutation occurred in and
resulting cells after division has occurred
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Mutagens – factors in the environment which affect mutation rate
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Variation is increased by crossing over, independent assortment and random fertilisation
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Identical/ monozygotic twins have exactly the same genotype so any differences between
them are likely to be environmental
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Any
differences between them are likely to be genetic
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There is variation in traits e
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some beetles are green, some brown
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There is difference in reproduction – since environment can’t support unlimited
population growth, not all individuals can reproduce to their full potential e
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green
beetles tend to get eaten more than brown beetles
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Heredity – surviving brown beetles have brown offspring, this trait has genetic bias
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End result – more advantageous trait allows brown beetles to have more offspring as
they are more common within the population
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Evolution – change in allele frequency
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Genetic drift – variation in allele frequencies
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The genetic diversity is reduced and inbreeding may occur which
reduces it further
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Pedigree – of known ancestry
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Inbreeding means closely related
individuals are more likely to have the same rare or recessive mutations
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Antibiotic resistance:
Antibiotics – kills bacteria in the body
Antiseptics – kill bacteria on the skin
Disinfectant – kills bacteria on surfaces
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Vertical gene transmission – bacteria pass on plasmid with resistant gene to offspring
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Bacterial conjugation – Pilus fuses with other bacterium, a copy of the plasmid is transferred
through the pilus, both cells have a copy of the plasmid containing antibiotic resistant gene
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MRSA – A bacteria that causes a range of illnesses from minor to life threatening
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Some individuals have a selective advantage making it more likely to survive than the
normal strain
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The fittest individuals are more likely to reproduce and
pass on its alleles to the next generation
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Selective breeding –
Inbreeding – mating between relatives
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Outbreeding – Mating between non-related individuals
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Decreases the chance of harmful double recessive genes being produced, hybrid vigour
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Select
offspring with the trait you desire and mate them with others who are also desirable
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Artificial selection produces high yielding as the plant with the largest seed is selected and
used to parent the next generation
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Ethical issues with selective breeding:
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Larger animals bred for their meat often suffer joint pain as bone size does not
increase as fast a muscle
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Chickens laying eggs everyday have a reduced lifespan
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Pets may suffer from inbreeding depression
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Elucidate – make clear
Composite – made up of several parts
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Classification is known as taxonomy
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When written they have to be in italics
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Phylogenetic system of classification – evolutionary history of groups of organisms which
tells is who is related and how closely related they are
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Diagram of taxonomic hierarchy:

Kingdom, phylum, class, order, family, genus, species
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Cytochrome C is an enzyme involved in respiration
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DNA hybridisation – Used to estimate the degree of similarity between two species
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DNA samples are taken from 2 species, cut into short sequences and heated to about
90*c
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2
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3
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4
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When two strands from the same species re-join they do so
at 87*C
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5
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Immunological studies:
1
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The animal will make
antibodies against the protein
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Extract and purify the antibody, then mix it with the same protein from species B
and measure the amount of precipitate formed
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If A and B are closely related, the proteins will be similar and there will be a strong
immune response and a lot of precipitate
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Strategies to ensure the same species mate include mating calls, pheromones,
displays/dances and nest building
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Courtship allows this space to be invaded without triggering
aggression
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Many species form pair bonds because it increases the chance of survival of the offspring
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? =

? (? − 1)
∑ ? (? − 1)

N = the total number of organisms of all species
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Two pieces of information needed to calculate a diversity index = what species are
present and how many of each species are present
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g
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Have specific sites of scientific interest that are protected
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It is done so timber can be sold and to make room for agriculture and roads
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Species become
extinct, or small populations become isolated and inbred
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Without massive transpiration rates of
forest means the water cycle gets disrupted which makes rainfall change
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Measures that could be taken place to prevent extinction:
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Ban on hunting
Create reserves
Captive breeding programmes
Ban international trade in endangered species e
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ivory

Many people believe having just one breeding pair of endangered species is enough, why is
this incorrect?
There is not enough genetic diversity
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Would
lead to inbreeding and genetic faults being paired up and expressed
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Some alleles are lost because they are not commercially useful at the time
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