The Golgi apparatus consists of a stack of flattened membrane-bound sacs called cisternae arranged parallel to one another, along with associated vesicles. Functions: it packages, modifies and sorts materials (such as proteins and lipids) received from the endoplasmic reticulum and secretes them in vesicles. It is also the site of formation of lysosomes and the synthesis of certain complex carbohydrates and cell wall materials.

Ribosomes are non-membrane-bound organelles made of RNA and protein; they are the site of protein synthesis. Lysosomes are single-membrane-bound vesicles containing powerful digestive (hydrolytic) enzymes that digest worn-out organelles, food and foreign material. They are called suicidal bags because on rupture their enzymes can digest the cell's own contents, causing the cell to die (autolysis).

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.

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.

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 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.

(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.

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

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).

The main photosynthetic pigments are chlorophyll a (the primary pigment, bluish-green), chlorophyll b (yellowish-green) and the carotenoids (carotenes and xanthophylls, which are yellow to orange). Chlorophyll a is the chief pigment that traps light and drives the light reaction. Chlorophyll b and the carotenoids are accessory pigments: they absorb light at wavelengths that chlorophyll a cannot and pass the energy to chlorophyll a, and carotenoids also protect chlorophyll from harmful photo-oxidation.

Blackman's law of limiting factors states that when a process is affected by more than one factor, its rate is limited by the factor that is nearest to its minimum (the limiting factor); it is that factor which directly determines the rate at that moment. The main factors affecting photosynthesis are external factors - light intensity, concentration of carbon dioxide, temperature and water - and internal factors such as the amount of chlorophyll.

Respiration is the process of breaking down food (glucose) to release energy stored as ATP. Aerobic respiration occurs in the presence of oxygen, completely oxidises glucose to carbon dioxide and water, and releases a large amount of energy (about 38 ATP per glucose). Anaerobic respiration occurs in the absence of oxygen, incompletely breaks down glucose into products such as ethanol or lactic acid, and releases only a small amount of energy (2 ATP per glucose).

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

The Krebs cycle (also called the citric acid cycle or TCA cycle) is the stage of aerobic respiration in which acetyl CoA (formed from pyruvic acid) is completely oxidised. It takes place in the matrix of the mitochondria. Each turn releases carbon dioxide and transfers energy to the reduced coenzymes NADH and FADH2, along with a small amount of ATP (GTP); the NADH and FADH2 then pass electrons to the electron transport chain to make more ATP.