Full set of revision notes for AQA Biology Unit 2

1. Unit 2 – Section 1
Variation
Sampling
• Involves taking measurements of individuals selected from the population of
organisms that's being investigated.
• In theory these individuals are representative of the population as a whole however
there are several reasons why this may not be the case:
▪ Bias – the selection process could be biased by the individuals taking the
sample by them making unrepresentative choices either deliberately or
unwittingly
▪ Chance – even if bias is avoided the individuals may be unrepresentative
due to pure chance
Prevent sampling bias by using random sampling:
• Divide the study area into a numbered grid – this can be achieved by using tape
measures
• Using a random numbers table obtain a set of coordinates
• Take a sample at the intersection of each pair of coordinates
Minimising the effect of chance:
• Use a large sample size
▪ The more individuals that are selected the smaller the probability that chance
will influence the result
▪ Greater the sample size the more reliable the data will be
• Analysis of the data collected
▪ Using stats tests to determine the extent to which the data has been
influenced by chance
▪ Tests allow us to decide whether any variation observed is the result of
chance or is more likely to have some other cause
Causes of variation:
• Mutations
▪ These sudden changes in DNA may or may not be passed on
• Meiosis
▪ This forms the gametes and mixes up which chromosomes end up in which
gamete so all are different
• Random fertilisation
▪ which sperm fuses with the egg in fertilisation is random therefore adds ot
the variety of characteristics the offspring have
Environmental influences:

2. • exerts an influence on all organisms
• Affect the way the organisms genes are expressed
• Environmental influences include:
▪ Climatic conditions (temperature, light intensity, rainfall etc.)
▪ Soil Conditions
▪ pH
▪ Food availability
Types of Variation
Variation due to genetic factors:
• All organisms fit into a few distinct groups (e.g. blood types A, B, AB and O)
• No intermediate types
• Usually controlled by a single gene
• Represented by a bar graph or pie chart
• Environmental factors have little influence on this type of variation
Variation due to environmental factors:
• Forms a continuum
• Examples in humans are height and mass
• Controlled by many genes
• Environmental factors determine where on the continuum the organism lies
• Represented by a normal distribution curve:
(image from tushar-mehta.com)
Mean and standard deviation & how these relate to the normal distribution curve:
• Mean
▪ maximum height of the curve
▪ average value
▪ Doesn't provide any information about the range of values
• Standard Deviation

3. ▪ measure of the width of the curve
▪ indication of the range of values either side of the mean
Standard Deviation → √∑ x − mean
2
n
don't forget the top line is actually: (x-mean)2
x= measured value from sample
n = total number of values in sample
Σ = sum of

4. Unit 2 : Section 2
DNA Nucleotide
made up of 3 components:
• Hexose sugar (deoxyribose)
• Phosphate group
• A base of which there are 4:
▪ Cytosine (C)
▪ Thymine (T)
▪ Adenine (A)
▪ Guanine (G)
Pairing of bases (the complimentary base-pair rule)
Base 1 Base 2 Number of hydrogen bonds
A T 2
G C 3
Remembering this:
A and T as letters are both made up of straight lines so go together. G and C are both made up of a
curved line so go together.
How we go from small nucleotides to the massive DNA double helix
• Nucleotides bond together in condensation reactions
• In these reactions the dexoyribose sugar from one nucleotide and the phosphate group from
another bond together
• This keeps happening and the structure formed from these linkages is called a sugar
phosphate backbone.
The overall structure
is a polynucleotide
• The bases stick out
from this and
following the
complimentary base-
pair rule another
polynucleotide joins to
the first one by
hydrogen bonding to
the bases.
• This other strand is
always upside down

5. Function of DNA
• Very stable so can pass from generation to generation without change
• Two separate strands joined by hydrogen bonds which allows them to separate for DNA
replication
• Very large molecule so carries a lot of genetic information
• The base pair rule protects the genetic information somewhat from corruption
What is a gene?
A section of DNA that codes for one polypeptide (protein)
Triplet code
This means that every 3 bases in DNA codes for one particular amino acid. Some amino acids have
multiple codes that make them. Every 3 bases is also called a codon.
The code is also non-overlapping as when the DNA code is read (for example in protein synthesis) 3
bases at a time are read and then you move on to the next set of 3 bases. You don't just move on
one base.
Chromosomes
• These are only visible as distinct structures during cell
division
• They are formed by the DNA helix combining with
proteins
• This then coils more and folds to form loops
• These loops then coil and pack together to form the
chromosome
Chromosome's occur in pairs. These are called homologous pairs as the chromosomes determine
the same genetic characteristics such as eye colour. This doesn't mean the chromosome’s are
identical however.
They occur in pairs as in sexual reproduction one chromosome comes from dad and one from mum
so they pair up. This is due to sex cell formation in meiosis as only half the number of chromosomes
are in the sex cells. There are normally 46 chromosomes in each body cell however there are only 23
in sex cells.
Alleles
These are different forms of the same gene. If the gene codes for eye colour then the alleles would

6. be blue, brown, green etc.
Each allele codes for a different polypeptide.

7. Unit 2 – Section 3
Genetic diversity
• Basically variation in the DNA of organisms
• All members of the same species have the same genes
◦ However there are different forms of these genes (eye colour – lots of different
colours = lots of different alleles) these different forms are called alleles
• The greater the number of different alleles a species has the greater the genetic
diversity of the species
◦ This means the species has a greater chance of adapting to survive change
The following are all factors that influence genetic diversity:
• Selective Breeding
• The founder effect
• Genetic bottlenecks
Selective Breeding
This involves selecting individuals with desired characteristics and mating them together.
Offspring that don't have these desired characteristics are killed or at least prevented from
breeding.
Impact on genetic diversity
This reduces genetic diversity as:
• Unwanted alleles are bred out of the population
• Therefore the variety of alleles in the population is deliberately reduced
• This leads to a population which has the desired characteristics but has much
reduced genetic diversity
This practice is commonly carried out by farmers to produce high yielding crops and
animals
The founder effect
• A few individuals from a population colonise a new region
• These individuals will only carry a small fraction of the gene pool (all the alleles a
species has collectively)
• The new population that develops will therefore show less genetic diversity than the
original population
• This is often seen on new volcanic islands for example
(c) myrevisionnotes

8. Genetic Bottlenecks
• Population suffers a drop in numbers (could be caused by a natural disaster)
• The survivors will only possess a small fraction of the gene pool
• The genetic diversity will therefore be less
• As the breed and re-establish the population the genetic diversity of this new
population will be restricted
• This means it is less likely the population can adapt and survive changes
(c) myrevisionnotes

9. (c) myrevisionnotes
Unit 2 – Section 4
Haemoglobin
Haemoglobin is a protein found in a wide variety of organisms. It has a primary, secondary,
tertiary and quaternary structure:
• Primary structure – consisting of amino acids peptide bonded together in 4 chains
• Secondary structure – in which each of the polypeptide chains are coiled into a helix
• Tertiary structure – each polypeptide chain is folded into a precise shape which is
important for carry oxygen
• Quaternary structure – in which all 4 polypeptides are linked together to form an almost
spherical structure
In addition to this each polypeptide is linked to a haem group which contains the Fe2+
ion. This
can bind to one oxygen molecule so each haemoglobin molecule can carry 4 oxygen
molecules in humans.
The role of haemoglobin:
The role of haemoglobin is to transport oxygen around the body to where it is needed. To be
efficient at this it must:
• Readily 'load' oxygen (association) at the gas exchange surfaces
• Readily 'unload' oxygen (dissociation) at the tissues where oxygen is required
Changing affinity and why this is important
Haemoglobin changes it's affinity for oxygen under certain circumstances this means that it
can load and unload oxygen where this is necessary:
Region of body Oxygen
concentration
Carbon dioxide
concentration
Affinity of
haemoglobin for
oxygen
Result
Gas Exchange
surface
High Low High Oxygen
association
Respiring tissues Low High Low Oxygen
Dissociation
Different organisms and Haemoglobin
There are different types of haemoglobin and these show different affinities for oxygen and
this can be an advantage to organisms living in different habitats
• Haemoglobin with a high affinity for oxygen – take up oxygen more easily but
release it less easily – would be an advantage for an organism in a low oxygen
environment

10. (c) myrevisionnotes
• Haemoglobin with a low affinity for oxygen – associates less easily with oxygen but
dissociates faster – advantageous for a very active organism
Oxygen dissociation curves
When haemoglobin is exposed to different partial pressures of oxygen haemoglobin doesn't
absorb oxygen evenly. The first molecule is difficult to absorb but then this forces the other 3
molecules to be absorbed a lot easier. The graph of this relationship is known as an oxygen
dissociation curve.
In the exam they could give any oxygen dissociation curve to be interpreted keep these in
mind:
Where is the curve? Affinity of
haemoglobin for
oxygen
Take up of oxygen Release of oxygen
Further to the left Higher Easier Harder
Further to the right Lower Harder Easier
Effects of carbon dioxide concentration
Haemoglobin has a reduced affinity for oxygen when carbon dioxide is present as the greater
the concentration of carbon dioxide the more readily the haemoglobin releases the oxygen it's
carrying.
• As the gas exchange surface (the lungs) – there are low levels of carbon dioxide as it
diffuses out the organism, this means that haemoglobin has a high affinity for oxygen
here. As there is also a high concentration of oxygen here means oxygen is readily
loaded
• In rapidly respiring tissues – the level of carbon dioxide is high therefore the affinity of
haemoglobin for oxygen is reduced and coupled with a low concentration of oxygen in
the muscles means that oxygen is readily unloaded from the haemoglobin into the
muscle cells
Loading, transport and unloading of oxygen
• At the gas exchange surface carbon dioxide is constantly being removed and due to
this the pH of the blood in this area is raised
• The higher pH changes the shape of haemoglobin meaning that it loads oxygen more
easily
• This shape also increases the affinity of haemoglobin for oxygen so that oxygen isn't
unloaded in the blood on it's way to the tissues
• In the tissues carbon dioxide is produced by respiration and as it diffuses into the blood
it makes the blood pH lower (makes the blood more acidic)

11. (c) myrevisionnotes
• This changes the shape of haemoglobin to a shape that means it has a lower affinity
for oxygen
• Haemoglobin therefore releases its oxygen into the respiring tissues
Starch, Glycogen and Cellulose

12. (c) myrevisionnotes
Starch
• Storage polysaccharide in plants it is found in many parts of the plant in the form of
small grains
• Adapted for this role as:
◦ Insoluble so doesn't dissolve in the water that is inside plant cells
◦ Compact – so a lot can be stored in a small space
◦ When hydrolysed forms α-glucose which can readily be transported around the
plant and used in respiration
• Storage molecule for plants and is never found in animal cells
Glycogen
• Very similar in structure to starch
• Same adaptations as starch
• Sometimes called 'animal starch' as it's the storage molecule in animals and is never
found in plants
Cellulose
• Unlike starch or glycogen it's a structural polysaccharide
• In order to bond together the adjacent β-glucose molecule has to rotate 180o
this
allows hydrogen bonding between the glucose chains and makes it strong
• major component of plant cell walls and provides rigidity
Comparison Table
Polysaccharide Monomers Bonding 3-D Structure
Starch α-glucose Glycosidic Coiled
Glycogen α-glucose Glycosidic Coiled
Cellulose β-glucose Glycosidic Sheets
They also all have a 1-4 linkage in the Glycosidic bond as on one glucose the bond starts
from carbon 1 and goes to carbon 4 on the neighbouring molecule.
Plant Cell Structure
Leaf palisade cell
• They are long thin cells that form a
continuous layer to absorb sunlight for
photosynthesis
• They have numerous chloroplasts that
arrange themselves in the best position to
collect the maximum amount of energy
• A large vacuole that pushes the cytoplasm
and chloroplasts to the edge of the cell

13. (c) myrevisionnotes
Chloroplasts
Main features of chloroplasts are:
• The chloroplast envelope – double plasma membrane that surrounds the organelle
and is highly selective as to what enters and leaves the organelle
• The grana – these are stacks of up to 100 disc like structures called thylakoids these
are where the first stage of photosynthesis takes place
• Thylakoids – these contain the photosynthetic pigment called chlorophyll, some of
these have extensions called lamella which join them to other thylakoids in adjacent
grana
• Stroma – this is a fluid filled matrix where the second stage of photosynthesis takes
place, within this there are a number of other structures such as starch grains
Cell wall
In plant cells this has the following features:
• Consists of a number of polysaccharides such as cellulose
• Thin layer called the middle lamella which marks the boundary between adjacent cells
and cements cells together
Functions of the cell wall are:
• To provide structure and strength to the cell to stop it bursting under the pressure
created when water enters by osmosis (osmotic pressure)
• To give structure to the plant as a whole
• To allow water to pass along it and therefore contribute to the passage of water
through the plant

14. (c) myrevisionnotes
Differences between plant and animal cells
Plant cells Animal cells
Cellulose cell wall and cell surface membrane Only a cell surface membrane
Chloroplasts are present in large numbers in
most cells
Chloroplasts are never present
Large single central vacuole filled with cell
sap
If there are vacuoles (rare) they are small and
scattered throughout the cell
Starch grains used for storage Glycogen granules are used for storage

15. Unit
2
–
Section
5
Cell
Division
Two
main
stages:
1. Nuclear
division
–
two
types
mitosis
and
meiosis
–
this
is
the
process
of
the
nucleus
dividing
2. Cell
division
–
follows
nuclear
division
and
is
the
process
of
the
whole
cell
splitting
into
two
Semi-­‐conservative
DNA
replication
• The
enzyme
DNA
helicase
breaks
the
hydrogen
bonds
linking
the
pairs
of
DNA
bases
• The
DNA
helix
then
separates
into
two
strands
and
unwinds
• Each
exposed
strand
then
acts
as
a
template
and
free
DNA
nucleotides
that
are
in
the
cytoplasm
join
to
these
template
stands
following
the
complimentary
base
pair
rule
(A-­‐T,
C-­‐G)
• Energy
is
needed
to
attach
these
free
nucleotides
to
the
template
strand
• These
are
then
joined
by
another
enzyme
called
DNA
polymerase
to
from
two
‘DNA
molecules’
from
the
one
that
was
present
in
the
cell
This
is
called
semi-­‐conservative
replication,
as
half
of
the
original
DNA
molecule
is
present
in
both
of
the
new
DNA
molecules
and
by
doing
this
the
two
strands
are
identical.
Mitosis
This
is
the
division
of
the
nucleus
of
the
cell
that
results
in
the
formation
of
two
identical
daughter
cells
with
each
of
them
having
an
exact
copy
of
the
parent
cell’s
DNA
5
Stages:
1. Interphase
–
The
DNA
replicates
and
the
cell
gets
ready
to
divide
by
synthesising
proteins
2. Prophase
–
The
chromosomes
become
visible
and
the
nuclear
envelope
disappears
3. Metaphase
–
The
chromosomes
arrange
themselves
at
the
centre
(equator)
of
the
cell
4. Anaphase
–
Spindle
fibres
pull
the
chromatids
towards
the
poles
of
the
cells
5. Telophase
-­‐
Nuclear
envelope
reforms
and
chromosomes
disappear
from
view
and
nucleolus
reforms

16.
Importance
of
mitosis
• Growth
–
When
a
sperm
and
egg
fuse
during
fertilisation
then
the
embryo
needs
to
grow.
All
cells
need
to
be
genetically
identical
as
they
all
need
to
have
a
full
set
of
genetic
information
to
form
the
new
organism
• Differentiation
–
Tissues
need
to
be
made
up
of
identical
specialised
cells
so
these
cells
divide
by
mitosis
• Repair
–
If
cells
are
damaged
it’s
important
they
are
replaced
with
identical
cells
that
have
the
same
structure
and
perform
the
same
function
The
Cell
Cycle
Cells
don’t
jut
keep
dividing
continuously;
rather
they
go
through
a
cycle
so
it
is
a
controlled
process

17.
• G1
–
Part
of
interphase
–
Proteins
to
synthesize
cell
organelles
are
produced
• S
phase
–
Part
of
interphase
–
when
DNA
replication
occurs
• G2
–
part
of
interphase
–
organelles
grow
and
divide
and
energy
stores
are
increased
• Mitosis

18. Unit
2
–
Section
6
Definitions
Term
Definition
Cell
differentiation/cell
specialisation
The
process
by
which
cells
become
adapted
for
their
job
within
the
body
Tissues
A
collection
of
similar
cells
that
perform
a
specific
function
Organs
A
combination
of
tissues
that
are
coordinated
to
perform
a
variety
of
functions,
however
they
usually
have
one
major
role
System
A
collection
of
organs
working
together
to
perform
a
particular
function
more
efficiently
How
do
cells
become
specialised?
• All
cells
have
all
your
genes
• However
only
some
of
these
are
switched
on
(expressed)
• Different
genes
are
switched
on
depending
on
what
type
of
cell
is
going
to
be
created
• Specialised
cells
have
different
shapes
but
also
different
numbers
of
organelles
such
as
mitochondria
–
as
muscle
cells
will
need
more
because
they
will
need
to
respire
more
to
produce
enough
energy
to
contract
and
relax
during
exercise
This
happens
because
cells
have
evolved
to
become
more
and
more
suited
for
a
particular
function
and
this
means
they
are
dependent
on
other
cells
to
carry
out
other
functions.
However
as
each
cell
is
adapted
for
its
particular
role
it
means
that
that
can
perform
this
role
more
effectively
so
the
organism
functions
more
efficiently.
Examples
of
Tissues,
Organs
and
Systems
Type
Examples
Tissue
Epithelial
tissues
Muscle
Connective
tissue
Organ
Heart
Lung
Stomach
Leaf
System
Circulatory
system
Gas
exchange
system
Digestive
system

19. 3.2.2 – Meiosis
Meiosis and Mitosis the difference
• Mitosis – Produces two genetically identical daughter cells with the same number of
chromosomes as the parent cell
• Meiosis – Produces 4 daughter cells each with half the number of chromosomes as
the parent cell
The stages of meiosis
1. The cell copies the chromosomes to form homologous pairs
2. These then line up at the centre of the cell and are pulled apart by spindle fibres
3. One copy of each chromosome goes into each of the two daughter cells
4. The chromosomes line up at the centre of these cells and are pulled apart again
5. This forms chromatids
6. One chromatid from each chromosome then goes into the two daughter cells
7. Don't forget this effectively happens twice as there are two cells initially with
chromosomes in
Meiosis 1