Carbohydrates are polyhydroxy aldehydes or ketones (or compounds that yield them on hydrolysis) and act as the main source of energy. They are classified as: (1) monosaccharides - simple single-unit sugars such as glucose and fructose; (2) oligosaccharides (mainly disaccharides) - such as sucrose, maltose and lactose, which yield 2 to 10 monosaccharides on hydrolysis; and (3) polysaccharides - large polymers such as starch, glycogen and cellulose.

Proteins are polymers of amino acids joined by peptide bonds. Their four levels of structure are: (1) Primary structure - the linear sequence of amino acids in a polypeptide chain; (2) Secondary structure - local folding into an alpha-helix or beta-pleated sheet held by hydrogen bonds; (3) Tertiary structure - the overall three-dimensional folding of the whole chain, giving the functional shape; and (4) Quaternary structure - the association of two or more polypeptide chains, as in haemoglobin.

Lipids are organic compounds that are insoluble in water but soluble in organic solvents; they include fats, oils, phospholipids and steroids and are generally esters of fatty acids and glycerol. Functions: (1) they are a concentrated store of energy (fats give about twice the energy of carbohydrates); (2) phospholipids form the structural basis of cell membranes; and they also act as thermal insulation and protect internal organs.

Nucleic acids are polymers of nucleotides and are of two types, DNA (deoxyribonucleic acid) and RNA (ribonucleic acid); they store and transmit genetic information. Each nucleotide has three components: (1) a nitrogenous base (adenine, guanine, cytosine, and thymine in DNA or uracil in RNA); (2) a pentose sugar (deoxyribose in DNA, ribose in RNA); and (3) a phosphate group.

Enzymes are biological catalysts, mostly proteins, that speed up biochemical reactions without being consumed. Properties: (1) they are highly specific, each acting on a particular substrate; (2) they work best at an optimum temperature and pH and are sensitive to changes in these. Mechanism: the substrate binds to a specific region of the enzyme called the active site, forming an enzyme-substrate complex (the lock-and-key model); the enzyme lowers the activation energy, converts the substrate into product, and is then released unchanged.

Essential amino acids are those that the body cannot synthesise and must be obtained from the diet (for example lysine and valine). Non-essential amino acids are those that the body can synthesise itself (for example glycine and alanine). A peptide bond is the covalent amide bond formed between the carboxyl group (-COOH) of one amino acid and the amino group (-NH2) of the next, with the removal of a water molecule.

The cell cycle is the sequence of events by which a cell duplicates its contents and divides into two daughter cells. It has two main phases: interphase (the preparatory, non-dividing phase) and the M phase (mitosis or division). Interphase is divided into three sub-phases: G1 phase (cell growth and normal metabolism), S phase (synthesis, in which DNA is replicated) and G2 phase (further growth and preparation for division).

Mitosis (M phase) has four stages: (1) Prophase - chromosomes condense and become visible as two sister chromatids, the spindle begins to form and the nuclear membrane and nucleolus start to disappear. (2) Metaphase - chromosomes align at the equator (metaphase plate) and spindle fibres attach to their centromeres. (3) Anaphase - the centromeres split and sister chromatids move to opposite poles. (4) Telophase - chromosomes reach the poles, decondense, and the nuclear membrane and nucleolus reappear, forming two nuclei.

Mitosis produces two daughter cells that are genetically identical to the parent cell, each with the same (diploid) number of chromosomes. Its significance: (1) it is responsible for growth of the body by increasing the number of cells; (2) it repairs and replaces old, worn-out or damaged cells and tissues; and (3) it maintains the constant chromosome number and enables asexual reproduction in some organisms.

Meiosis is a type of cell division that occurs in the reproductive cells and consists of two successive divisions, meiosis I and meiosis II, but only one round of DNA replication. It produces four haploid daughter cells from one diploid parent cell, so it is also called reductional division. Significance: (1) it halves the chromosome number so that fertilisation restores the diploid number in the next generation; and (2) crossing over during meiosis I produces genetic variation, which is important for evolution.

(1) Mitosis involves a single division producing two daughter cells, whereas meiosis involves two successive divisions producing four daughter cells. (2) In mitosis the daughter cells have the same (diploid) chromosome number as the parent and are genetically identical, whereas in meiosis the chromosome number is halved (haploid daughter cells) and genetic variation is introduced through crossing over. Mitosis occurs in body (somatic) cells while meiosis occurs in reproductive cells.

Cytokinesis is the division of the cytoplasm of a cell that follows nuclear division (karyokinesis) to form two separate daughter cells. In animal cells it occurs by the formation of a cleavage furrow that deepens and constricts the cell from the outside inward. In plant cells, because of the rigid cell wall, it occurs by the formation of a cell plate that grows from the centre outward to divide the cell.

Diffusion is the net movement of molecules of a substance from a region of higher concentration to a region of lower concentration until they are evenly distributed; it does not require a membrane or energy. Osmosis is the movement of solvent (water) molecules from a region of higher water potential to a region of lower water potential across a semipermeable membrane. Its two types are endosmosis (water entering the cell) and exosmosis (water leaving the cell).

Plasmolysis is the shrinkage of the protoplasm (cell contents) away from the cell wall when a plant cell is placed in a hypertonic solution, due to loss of water by exosmosis. Turgor pressure is the pressure exerted by the cell contents (the water-filled protoplast) against the cell wall when the cell is fully turgid (swollen with water); it keeps herbaceous plant parts firm and erect.

Transpiration is the loss of water in the form of water vapour from the aerial parts of a plant, mainly through the stomata of leaves. Significance: it creates a transpiration pull that helps in the upward transport (ascent) of water and minerals, cools the plant surface, and helps in the absorption of water by roots. Factors affecting it include light, temperature, humidity of the air, and wind speed - transpiration increases with light, higher temperature and wind, but decreases with high humidity.

Macronutrients are mineral elements required by plants in large amounts (more than 10 millimole per kg of dry matter); they include carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium and sulphur. Micronutrients (trace elements) are required in very small amounts (less than 10 millimole per kg of dry matter); they include iron, manganese, copper, zinc, boron, molybdenum and chlorine.

Biological nitrogen fixation is the conversion of atmospheric nitrogen (N2) into ammonia (a usable form) by living organisms using the enzyme nitrogenase. Rhizobium is a symbiotic bacterium that lives in the root nodules of leguminous plants (such as pea and gram); it fixes atmospheric nitrogen into ammonia, which the plant uses to make amino acids, while the plant provides the bacterium with food and shelter.

Photosynthesis is the process by which green plants and some other organisms synthesise their own food (glucose) from carbon dioxide and water using light energy trapped by chlorophyll, releasing oxygen as a by-product. Overall equation: 6CO2 + 12H2O -> (in the presence of light and chlorophyll) C6H12O6 + 6H2O + 6O2. It takes place in the chloroplast.

6CO2 + 12H2O -> C6H12O6 + 6H2O + 6O2

The light reaction takes place on the thylakoid membranes of the grana in the chloroplast. Chlorophyll absorbs light energy and gets excited, and this energy is used to split water molecules (photolysis of water) into hydrogen ions, electrons and oxygen; the oxygen is released. The energy is used to synthesise the energy-rich molecules ATP (by photophosphorylation) and NADPH. Thus the products of the light reaction are ATP, NADPH and O2, which are then used in the dark reaction.

In non-cyclic photophosphorylation both photosystems (PS I and PS II) take part, the electron travels in a one-way path, water is split (releasing oxygen), and both ATP and NADPH are produced. In cyclic photophosphorylation only PS I is involved, the electron is recycled back to the same photosystem, no water is split (so no oxygen is released) and only ATP is produced (no NADPH).