Reproduction.

No organism lives forever — yet life itself is unbroken. Reproduction is the process by which living things make new individuals of their own kind, passing on a copy of their genetic information so the species continues. From a single budding yeast cell to the carefully timed monthly cycle of the human body, it is the one process without which life would simply stop.

1 · Asexual vs sexual reproduction

There are two broad ways to make new life. In asexual reproduction a single parent produces offspring by mitosis alone — no gametes are made and no fusion occurs. The offspring are genetically identical to the parent and to each other; they form a clone. It is fast, needs only one parent, and is common in bacteria (binary fission), yeast (budding), Hydra (budding), Amoeba, and in plants by vegetative propagation (runners, tubers, bulbs, cuttings).

In sexual reproduction two parents each contribute a sex cell, or gamete, made by meiosis. A male gamete and a female gamete fuse (fertilisation) to form a zygote, which grows into the new individual. Because the gametes are genetically varied and combine at random, the offspring show genetic variation — they differ from their parents and from one another.

2 · Reproduction in flowering plants — the flower

The flower is the reproductive organ of a flowering plant (angiosperm). A typical flower is built from four whorls attached to the swollen tip of the stalk, the receptacle:

• Sepals (calyx) — the outer green leaf-like ring that protects the bud.
• Petals (corolla) — often large and coloured to attract insects in insect-pollinated flowers.
• Stamens (androecium) — the male parts. Each stamen is an anther (which makes pollen grains, containing the male gametes) on a stalk, the filament.
• Carpel / pistil (gynoecium) — the female part: a sticky stigma that receives pollen, a style that supports it, and an ovary at the base containing one or more ovules. Each ovule holds the female gamete (the egg cell) inside an embryo sac.

3 · Pollination

Pollination is the transfer of pollen grains from an anther to a stigma. It is not the same as fertilisation — it only delivers the pollen; fusion of gametes comes later.

• Self-pollination — pollen lands on the stigma of the same flower (or another flower on the same plant). It guarantees seed but gives little variation.
• Cross-pollination — pollen is carried to a stigma on a different plant of the same species, giving greater genetic variation.

Pollen is moved by insects (bright petals, scent, nectar, sticky sculptured pollen) or by wind (small dull flowers, no nectar, large feathery stigmas and loosely held anthers releasing light, smooth pollen in huge amounts).

4 · Double fertilisation, seed & fruit

Once a compatible pollen grain lands on the stigma it germinates and grows a pollen tube down through the style toward the ovule, guided chemically. The tube carries the two male gametes to the embryo sac. There, the event unique to flowering plants takes place — double fertilisation:

• One male gamete fuses with the egg cell → a diploid (2n) zygote, which grows into the embryo of the seed.
• The second male gamete fuses with the two polar nuclei → a triploid (3n) endosperm, the food store that nourishes the growing embryo.

After fertilisation the parts change function: the ovule becomes the seed (its wall hardening into the seed coat / testa), and the whole ovary becomes the fruit, which protects the seeds and aids their dispersal. The petals, stamens, style and stigma wither and fall.

5 · The human male reproductive system

The male system makes sperm and delivers them. Its parts:

• Testes — paired organs in the scrotum (held outside the body so they stay a little below 37 °C, which sperm production needs). Inside are coiled seminiferous tubules where sperm are made, and interstitial (Leydig) cells that secrete the male hormone testosterone.
• Epididymis — a coiled tube on each testis where sperm mature and are stored.
• Sperm duct (vas deferens) — carries sperm from the epididymis up toward the urethra.
• Accessory glands — the seminal vesicles, prostate gland and Cowper's (bulbourethral) glands add fluids (rich in fructose for energy, alkaline to neutralise acidity) that, with the sperm, form semen.
• Urethra — runs through the penis and carries semen (and, at other times, urine) out of the body.

6 · The human female reproductive system

The female system makes eggs, receives sperm, and supports the developing embryo. Its parts:

• Ovaries — paired organs that contain the follicles; each month they release one secondary oocyte (egg) and secrete the hormones oestrogen and progesterone.
• Oviducts (Fallopian tubes) — funnel-shaped tubes that catch the released egg; their ciliated lining sweeps it toward the uterus, and fertilisation normally happens here.
• Uterus (womb) — a muscular organ with a thick, blood-rich lining, the endometrium, in which the embryo implants and develops.
• Cervix — the narrow neck of the uterus opening into the vagina.
• Vagina — the muscular canal that receives the penis and semen, and forms the birth canal.

7 · Gametogenesis

Gametogenesis is the formation of gametes from diploid (2n) germ cells by meiosis, which halves the chromosome number to haploid (n) so that fertilisation restores the diploid number.

Spermatogenesis (in the testes)

A diploid spermatogonium grows into a primary spermatocyte (2n). Meiosis I gives two secondary spermatocytes (n); meiosis II gives four haploid spermatids, which mature into four functional sperm. So one germ cell → four sperm, and production runs continuously from puberty onward.

Oogenesis (in the ovaries)

A diploid oogonium becomes a primary oocyte (2n). Meiosis is unequal: cytoplasm is kept in one large cell so the egg has a food store. Meiosis I gives one secondary oocyte and a tiny first polar body; meiosis II (completed only if a sperm enters) gives one ovum and a second polar body. So one germ cell → one ovum + (up to three) polar bodies, and only one egg is usually released per month.

8 · The menstrual cycle

The menstrual cycle is the roughly 28-day cycle that prepares the female body for pregnancy each month. It is controlled by four hormones — two from the anterior pituitary (FSH and LH) and two from the ovary (oestrogen and progesterone) — working together as a feedback system. It has three phases:

Hormonal control by feedback

• FSH (pituitary) → starts a follicle growing and makes it secrete oestrogen.
• Oestrogen (follicle) → thickens the uterine lining; at its peak it triggers the LH surge (positive feedback).
• LH (pituitary) → causes ovulation and turns the follicle into the corpus luteum.
• Progesterone (corpus luteum) → maintains the lining and, with oestrogen, inhibits FSH and LH (negative feedback) so no new follicle starts while one egg is in play.

9 · Fertilisation & early development

Fertilisation is the fusion of a sperm with the secondary oocyte, normally in the oviduct. One sperm penetrates the egg; their haploid nuclei fuse to restore the diploid zygote (2n). The zygote divides by mitosis as it travels down the oviduct, becoming a hollow ball of cells (a blastocyst), which then implants in the prepared endometrium about a week later — the start of pregnancy.

10 · Contraception & reproductive health

Contraception is the deliberate prevention of pregnancy. Methods work by different routes:

• Barrier methods (condom, diaphragm) physically stop sperm reaching the egg; condoms also reduce sexually transmitted infections.
• Hormonal methods (the contraceptive pill) supply oestrogen/progesterone that inhibit FSH and LH, so no follicle matures and ovulation is prevented.
• Intrauterine devices (IUDs) prevent implantation in the uterus.
• Surgical methods (vasectomy, tubal ligation) permanently block the sperm duct or oviduct.

Reproductive health also means avoiding and treating sexually transmitted infections, sound maternal nutrition and care during pregnancy, and family planning — all part of the BIEK syllabus' focus on healthy populations.