The resting potential is the electrical potential difference across the membrane of a neuron when it is not conducting an impulse; the inside is negatively charged relative to the outside (polarised), maintained by the sodium-potassium pump keeping more sodium ions outside and potassium ions inside. The action potential (nerve impulse) is the rapid reversal of this charge when a stimulus is applied: sodium ions rush in and the membrane becomes depolarised (positive inside), and this change travels along the axon as the nerve impulse before the resting state is restored.

A synapse is the junction between the axon terminal of one neuron and the dendrite (or cell body) of the next neuron, with a tiny gap called the synaptic cleft between them. When a nerve impulse reaches the axon terminal, it triggers the release of chemical messengers called neurotransmitters (such as acetylcholine) into the synaptic cleft. These diffuse across the gap and bind to receptors on the next neuron, generating a new impulse in it. Thus transmission across a synapse is chemical and one-directional.

Insulin is a hormone secreted by the beta cells of the pancreas; it lowers the level of glucose in the blood by promoting the uptake of glucose by cells and its conversion into glycogen in the liver. A deficiency of insulin causes the disease diabetes mellitus. Adrenaline (epinephrine) is secreted by the adrenal medulla; it is the emergency or fight-or-flight hormone that prepares the body for stress by increasing heart rate, blood pressure, breathing rate and blood glucose level.

The two reproductive whorls of a flower are the androecium and the gynoecium. The androecium is the male reproductive whorl made up of stamens; each stamen has an anther that produces pollen grains (which contain the male gametes). The gynoecium (pistil) is the female reproductive whorl made up of one or more carpels; each carpel has an ovary containing ovules (which contain the female gamete, the egg), a style and a stigma that receives the pollen.

A typical mature embryo sac (female gametophyte) is 7-celled and 8-nucleate. At the micropylar end there is the egg apparatus consisting of one egg cell and two synergids (with special thickenings called filiform apparatus). At the chalazal end there are three antipodal cells. In the centre there is a large central cell containing two polar nuclei. Thus, although there are 8 nuclei, there are only 7 cells because the central cell has two nuclei.

Pollination is the transfer of pollen grains from the anther to the stigma of a flower. Self-pollination (autogamy) is the transfer of pollen from the anther to the stigma of the same flower or another flower of the same plant; it maintains parental characters. Cross-pollination (allogamy) is the transfer of pollen to the stigma of a flower on a different plant of the same species; it brings about variation. Cross-pollination is carried out by agents such as wind, water, insects and birds.

After fertilisation: (1) the zygote develops into the embryo; (2) the primary endosperm nucleus develops into the endosperm, which nourishes the developing embryo; (3) the ovule develops into the seed and its integuments harden into the seed coat; and (4) the ovary develops into the fruit and its wall becomes the pericarp. Other parts of the flower such as the petals, stamens, style and stigma usually wither and fall off.

Apomixis is the formation of seeds without fertilisation; it is a form of asexual reproduction that mimics sexual reproduction (for example, in some species of grasses and Citrus). The seeds so formed are genetically identical to the parent. Polyembryony is the occurrence of more than one embryo in a single seed; it is commonly seen in Citrus and mango, where the extra embryos may develop from the cells of the nucellus.

Many flowering plants have devices to encourage cross-pollination (outbreeding) because continued self-pollination leads to inbreeding depression, and cross-pollination introduces genetic variation, which is important for adaptation and evolution. Two such outbreeding devices are: (1) unisexuality - the male and female flowers are separate (or on separate plants), so self-pollination is not possible; and (2) different timing of maturation of the anther and the stigma (dichogamy - protandry or protogyny), so that they are not receptive at the same time. Another device is self-incompatibility.

The human male reproductive system consists of: a pair of testes (the primary sex organs, which produce sperms and the male hormone testosterone), housed in the scrotum; a system of ducts, namely the epididymis, vas deferens, and urethra, that carry and store the sperms; and the accessory glands - the seminal vesicles, the prostate gland and the bulbourethral (Cowper's) glands - whose secretions form the seminal fluid. The penis is the copulatory organ.

The human female reproductive system consists of: a pair of ovaries (the primary sex organs, which produce ova/eggs and the hormones oestrogen and progesterone); a pair of oviducts (fallopian tubes), which carry the egg and are the usual site of fertilisation; the uterus (womb), where the embryo develops; the cervix, which connects the uterus to the vagina; and the vagina, which is the copulatory canal and birth canal.

Oogenesis is the process of formation of the mature female gamete (ovum) from the germ cells in the ovary; it begins during the embryonic stage. An oogonium forms a primary oocyte, which undergoes the first meiotic division to form a secondary oocyte and a small first polar body; the second meiotic division (completed only after fertilisation) gives one large ovum and a second polar body. Thus, one primary oocyte forms only one functional ovum, whereas in spermatogenesis one primary spermatocyte forms four functional sperms.

A human sperm is a microscopic, motile cell with three main parts: (1) a head, which contains the haploid nucleus and is capped by the acrosome (containing enzymes that help the sperm penetrate the egg); (2) a middle piece, which contains many mitochondria that provide the energy for movement; and (3) a tail (flagellum), which lashes to propel the sperm. A plasma membrane covers the whole body of the sperm.

Fertilization is the fusion of a sperm with the ovum to form a diploid zygote; it normally takes place in the ampullary region of the fallopian tube. The zygote then undergoes repeated cell divisions (cleavage) as it moves down the tube to form a hollow ball of cells called the blastocyst. Implantation is the attachment and embedding of the blastocyst into the thick, prepared wall (endometrium) of the uterus, where the embryo then develops further.

The placenta is a special disc-shaped structure that connects the developing foetus (embryo) to the wall of the mother's uterus; it is formed from the tissues of both the foetus and the mother and is connected to the foetus by the umbilical cord. Functions: (1) it allows the exchange of materials - it supplies oxygen and nutrients from the mother to the foetus and removes carbon dioxide and wastes from the foetus; and (2) it acts as an endocrine gland, secreting hormones such as hCG, oestrogen and progesterone to maintain the pregnancy.

Oestrogen is secreted mainly by the growing ovarian follicle; it stimulates the repair and thickening of the uterine lining after menstruation, develops secondary sexual characters, and (at high levels) triggers the LH surge that causes ovulation. Progesterone is secreted mainly by the corpus luteum after ovulation; it maintains and further thickens the uterine lining (endometrium) to prepare it for implantation and, along with other hormones, maintains pregnancy if fertilisation occurs.

Reproductive health means a total well-being in all aspects of reproduction - physical, emotional, behavioural and social. A reproductively healthy society is one whose people have physically and functionally normal reproductive organs and healthy attitudes towards reproduction. Two measures to achieve it are: (1) creating awareness through education about reproduction, safe practices and available facilities (for example the RCH - Reproductive and Child Health - programmes); and (2) providing medical facilities and care for pregnancy, contraception, birth control and treatment of sexually transmitted infections.

Contraceptive methods are used to prevent unwanted pregnancy. The main types are: (1) natural methods, such as the rhythm method (periodic abstinence) and withdrawal; (2) barrier methods, such as condoms and diaphragms, which prevent the sperm and ovum from meeting; (3) intrauterine devices (IUDs), such as the copper-T; (4) hormonal methods, such as oral contraceptive pills; and (5) surgical methods (sterilisation), such as vasectomy in males and tubectomy in females. An ideal contraceptive should be safe, effective and reversible.

Sexually transmitted diseases (STDs), also called venereal diseases, are infections that are transmitted mainly through sexual contact. Examples include gonorrhoea, syphilis, genital herpes, hepatitis B and AIDS (caused by HIV). They can be prevented by: avoiding sexual contact with unknown or multiple partners, using condoms, avoiding sharing needles, and getting early diagnosis and complete treatment. Awareness and safe practices are the best prevention.

Assisted reproductive technologies (ART) are special medical techniques used to help infertile couples have a child. In IVF (in vitro fertilisation), the ovum and sperms are collected and fertilisation is carried out outside the body in the laboratory (in a culture dish); the resulting early embryo is then transferred into the uterus (embryo transfer). A baby born by such a technique is commonly (though misleadingly) called a test-tube baby. Other ART methods include GIFT, ZIFT and ICSI.