BIOLOGY

AS LEVEL REVISION BIOL2

Exchange Systems
Size and Surface Area
The larger an organism, the smaller its surface area to volume ratio is.
This means that larger organisms cannot rely on simple diffusion of nutrients into their cells from
the surroundings; they need dedicated gaseous exchange systems to provide nutrients for their
large volume of living cells.
Smaller organisms, like flatworms, can acquire all the oxygen and nutrients it needs by diffusion
from its surroundings due to its greater surface area to volume ratio.

Gas Exchange
Gas exchange in insects
Insects have a smaller surface area to volume ratio as they have a rigid exoskeleton covered in a
waterproof cuticle in order to minimise water loss.
This means that they require an internal system for gaseous exchange.

The tracheal system:
The trachea is supported by strengthened rings to
stop collapsing due to pressure when taking in air.
The spiracles are pores in the cuticle and have
valves to control water loss; they remain closed
mostly but open periodically for gas exchange.
The diffusion gradient is maintained by closing
the spiracles. When all the oxygen is used in
respiration, the oxygen concentration in the
tracheoles falls. This creates a diffusion gradient
for oxygen to diffuse in form the atmosphere
along the trachea to the tissues.
Tracheoles increase the surface area, and their walls are thin for a small diffusion pathway.
Insects have muscles which move air in and out of the trachea, creating a ventilation system which
speeds up diffusion.
This system relies on the diffusion of gases straight into respiring tissue. This limits the growth of the
insect, as the diffusion pathway must be very short for this system to function.
The build-up of a waste product of respiration, CO
2
, provokes the spiracles to open and release it.
Gaseous exchange in fish
Like insects, fish have waterproof outer coverings and a small surface area to volume ratio.

Fish therefore have gills as an internal gas exchange system.

Gills are ventilated as water enters the mouth through the
buccal cavity and the fish closes it and raises the floor of the
mouth. This decreases volume and increases the pressure,
eventually forcing water through the opercular cavity and over
the gills.

Water flows through the gills in one direction as opposed to the tidal system in humans. This is
because it requires less energy, as water is relatively dense compared to air and would require more
energy to force out again.
In the gill there are many gill filaments. This
gives the gills a large surface area to volume
ratio for faster gaseous exchange.

There are also many lamellae, thin structures
at right angles to the filaments, giving them a
larger surface area.

There are a large number of capillaries
adjoined to the gills, carrying away
oxygenated blood and maintaining a diffusion
gradient.

These all have thin epithelial walls, reducing the diffusion pathway for faster gaseous exchange.

Water passes through the gills in a counter-current system which maintains a concentration
gradient across the gill. Water moves across the gills in the opposite direction to the flow of blood,
so blood with the highest O
2
content lines up with water with the highest O
2
content and vice versa:
there is always more oxygen in the water than in the blood directly beneath it, and so equilibrium is
never reached diffusion is ongoing.







Gaseous exchange in plants

Plants require gas exchange for two main processes photosynthesis and respiration.

Gaseous exchange occurs through stomata, microscopic pores on the surfaces of leaves. These lead
into air spaces in the mesophyll layer of the cell.

The stomatal aperture can be controlled by the plant. Around the stomata are two guard cells,
specially adapted epidermal cells which have outer walls which are thinner and more elastic than the
inner walls. Thus, when the cells are turgid, they bend and open the stomata; when they are flaccid
the stomata are closed.

Leaves have thin, flat shapes to increase the
surface area to volume ratio, and also to
give a shorter diffusion pathway for gases.

In sunlight the stomata open fully to allow
CO
2
to diffuse in for photosynthesis. At
night the stomata are closed, as
photosynthesis cannot occur and open
stomata lead to water loss.

Water leaves plants through the stomata by
evaporating from the mesophyll cells in
transpiration.

Temperature, humidity and wind speed all affect the rate of transpiration.

Plants adapted to live in dry conditions are known as xerophytes. The features of xerophytes to suit
dry conditions are known as xeromorphic features.

Plants in cold conditions often have xeromorphic features: coniferous plants also have spines and
waxy cuticles. Deciduous plants lose their leaves in winter as low light intensities render the extra
photosynthesis from them inefficient.
















Mammalian blood transport

The circulatory system is used to transport nutrients and gases to cells by mass transport: the bulk
transport of substances from one area to another by pressure changes (initiated by the heart). It also
maintains concentration gradients and removes waste products.

Mammals have closed circulatory systems with a network of blood vessels: arteries carry blood away
from the heart, and lead to arterioles small, narrow-walled vessels connecting arteries to
capillaries. Capillaries are microscopic vessels forming networks in the bodys tissues. They are
connected via venules (narrow, vein-like vessels) to veins, which carry blood back to the heart.

Mammals also have double circulatory systems: blood passes through the heart twice in a single
circuit of the body. The pulmonary circulatory system transports between the heart and lungs; the
systemic circulatory system between the heart and the rest of the body. Each organ has a major
artery and vein connecting it to the heart:

Brain = jugular vein/carotid artery.
Liver = hepatic vein/artery.
Stomach and intestines = gastric and mesenteric arteries; deoxygenated blood from the stomach
and intestines passes through the hepatic portal vein to the liver before returning to the heart.
Kidneys = renal vein/artery.

Arteries, capillaries and veins

Arteries carry blood under high pressure away from the heart. They have the following features:

The lumen is relatively narrow to maintain a high pressure.
Thick layer of muscle fibres so that smaller arteries can be constricted (vasoconstriction) or
dilated (vasodilation) to control the blood-flow through them.
Thick layer of elastic fibres to maintain blood pressure in order for blood to reach
extremities. It stretches in time with each contraction of the heart (systole) and recoils when
the heart relaxes (diastole). This helps keep a relatively constant pressure.
No valves; the pressure of blood is too high for backflow.
Folded endothelial lining, to accommodate for the constriction and dilation of the arteries.

Arterioles are similarly structured, except they transport blood under
lower pressure than the arteries in order to control the blood-flow into
capillaries. This means their muscle layer is relatively thicker than the
arteries, as when it contracts, blood flow into capillaries is restricted and
controlled. The elastic layer is relatively thinner due to the lower pressure.

Veins carry blood back to the heart. They have the following features:

Wide lumen to allow least resistance to blood-flow.
Thin muscle layer and elastic layer, as blood-flow is under lower pressure, and controlling it
is not usually necessary.
Valves present to prevent backflow of blood.
Skeletal muscles which contract to raise pressure and force blood along.
Smooth endothelial lining to reduce friction against the blood-flow.
Capillaries allow metabolic materials to be exchanged between blood and tissues of the body. The
flow of blood in the capillaries is much slower to allow this to happen. It has the following features:

Very small lumen, wide enough to allow one red blood cell through at a time.
o The blood cells are squeezed against the ride of the capillary, reducing diffusion
pathway and greater friction allows more time for diffusion.
o Increases the surface area of the red blood cell with the capillary for faster diffusion
of oxygen.
Endothelial lining is one cell thick, reducing the diffusion pathway for faster diffusion.
Lining made up of flattened endothelial cells which again reduces the diffusion pathway.
Fenestrations, small gaps in the walls, increase permeability for molecules passing out of
capillaries.
Numerous there are large numbers of capillaries throughout the bodys tissues, giving a
large combined surface area.

Tissue fluid and lymph

Tissue fluid forms the immediate environment of most mammalian cells. It has specific chemical
makeup and temperature etc. to provide optimum conditions for the cells.

Tissue fluid forms because of the high hydrostatic pressure at the arterial end of capillaries. This
squeezes water and small molecules out of the capillaries in a process known as ultrafiltration.

Tissue fluid, because it comes from blood, has a similar composition to plasma, but without the
globular proteins these are too large to be squeezed out of the capillaries. This establishes a water
potential gradient between the capillaries and the tissue fluid, known as osmotic pressure.

At the venous end, the osmotic pressure is greater than the hydrostatic pressure, and fluid moves
back into the capillaries. Any excess tissue fluid drains into the lymphatic system.

















The lymphatic system is a second circulatory system made up of lymph vessels which are similar to
veins. Lymph, the fluid in the lymph vessels, is similar to tissue fluid but contains more fatty
substances and white blood cells. Lymph flows slowly through the lymphatic system by external
pressures such as from breathing or contraction of skeletal muscles. The lymph system re-joins the
circulatory system through ducts at the vena cava.
Roots and water transport

The roots of a plant extend underground in order to absorb vital water and nutrients.

The structure of a typical root is as follows:
A thin walled outer layer known as the
epidermis, upon which there are many
root hair cells. Water enters these cells
from the surrounding soil by osmosis.

The inner part of the cortex is the
endodermis, which is a single layer of
cells around the vascular bundle
containing the xylem and phloem. The
walls of the endodermal cells are
impermeable to water due to a waxy
waterproof lining this is called the
Casparian strip.

Water can take two possible routes from the root hair cells to the xylem:

The apoplastic pathway:

Water moves only through the cellulose cell walls of the cortex by cohesion tension of
water molecules.
The cellulose is made of fibres with water filled spaces to give little resistance.
However, once the water reaches the impermeable Casparian strip it cannot pass and is
forced to pass through the protoplast to join the symplastic pathway.

The symplastic pathway:

Water passes through the cytoplasm from cell to cell along the water potential gradient
maintained by root pressure by osmosis.
The cytoplasms of neighbouring cells are joined by fine tubes known as plasmodesmata
which form an unbroken water transport pathway.

Found within the cell membrane, are carrier proteins which allow ions, such as nitrate, to be moved
from the cortex into the xylem by the process of active transport.

As the dissolved ions are now found within the xylem vessel, this can be termed a hypertonic
solution with a higher water potential than the cortex cells. As a result, water moves from the
cortex into the xylem by the process of osmosis.

Cohesion tension

When water molecules evaporate from the stomata, the hydrogen bonds between water molecules
pull water up the xylem. This pull is transmitted all the way down through the roots, forming an
unbroken stream of water molecules. It is unbroken through the xylem due to adhesion from the
xylem walls.

Transpiration

The transpirational stream is the continuous passage of water from the roots of a plant up to the
leaves. The speed of the transpiration stream is dictated by the rate of evaporation of water out of
the plants exchange surfaces.

The rate of transpiration is affected by:

Temperature:

Increased temperature increases the kinetic energy of water molecules. This makes water
molecules move faster.
This increases the rate of evaporation and diffusion of water molecules out of the leaf.

Light intensity:

More stomata open to allow more carbon dioxide to diffuse the leaf for photosynthesis.
This means more water is able to diffuse out of the leaf as there is an increased surface area
for diffusion.

High wind speed:

Water vapour is removed quickly from outside the leaf by air movement.
This increases the water potential gradient between the moist air in the air spaces and
outside of the leaf. This increases the rate of diffusion of water out of the leaf.

High humidity:

Diffusion of water vapour out of the leaf slows down as the leaf is already surrounded by
moist air.
This is because the water potential gradient has been reduced (or removed).
This decreases the rate of diffusion of water out of the leaf.

Transpiration during the day

During the day there is increased light intensity and temperature, increasing the rate of
transpiration as detailed previously.

Increased evaporation and diffusion of water out of the leaves decreases the water potential of
mesophyll cells in the leaves. This increases the rate of osmosis between the xylem and mesophyll
cells.

This means there is more tension in the xylem as more water molecules are being drawn up to
replace them; this tension is caused by the cohesion between water molecules (due to hydrogen
bonds).

This reduces the diameter of the xylem vessels as there is more adhesion of water molecules to the
walls of the xylem, pulling them together and making them narrower.
Adaptations of the xylem vessel:

They are long cells/tubes with no end walls. This maintains continuous water columns.
They have no cytoplasm and no organelles which allows easier water flow.
They have lignin which provides support to withstand tension.
The lignin is waterproof to keep water in cells;
There are pits in walls to allow lateral movement.

Transpiration is not just used to provide water for photosynthesis; only 1% is used for this purpose.
The water can also be used for:

Keeping the cells turgid.
Acts as a solvent for transport.
Component of cell cytoplasm.
Medium for chemical reactions.
Keeping plant cells cool.

The rate of transpiration can be measured using a potometer:

A cut shoot is placed in one end of a capillary tube.
A single air bubble in the capillary tube is formed.
As water is drawn up through the shoot, the air bubble moves. The speed at which the air
bubble moves can be used to determine the rate of transpiration.

Xerophytes
Plants which live in hot/dry conditions and have adaptations to minimise water loss are known as
xerophytes. These adaptations include:
Adaptation How it reduces water loss by transpiration
Reduced number of
stomata
Reduces the surface area through which water can diffuse.
Leaves curled
Reduces surface area for water vapour loss & reduces the water
potential gradient by keeping moist air near the leaf
Thick waxy cuticle or stems
and leaves

Reduces evaporation through cuticle

Stomata in sunken pits
or grooves
Moist air is trapped close to leaf - reduces water potential gradient
Fleshy or succulent leaves
& stems
Water is stored in leaf and stem tissue so it is readily available it times
of low water availability
Leaf surface covered with
fine hairs
Moist air is trapped close to leaf - reduces water potential gradient
Leaves reduced to
spines or scales
Reduces surface area to volume ratio for evaporation and diffusion of
water vapour to occur over.
Deep root system
Deep roots to tap into lower water tables. Shallow roots to quickly
absorb surface moisture from overnight condensation
Cell Replication
Cell Cycle
This is a sequence of events carried out by every dividing eukaryotic cell after the cell cycle two
genetically identical copies are formed, used either for growth and repair or asexual reproduction.

The first three stages of the cell cycle are known as interphase and their key events are:

G1 phase Protein synthesis occurs.
S phase DNA is replicated, duplicating the chromosomes.
G2 phase Organelles grow.

Following interphase is mitosis the division of the cell to from two diploid daughter cells.

Mitosis
This is the process of nuclear division which forms two genetically identical daughter cells, carrying
the same number of chromosomes as the parent cells (diploid).

It is a continuous process, but is often arbitrarily divided into four phases:

Prophase:
The chromatin in the nucleus shortens and thickens (supercoils) into chromosomes.
They are visible as two sister chromatids joined by centromeres.
The nuclear membrane disintegrates and disappears.
The centriole divides in two and moves to the opposite poles. The daughter
centrioles form the spindle apparatus: this is a network of microtubules which help
to carry out mitosis.

Metaphase:
The centromeres of the sister chromatids line up along the equator of the cell.
The spindle fibres attach to the sister chromatids at the centromeres.

Anaphase:
The spindle fibres attached to the chromosomes contract and shorten.
This pulls the centromeres apart, causing them to split.
This causes the sister chromatids to separate and migrate as individual
chromosomes to the opposite poles of the cell.

Telophase:
Then the two sets of chromosomes reach opposite poles the nuclear envelopes
reform around them; the spindle apparatus disappears.
The chromosomes uncoil and are no longer visible.

Mitosis is normally followed by cytokinesis. The cell membrane pinches and separates into two
separate diploid cells.


Identifying stages of mitosis:











Mitosis and cancer

Cancer is a group of diseases caused by uncontrolled growth of cells.

It is caused by damage to the genes regulating mitosis and the cell cycle.


Treating cancer

Block the cell cycle to disrupt the cell division of new cancer cells using chemotherapy:

Prevent DNA from replicating (interphase) e.g. cisplatin.

Inhibit metaphase stage of mitosis by interfering with spindle formation e.g. vinca alkaloids.

Drug that prevents spindle fibres shortening (anaphase).
Meiosis

Meiosis is cell division to form haploid gametes for sexual reproduction a diploid cell divides twice
to form four gametes which are genetically different.

Meiosis begins with prophase 1 which is similar to
mitotic prophase. However, when the
chromosomes meet at the centre, the homologous
chromosomes cross to form bivalents, where
genetic information is exchanged between them
in a process known as genetic recombination.

The rest of the first division in meiosis is similar to
mitosis, except the chromosomes do not divide.
Instead, the homologous chromosomes separate
into two nuclei so that each cell contains only one
allele for each gene.

The process then repeats itself in prophase 2; the
nuclear envelopes disintegrate again and the cells
divide once more. This time the chromosomes
separate; four gametes are formed.

Meiosis is important as it ensures that when the two
haploid gametes fuse together, the resulting zygote is
diploid and retains the chromosome number of the
previous generation.

Genetic variation is ensured in the following ways:

Genetic recombination the genes recombine randomly to so each chromosome in the
gametes is different to those of the parent.

Independent assortment when the homologous chromosomes line up during the first
division they position themselves randomly there are millions of possible combinations of
chromosomes in each nucleus.

Random fusion of gametes a single organism produces millions of gametes, which could
hypothetically fuse with the gametes of any member of the same species, giving billions of
possible combinations.










DNA

What is DNA?

DNA stands for deoxyribonucleic acid, and is the chemical that determines our inherited
characteristics.

It contains huge amounts of information in the form of the genetic code.

DNA is found inside the nucleus of cells and is packed into chromosomes in eukaryotic cells.











DNA Structure

DNA is a made up of two strands of nucleotides, (monomers). These repeat many times to form
each polypeptide strand.
These strands form a double helix, with the backbone being composed of deoxyribose and
phosphate, and the internal rungs between the two chains are formed of nitrogenous base pairs.
DNA base pairs
DNA backbone
The double helix is coiled, and so a large amount of information can be stored in a small space. This
helps to ensure a uniform rung length.
The double helix structure is very stable. This means genetic information is protected from
corruption by chemicals and external forces by the deoxyribose-phosphate backbone.
Each nitrogenous base is complementary in shape to another:
Adenine always pairs with thymine.
Cytosine always pairs with guanine.
Nucleotides
Nucleotides are monomers of DNA; they join
adjacent nucleotides to form a polypeptide chain.
This occurs in condensation reactions.
When two strands of DNA come together to form a
double helix, the two complementary nitrogenous
bases on the nucleotides are joined together with
hydrogen bonds.
The A-T bond involves two hydrogen bonds forming.
The C-G bond involves three hydrogen bonds forming.
The nucleotides being bonded by hydrogen bonds is important as they are relatively weak, and so
can easily be broken to separate the strands for DNA replication or protein synthesis.

Base pairing rules
As stated previously, adenine always bonds with thymine, and cytosine with guanine.
Therefore in a section of DNA, the number of each complementary base is the same.
e.g. If 20% of the bases within a section of DNA are thymine, calculate the percentage of cytosine in
this section.
20% are thymine, and therefore 20% must be adenine.
This leaves 60% for cytosine and guanine; they must each be 30%.
Difference in DNA between eukaryotic and prokaryotic cells
Eukaryotic cells Prokaryotic cells
DNA molecules long DNA molecules relatively short
Form a line (linear) Form circles of DNA (plasmids)
Associated with proteins Not associated with protein molecules
Form chromosomes No chromosomes
Introns present No non-coding regions in DNA
Chromosomes
DNA is wound around proteins called histones, forming DNA-protein complexes. These are then
coiled and looped, and finally packed into chromosomes. They are found as pairs of homologous
chromosomes.
Homologous pairs contain the same type of genes; however they may have a different version of the
gene. Each different version of a gene is called an allele.

Key definitions:

Genes Short sections of DNA which code for particular characteristics.
Alleles Different forms of each gene.

Coding for proteins

Genes are sections of DNA which determine the sequence (and number) of amino acids in a
polypeptide chain.

This determines the hydrogen and ionic bonding of the protein, and how it folds, and hence the
tertiary structure of the protein.





















Sections of DNA are transcripted on to messenger RNA (mRNA).
This is then transported to the cytoplasm where the DNA is decoded by ribosomes.
The ribosomes translate the DNA code into amino acids, which then bond and fold to form proteins.
The sequence of amino acids determines the tertiary shape of the proteins.


A sequence of three bases in DNA is called a codon. Each codon codes for one amino acid in a
protein.











The genetic code



G A U U C G
G G A G C U
Asp
Gly
Ser
Ala
Mutations
Sometimes mutations can occur when translating the triplet code.
If the mutation means that a different amino acid is coded for an enzyme:
There is a change in sequence of amino acids.
Different, primary structure.
Different hydrogen/ionic bonding.
Alters tertiary structure of enzyme (and active site).
Therefore, substrate and active site no longer complementary in shape and cannot form E/S
complexes.
DNA replication
This occurs during S phase of interphase, and produces DNA in new cells which are identical to the
parental DNA.
When describing DNA replication the semi-conservative model is used:
Strands of DNA separate, as the hydrogen bonds are broken by DNA helicase enzyme.
Both/each strand act as template.
Free nucleotides attach to the strand, following complementary base pairing rules: A with T
and G with C.
DNA polymerase joins nucleotides together on the new strand.
DNA ligase joins the two strands together.
Hydrogen bonds reform.
This forms hybrid DNA one strand is parental whilst the other is new.
Experimental evidence for semi-conservative replication










Cells
Cells are the building blocks of every organism.
Cells can differentiate in multi-cellular organisms in order to carry out a particular function.
Groups of cells which work together to carry out the same function are known as tissues. Organs are
aggregations of tissues, and these organs are organised into systems.

Differences between plant cells and animal cell
As eukaryotic organisms, plants and animals share the following features in the ultrastructure of
their cells:
Membrane bound organelles e.g.
Nucleus
Mitochondria
Golgi apparatus
80s ribosomes
Plant cell Animal cell
Cellulose cell wall surrounding cell
alongside cell-surface membrane
Only cell-surface membrane present
Chloroplasts present in large numbers in
many leaf cells
Chloroplasts never present
Large, permanent vacuole filled with cell
sap
If vacuoles are present they are small and
scattered throughout the cell
Starch grains used for storage Glycogen granules are used for storage

The structure of a palisade cell
Palisade cells are found in the leaves and are used in photosynthesis, the conversion of carbon
dioxide and water into oxygen and glucose for use in respiration. It has the following adaptations:
Cell wall is thin and has a large surface area to volume ratio
in order to maximise CO
2
absorption.
Thin, elongated cells which form a continuous layer to
maximise sunlight absorption.
These cells contain chloroplasts, inside which
photosynthesis is carried out. By having many chloroplasts
in each cell, palisade cells have the following adaptations:
Many chloroplasts in order to maximise the surface area for
CO
2
absorption.
Many chloroplasts which can move in order to maximise light absorption.
Many chloroplasts with a range of pigments which absorb a range of wavelengths in order to
maximise light absorption.
Chloroplasts
Chloroplasts consist of:
Chloroplast envelope which is a double plasma membrane, highly selective above what can enter
and leave the chloroplast.
Grana are stacks of disc-like structures called thylakoids which:
Contain the photosynthetic pigment chlorophyll in order to trap sunlight.
Have a large surface area for the attachment of chlorophyll, electron carriers and enzymes
for the first light dependent stage of photosynthesis.
Stroma refers to the fluid-filled matrix where the second light independent stage of photosynthesis
occurs. It contains all the enzymes needed for carbon dioxide fixation as well as starch grains and
ribosomes etc.








Cell wall
Cell walls are made up of microfibrils of the polysaccharide cellulose.
There is a thin layer called the middle lamella which marks the boundary between adjacent cell
walls. This forms a strong material suited to its role to:
Provide mechanical strength to the plant.
Prevent cell bursting due to osmotic damage.
To allow water to pass along it (apoplastic pathway).





Carbohydrates
-glucose
This is a -glucose molecule:




They form glycosidic bonds in condensation reactions to form cellulose, a polysaccharide.



Polysaccharides
There are three key polysaccharides in eukaryotic organisms:
Starch
Cellulose
Glycogen
Animal cells Plant cells
Glycogen Starch
Cellulose

Starch
Starch is used as energy storage in plant cells, and is often found in the form of granules in the
cytoplasm.
It is formed of -glucose monomers, linked by glycosidic bonds in condensation reactions.
It is insoluble, and so does not affect the water potential of the cell leading to water loss by
osmosis.
It also forms a helical/spiral shape so that the long, unbranched chains of starch can be
compact, and store a lot of energy in a small amount of space.



Cellulose
Cellulose is used as energy storage in plant cells, but more importantly is also a major component in
cell walls, providing rigidity to cells.
It is formed of -glucose monomers, linked by glycosidic bonds in condensation reactions.
It also forms long, unbranched chains which run parallel to one another.
The position of the OH group is above the ring, meaning the molecules must rotate by 180
to form a glycosidic bond. This allows the CH
2
OH group to alternate between being above
and below each chain.
This allows many hydrogen bonds to form.
Individual hydrogen bonds are weak, however many bonds forming cross-linkages
between adjacent chains gives the microfibrils strength.
This structure is important for cellulose as it prevents plant cells bursting during osmosis, instead
enabling them to be turgid and maximise surface area for photosynthesis.

Glycogen
Glycogen is used as energy storage in animal cells, often found in the form of granules in the
cytoplasm.
It is formed of -glucose monomers, linked by glycosidic bonds in condensation reactions.
It is insoluble, and so does not affect the water potential of the cell leading to water loss by
osmosis.
It forms short, branched chains of -glucose, enabling it to be more readily hydrolysed into
glucose for respiration.
It also forms a compact structure, and can store a lot of energy/glucose in a small amount of
space.
The properties of the three carbohydrates are summarised below:
Carbohydrate Found in Monomer Structure Function
Starch
Plant cells
(cytoplasm)
-glucose
Long, straight, unbranched chains of
-glucose.
Chains linked by glycosidic bonds.
Wound into tight helical coils.
Energy storage
in plant cells.
Cellulose
Plant cells
(cell walls)
-glucose
Long, straight, unbranched chains of
-glucose running parallel.
Chains linked by glycosidic bonds.
Hydrogen bonds form between chains.
Energy storage
in plant cells
and also
provide rigidity
(cell wall).
Glycogen
Animal cells
(cytoplasm)
-glucose
Short, branched chains of -glucose.
Chains linked by glycosidic bonds.
Energy storage
in animal cells.
Haemoglobin
Haemoglobin is a quaternary structure protein comprised of four polypeptide chains linked
together to form a spherical molecule.
Each polypeptide has an associated haem group containing an Fe
2+
ion. Each can combine with one
molecule of oxygen to form oxyhaemoglobin.

Oxygen transportation
The role of haemoglobin is the transport of oxygen.
In order to do this it must have an affinity to oxygen: a tendency to associate with oxygen at the gas
exchange surface in order to bind to it.
Substances such as carbon dioxide change the shape of the haemoglobin, loosening its bond with
oxygen so that it is released at respiring tissues.

Partial pressure
The partial pressure is a measure of the concentration of a gas.
In relation to oxygen, partial pressure (p.O
2
) is important in determining whether oxygen is loaded or
unloaded from the haemoglobin.
At gas exchange surfaces, e.g. the alveoli: At respiring tissues, e.g. muscle tissue:
The p.O
2
is high.
The p.CO
2
is low.
Affinity of haemoglobin for oxygen is high.
Oxygen is loaded onto the haemoglobin.
The p.O
2
is low.
The p.CO
2
is high.
Affinity of haemoglobin for oxygen is low.
Oxygen is unloaded from the haemoglobin.

The loading, transport and unloading of oxygen can be shown on an oxygen dissociation curve:

The shape is sigmoid, as after the first O
2

molecule is loaded, the shape of
haemoglobin alters to increase its affinity
to oxygen this means the remaining
three oxygen molecules are more easily
loaded, making the graph steeper.
The graph then plateaus at higher partial
pressures as the haemoglobin is
becoming saturated with oxygen, making
it harder for oxygen molecules to bind.
Variation in haemoglobin
Different species have different types of haemoglobin that have different affinities to oxygen.
A reminder:
High affinity oxygen is taken up easily, but not released readily.
Low affinity oxygen is taken up less easily, but released more readily.
How is this caused?
Different animals have different DNA (sequence of bases).
Therefore a different sequence of amino acids.
Different primary structure.
Different folding and hydrogen and ionic bonding.
Different tertiary structure of polypeptide chains in haemoglobin.
Why would an animal need:
Low affinity:
Organisms with a high metabolic rate need to release more oxygen readily into tissues in
order to respire (provided there is a high p.O
2
).
Organisms with a large surface area to volume ratio, i.e. smaller organisms lose heat
rapidly, therefore need to release more oxygen readily into tissues in order to respire and
generate heat energy.
High affinity:
Organisms living in low partial pressures of oxygen need to bind with oxygen easily, and
release more slowly into tissues.
Haemoglobin saturated at lower p.O
2
so sufficient oxygen reaches tissues.
These haemoglobins would have different oxygen dissociation curves; higher affinity to the left,
lower affinity to the right.








Bohr Effect
The higher the levels of carbon dioxide in the blood, the lower the affinity to oxygen; this is the Bohr
Effect.
This shifts the oxygen dissociation curve right as it is harder for oxygen to be associated with
haemoglobin, but easier for it to be released.
This allows the release of oxygen at respiring tissues, where the level of carbon dioxide is high.









Explaining the Bohr Effect:
At the aerobically respiring tissues carbon dioxide is constantly being produced by respiring
cells.
When in solution, carbon dioxide forms carbonic acid which dissociates into H
+
and HCO
3
-

ions.
This increases the H
+
ion concentration (lactate produced in anaerobic respiration has the
same effect).
The blood becomes acidic as the pH is lower in the blood within the tissues.
The lower pH alters the shape of the haemoglobin into one which has a lower affinity for
oxygen.
Therefore, haemoglobin releases its oxygen into the respiring tissues (unloading) and the
oxyhaemoglobin dissociates at higher oxygen concentrations.





Principles of taxonomy
Taxonomy is the theory and practice of biological classification.
Classification is the organisation of living organisms into groups, which is necessary considering the
tens of millions of different species on Earth.

Hierarchical classification
This is a system of grouping organisms where larger groups are split into smaller groups based on
shared characteristics, and these groups do not overlap.
When classifying different organisms, they are placed into taxonomic groups in the following
hierarchy:
Kingdom
Phylum
Class
Order
Family
Genus
Species
K P C O F G S
The name of the species is the genus followed by the species in italics.
The first name, the generic name, is capitalised, but the specific name is lowercase.
e.g. Dasypus novemcinctus
Homo sapiens

Phylogeny
Phylogeny is the study of how closely related species are by looking at their evolutionary line of
descent/relationship.







Classification
levels become
more specific
towards the
bottom.
Species
A species may be defined in terms of observable similarities and the ability to produce fertile
offspring.
Members of a single species:
Are similar individuals/organisms to each other, but are different from members of the
same species.
Are capable of breeding to produce living, fertile offspring.
Possess similar genes, therefore closely resemble each other biochemically and physically.
Similar patterns of development and immunity.
Occupy the same ecological niche.
How to check if two organisms are of the same species:
See if they produce fertile offspring when mated.
Look at DNA and biochemical/physical characteristics.
Look at form and number of chromosomes.
See if they occupy the same ecological niche.
Look at fossil records or evolutionary history (phylogeny).















Genetic Comparisons
In evolution one species gives rise to another due to mutations in the sequence of bases in the DNA.
This means that different proteins are encoded for, and the features of the organism changes.
Therefore by comparing the DNA and proteins of different species we can determine the
evolutionary relationships between them.

DNA comparison
DNA sequencing can be used to directly compare organisms by looking for similarities in the order of
bases in their DNA.
Similarities in DNA may also be determined by DNA hybridisation:
Hybrid strands are formed of the same genes from different species. These are then heated to break
hydrogen bonds between them.
The more closely related the species are the more complementary nucleotide bases that they will
share.
The more complementary bases that they share the more hydrogen bonds will form.
The more hydrogen bonds that form the stronger the hybrid strand will be and the more
temperature it will take to separate the strand.
Therefore, the higher the temperature that the strands split, the more closely related that the
species are.









Protein comparison
The sequence of amino acids in two different species can reflect how closely related two species are.
The closer the amino acid sequence, the closer the relationship, as the amino acid sequence is
determined by the DNA/base sequence.

However comparing the sequence of bases is more useful as:
There are more bases than amino acids.
Degeneracy of the code there is more than one code for some amino acids.
There are introns in DNA which do not code for any amino acids.

Immunological comparisons may be also be used to compare variations in specific proteins.
Serum albumin from species A is injected into species B.
Species B will produce antibodies specific to all the antigen sites on the albumin from species A.
Serum is extracted from species B; this serum contains antibodies specific to antigens for species A.
Serum from species B is then mixed with the blood of another species.
The antibodies respond to the corresponding antigens on the albumin for this species.
The response is the formation of a precipitate.
The greater the number of similar antigens, the more precipitate formed and the more closely the
species are related.
The fewer the number of similar antigens, the less precipitate formed and the more distantly the
species are related.












In the above example we can tell that the baboon is more closely related as more precipitate has
formed, meaning more of its surface antigens are similar to species A.


Specific antibodies to
antigens on species A
Courtship Behaviour
Courtship behaviour as a necessary precursor to successful mating in order to produce offspring and
pass on their DNA for the survival of the species.
It is necessary as females only produce eggs at specific times, and courtship behaviour maximises
the chance of fertilisation.
How does courtship behaviour increase the probability of successful mating?
It allows species recognition only members of the same species can reproduce to make fertile
offspring.
Attracts and recognises a mate capable of breeding sexually mature, fertile and receptive to
mating.
Forms a pair bond which leads to successful mating and raising of offspring.
Synchronisation of mating maximising the chance of egg meeting sperm, stimulating the release of
gametes.
The male carries out an action (stimulus) specific to that
particular species (e.g. mating call). The response can either be:
Female responds with own action.
Male responds again (following courtship sequence).
The longer this ensues the more likely that mating will result.
Female does not respond.
Female is not receptive (may be different species/no fertile), so mating does not occur.
Male moves onto another female and this continues until mating ensues.
Courtship displays are also similar in different species which have close evolutionary relationships.










Variation
Variation can exist between different species this is interspecific variation.
It can, however, also exist between organisms of the same species this is intraspecific variation.
This can be caused by:
Meiosis formation of gametes:
Crossing over
Random assortment of chromosomes
Mutations of genes.
Fusion of gametes in sexual reproduction is random.
Variation can be caused by factors which are inherited, as a result of the environment, or both.
e.g. Eye colour is an inherited characteristic.
Accent or dialect is largely caused by the environment.
Height and weight is influenced by both inherited characteristics and environmental factors.

Investigative data
Data can either be discontinuous or continuous. Each type of data presents different forms of
variation:
Continuous variation Discontinuous variation



Continuous data has continuous variables on the
x-axis and takes the form of a histogram. The
data is grouped into different ranges.

This graph shows a normal distribution curve.
Continuous variation is:
Controlled by several genes.
Many phenotypes.
Influenced by the environment.

Discontinuous data has discrete variables on the
x-axis, and takes the form of a bar-chart.


Discontinuous variation is:
Controlled by one or very few genes.
Limited phenotypes
Little influence from the environment.
Most data has variation. It generally has a normal distribution, shown by the curve in a bell shape.







Standard deviation and range
Standard deviation shows the spread of data about the mean.
A large standard deviation is a large spread of data about the mean (a lot of variation).
A small standard deviation is a small spread of data about the mean (not much variation).
Range is the largest value minus the smallest value.
Using S.D. is preferable to range as:
Shows the spread of all data values about the mean.
Range just shows the highest and lowest values.
The range is affected by outliers.
Standard deviation allows for statistical use this allows you to tell if differences are
significant.

Sampling
Sampling involves taking measurements from individuals selected from the whole population.
The individuals selected must be representative of the whole population. To achieve this the
selection must be unbiased.
e.g. Random number generator to find co-ordinates for a section of a field to be sampled.

Twin studies
Twins are often used to show environmental and genetic differences.
Identical twins show genetic influence and any differences between them show environmental
influence.
Non-identical twins show an environmental / non-genetic influence.
If studies show that it is more common for both identical twins to get a disease, but not non-
identical, this shows that genes play greater role than environmental factors.
If studies show that it is more common for both non-identical twins to get a disease, but not
identical, this shows that the environment play a greater role than genetic factors.
























Genetic diversity
Genetic diversity is the number of different alleles in a population.
The greater the number of alleles between members of the same species, the greater the diversity
of the species. This means the more likely the species will survive in a changing environment.
Factors affecting genetic diversity
Selective breeding:
Individuals with desired alleles for the desired physical characteristics are selected and bred
to produce offspring.
These alleles increase in frequency in the population as some of the offspring acquire them.
The offspring which do not show the desired characteristics are killed/not allowed to breed,
deliberately eliminating non-selected alleles.
This reduces the variety/range of alleles in the population, meaning less genetic diversity.
This can be used to select high-yield bearing animals or plants on farms in order to maximise profit.
e.g. Cows which produce large volumes of milk.

The founder effect:
A few individuals from a population migrate and
colonise a new region, setting up a new population.

As there are a limited range of alleles from the old
population carried into this new population, this
population will have less genetic diversity.

If separated they may develop into a separate species over time.

Genetic bottlenecks:
The population of a species may suddenly drop in
numbers (e.g. due to volcanic activity) and only
certain members survive.

Survivor population have fewer alleles than the
original population.

This decreases the genetic diversity of the new
population stemmed from the survivors.


The impact of reducing genetic diversity is that if there is a change in conditions in the future, the
population is less likely to be able to survive and more likely to become extinct.
Arguments for and against selective breeding
For Against
It produces high-yielding animals fewer
need to be cultivated to meet demand.
It reduces genetic diversity, so that if there is a
change in conditions in the future e.g. diseases,
the population is less likely to be able to survive
and more likely to become extinct.

It can be used to produce animals and plants
with an increased resistance to disease.
Farmers need to use fewer drugs and
pesticides.
It can cause health problems; dairy cows have
shorter life expectancy due to problems caused
by carrying and producing so much extra milk.
Animals and plants could be bred to have
increased tolerance of extreme conditions
such as drought.




















Genetic variation in bacteria
DNA is the genetic material in bacteria as well as in most other organisms, and there is variation
amongst bacteria due to differences in DNA. This can either be caused by mutations or conjugations.
Mutations
Mutations are changes in the sequence of DNA due to one or more bases in DNA being deleted,
added or replaced during DNA replication.
This causes:
Sequences of bases to change.
Sequence of amino acids changes.
Different tertiary structure protein.
Different characteristics of the bacterium.
Conjugations
Genetic diversity in bacteria can also occur in conjugation, which is the transfer of DNA from one
bacterial cell to another.
One cell produces a thin projection (pilus)
which meets another bacterium and forms a
conjugation tube.

The conjugation tube joins the two cells, and
the plasmid of the donor cell starts to
replicate.

The circular DNA is broken to make it linear
before the replicated plasmid is transferred
through the conjugation tube and the strand
enters the recipient cell.

The linear strand forms a new plasmid, and
thus acquires new characteristics from the
donor cell.






Antibiotics
Antibiotics are drugs which prevent bacterial growth by:
Disrupting the formation of the cell wall (e.g. penicillin).
Interfering with protein synthesis.
Interfering with DNA replication.
Penicillin destroys bacterial cell walls and stops them from forming. This means bacteria no longer
can resist osmotic effect and swell and burst (lysis). This means that human cells, which have no cell
walls, are unaffected.

Antibiotic resistance
Bacteria can become resistant to antibiotics due to mutations or conjugations.
In the case of mutations:
There is a chance mutation in a bacterium which gives it an allele giving resistance to the
antibiotic.
This gives the bacterium a selective advantage.
When the antibiotic is applied, the other bacteria will be killed by the antibiotic, but that
bacterium with the chance advantageous mutation will survive.
The surviving resistant bacterium will reproduce to pass on the resistant allele to the next
generation via vertical gene transmission.
These antibiotic-resistant bacteria will gradually predominate in the population.
The frequency of the allele for antibiotic resistance increases in the population.
Repeated use of the same antibiotic becomes gradually less effective and increases the
proportion of bacteria with the resistant allele.
In the case of conjugations:
The allele for antibiotic resistance is carried in the
DNA plasmids.
These plasmids can be transferred from one
bacterium to another via conjugation (horizontal
gene transmission).
This can even lead to some bacteria accumulating
genes and give rise to superbugs which are
resistant to many antibiotics.
Bacteria can develop resistance to penicillin by producing the enzyme penicillinase which breaks
down penicillin in the antibiotic. It may instead develop a capsule and membrane complex which
does not allow penetration, and pumps out any penicillin.


MRSA is any strain of the Staphlococcus aureus bacterium which is resistant to multiple antibiotics.
This makes it highly prevalent and dangerous in hospitals as:
Patients are in close proximity and the bacteria can spread from person to person.
People are older/sick.
Many antibiotics are being used, so strains quickly become resistant to them all.
In order to prevent this from causing a problem:
Antibiotics should be used sparingly and only when necessary.
Patients must always complete their course of antibiotics.
Patients should not stockpile antibiotics from previous illnesses to use later on.




















Biodiversity
Biodiversity describes the variety of living organisms.
Species diversity refers to:
The number of different species.
The number of individuals of each species within any one community (i.e. the proportion of
the community that is made up of an individual species).
The higher the species diversity, the more stable the ecosystem and the more likely it is that it will
be able to cope with a change in climate.

Index of diversity
An index of diversity describes the relationship between the number of all species and the number
of individuals of each species in a community.
Calculation of an index of diversity from the formula:
d = N (N 1)
n (n 1)
N = total number of organisms of all species
n = total number of organisms of each species
= the sum of

Activities affecting species diversity
Activity How it affects species diversity
Agriculture
Less variety of plant species
Less habitats
Less ecological niches
Less food varieties
Less protection from predators
Deforestation
Less variety of plant species
Less habitats
Less ecological niches
Less food varieties
Less protection from predators
Planting
hedgerows
Greater variety of plant species
More habitats
More ecological niches
More food varieties
More protection from predators

Arguments for maintaining species diversity:
Prevents extinction of species.
Prevent loss of populations of species.
Some organisms may have medical/pharmaceutical uses.
Some organisms may have commercial uses.
Preserving organisms for future generations.
Prevents loss of habitats.
Useful for tourism.
Saving local forest communities.