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).

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

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

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.

Fermentation is the incomplete breakdown of glucose (anaerobic respiration) that occurs without oxygen. In alcoholic fermentation, pyruvic acid is converted into ethanol and carbon dioxide (for example by yeast), which is used in brewing and baking. In lactic acid fermentation, pyruvic acid is converted into lactic acid (for example in our muscles during vigorous exercise and by Lactobacillus in curd formation). Both release only a small amount of energy.

The respiratory quotient (RQ) is the ratio of the volume of carbon dioxide released to the volume of oxygen consumed during respiration. RQ = volume of CO2 evolved / volume of O2 consumed. For carbohydrates, which are completely oxidised, the value of RQ is 1. For fats the RQ is less than 1 (about 0.7) and for organic acids it is greater than 1.

RQ = CO2 evolved / O2 consumed

Growth is an irreversible and permanent increase in the size, mass or number of cells of an organism or its parts. In plants, growth has three phases along the growing region: (1) the phase of meristematic (cell division) activity at the tips of roots and shoots; (2) the phase of elongation, in which cells just behind the meristem enlarge; and (3) the phase of maturation, in which the enlarged cells differentiate and attain their final form and function.

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.

Abscisic acid (ABA) is a growth inhibitor: it promotes dormancy of seeds and buds, causes closure of stomata during water stress, and promotes abscission (shedding) of leaves and fruits; it is often called the stress hormone. Ethylene is a gaseous hormone that promotes the ripening of fruits, causes ageing and abscission of leaves and flowers, and breaks the dormancy of seeds and buds.

Photoperiodism is the response of plants to the relative lengths of day and night (the photoperiod), especially with regard to flowering; on this basis plants are grouped as short-day, long-day and day-neutral plants. Vernalisation is the promotion of flowering in certain plants by exposing them to a period of low temperature (cold treatment); it prevents premature flowering and enables the plant to flower at the proper time.

The human alimentary canal is a continuous tube from the mouth to the anus. In sequence it consists of: the mouth (buccal cavity), pharynx, oesophagus, stomach, small intestine (duodenum, jejunum and ileum) and large intestine (caecum, colon and rectum), ending at the anus. Associated digestive glands are the salivary glands, liver and pancreas.

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.

The dental formula of an adult human is 2123/2123, which represents, for one half of each jaw, 2 incisors, 1 canine, 2 premolars and 3 molars. Since this is for one half, the total number of teeth is 32. The four types of teeth are incisors (for cutting/biting), canines (for tearing), premolars and molars (for grinding and chewing). Human teeth are thecodont (fixed in sockets), diphyodont (two sets in life) and heterodont (different types).

2123/2123 (total 32 teeth)

Most of the absorption of digested food takes place in the small intestine, mainly the ileum. Its inner surface is highly folded and bears numerous finger-like projections called villi, and each villus has microvilli on its cells. These greatly increase the surface area for absorption. Each villus contains a network of blood capillaries and a lymph vessel (lacteal); simple sugars and amino acids pass into the blood capillaries while fatty acids and glycerol enter the lacteals.

Bile is a greenish-yellow fluid secreted by the liver and stored in the gall bladder; it is released into the duodenum. Although it contains no digestive enzymes, bile salts emulsify fats, that is, they break large fat globules into tiny droplets, thereby increasing their surface area for the action of the enzyme lipase. Bile also makes the medium alkaline (neutralising the acidic food from the stomach) so that intestinal enzymes can work.

Breathing is the physical process of taking in oxygen-rich air (inspiration) and giving out carbon dioxide-rich air (expiration). The human respiratory system consists, in sequence, of the external nostrils, nasal cavity, pharynx, larynx, trachea (windpipe), two bronchi, bronchioles and finally the alveoli of the lungs, where the exchange of gases takes place.

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.