OCR Categories Archives: A Level

Transport in Animals #2

Transport in Animals

Transport – the movement of substances such as oxygen, nutrients, hormones, waste and heat around the body

Three factors influence the need for transport systems:

  • Size
  • Surface area/volume ratio
  • Level of metabolic activity

Features of a good transport system:

  • A fluid or medium to carry substances around the body e.g. blood, lymph
  • A pump to create pressure to push fluid round the body
  • Exchange surfaces that enable substances to enter the fluid (blood) e.g. capillaries
  • Tubes or vessels to carry the fluid by mass flow
  • Two circuits, one for collection of oxygen and another for the delivery of oxygen

Single circulatory system – one in which the blood flows through the heart only once for each circuit of the body e.g. heart -> gills -> body -> heart

Double circulatory system – one in which the blood flows through the heart twice for each circuit in the body e.g. heart -> body -> heart -> lungs -> heart


  • The blood pressure must not be too high in the pulmonary circuit so as not to damage the delicate capillaries
  • The heart can increase the pressure after it returns to the lungs so that the blood circulates through the body quickly
  • The systemic circulation can carry higher pressured blood than the pulmonary circulation

Blood Vessels


Artery Adaptions:

  • Artery walls are thick to withstand high pressure
  • The lumen is small to maintain the high pressure
  • The inner layer consists of a thin layer of elastic tissue that allows the walls to stretch and recoil
  • The middle layer consists of a thick layer of smooth muscle
  • The outer layer is a thick layer of collagen and elastic tissue for strength and to support the recoil for maintaining pressure.


  • Small blood vessels that distribute the blood from arteries to the capillaries
  • They have a layer of smooth muscle which contracts to increase resistance to flow and reduces the rate of blood flow
  • Constriction of arteriole walls is used to divert blood to regions of the body that are demanding oxygen


  • Very thin walls consisting of a single layer of endothelium
  • Narrow lumen to squeeze red blood cells against the wall to aid in the transfer of oxygen and reduce the diffusion distance
  • Leaky walls to allow blood plasma and dissolved substances to leave the blood (e.g. in lymph)


  • These collect blood from the capillary bed and lead into the veins
  • Consists of thin layers of muscle and elastic tissue as the pressure is relatively low


  • Carry blood back to the heart
  • Large lumen in order to ease the flow of blood
  • Walls are thin as they do not need to stretch and recoil and are not actively constricted in order to reduce the blood flow
  • They contain valves to prevent back flow of blood and help the blood flow into the heart properly

Blood plasma and tissue fluid

Plasma is the fluid portion of the blood which may move into the body, containing dissolved substances such as O2 and CO2. Tissue fluid is formed by blood plasma leaking from the capillaries into the tissues.

  1. t the arteriole end of the capillary bed the blood is at high hydrostatic pressure (the pressure a fluid exerts when pushing against the sides of a vessel or container
  2. This high hydrostatic pressure pushes the plasma out of the capillaries between the tiny gaps between the cells in the walls
  3. The fluid contains dissolved nutrients and oxygen but the cells and the platelets are too large to leave the blood system
  4. Not all the fluid returns to the blood, some is directed into the lymphatic system, which drains the excess tissue fluid out of the tissues and returns it to the blood system in the subclavian vein
  5. The lymphatic system contains many more lymphocytes which are produces in the lymph nodes

Oncotic pressure – the pressure created by the osmotic effects of the solutes and causes the movement of tissue fluid into the blood (it has a negative figure and is also measured in kPa)

Structure of the Heart


Blood Pressure

Atria – These chambers have relatively thin walls as they do not need to create much pressure

Right ventricle – thicker walls than the atria but still not as thick as the left ventricle as the blood only needs to be pumped as far as the lungs which lie next to the heart in the chest cavity

Left ventricle – two or three times thicker than the right ventricle as an inordinate amount of pressure must be created here to overcome the resistance of the systemic circulation

Cardiac Muscle Structure


  • They are divided into contractile units called sarcomeres
  • There are numerous mitochondria between the myofibrils (muscle fibres)
  • The intercalated discs facilitate synchronised contraction

The Cardiac Cycle

Blood enters the aorta and pulmonary artery in a rapid spurt but must be delivered in an even flow to prevent damage so the structure of the artery walls come into play. The smooth muscle and elastic fibre layers allow stretch and recoil with each beat of the heart which them lowers the pressure. The further along the blood flows into the arterioles the less obvious the fluctuations tend to be. It is important to maintain blood pressure so that the blood can travel all the way around the body and deliver nutrients to all the tissues.

Coordination of the cardiac cycle


The heart is myogenic as it can initiate its own contraction.

If the contractions of the atria and ventricles are not synchronised it is known as fibrillation.

The sino-atrial node generates a wave of excitation at regular intervals (55-80x per min).

The wave spreads over the walls of both atria causing the cardiac muscle cells to contract during atrial systole.

At the top of the atrioventricular septum, the atrioventricular node delays the signal before conducing it down the purkyne tissue and spreads out through the walls of both ventricles.

This means that they contract from the apex upwards, forcing the blood in the correct direction.


  • Wave P shows the excitation of the atria
  • QRS indicates the excitation o the ventricles
  • T shows diastole


Sinus rhythm – normal

Bradycardia – slow heart rate

Tachycardia – fast heart rate

Atrial fibrillation – atria beating faster than the ventricles

Ectopic heartbeat – irregular heart beat

Transport of Oxygen

Haemoglobin + oxygen à oxyhaemoglobin

Haemoglobin has a high affinity for oxygen and each of the four haem groups binds with one oxygen molecule

Oxygen is absorbed in the blood as it passes the alveoli in the lungs. The oxygen binds reversibly to the haemoglobin.

In the body tissue dissociation takes place and the oxyhaemoglobin releases the oxygen for the cells to use in aerobic respiration

The ability of haemoglobin to take up and release oxygen depends on the oxygen partial pressure. Association of oxygen and haemoglobin takes place when the partial pressure is high whereas the dissociation of oxygen and haemoglobin takes place when the partial pressure of oxygen is low.

Partial pressure symbol = pO2

Haemoglobin dissociation curve:









After the first oxygen binds to the haemoglobin the haemoglobin undergoes a slight conformational change which allows other oxygen molecules to more easily bond to its remaining haem groups.

Fetal haemoglobin lies to the left of the normal haemoglobin dissociation curve as fetal Haemoglobin has a higher oxygen affinity that normal haemoglobin as the fetal haemoglobin must cause the dissociation of the mother’s haemoglobin in the placenta and absorb oxygen from the surrounding fluid as well.

Transport of CO2

Carbon dioxide is transported in three ways:

  • 5% is dissolved directly in the plasma
  • 10% is combined directly with haemoglobin to form carbaminohaemoglobin
  • 85% is transported in the form of hydrogencarbonate ions

  1. Carbon dioxide diffuses into the red blood cell and combines with water to form carbonic acid (catalysed by carbonic anhydrase) CO2 + H2O à H2CO3
  2. The carbonic acid releases H+ ions and hydrogencarbonate ions.
  3. The hydrogen carbonate ions diffuse out of the cell into the plasma.
  4. Charge of the RBC is maintained by the chloride shift, where negative chloride ions move into the cell to adjust the charge.
  5. The hydrogen ions have the potential to make the RBC very acidic so the haemoglobin acts as a buffer and binds with the H+ ions to make haemoglobinic acid.

The Bohr Effect

Carbon dioxide concentration increases

The following increase in cell cytoplasm acidity causes changes in the tertiary structure of the haemoglobin and reduced its affinity for oxygen

This causes the dissociation of the oxygen to the tissues in order to provide them with it for respiration

This ensures that tissues that are respiring more such as contracting muscles get more oxygen in comparison to those not respiring as quickly/actively

The Bohr shift refers to the change in the haemoglobin dissociation curve which moves down and right when more CO2 is present.


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Classification and Evolution

4.3 Classification and Evolution

Biological Classification

Binomial system – a system that uses the genus name and the species name to avoid confusion when naming organism

Classification – the process of placing living things into groups

Reasons for classification:

  • For convenience
  • To make the study of living things easier
  • For easier identification
  • To show the relationships between species

Modern Classification Hierarchy

As you descend the taxonomic ranks from Domain à Species it becomes harder to distinguish and separate closely related organisms from each other and to place them accurately.

Reasons for the binomial naming system:

  • The same organism may have a completely different common name in different parts of a country
  • Different common names are used in different countries
  • Translation of languages and dialects may give different names
  • The same common name may be used for a different species in a different part of the world

Using observable features for classification

Species – a group of organisms that can freely interbreed to produce fertile offspring

This definition does not work for organisms that reproduce asexually and is very hard to apply to organisms known only from fossil records and the like.

Phylogenetic definition of species – a group of individual organisms that are very similar in appearance, anatomy, physiology, biochemistry and genetics

Early classification systems by Linnaeus and Aristotle were based solely on appearance and features which limited the classification to observable features only.

The Five Kingdoms

Evidence used in Classification

  • Biological Molecules

Some biological molecules, such as those for DNA replication and respiration are essential for life and therefore all living things have a variant that can be compared to show how closely related they are. If we assume that the earliest living common ancestors to living things had the same version of these molecules then any changes are a direct result of evolution.

  1. Cytochrome C

The protein cytochrome C is essential in respiration but is not identical in all species due to evolution. The sequences of amino acids in the protein can help draw conclusions about how closely related they are. If the sequences are the same then the two species must be closely related and if they are different they are not so closely related. The more differences found between the sequences, the less closely related the two species.

  1. RNA Polymerase

RNA Polymerase is also used as an indicator of evolution because of its essential role in protein synthesis


  • DNA
  1. Genome Sequencing

Advances in genome sequencing have meant that the entire base sequence of an organism’s DNA can be determined. The DNA sequence of one organism can then be compared to the DNA sequence of another organism. This will show you how closely related they are to each other.

  1. Comparing amino acid sequences

Proteins are made of amino acids. The sequence of amino acids in a protein is coded for by a base sequence in the DNA. Related organisms have similar DNA sequences and so similar amino acid sequences in their proteins.

 3. Immunological Comparisons

Similar proteins will bind to the same antibodies. So, if antibodies to a human version of a protein were added to isolated samples from other species, then any protein similar to the human version will be recognised and bind to the antibody.

Artificial Classification – classification for convenience, e.g. in plant identification books, sorting by flower colour

Natural Classification – Biological classification involving a detailed study of the individuals in a species, it uses many characteristics, reflects evolutionary relationships and may change with advancing knowledge

Phylogeny– the study of the evolutionary relationships between organisms

Divergent evolution is where another species has evolved from the original common ancestor and the two species get progressively less similar.

Convergent evolution is where two species, who may share the same environment and therefore the same factors that affect survival, evolve similar characteristics.

Natural Selection

Natural Selection – the term used to explain how features of the environment apply a selective force on the reproduction of individual in a population

Charles Darwin did not invent the theory of evolution but he proposed natural selection as a mechanism towards the theory. It was controversial at the time as it countered the popular religious beliefs.

Darwin developed his ideas from the expedition he sailed with the HMS Beagle around the Galapagos Islands. Wallace was another naturalist who came to the same conclusion as Darwin.

  1. Offspring generally appear similar to their parents
  2. No two individuals are identical
  3. Organisms have the ability to produce large numbers of offspring
  4. Populations in nature tend to remain fairly stable in size
  5. There is a struggle to survive
  6. Better adapted individuals survive and pass on their characteristics
  7. Over time a number of changes may give rise to a new species

Fossil Evidence

In the past the world was inhabited by species that were different from those present today.

Old species have dies out and new species have arisen.

The new species that have appeared are often similar to the older ones found in the same place.

One of the most complete fossil records is that of the horse.



Variation – the presence of variety – the differences between individuals

Intraspecific variation – variation between members of the same species

Interspecific variation – the differences between species

Continuous variation – variation where there are two extremes and a full range of values in between

Discontinuous variation – variation where there are distinct categories and nothing in between

Causes of Variation

  1. Inherited or genetic variation

This includes the combination of alleles that is inherited from our parents which is completely unique to us (unless there is an identical twin).

  1. Environmental variation

Many characteristics are brought out by environmental changes. For example, an overfed pet can become obese and a person’s skin tone may change due to exposure to the sun.

  1. Combined effects

Humans have become taller as the result of a better overall diet but however well your diet, you are unlikely to grow as tall as other people if your family is short.

Not all genes are active at any one time e.g. puberty is a time when many different genes are activating.

Changes in the environment can also directly affect which genes are active.


  1. Sample a population – this has to be random
  1. Mean – to show variation between samples

3. Standard Deviation – to show the spread of values about the mean

4. Spearman’s Rank Correlation Coefficient – to consider the relationship of the data

5. Student’s t-test – used to compare two means


Adaptation – a characteristic that enhances survival in the habitat

Anatomical adaptations – structural features

Behavioural adaptations – the ways that behaviour is modified for survival

Physiological adaptations – affect the way that processes work (also called biochemical)

Marram Grass (example)

Natural Selection

  1. Mutation creates an alternative version of a gene (alleles)
  2. This creates genetic variation between the individuals of a species (intraspecific variation)
  3. When resources are scarce, the environment will select those variations (characteristics) that give an advantage. There is a selection pressure.
  4. Individuals with an advantageous characteristic will survive and reproduce
  5. Therefore they pass on their advantageous characteristics (inheritance)
  6. The next generation will have a higher proportion of individuals with the successful characteristics. Over time, the group of organisms becomes well adapted to their environment.

Pesticide Resistance in insects

An insecticide applies a very strong selection pressure. If the individual insect is susceptible then it will die, but if it has resistance it will survive and reproduce, spreading the resistance through the entire population.

e.g. Mosquitos have developed and enzyme that can break down the pyrethroids used to treat mosquito nets.

e.g. Insect populations have become resistant to the insecticide DDT which binds to a receptor on the plasma membrane of certain cells in insects. This is due to mutations in the genes coding for cell surface receptors.

When insects become resistant it leads to pesticides accumulating in the food chain. When predators eat the insect they may get a large dose of the insecticide. This is why DDT is banned in many areas.


The use of antibiotics is a strong selection pressure on bacteria. MRSA is a very resistant bacteria that has cropped up because of over prescription of antibiotics. Medical researchers are struggling to develop new and effective drugs as the bacterial populations rapidly become resistant to them.

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2.1.3 Enzymes

(a) state that enzymes are globular proteins, with a specific tertiary structure, which catalyse metabolic reactions in living organisms

All enzymes are globular proteins with a specific tertiary structure, which catalyse metabolic reactions in all living organisms. This means that they speed up chemical reactions, but are not ‘used-up’ as part of the reaction.

Enzymes are relatively large molecules, consisting of hundreds of amino acids which are responsible for maintaining the specific tertiary structure of the enzyme. Each enzyme has a specific active site shape, maintained by the specific overall tertiary structure. Therefore the tertiary structure must not be changed.

(b) state that enzyme action may be intracellular or extracellular

Extracellular enzyme action occurs outside the cell, which produces the protein. For example, some enzymes in digestive systems are extracellular as they are released from the cells that make them, onto food within the digestive system spaces.

Intracellular enzyme action occurs inside the cell, which produces the enzyme. For example, some enzymes in digestive systems are found in the cytoplasm of cells or attached to cell membranes and the reaction takes place inside the cell.


(c) describe, with the aid of diagrams, the mechanism of action of enzyme molecules, with reference to specificity, active site, lock and key hypothesis, induced-fit hypothesis, enzyme-substrate complex, enzyme-product complex and lowering of activation energy.


The activation energy is the minimum level of energy required to enable a reaction to take place. Enzymes work by lowering the activation energy of reactions. This means reactions can proceed quickly at temperatures much lower than boiling point as less energy is required for the reaction.


(d) describe and explain the effects of pH, temperature, enzyme concentration and substrate concentration on enzyme activity

(e) describe how the effects of pH, temperature, enzyme concentration and substrate concentration on enzyme activity can be investigated experimentally

(f) explain the effects of competitive and non-competitive inhibitors on the rate of enzyme-controlled reactions, with reference to both reversible and non-reversible inhibitors

An enzyme inhibitor is any substance or molecule that slows down the rate of an enzyme-controlled reaction by affecting the enzyme molecule is some way.

Reversible inhibitors are inhibitors that bind to the active site for a short period and then leave. The removal of the inhibitor from the reacting mixture leaves the enzyme molecules unaffected.


Irreversible inhibitors are inhibitors that bind permanently to the enzyme molecule. Any enzyme molecules bound by inhibitor molecules are effectively denatured.



(g) explain the importance of cofactors and coenzymes in enzyme controlled reactions

A cofactor is any substance that must be present to ensure enzyme-controlled reactions can take place at the appropriate rate. Some cofactors are part of the enzymes (prosthetic groups); others affect the enzyme on a temporary basis (coenzymes and inorganic ion cofactors).

(h) state that metabolic poisons may be enzyme inhibitors, and describe the action of one named poison

(i) state that some medicinal drugs work by inhibiting the activity of enzymes



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Nucleic Acids

Nucleic Acids

  • The monomer units in nucleic acids are called nucleotides.
  • They consist of a pentose sugar (deoxyribose/ribose) a phosphate group and a nitrogenous base that is either a pyrimidine or a purine.
  • These three components are joined in condensation reactions by covalent bonds.

Nucleotide derivatives

ATP and ADP are nucleotide derivatives and are often present in co-enzymes such as NADP – nicotinamide adenine dinucleotide phosphate which is used in photosynthesis.






DNA structure

  • DNA is made of two polynucleotide strands, antiparallel to each other.
  • The monomers in DNA include deoxyribose sugar, a phosphate group and the bases cytosine, thymine, adenine and guanine.
  • The monomers are bonded by covalent phosphodiester bonds.
  • The bases are bonded with hydrogen bonds in complementary base pairing. These hydrogen bonds are weak but together are stronger as there is a large number of them.
  • It stores genetic information and hereditary material in the sequence of its bases.
  • DNA coils into a double helix shape.
  • DNA is a macromolecule and is very long to store all the information.
  • Purines and pyrimidines always pair together to provide equal sized rungs on the ladder like structure of DNA. A pairs with T, C pairs with G and vice versa.
  • The 5’ end is where the phosphate is attached to the fifth carbon and the 3’ end is where the phosphate is attached to the third carbon.
  • In eukaryotes, DNA is stored in the nucleus and is wound around histone proteins to form chromosomes. Each chromosome is one molecule of DNA. There is also a loop inside mitochondria and chloroplasts.
  • In prokaryotes, DNA is in a loop floating free within the cytoplasm and is not wound around histone proteins. It is therefore naked.


DNA Replication

DNA uses semi-conservative replication as both of the original strands are used in the two daughter strands.

DNA is a self-replicating molecule and its replication takes place during interphase in cell division.

The DNA in mitochondria and chloroplasts also replicates each time these organelles divide.


  • DNA unwinds (catalysed by gyrase enzyme) and unwinds (catalysed by DNA helicase)
  • Free DNA nucleotides form hydrogen bonds to the exposed bases (using complementary base pairing) this is catalysed by DNA polymerase and occurs in the 5’ – 3’ direction ONLY
  • Discontinuous okazaki fragments are made on the lagging strand of DNA that flows from 3’ – 5’ which are then connected up by DNA ligase
  • The nucleotides join together to form a new molecule of DNA

Mutations may occur in replicating the genetic code. These mutations occur when DNA is not accurately copied and when the wrong nucleotide is inserted into the strand. Some mutations are harmful, such as achondroplasia (a form of dwarfism), Marfan syndrome (a connective tissue disorder), and Huntington disease (a degenerative disease of the nervous system). Some are not, such as red hair.

Mutations can be passed on and inherited from parents or can occur spontaneously.



Protein synthesis


  • In the nucleus, DNA unwinds and unzips

  • Hydrogen bonds between the complementary nucleotides break
  • RNA polymerase catalyses the pairing of free RNA nucleotides to the template strand of the DNA
  • The RNA strand is identical to the other coding strand of DNA
  • The mRNA chain passes out of the nucleus through a nuclear pore
  • Exons are regions of the RNA strand that code for the amino acids of the protein being synthesized whereas introns are segments that do not code for the protein and are not useful
  • Introns are cut out and the exon regions are spliced together to form a long chain of codons
  • The mRNA chain progresses out of the nucleus and over onto the rough endoplasmic reticulum that has the ribosomes on it
  • Ribosomes are made up of two subunits

  • Transfer RNA molecules bring amino acids to the ribosome and pair up their anticodons with the codons in the mRNA molecule.
  • Peptide bonds form between the adjacent amino acids forming the polypeptide
  • ATP is required for translation
  • When the polypeptide has been assembled the RNA chain breaks down to be recycled again
  • The newly synthesized polypeptide is folded into its shape to form the protein






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Communicable Diseases

Communicable Diseases

Organisms that cause disease

Pathogen – a microorganism that causes disease

Transmission of Pathogens

Lifecycle of a pathogen:

  • Travel from one host to another (transmission)
  • Entering the hosts tissues
  • Reproducing
  • Leaving the hosts tissues

Direct Transmission

Overcrowding, poor ventilation, poor health, poor diets, homelessness and living and working with people who have migrated from areas where disease is more common, all may affect the likelihood of catching a disease.

Indirect Transmission

Some pathogens, like the protoctista plasmodium that causes malaria, use vectors for transmission.

Indirect transmission of plant pathogen occurs as a result of insect attack.

The fungus that causes Dutch elm disease is carried by the beetle Scolytus multistriatus

Plant Defences against pathogens

Passive Defences

  • Physical Defences
  1. Cellulose cell wall – this acts as a physical barrier but also contains many chemical defences that can be activated when a pathogen is detected
  2. Thickening of the cell wall with lignin – lignin is a phenolic compound and completely waterproof as well as largely indigestible
  3. Waxy cuticles – these prevent water collecting on the cell surfaces which removes the water that the pathogenic cells need to survive
  4. Bark – most back also contains a variety of chemical defences as well as being a physical barrier to disease
  5. Stomatal closure- stomata are possible points of entry for pathogens and so the guard cells can close them when pathogenic organisms are detected
  6. Callose – callose is a large polysaccharide that is deposited in the sieve tubes at the end of the growing season around the sieve plates and blocks the flow in the tube. This can prevent a pathogen spreading around the plant
  7. Tylose formation – tylose is a balloon like swelling or projection that fills the xylem vessel, when fully formed it can completely block off that part of the xylem vessel. It also contains a high concentration of chemicals such as terpenes that are toxic to pathogens
  • Chemical Defences
  1. Plant tissues contain a variety of chemicals that have anti-pathogenic properties including terpenoids, phenols, alkaloids and hydrolytic enzymes.
  2. Some of these chemicals such as the terpenes in tyloses and the tannins in bark are present before infection. However, because the production of chemicals requires a lot of energy, many chemicals are not produce until after an infection is discovered.

Active Defences

  • Cell walls become thickened and strengthened with additional cellulose
  • Deposition of callose between the plant cell wall and cell membrane near the pathogen. Callose deposits impeded cellular penetration at site of infection, strengthen the cell wall and block of plasmodesmata.
  • Oxidative bursts that produce highly reactive oxygen molecules capable of damaging the cells of invading pathogens
  • An increase in the production of chemicals
  • Necrosis – deliberate cell suicide
  • Canker – a sunken necrotic lesion in the woody tissue such as the main stem or branch that causes the death of the cambium tissue in the bark.

Primary Defences against Disease

Primary defences are the defences in place that prevent pathogenic material from entering the body.

The Skin

The skin is the main primary defence. The outer layer of skin is called the epidermis and consists of layers of keratinocytes. The keratinocytes are produces at the base of the epidermis and migrate out to the surface of the skin, slowly drying out and their cytoplasm in replaced by the protein keratin in the process of keratinisation. By the time the cells reach the surface they are dead. The layer of dead keratinised cells act as effective layer of disease prevention.

Blood Clotting and skin repair

  1. Abrasions or lacerations damage the skin and open the body to infection
  2. The body prevents excess blood loss by forming a clot and making a temporary seal to prevent infection
  3. Calcium ions and 12 clotting factors are released from the platelets and damaged tissue
  4. Damage to blood vessel exposes collagen
  5. Platelets bind to collagen fibres and release clotting factors, a temporary plug is formed
  6. Inactive thrombokinase in blood (factor X) is turned into active thrombokinase (an enzyme)
  7. Prothrombin in blood and thrombonkinase and Ca2+ ions make active thrombin
  8. Active thrombin turns the soluble fibrinogen in the plasma into insoluble fibrin which attach to the platelets in the plug and clot, trapping more red blood cells and platelets.
  9. As the skin grows and the scab shrinks the edges of the laceration are pulled together.


Mucous Membranes

The epithelial layer contains mucus-secreting cells called goblet cells and also mucus secreting glands under the epithelium.

The mucus traps any pathogens that may be in the air

The epithelium is also ciliated

Cilia are tiny hair-like organelles that can move in a coordinated fashion to waft the mucus along.

Coughing and sneezing – areas that are prone to microorganism attack are sensitive and respond to irritations by coughing sneezing and vomiting in the hope that the expulsion of air will propel the pathogen from the body.

Inflamation – the tissue may be hot and painful as the presence of harmful microorganisms has been detected by mast cells which release a cell signalling substance called histamine which causes vasodilation to make the capillary walls more permeable to white blood cells and proteins. The increased production of tissue fluid causes the swelling (oedema)

Eyes are protected by antibodies and enzymes in tear fluid

The ear canal is lined with wax

The female reproductive system is protected by a mucus plug in the cervix and by maintaining relatively acidic conditions in the vagina.

Specific Immune Response

Antibodies – specific proteins released by plasma cells that can attach to pathogenic antigens

Clonal expansion – an increase in the number of cells by mitotic cell division

Interleukins – signalling molecules that are used to communicate

Specific immune response involves B lymphocytes (B cells) and T lymphocytes (T cells) which are white blood cells with specialised receptors on their cell surface membranes. Antibodies are produced by B lymphocytes, and these neutralise foreign antigens. Long term disease protection is provided. An immunological memory is produced as B memory cells are released and circulate in the body for a number of years.




  1. Pathogen enters the body
  2. The antigens on the pathogen are presented on the pathogens cell membrane as it travels in the body fluids, on infected cells, and on the plasma membrane of macrophages that have engulfed the pathogens during the secondary non-specific response.
  3. T cells (from thymus) and B cells (from bone marrow) must detect the antigen from one of these three sources
  4. The detection of the pathogenic antibodies triggers clonal expansion in both T and B cells.
  5. T helper cells release cytokines that further stimulate the development of B cells
  6. Proliferation occurs once the correct lymphocytes have been activated.
  7. Cells differentiate and T & B memory cells are produced to remain in the blood should the body ever come under attack from the same pathogen again.
  8. T killer cells attack infected host cells and plasma cells make antibodies (the differentiation for this is triggered by cytokines from the macrophages
  9. Finally T regulator cells end the immune response to prevent the attack of the body’s own cells.


T lymphocytes

Come from the bone marrow and develop in the thymus

T helper cells (Th) – release cell signalling molecules (cytokines) that stimulate the immune response of B cells to develop and stimulate phagocytosis by phagocytes

T killer cells (Tk) – attack and kill host-body cells that display the foreign antigen as well as infected body cells

T memory cells (Tm) – provide long term immunity by staying in the blood for a long time

T regulator cells (Tr) – inhibit and end the immune response, preventing autoimmunity

They are involved in cell-mediated response (combat microorganisms)

They are a complementary shape to the antigen of pathogens and once the T cell has found a complementary antigen clonal expansion takes place produced by mitosis


B lymphocytes

Grow completely in the bond marrow

Plasma cells – derived from the B lymphocytes, these cells manufacture antibodies

B memory cells – cells that remain in the blood for a long time, providing long-term immunity

They are involved in the humoral response (producing antibodies)


Cell signalling

Macrophages release monokines which attract neutrophils (by chemotaxis – the movement of cells towards a particular chemical) and stimulate differentiation of B cells (and the release of antibodies)

T cells and macrophages release interleukins which stimulate clonal expansion (proliferation) and the differentiation of B & T cells

Many cells release interferon which inhibits virus replication and stimulates T killer cells


Autoimmune diseases

A disease that occurs when the immune system attacks a part of the body

Arthritis – a painful inflammation of a joint that starts with antibodies attacking the membranes around the joint

Lupus – swelling and pain in any part of the body, antibodies attack certain proteins in the nucleus of cells and affected tissue


Antigens are molecules that stimulate an immune response, usually proteins/glycoproteins in the pathogen’s plasma membrane, and when detected the production of antibodies is commenced.

Antibodies are specific to the antigen as antigens are specific to the organism. Our own antigens are recognised as ‘self’ by the immune system and do not provoke a response

Antibodies are immunoglobins (complex proteins produces by the plasma cells) and are released in response to an infection, they have a region with a specific shape to the antigen, antibodies attach to antigens and render them harmless

4x polypeptide chain, 2x light chains & 2x heavy chains

The tips of the y are the variable region but is the same for every type of antibody


A group of antibodies that bind to pathogen antigens and then act as binding sites for phagocytic cells

Some are non-specific

Some are produced as part of a specific immune response and bind to specific antigens

Antibodies flag up a pathogen for the phagocyte/attach to antigen which has a use to the pathogen, disabling it.

They also prevent the pathogen to enter the host cell.

Neutralise pathogens that use their antigens to bind to host cells etc.

By attaching onto the pathogen they make them easier to identify and easier for the phagocytes to bind to them/engluf them.

Antibodies that cause the pathogens to stick together (agglutinate) by making crosslinks between their antigens

This makes the pathogen non-effective and easily phagocytosed



Some antibodies bind to molecules that are release by pathogenic cells. These molecules may be toxic and the action of anti-toxins render them harmless.

Primary and Secondary Responses

Primary immune response – initial response caused by a first infection
Secondary immune response – more rapid and vigorous response caused by a second or subsequent infection by the same pathogen

  1. infection by pathogen
  2. lag phase
  3. antibodies produced
  4. antibody level rises to combat infections
  5. pathogen dealt with
  6. antibody level declines – short lived
  7. secondary immune response is much faster



Vaccination – a way of stimulating an immune response so that immunity is achieved, provides immunity to specific disease by deliberate exposure to a weakened/dead strain of antigenic material

Antigenic material takes many forms:

  • Whole live microorganisms (usually not very harmful g. smallpox which prevents cowpox virus, a much nastier disease)
  • Harmless attenuated version of the pathogenic organism e.g. measles & TB
  • Dead pathogen e.g. typhoid
  • Antigen preparations only, no actual pathogen e.g. hep B
  • Toxoids – harmless version of a toxin e.g. tetanus



  • Herd vaccination – providing the vaccine to all or almost all of the population so that the pathogen cannot spread, it is necessary to vaccinate 80 – 95% of the population to completely immunise the population
  • In the UK young children are immunised against the following diseases: diphtheria, tetanus, whooping cough, polio, meningitis, measles, mumps and rubella.
  • Ring vaccination – used when a new disease case is reported, vaccinated all people in the immediate vicinity of the case, also used to control livestock disease
  • Epidemic – a rapid spread of disease through a high proportion of the population
  • Influenza – a killer disease caused by a virus, people aged 65+ are most at risk as well as people with respiratory tract problems, the swine flu pandemic is an example of this virus


Types of immunity

Active immunity – immune system activated and own antibodies manufactured

Artificial immunity – immunity achieved as a result of medical intervention

Natural immunity – immunity achieved through normal life processes
Passive immunity – immunity achieved when antibodies are passed to the individual through breast feeding or injection

Development of Drugs

Antibiotic – a chemical which prevents the growth of microorganisms, can be antibacterial or antifungal

Personalised medicine – development of designer medicines for individuals

Synthetic biology – re-engineering of biology, from the production of new molecules that mimic a natural process to the use of natural molecules to produce new biological systems that do not exist in nature

The antibiotic penicillin was discovered by Alexander Fleming accidentally.


Traditional remedies

Morphine originated in the use of sap from unripe poppy seed heads in Neolithic times, in the 12th century the opium from poppies was used as an anaesthetic and by the 19th century morphine and opium were used to reduce nervous action in the central nervous system

Willow bark is used to relieve pain and fever. Its active ingredient was found to reduce the side effect of stomach bleeding by adding an acetyl group which lead to the development of aspirin and ibuprofen

Wildlife remedies

Monkeys, bears and other animals rub citrus oils on their coats as insecticides and antiseptics to prevent insect bites and infection. Birds line their nests with medicinal leaves to protect young from blood sucking mites. Chimps swallow leaves folded in a particular way to remove parasites from the digestive tract

Further plant research

Scientists use the traditional plant medicines as a starting point for new medicines and then try to isolate their active ingredient.

Pharmaceutical companies also research the way that microorganisms cause disease so that they can model ideal proteins and glycoproteins to act as medicines on these drugs. E.g. the HIV virus binds to the CD4 and CCR5 receptors on T helper cells. In order to cure this scientists are experimenting with blocking this binding with other similarly shaped proteins generated in computer models.

Overuse and misuse of antibiotics have enabled microorganisms to develop resistance which limits the effectiveness of current medicines.






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Cellular Control

Cellular Control

  • state that genes code for polypeptides including enzymes

Gene – a length of DNA that codes for one or more polypeptides, including enzymes.

Polypeptide – a polymer consisting of a chain of amino acids residues joined by peptide bonds.

Protein – a large polypeptide – usually 100 or more amino acids. Some proteins consist of one polypeptide chain and some consist of more than one polypeptide chain.

Genome – the entire DNA sequence of that organism. The human genome consists of about 3 billion nucleotide base pairs.




  • explain the meaning of the term genetic code

Genetic Code – the sequence of nucleotide bases on a gene that provides the codes for the construction of a polypeptide of a protein. The characteristics of the genetic code includes:

  • Triplet code – a sequence of three nucleotide bases codes an amino acid.
  • Degenerate code – all amino acids (except methionine) have more than one code.
  • Stop codes – indicates the end of a polypeptide chain (doesn’t correspond to an amino acid).
  • Widespread but not universal – where the same base sequence codes for similar polypeptides in different organisms, but are not always identical



  • describe, with the aid of diagrams, the way in which a nucleotide sequence codes for the amino acid sequence in a polypeptide

Transcription – the creation of a single-stranded mRNA copy of the DNA coding strand.

Messenger RNA (mRNA)activated nucleotides that uses complementary base pairing to the template strand to be arranged identically to the coding strand, other than uracil being present and thymine being absent. They carry the codons that are used to be make the polypeptide.



  1. The gene unwinds and unzips, by the length of the DNA that makes up the gene dips into the nucleolus. The hydrogen bonds between the complementary bases break forming two
  • Template strand – the strand of DNA that is used to help make mRNA. RNA nucleotides use the template strand to make mRNA through complementary base pairing between the template strand and mRNA.
  • Coding strand – the strand of DNA that is identical to mRNA, other than the base uracil being present in preference to thymine.
  1. Activated RNA nucleotides bind, using hydrogen bonds, to the exposed bases on the template strand, through complementary base pairing (U binds with A, C binds with G), catalysed by the enzyme RNA polymerase.
  2. Two extra phosphoryl groups are released, which releases energy for bonding adjacent nucleotides.
  3. mRNA is formed and passes out of the nucleus through a pore in the nuclear envelope, to a ribosome.
  • describe, with the aid of diagrams, how the sequence of nucleotides within a gene is used to construct a polypeptide, including the roles of messenger RNA, transfer RNA and ribosomes

Translation – the assembly of polypeptides (proteins) at ribosomes.

Transfer RNA (tRNA)lengths of RNA that fold into hairpin shapes and have three exposed bases at one end where a particular amino acid can bind. At the other end of the molecule are three unpaired nucleotide bases, known as an anticodon.



  1. mRNA binds to a ribosome. Two codons are attached to the small subunit of the ribosome. The first codon is always AUG, which codes for methionine. A tRNA with methionine and the anticodon UAC forms hydrogen bonds with this codon.
  2. The next tRNA, with another amino acid, binds to the second exposed codon on the mRNA with its complementary anticodon.
  3. A peptide bond forms between the two adjacent amino acids. This repeats forming a polypeptide.
  4. The last codon on the mRNA is called the stop codon (UAA, UAC or UGA), which stops translation.
  5. The rough endoplasmic reticulum packages the polypeptide into a vesicle, which moves to the Golgi apparatus to give the protein its final secondary/tertiary structures.


  • state that mutations cause changes to the sequence of nucleotides in DNA molecules

Mutation – a change in the amount of or arrangement of the genetic material in a cell, by base deletion, addition, substitution or by inversion or repeat of a triplet. Mutations cause changes to the sequence of nucleotides in DNA molecules.


Mutations may occur during DNA replication. Certain substances (mutagens) may cause mutations, including tar found in tobacco, UV light, X-rays and gamma rays. Mitotic mutations are somatic mutations and are not passed on to offspring. Meiotic mutations and gamete formation can be inherited (passed on to offspring).


What types of mutation are there?

  1. Insertion/deletion mutations – in which one of more nucleotide pairs are inserted or deleted from a length of DNA, causing a frameshift – the amino acid sequence is altered after the insertion/deletion point.
  2. Point mutations/Substitution – in which one base pair replaces
  • Nonsense – introduces a premature stop codon, stopping translation early, giving a truncated
  • Missense – changes the codon, changing the amino acid produced, so there is a change in the tertiary structure.
  • Silent – changes the codon, but the amino acid produced is not changed, so the amino acid sequence remains the same.

  • explain how mutations can have beneficial neutral or harmful effects on the way a protein functions



Allele – an alternative version of a gene. It is still at the same locus on the chromosome and codes for the same polypeptide but the alteration to the DNA base sequence may alter the protein’s structure.

Mutations with Neutral Effects:

If a gene is altered by a change to its base sequence, it becomes another version of the same gene – an allele. It may produce no change to the organism if:

  • The mutation is in a non-coding region of the DNA.
  • It is a silent mutation – the base triplet changes but still codes for the same amino acid, so the protein is unchanged.

If a mutation does cause a change to the structure of the protein, and therefore different characteristics, but the changed characteristic gives no particular advantage or disadvantage to the organism, then the effect is also neutral.

Mutations with Harmful or Beneficial Effects:

Early humans in Africa almost certainly had dark skin. The pigment melanin protected from the harmful effects of ultraviolet light. However, they could still synthesise vitamin D from the action of the intense sunlight on their skin. This is an important source of vitamin D, because much of the food that humans eat contains very little vitamin D.

The Inuit people have not lost all their skin pigments, although they do not live in an environment that has intense sunlight. However, they eat a lot of fish and seal meat, including the blubber, both rich sources of dietary vitamin D. Depending on the environment, the same mutation for paler skin can be beneficial or harmful. Individuals within a population who have a certain characteristic may be better adapted to the new environment. The well-adapted organisms can out-compete those in the population that do not have the advantageous characteristics. This is natural selection, the mechanism for evolution. Without genetic mutations there would be not evolution.


  • state that cyclic AMP activates proteins by altering their three-dimensional structure

Some proteins have to be activated by a chemical, cyclic AMP that, like ATP, is a nucleotide derivative. Cyclic AMP activates proteins by altering their three-dimension structure, so that their shape is a better fit to their complementary molecules.


  • explain genetic control of protein production in a prokaryote using the lac operon
  1. coli normally respires glucose but it can also use lactose as a respiratory substrate. E. coli grown in a culture medium with no lactose can be placed in a medium with lactose. At first they cannot metabolise the lactose because they only have tiny amounts of the two enzymes needed to metabolise it. A few minutes after lactose is added to the culture medium, E. coli bacteria increases the rate of synthesis if the enzymes by about 1000 times. Lactose must trigger the production of the two enzymes, and is known as the inducer.


The lac operon is a section of DNA within the bacterium’s DNA, consisting of:

  • Structural genes – the enzymes:
  • β-galactosidase – breaks down lactose to glucose and galactose.
  • Lactose permease – helps the cell to absorb lactose/increase uptake of lactose.
  • Control sites:
  • Operator region – binds to the repressor and can switch on and off the structural genes.
  • Promoter region – binds to RNA polymerase and controls transcription.


The regulator gene controls the production of repressor protein. This repressor molecule binds to the operator region, preventing RNA polymerase binding to the promoter region and preventing transcription. Therefore the structural genes are switched off and lactose is not broken down.

  • explain that the genes that control the development of the body plans are similar in plants, animals and fungi, with reference to homeobox sequence

Homeobox genesregulatory genes that codes for proteins that controls the development of body plans.


Homeobox genes each contain a sequence of 180 base pairs (homeobox) coding for the homeodomain. The homeodomain on the protein is able to bind to DNA, switching it on or off controlling transcription (the protein is the transcription factor). Homeobox genes are arranged into clusters known as Hox clusters. Some organisms have more Hox clusters than others.

Homeobox genes genetically mediate development of organisms:

  • Maternal-effect genes – determine the embryo’s polarity (which end is the head (anterior) and which end is tail (posterior)).
  • Segmentation genes – specify the polarity of each segment.
  • Homeotic selector genes – specifies the identity of each segment and direct the development of individual body segments. These are the master genes in the control networks of regulatory genes. There are two gene families:
  • The complex that regulates development of thorax and abdomen
  • The complex that regulates development of head and thorax

Mutations of these genes can change one body part to another. This can be seen in the condition known as antennapedia – where the antennae of Drosophila look more like legs.


  • outline how apoptosis (programmed cell death) can act as a mechanism to change body plans

Apoptosisprogrammed cell death that occurs in multicellular organisms. Cells should undergo about 50 mitotic divisions (the Hayflick constant) and then undergo a series of biochemical events that leads to an orderly and tidy cell death. This is in contrast to cell necrosis, an untidy and damaging cell death that occurs after trauma and releases hydrolytic enzymes. The apoptosis process occurs very quickly.

  1. Enzymes break down the cell cytoskeleton.
  2. The cytoplasm becomes dense, with organelles tightly packed.
  3. The cell surface membrane changes and small bits called blebs
  4. Chromatin condenses and the nuclear envelope breaks. DNA breaks into fragments.
  5. The cell breaks into vesicles that are taken by phagocytosis. The cellular debris is disposed of and does not damage any other cells or tissues.


Apoptosis is controlled by a diverse range of cell signals, including cytokines made by cells of the immune system, hormones, growth factors and nitric oxide. Nitric oxide can induce apoptosis by making the inner mitochondrial membrane more permeable to hydrogen ions and dissipating the proton gradient. Protons are released into the cytosol. These proteins bind to apoptosis inhibitor proteins and allow the process to take place.


Apoptosis is an integral part of plant and animal tissue development. The excess cells shrink, fragment and are phagocytosed so that the components are reused and no harmful hydrolytic enzymes are released into the surrounded tissue. Apoptosis is tightly regulated during development, and different tissues use different signals for inducing it. It weeds out ineffective or harmful T lymphocytes during the development of the immune system.

During limb development apoptosis causes the digits (fingers and toes) to separate from each other.


Children between the ages of 8-14 years, 20-30 billion cells per day undergo apoptosis. In adults, 50-70 million cells per day undergo apoptosis. If the rates are not balanced:

  • Not enough apoptosis leads to the formation of tumours.
  • Too much apoptosis leads to cell loss and degeneration.

Cell signalling plays a crucial role in maintaining the right balance.



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Practical Biochemistry Tests

Practical Biochemistry Tests


  • Add a solution of iodine to sample
  • If starch is present, iodine changes colour from yellow-brown to blue-black


  • Add Benedicts solution (alkaline copper sulphate) to sample
  • Heat in a water bath of 80oc
  • If reducing sugar is present it turns brick-red – if not present no change
  • Compare sample to know sugar contents in a colorimeter
  • If not present – do non-reducing sugar test
  • If present – compare on a graph to the know sugar contents solutions


  • Make sure no reducing sugars in sample
  • Boil the sample with Hydrochloric acid
  • Cool solution and neutralise by adding sodium hydrogen carbonate solution
  • Repeat reducing sugar test again


  • Add biuret reagent to sample
  • If colour changes to lilac proteins are present


  • Mix sample with ethanol – dissolves any present lipid
  • Then pour liquid into water contained in another clean test tube
  • If lipid present – cloudy white emulsion with form near top of water


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Cell Structure #2


Light microscope – Light microscopes shine light through a specimen and then uses focused lenses to magnify it.

Laser scanning microscope – These use laser light to scan an object, point by point, before using the information to assemble an image on the computer. They create very high definition, high contrast images and have depth selectivity (they can focus on structures at different depths in the specimen). They are used for biological research and in the medical progression e.g. to use on a patient with a fungal corneal infection as the fungal filaments within the cornea will be visible on the microscope image. It helps give faster diagnosis so the treatment is given earlier and is therefore more effective.

  1. Nerve Cell

  1. Lung tissue

  1. Plant vascular bundle

  1. E.Coli bacteria

  2. Mitochondria


The magnification is how many times bigger an image is compared to the original specimen. Microscopes produce linear magnification. (it’s width and length are magnified by the same amount)

Magnification = Size of Image/Size of Object

Size of Object = Size of image/Magnification

Size of image = Size of Object x Magnification



Resolution is the microscopes ability to show fine detail and produce clear, sharp images.

An eyepiece graticule is an eye piece that fits on a microscope with a ruler etched into it. The ruler has 100 divisions which are measured in Eye Piece Units or EPU’s. To calibrate the eyepiece graticule a graticule slide is placed under the light microscope with another ruler displaying either 1cm or 1mm in 100 divisions. Using the graticule slide, we can calibrate the lens to work out how long each division on the eyepiece would be after the image is magnified. The image of the ruler is then superimposed over your slide sample so that you can use the scale you have worked out with the graticule slide to measure samples and specimens.

Stage graticules are available in two types

  • (book) 1mm long graticule with 100 divisions. 1mm = 1000μm so each division = 10µm
  • (class) 1cm long graticule with 100 divisions. 1cm = 10000µm so each division = 100µm



Eukaryotic Cells

  • Robert Hooke 1665 discovered cells
  • Cilia/flagellum – long whip like tail made of microtubules
  • Cell membrane – encloses the cell contents, selective permeability, flexible
  • Cytoplasm – water and nutrients, contains the cytoskeleton and centrosomes which assemble microtubules
  • Endoplasmic reticulum – phospholipid bilayer: Rough – ribosomes and create protein: Smooth – enzymes create lipids cell detox and stores ions such as sodium ions
  • Ribosomes assemble amino acids into poly peptides
  • Nucleus – nucleoplasm inside, stores the cell’s DNA, chromatic holds the DNA, Cell division means the chromatin makes chromosomes Nucleolus makes rRNA combines with proteins which makes ribosomes
  • Golgi apparatus – processes and packaging and sends them out, contains Golgi bodies – these are the Golgi apparatus layers, proteins cut up into smaller hormones and combine proteins with carbs to make various molecules e.g. mucus
  • Lysosomes – cell digestion, enzyme sacks, cell waste >> simple compounds used for building materials
  • Vesicles – phospholipid bilayer, responsible for transporting substances around the body
  • Mitochondria – respiration – makes energy adenosine tri-phosphate, muscle cells need more energy so have more mitochondria, they used to exist as prokaryotes and contain a small amount of DNA

Eukaryotic Animal Cell


Mitochondria are double membrane structures enclosing a semi-fluid matrix, in which aerobic respiration (ATP synthesis) occurs, producing the energy the cell needs to move, contract, divide, and produce secretory products. The outer membrane of a mitochondrion is smooth, whereas the inner membrane forms cristae (folds) and is very convoluted. On the cristae, simple sugars such as glucose are combined with oxygen to produce ATP. Both the cristae and the semi-fluid matrix contain the enzymes that allow for aerobic respiration.

Vesicle – Lysosome

Lysosomes assist in intracellular digestion as they contain hydrolytic enzymes that break down bacteria. In a white blood cell, lysosomes are released into the vacuole to kill and digest the bacteria the cell has engulfed. Having to many lysosomes in the cytoplasm can cause necrosis.

Plasma Membrane/Cell Surface Membrane

The plasma membrane is a bilayer of phospholipid molecules with hydrophilic heads and hydrophobic tails, there are protein channels set into the barrier to transport substances into the cell. It’s function is to be a partially permeable membrane that provides protection and physical allows the import and export of chemicals and substances to and from the cell.

Golgi Apparatus

The Golgi apparatus is membrane-bound structure composed of a single membrane that binds a stack of large vesicles in a network, where they can modify and package proteins for transport. The stack of large vesicles

is surrounded by many smaller vesicles that contain the packaged protein macromolecules. The enzymatic/hormonal contents of lysosomes, peroxisomes and secretory vesicles are also packaged in vesicles at the periphery of this organelle. The vesicles can become detached from the Golgi which allows chemicals to be kept isolated from each other in the cytoplasm. These detached vesicles allow for secretion by merging with the plasma membrane of the cells surface.


These are membrane bound packets of oxidative enzymes responsible for turning hydrogen peroxide into harmless water and oxygen.


The cytoplasm is the semi-liquid medium that the animal cell’s organelles are suspended in. It makes up the main body of the cell, contains many dissolved ions and is the site of most chemical reactions within the cell.


The cytoskeleton is composed of microfilaments, microtubules and intermediate filaments and functions for both movement and support in the cell.

Smooth Endoplasmic Reticulum (SER)

The smooth endoplasmic reticulum is a continuation of the outer nuclear membrane. It assists the synthesis and transport of lipids. Unlike the rough endoplasmic reticulum, the smooth ER doesn’t have ribosomes.

Nuclear Pore

Nuclear pores are protein lined channels that regulate the transport of chemicals and substances to and from the nucleus. mRNA moves in and out of these entrances.


Centrioles are the two short cylinders which contain a ring of nine groups of fused microtubules, with three in each group. These migrate to the opposite poles to aid cell division when the spindle begins to develop. Microtubules and centrioles are part of the cytoskeleton.

Secretory Vesicles

Hormones and neurotransmitters and other cell secretions are packaged in the secretory vesicles at the Golgi apparatus. The vesicles are then transported to the cell membrane where they can release the cell secretions outside the cell.


The nucleolus is where RNA and ribosomes are made within the nucleus. It is made of proteins and unlike many of the cell organelles in an animal cell, is not a membrane bound structure


Vacuoles are membrane bound sacks that aid in intracellular digestion, storing nutrients and waste products and increasing the size of the cell during growth. In animal cells, vacuoles are generally small, unlike in plant cells where they are generally larger.

Rough Endoplasmic Reticulum (RER)

A folded membrane structure and a continuation of the outer nuclear membrane, the rough endoplasmic reticulum is very similar to the smooth. However the RER has ribosomes which synthesize proteins, before they are collected in the RER for transport around the cell.


Centrosome/Microtubule Organizing Centre (MTOC)

A centrosome/microtubule organizing centre (MTOC) is the area of the cell that produces microtubules. A complete animal cell centrosome is a pair of centrioles held perpendicular to each other. At cell division, the centrioles replicate and the centrosome divides, resulting in two centrosomes with a pair of centrioles each. They migrate to the cells poles and there microtubules grow into a spindle, responsible for separating replicated chromosomes into the two daughter cells the original animal cell will divide into.


Ribosomes are the site of protein synthesis. The protein chains are lengthened by transfer RNA adding individual amino acids to the messenger RNA strand that the nucleus has sent to the ribosomes. Ribosomes are structured in two parts, a large subunit and a small subunit. They are full of RNA.


Cell Wall

In plants this is made of cellulose and is strong to prevent cells from bursting when turgid. They provide support, maintain the cells shape and allow solutions to pass through. Fungi have cell walls of chitin and not cellulose.

Prokaryotic Cells

Prokaryotic cells are much smaller than eukaryotic and do not contain some of the organelles present in eukaryotes. They do not have centrioles or a nucleus and so divide by binary fission. They also do not have membrane bound organelles (e.g. ER and Golgi). Around their edge they sometimes have a waxy capsule and a cell wall made of peptidoglycan as well as pili and flagella. Instead of a nucleus the cell stores its genetic material in the plasmid and a nucleoid. Its ribosomes are smaller and it contains starch granules as well.




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Maintaining Biodiversity

Maintaining Biodiversity

(a) outline the reasons for the conservation of animal and plant species, with reference to economic, ecological, ethical and aesthetic reasons

How are humans reducing biodiversity?

  • Disruption of food chains – over-harvesting
  • Killing to remove competitors for our food – using pesticides to kill insects, fungi and other pests
  • Pollution – especially industrial pollution causing climate change
  • Habitat destruction – e.g. deforestation for agriculture
  • Inadvertent introduction of new predators and competition to areas
  • Killing for protection – attempting to kill insects that are vectors of disease (e.g. Anopheles mosquitoes) or to remove the threat of a predator
  • Agriculture – only harvesting one species (monoculture) creates a low species diversity



(b) discuss the consequences of global climate change on the biodiversity of plants and animals, with reference to changing patterns of agriculture and spread of disease

Climate change has a major impact on the biodiversity of plants and animals. As the climate changes, species that have lost their genetic variation and are unable to evolve, will be unable to adapt to the changes in temperature and rainfall in the area where they live. The only alternative for them is to migrate. However, there will be obstructions to this migration:

  • major human developments
  • agricultural land
  • large bodies of water
  • humans

For example, the golden toad of the Costa Rican cloud forest moves uphill as the climate warms up to stay in the most suitable habitat. However when it reaches the top of the hill it has nowhere to go. Therefore this species is becoming extinct as the climate changes.

(c) explain the benefits for agriculture of maintaining the biodiversity of animals and plant species

Allowing biodiversity to increase means that genetic diversity also increases. Wild animals and plants may hold the answer to problems caused by climate change as they have adapted to overcome the problems presented by the environment over many years as well as pests and disease. By careful selection and breeding from wild species, we may be able to breed new crop varieties that can cope with the new conditions created by climate change. Genetic engineering to produce transgenic species can be used.

The number of potential new medicines and the range of possible vaccines that could be developed from wild plants and microorganisms is unknown. It’s important to maintain genetic diversity of wild plants and animals because of the potential that exists in the wide range of species currently alive.

(d) describe the conservation of endangered plant and animals species, both in situ and ex situ, with references to the advantages and disadvantages of these two approaches

(e) discuss the role of botanic gardens in the ex situ conservation of rare plant species or plant species extinct in the wild, with reference to seed banks

(f) discuss the importance of international co-operation in species conservation with reference to The Convention in International Trade in Endangered Species (CITES) and the Rio Convention on Biodiversity

(g) discuss the significance of environmental impact assessments (including biodiversity estimates) for local authority planning decisions


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Cell Membranes

Cell Membranes

(a) outline the roles of membranes within cells and at the surface of cells


(b) state that plasma (cell surface) membranes are partially permeable barriers

Plasma membranes are partially permeable meaning they let some molecules through but not others.

(c) describe, with the aid of diagrams, the fluid mosaic model of membrane structure

The phospholipid bilayer is the basic structural component of plasma membranes. It consists of 2 layers of phospholipid molecules. The centre of the bilayer is hydrophobic so the membrane doesn’t allow water-soluble substances (like ions) through it – it acts as a barrier to these dissolved substances

In the fluid mosaic model, phopholipid molecules form a continuous double layer (bilayer). The bilayer is ‘fluid’ because the phospholipids are constantly moving. The fluid mosaic model also contains cholesterol molecules, proteins, glycoproteins and glycolipids.

(d) describe the roles of components of the cell membrane:

(e) outline the effect of changing temperature on membrane structure and permeability

(f) explain the term cell signalling

(g) explain the role of membrane-bound receptors as sites where hormones and drugs can bind

Cell signalling is when cells communicate with one another by signals. In order to detect signals, cells must have on their surface ‘sensors’ capable of receiving signals, these are known as receptors and are often protein molecules or modified protein molecules (e.g. glycoproteins). In multicellular organisms, communication is often mediated by hormones between cells. Hormones are chemical messengers, produced in specific tissues and then released. Any cell with a receptor for the hormone molecule is called a target cell.

Cells communicate with each other using messenger molecules:

  1. One cell releases a messenger molecule (e.g. hormone)
  2. This molecule travels to another cell (e.g. in the blood)
  3. The messenger molecule is detected by the cell because it binds to a receptor on its cell membrane

Receptor proteins have specific shapes – only messenger molecules with a complementary shape can bind to them. Different cells have different types of receptors – they respond to different messenger molecules. A cell that responds to a particular messenger molecule is called a target cell.

Glycoproteins have receptors. They have a role in:

  • cell adhesion – bind cells together in a tissue
  • acting as antigens on the surface of cells. Cells of the immune system have receptors that detect the glycoproteins and can determine whether they are ‘self’ or ‘non self’

Many drugs work by binding to receptors in cell membranes. They either trigger a response in the cell, or block the receptor and prevent it from working e.g. cell damage causes the release of histamine. Histamine binds to receptors on the surface of other cells and causes inflammation. Antihistamines work by blocking histamine receptors on cell surfaces. This prevents histamine from binding to the cell and stops inflammation.

(h) explain what is meant by passive transport (diffusion and facilitated diffusion including the role of membrane proteins), active transport, endocytosis and exocytosis

Substances can move across a membrane through 2 processes: passive and active

Active Transport:

(i) explain what is meant by osmosis, in terms of water potential

Osmosis is the movement of water molecules by diffusion from a region of high water potential to a region of low water potential across a partially permeable membrane

Water potential is a measure of the concentration of water molecules that are ‘free’ to diffuse.

Adding solutes to water means the water molecules cluster around the solute molecules, lowering the concentration of ‘free’ water molecules and therefore lowers the water potential.

(j) recognise and explain the effects of solutions of different water potentials can have upon plant and animal cells

In pure water, water moves into a cell by osmosis down a water potential gradient.

  • Animal Cell – becomes haemolysed (bursts open)
  • Plant Cell – the cell wall prevents bursting. The membrane pushes against the cell wall and the cell becomes turgid

In a very low water potential solution (e.g. concentrated sugar solution), water moves out of a cell by osmosis down a water potential gradient.

  • Animal Cellshrinks and appears wrinkled and the cell becomes crenated
  • Plant Cell – the membrane pulls away from the cell wall and the cell becomes plasmolysed



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