The nucleus is a spherical, membrane-bound organelle that controls the activities of the cell. It is bounded by a double membrane called the nuclear envelope, which has nuclear pores for exchange of materials. Inside is the nucleoplasm containing chromatin (DNA plus proteins) and one or more nucleoli. Functions: it stores the genetic material (DNA), controls cell metabolism and heredity, and the nucleolus is the site of ribosome (rRNA) synthesis. During cell division the chromatin condenses into chromosomes.

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.

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.

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.

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.

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.

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.

The Calvin cycle (C3 pathway) occurs in the stroma of the chloroplast and does not directly need light. Its three phases are: (1) Carboxylation - carbon dioxide combines with a 5-carbon acceptor RuBP, catalysed by the enzyme RuBisCO, to form two molecules of 3-carbon 3-phosphoglyceric acid (3-PGA). (2) Reduction - 3-PGA is reduced to G3P (a sugar) using ATP and NADPH from the light reaction. (3) Regeneration - some G3P is used to regenerate the acceptor RuBP so the cycle can continue, using ATP. Glucose is formed from the G3P produced.

(1) In C3 plants the first stable product of carbon fixation is the 3-carbon compound 3-PGA, whereas in C4 plants it is the 4-carbon compound oxaloacetic acid (OAA). (2) C3 plants use only the Calvin cycle, while C4 plants use the Hatch-Slack pathway in addition to the Calvin cycle. (3) C4 plants have a special leaf anatomy called Kranz anatomy with bundle-sheath cells and are more efficient photosynthetically (e.g. maize, sugarcane), while C3 plants lack Kranz anatomy (e.g. rice, wheat).

Glycolysis (the EMP pathway) is the first step of respiration in which one molecule of glucose (6 carbon) is partially broken down into two molecules of pyruvic acid (3 carbon). It occurs in the cytoplasm of the cell and does not require oxygen. Its net products from one glucose molecule are 2 molecules of pyruvic acid, a net gain of 2 ATP and 2 molecules of NADH (reduced coenzyme).

glucose -> 2 pyruvic acid + 2 ATP + 2 NADH

Auxin promotes cell elongation, apical dominance (suppression of lateral buds by the apical bud), and is used in rooting of stem cuttings and preventing premature fruit drop. Gibberellin promotes stem elongation (increases the length of the internodes), breaks seed and bud dormancy, and can induce bolting in rosette plants. Cytokinin promotes cell division (cytokinesis), delays the ageing (senescence) of leaves and helps in the growth of lateral buds.

Salivary amylase (ptyalin) is present in saliva in the mouth and acts on starch, breaking it down into maltose. Pepsin is secreted by the stomach (active in acidic medium) and acts on proteins, breaking them into peptones and proteoses. Trypsin is secreted by the pancreas and acts in the small intestine on proteins and peptones, breaking them into smaller peptides and amino acids.

During inspiration, the diaphragm contracts and flattens and the external intercostal muscles contract, raising the ribs and sternum. This increases the volume of the thoracic cavity and lowers the pressure inside the lungs below atmospheric pressure, so air rushes in. During expiration, the diaphragm and intercostal muscles relax, so the diaphragm becomes dome-shaped and the ribs move down and in. This decreases the volume of the thoracic cavity and raises the pressure in the lungs above atmospheric pressure, so air is pushed out.

The human heart is a muscular organ with four chambers: two upper thin-walled atria (right and left) and two lower thick-walled ventricles (right and left). The right side handles deoxygenated blood and the left side handles oxygenated blood, kept separate by a septum. Double circulation means the blood passes through the heart twice in one complete cycle: in pulmonary circulation deoxygenated blood is pumped from the right ventricle to the lungs and returns oxygenated to the left atrium; in systemic circulation oxygenated blood is pumped from the left ventricle to the body and returns deoxygenated to the right atrium.

The cardiac cycle is the sequence of events that occurs during one complete heartbeat, lasting about 0.8 second. It has two main phases: systole, the contraction phase, in which the heart muscle contracts and pumps blood out (atrial systole pushes blood into the ventricles, and ventricular systole pumps blood into the aorta and pulmonary artery); and diastole, the relaxation phase, in which the heart muscle relaxes and the chambers fill with blood. The rhythmic contraction and relaxation ensure continuous circulation of blood.

The nephron is the structural and functional unit of the kidney; each kidney has about one million nephrons. Each nephron has two main parts: (1) the Malpighian body (renal corpuscle), consisting of a cup-shaped Bowman's capsule enclosing a bunch of capillaries called the glomerulus; and (2) the renal tubule, which continues from Bowman's capsule as the proximal convoluted tubule (PCT), then the U-shaped loop of Henle, then the distal convoluted tubule (DCT), which opens into a collecting duct. Blood is filtered in the glomerulus and the filtrate is processed along the tubule to form urine.

Urine formation involves three steps: (1) Glomerular filtration - blood is filtered under pressure in the glomerulus, and water, salts, glucose, urea and other small molecules pass into Bowman's capsule as the glomerular filtrate (blood cells and proteins are not filtered). (2) Tubular reabsorption - as the filtrate passes along the tubule, useful substances such as glucose, amino acids, most water and essential salts are reabsorbed back into the blood. (3) Tubular secretion - additional waste substances such as extra hydrogen ions, potassium ions and ammonia are actively secreted from the blood into the filtrate. The remaining fluid is urine.

According to the sliding filament theory, muscle contraction occurs when the thin actin filaments slide over the thick myosin filaments, so the length of the filaments does not change but the sarcomere shortens. When a nerve impulse arrives, calcium ions are released and expose binding sites on actin; the myosin heads attach to actin forming cross-bridges, and using energy from ATP they pull the actin filaments towards the centre of the sarcomere. As a result the Z-lines come closer and the muscle shortens (contracts). When the impulse stops, calcium is pumped back and the muscle relaxes.

A reflex action is a sudden, rapid, automatic (involuntary) response to a stimulus that is controlled by the spinal cord without involving conscious thought by the brain (for example, quickly withdrawing the hand from a hot object). The pathway is called the reflex arc, whose components are: a receptor (detects the stimulus), a sensory (afferent) neuron (carries the impulse to the spinal cord), an interneuron in the spinal cord (relays the impulse), a motor (efferent) neuron (carries the impulse to the effector), and an effector such as a muscle or gland that produces the response.

Endocrine glands are ductless glands that pour their secretions (hormones) directly into the blood. Four important ones are: (1) the pituitary gland, which secretes growth hormone (and is called the master gland); (2) the thyroid gland, which secretes thyroxine; (3) the pancreas (islets of Langerhans), which secretes insulin and glucagon; and (4) the adrenal gland, which secretes adrenaline (epinephrine).