 Enzymes are biological catalysts, that is they speed up the rate of reactions in biochemical pathways both within and outwith the cell.
 Enzymes are made of protein and are specific, that is one enzyme will catalyse only one type of reaction.
 The substrate molecule fits into the 'active site' of the enzyme molecule in order that the reaction can occur.
 A metabolic pathway is a sequence of reactions controlled by enzymes which change one metabolite to another.
 Genes code for proteins, including enzymes.
 If, as a result of mutation, a gene is changed, it may not code for the correct enzyme. If this enzyme is needed in a metabolic pathway, the pathway is blocked and cannot continue. Intermediate metabolites build up which can cause problems in the body.
 The inability to produce an enzyme as a result of gene mutation is known as an
'inborn error of metabolism'.
 Various factors can affect enzyme activity, including substrate concentration, enzyme concentration and inhibitors.
 As substrate concentration increases, enzyme activity increases up to a maximum, after which the activity levels off.
 As enzyme concentration increases, enzyme activity increases, again up to a maximum after which it levels off.
 An inhibitor is a substance which slows down, or stops, the activity of an enzyme.
 A competitive inhibitor competes with the substrate for the enzyme's active site.
 A non-competitive inhibitor combines with a part of the enzyme other than the active site. This changes the shape of the enzyme and prevents the substrate from combining with it.
 Some enzymes are produced in an inactive form and must be activated before they can function. Examples of activators are mineral ions (zinc, iron), vitamins
(the vitamin B complex) and other enzymes (for example the inactive trypsinogen is changed by the enzyme enterokinase to its active form trypsin).

 Proteins are composed of building blocks called amino acids.
 Amino acids are linked together by strong bonds called peptide bonds to form polypeptide chains.
 The diversity of structure and function among proteins is due to: o the arrangement of amino acids in polypeptide chains; o the association, in many cases, of more than one polypeptide chain in the mature protein.
 Proteins are organised at four different structural levels: o primary (sequence of amino acids); o secondary (weak hydrogen bonds between amino acids cause the polypeptide chain to fold); o tertiary (strong bonds form between amino acids causing more folding to take place); o (two or more polypeptide chains often associate together to form the final protein structure).
 Proteins have many functions including: enzymes, some hormones, muscular contraction, transport of substances, antibodies and structural proteins.
 All enzymes are globular proteins. The 3D structure of the enzyme is vital to its function. An area of the enzyme is folded so that an "active" site is exposed which readily combines with its substrate.
 Hormones are chemical messengers produced by special glands. Many hormones, for example insulin and ADH, are proteins.
 The myofibrils of skeletal muscle consist of two types of protein filaments: thin actin and thick myosin filaments. The areas where actin and myosin filaments overlap are darker than the areas where there is no overlap. This leads to the striations (or stripes) associated with this type of muscle. During muscle contraction the thin actin and the thick myosin filaments slide past one another and so the muscle gets shorter.
 Many proteins transport substances around the body. Myoglobin and haemoglobin, for example, are involved in the transport of oxygen. Other proteins found in cell membranes transport chemicals across the membrane.
 Antibodies are proteins produced by white blood cells called lymphocytes and are very important in preventing and fighting disease.
 Bones, tendons, ligaments, skin and hair are all mainly composed of structural proteins such as collagen, elastin and keratin.

 DNA is found on chromosomes inside the nucleus of the cell.
 Chromosomal DNA is divided up into regions containing genes.
 DNA consists of two strands twisted into a double helix.
 DNA strands are made of nucleotides joined together.
 There are four types of nucleotides.
 Each nucleotide consists of deoxyribose sugar, a phosphate group and one of four types of organic base - adenine, guanine, cytosine or thymine.
 Base pairing rules - A always pairs with T and G always pairs with C.
 The information to determine the sequence of amino acids in a protein is contained in the sequence of bases in DNA.
 RNA is single-stranded.
 RNA is made of nucleotides similar to DNA.
 In RNA, the base uracil replaces thymine (which is found in DNA).
 RNA nucleotides contain the sugar ribose instead of deoxyribose.
 Genetic information is transcribed from DNA to mRNA in the nucleus.
 mRNA is translated into protein at the ribosomes in the cytoplasm.
 The genetic code consists of triplet codons which code for different amino acids.
 Different tRNA molecules become attached to specific amino acids.
 tRNA molecules contain anticodons which temporarily become attached to complementary mRNA codons in ribosomes during translation. Thus amino acids are lined up in a particular sequence.
 Adjacent amino acids are joined together by peptide bonds.
 The process of translation produces a protein or polypeptide chain whose amino acid sequence is determined by the sequence of bases on the DNA molecule.

 ATP is a means of transferring chemical energy in cells.
 ATP is regenerated from ADP and inorganic phosphate.
 The quantity of ATP in the body stays more or less constant.
 Glycolysis is the breakdown of the 6C sugar glucose to two 3C molecules of pyruvic acid.
 Glycolysis takes place in the cytoplasm of the cell.
 During glycolysis there is a net gain of 2 ATP molecules.
 Glycolysis does not require oxygen; it is an anaerobic process.
 The inner membrane of a mitochondrion is highly folded into cristae which project into an inner fluid-filled matrix.
 The Krebs cycle reactions take place in the matrix of the mitochondria.
 The cytochrome system reactions take place on the cristae of the mitochodria.
 The Krebs (citric acid, tricarboxylic acid) cycle consists of the following stages: o pyruvic acid (produced by glycolysis) diffuses into the matrix of the mitochondrion where it is converted into a 2C compound called acetyl coenzyme A (acetyl CoA); o the conversion of pyruvic acid to acetyl CoA releases hydrogen which is bound to NAD to form NADH
2
; o each acetyl CoA combines with a 4C molecule to form a 6C molecule called citric acid; o during the Krebs cycle, citric acid is converted through a series of enzyme-catalysed reactions back into the 4C molecule. In the process, both carbon (in the form of carbon dioxide) and hydrogen are released; o hydrogen becomes bound to NAD to form NADH
2
. This will be used in the next stage of respiration to release energy for ATP production; o carbon dioxide diffuses out of the cell as a waste product that is removed from the organism by breathing out or by diffusion over the body surface.
 The cytochrome (hydrogen transfer) system consists of the following stages: o each NADH
2
molecule passes two hydrogens (or electrons) to a set of carriers (mostly composed of proteins) called the cytochrome system; o the energy released from the hydrogens (electrons), as they pass through the cytochrome system is used to produce ATP. This process is called oxidative phosphorylation; o enough energy is released to produce three molecules of ATP from each molecule of NADH
2
. o the final acceptor for the hydrogens (electrons) which are passed along the cytochrome system is oxygen o the hydrogen and oxygen combine to form water. o
The Krebs cycle and the cytochrome system require oxygen, that is they are aerobic processes.
 Anaerobic respiration occurs in the cytoplasm, producing 2 ATP molecules for every glucose molecule respired. The final metabolic product is lactic acid.
 Aerobic respiration takes place in the mitochondria, producing 38 ATP molecules for every glucose molecule respired. The final metabolic products are carbon dioxide and water.

 Carbohydrates are grouped into monosaccharides, for example glucose and fructose; disaccharides, for example maltose and sucrose; polysaccharides, for example starch, cellulose and glycogen.
 The main source of carbohydrate energy in the body is glucose. This is known as the main respiratory substrate.
 Lipids and proteins are known as alternative respiratory substrates, since they are used after glucose levels have been depleted.
 Fatty acids are converted to acetyl coA and enter the Krebs cycle.
 A small amount of energy is derived from excess dietary protein. Excess amino acids are broken down in a process called deamination to urea and other substances such as pyruvic acid and Krebs cycle intermediates. These substances can act as respiratory substrates.
 During prolonged starvation tissue protein such as skeletal muscle is broken down and used as a respiratory substrate.
 During marathon running glycogen stored in muscles is converted to glucose and used as an energy source. Then glycogen from the liver is converted to glucose which is transported by the blood to the muscles. Finally fats stored in fat cells are broken down to fatty acids which are transported by the blood to the muscles. These provide a sustained energy supply for muscle cells.
 Lipids have many functions within the body, including: o energy store; o heat insulation; o nerve insulation; o protection from physical damage by fat pads on feet and hands; o essential components of cell membranes (phospholipids); o transport of water insoluble vitamins A and D; o waterproofing and protection of skin (sebum); o hormones, such as oestrogen and testosterone, are lipids (steroids).

 Membranes are composed of phosphlipids and proteins.
 Membrane structure is described as fluid-mosaic - the phospholipids form two layers which are mobile. The proteins are found scattered as a mosaic in and on the lipid layers.
 One end of the phospholipid molecule is repelled by water (hydrophobic) and the other is attracted to water (hydrophilic). In the company of other similar molecules, phospholipid molecules arrange themselves into a bi-layer.
 The arrangement of the phospholipid molecules is fluid yet at the same time it forms a stable and effective barrier around the cell.
 Membrane proteins have many functions, including the following: o some proteins are enzymes; o some proteins are receptor sites for hormones ; o some proteins act as a skeleton which support the membrane and allow it to move; o some carrier proteins actively transport materials across the membrane; o some proteins form pores or channels in the membrane allowing small molecules to pass through; o some proteins act as markers for self-recognition.
 The cell membrane maintains a constant environment within the cell by regulating the passage of molecules between the cell and its environment by diffusion, osmosis and active transport.
 In diffusion, molecules always move 'down' a concentration gradient from a high concentration to a low concentration. It is a passive process.
 Osmosis is the net movement of water molecules across a selectively permeable membrane from a high water to a low water concentration. It is a passive process.
 Active transport is the movement of molecules against a concentration gradient.
It is a process that requires energy.
 Endocytosis is a movement of the whole plasma membrane to engulf either solid
(phagocytosis) or liquid (pinocytosis) material.
 Exocytosis is the means by which molecules synthesised by the cell can be secreted.

 An antigen is defined as a molecule recognised as foreign by an organism and which elicits an immune response.
 Cells bearing non-self antigens are attacked by the immune system.
 There are four possible blood groups among humans: A, B, AB and O.
 The antigens on the red blood cells of different blood groups are as follows: o blood group individuals A possess A antigens; o blood group B individuals possess B antigens; o blood group AB individuals possess both A and B antigens; o blood group O individuals possess no antigens - since they have no antigens to stimulate an immune response blood group O can be given to any other blood group and is known as the universal donor.
 If exposed to different blood groups the following antibodies will be produced: o A individuals will produce anti-B antibodies; o B individuals will produce anti-A antibodies; o AB individuals will not produce any antibodies - they are said to be universal recipients because they can receive blood from any other blood group without producing antibodies; o
O individuals will produce both types of antibody and can therefore only receive blood from other O individuals. o
Foreign, or non-self, antigens stimulate B lymphocytes to produce proteins called antibodies, which are Y - shaped. o Each antibody has binding sites for a specific antigen on the tip of each of the "Y" arms so it only recognises one type of antigen. o Antibodies and antigens bind together like a lock and key. When this happens, a complex forms making the antigen harmless.
 B lymphcytes are produced and mature in the bone marrow and carried by the blood to the lymph nodes.
 When they come into contact with a foreign antigen the B cells differentiate and secrete large quantities of antibodies into the lymph and blood. The antibodies bind to the antigens of the foreign cells, making them temporarily inactive.
 Because the antibodies act at a distance from the parent B-lymphocytes, this response is known as the humoral response.
 T lymphocytes are produced in the bone marrow and mature within the thymus gland.
 Killer T cells detect and kill cells in the body which are harbouring pathogens.
They do this by secreting toxic chemicals into the cells.
 Because the T cells themselves are directly involved in the immune response, this is called the cellular response.
 T helper cells produce chemicals which activate B lymphocytes so that they produce antibodies. They also increase the rate of phagocytosis.
 Macrophages are white blood cells which destroy foreign particles such as bacteria and viruses by a process called phagocytosis.
 The macrophage detects chemicals from the bacterium and moves towards it.
The cell membrane of the macrophage surrounds the bacterium, enclosing it within a vacuole.

 Lysosomes in the cytoplasm of the macrophage contain digestive enzymes. Some of the lysosomes fuse with the vacuole and release their enzymes into it. The bacterium is digested and the breakdown products are absorbed by the macrophage.

 Innate immunity is an inborn, non-specific ability to resist infection by a pathogen. Examples include: o phagocytosis; o skin epithelial cells; o mucus membranes; o ciliated cells; o lysozyme in tears.
 Naturally immunity is acquired when an individual is infected by a pathogen.
When the body is exposed to foreign antigens the production of B cells and T cells is stimulated.
 Artificially acquired immunity occurs when an individual is deliberately exposed to a pathogen. Again, the presence of foreign antigens stimulates the production of B cells and T cells that produce antibodies to combat the pathogen Artificial immunity is acquired by vaccination with a weakened or killed version of the pathogen.
 Active immunity occurs if a person produces their own antibodies. This can occur following either natural or artificial exposure to the pathogen's antigens.
 Passive immunity occurs when an individual receives ready made antibodies. This process occurs naturally when antibodies are passed through the placenta from the mother to the foetus or in milk colostrum shortly after birth. It can also occur artificially by injecting antibodies isolated from the serum of humans who have recovered from a disease.
 Autoimmunity occurs when the immune system fails to recognise its own cells as
'self' and treats them as if they were foreign, 'non-self' cells by attacking and destroying them.
 An allergy occurs when the immune system makes a mistake and misidentifies a harmless substance as a harmful one. The B cells cause the body to produce large quantities of antibodies which attach themselves to specialised cells in the connective tissue. This sensitises the body to the allergen and causes an allergic reaction when the allergen next enters the body.
 The primary immune response occurs when the body is infected by a pathogen for the first time. The secondary immune response occurs when the body is subsequently infected by the same pathogen. The primary response is slower and produces fewer antibodies than the secondary response.
 Viruses are not cells; are very small and consist of a nucleic acid enclosed in a protein coat. They cannot replicate by themselves, but infect cells and alter the host cell's genetic makeup to produce more viruses.
 Viruses infect cells in the following way: o Viruses latch onto specific receptors on the host cell surface. o Part or all of the virus is injected into the host cell. o Viral nucleic acid provides the instructions for the production of new viral particles. The host cell provides nucleotides; enzymes; ribosomes; tRNAs; amino acids and ATP. o
Once viral nucleic acids and protein components are made, new viral particles assemble spontaneously. o New viral particles are released as the host cell bursts (or lyses).

 Genes are regions of chromosomal DNA.
 DNA replication occurs in the following stages: o the DNA molecule uncoils; o nucleotide chains separate by the breakage of the weak hydrogen bonds between the nucleotides of the two strands; o free DNA nucleotides line up against the two exposed strands; o the base pairing rule ensures that A bases pair with T and C bases with
G; o chemical bonds form between adjacent free nucleotides and between the complementary bases.
 Appropriate enzymes and ATP as an energy are required for DNA replication.
 DNA replication is important because it ensures that an exact copy of the individual's (and therefore the species') genetic code is passed onto every new cell during growth as well as from one generation to the next during sexual reproduction.
 DNA replication occurs before nuclear division in both mitosis and meiosis.
 Every species has a characteristic number of chromosomes, called the chromosome complement, present in the nucleus of its body cells. In humans the normal chromosome complement is 46 chromosomes which are arranged into 23 homologous pairs.
 Homologous chromosomes possess the same genes which are found at the same place (locus) on each chromosome.
 The first 22 pairs in a human karyotype are completely identical and are called autosomes. The last pair is known as the sex chromosomes. In females (XX) the sex chromosomes are completely homologous, but in males (XY) the Y chromosome is much smaller and only homologous with a small part of the X chromosome.
 A diploid cell contains two sets of chromosomes while a haploid cell contains only one set.
 Meiosis is the type of nuclear division which produces haploid gametes.
 Meiosis consists of two successive divisions. In the first meiotic division the number of chromosomes is reduced. In the second meiotic division the double stranded chromosomes are separated into single stranded ones.
 Variation in gametes is increased by two processes: independent assortment and crossing over.
 Independent assortment describes how the homologous pairs of chromosomes assort themselves onto a spindle fibre independently of any other pair. Crossing over occurs at chiasmata which form when the homologous chromosomes move close together and the chromatids intertwine.
 Fertilisation involves the joining of two haploid gametes, from two parents, which results in the formation of a zygote with the diploid number of chromosomes.
 This mixing of genotypes results in an individual who is genetically unique and leads to variation in a population.

 You should know the meanings of the following terms: haploid, diploid, gene, allele, homozygous, heterozygous, genotype, phenotype, dominant, co-dominant, incompletely dominant, F1 generation, F2 generation.
 An individual has two alleles for each gene (except for some sex-linked genes).
 In a pair of alleles where one is dominant and one recessive, the dominant allele is represented by an upper case letter and the recessive allele by the same letter in lower case. Tongue rolling, for example, is controlled by a single gene.
An individual who is homozygous dominant (TT) or heterozygous (Tt) will be able to roll their tongue. A homozygous recessive individual (tt) will not be a tongueroller.
 Huntington's chorea is a condition caused by the presence of the dominant allele of a gene. HH or Hh individuals will eventually develop the disorder in middle age. hh individuals will not develop it at all.
 An example of incomplete dominance is sickle cell anaemia. Since both alleles have an effect on the phenotype, each one is represented by a different upper case letter. H represents the normal allele, while S represents the sickle cell allele. HH individuals have normal haemoglobin, SS individuals have sickle shaped red blood cells and suffer from sickle cell anaemia. HS individuals have a mixture of normal and sickle shaped red blood cells and suffer from the milder sickle cell trait. The HS genotype offers a selective advantage over HH in regions where malaria is prevalent.
 An example of co-dominance is the MN blood group. MM individuals have M antigens on the surface of their red blood cells, NN individuals have only N antigens, while MN individuals have both M and N antigens present.
 Some genes have three or more alleles; however any one individual can only possess two of these. The presence of multiple alleles in a population increases the amount of genetic variation. The ABO blood group is an example of multiple alleles. The alleles A and B are co-dominant and both are completely dominant to the O allele. Thus there are more possible genotypes and phenotypes for this characteristic than there would be if there were only two alleles. AA and AO individuals are blood group A; BB and BO individuals are group B; AB individuals are blood group AB while OO individuals are group O.
 Although 22 of the 23 pairs of chromosomes in humans are homologous, the sex chromosomes are not so in males. Females are XX which are completely homologous. Males, however, are XY. Since the Y chromosome is much smaller, it is homologous only will a small part of the X chromosome. Those genes present on the non-homologous part of the X chromosome are said to be sex-linked. A male only has one allele of these genes.
 Some human sex-linked conditions are red-green colourblindness, haemophilia and Duchennes muscular dystrophy.
 Males inherit sex-linked conditions from their mother, since they receive a Y chromosome from their father. If their mother is heterozygous for the gene (Cc for colour vision), then her son has a 1 in 2 chance of receiving the abnormal allele. The mother will not be affected since she has a normal allele in addition to the recessive, abnormal one.

 Human characteristics which are controlled by a single pair of alleles produce discontinuous variation in a population. However, most characteristics are controlled by two or more pairs of alleles, for example, height and skin colour.
These characteristics show continuous variation in a population.
 When graphed, the range of values of a polygenic characteristic shows a normal distribution pattern of inheritance.
 A gene mutation is a random change in the number or sequence of bases.
 A substitition mutation occurs when a one base is substituted for another while an inversion mutation involves the reversal of the sequence of bases. These two mutations do not usually have a large effect on the amino acids sequence coded for and are known as point mutation.
 A deletion mutation involves the removal of a base while an insertion mutation involved the addition of a base. These types of mutation are known as frame shift mutations because they alter the entire sequence of amino acids coded for.
 Chromosome mutations are due to changes in chromosome structure or changes in the number of chromosomes in a cell.
 Non-disjunction occurs when the spindle fails during meiosis; some gametes end up with both homologous chromosomes, while others are missing a chromosome.
Non-disjunction can be seen in an individual's karyotype.
 Examples of conditions caused by non-disjunction are: Down's syndrome (an extra copy of chromosome 21); Turner's syndrome (lacking an X chromosome) and Klinefelter's syndrome (an extra X chromosome).
 Looking at the phenotypes in many generations of a family by using a family tree can enable genotypes of individuals to be determined. This information can be used to assess the risk of a child inheriting a genetic disorder. However this type of genetic screening is only useful for disorders caused by one gene.
 In polygenic disorders, for example schizophrenia and epilepsy, risk assessment is usually based on the actual incidence of the disorder within a population.
However family history can also be useful in such cases, showing the number of individuals affected through several generations. The more individuals in a family tree who are affected, the greater is the risk, although it would be impossible to say with certainty that any one individual would suffer from the disorder.
 If the risk of a genetic disorder is high, fetal material can be taken from the amniotic fluid by a process called amniocenteses. Karyotypes of the fetal cells can be produced and chromosome abnormalities detected.
 Post-natal screening is also used for conditions which have a genetic basis. Blood samples are taken from newborns and tested for phynylketonurea (PKU). Any affected infants can be treated immediately thus reducing the risk of brain damage.