How do Organisms Reproduce?

In asexual reproduction, a single organism produces offspring that are genetically identical (clones). This is faster but produces no genetic variation. Methods include: (1) Fission — the organism splits into two or more parts. Binary fission in Amoeba (splits into two equal halves); multiple fission in Plasmodium (splits into many cells inside a cyst). (2) Budding — a small bud grows on the parent, develops, and detaches (Hydra, yeast). (3) Spore formation — organisms produce tiny spores with thick protective walls that germinate under favourable conditions (Rhizopus/bread mould). (4) Fragmentation — the organism breaks into pieces, each growing into a new individual (Spirogyra). (5) Regeneration — an organism regrows from a cut part, using specialised cells (Planaria, Hydra). (6) Vegetative propagation — new plants grow from vegetative parts: roots (sweet potato), stems (potato tuber — eyes are buds), leaves (Bryophyllum — buds on leaf margins). Artificial methods include layering, cutting, and grafting.

Flowers are the reproductive organs of plants. A complete flower has four whorls: sepals (protect bud), petals (attract pollinators), stamens (male: filament + anther containing pollen grains), and pistil/carpel (female: stigma + style + ovary containing ovules). Pollination is the transfer of pollen from anther to stigma — self-pollination (same flower) or cross-pollination (different flower, by wind/water/insects/birds). After pollination, the pollen grain germinates on the stigma, grows a pollen tube down the style to the ovary, and releases male gametes. Fertilisation occurs when a male gamete fuses with the egg cell inside the ovule. After fertilisation, the ovule develops into a seed (containing the embryo) and the ovary develops into a fruit. The seed has a seed coat, stored food (cotyledons or endosperm), and an embryo (radicle + plumule).

The male reproductive system consists of: testes (paired, produce sperm and testosterone, located in scrotum outside body for lower temperature), vas deferens (carries sperm), seminal vesicles and prostate gland (add fluids to make semen), and urethra (common passage for urine and semen). The female reproductive system consists of: ovaries (paired, produce eggs/ova and oestrogen), fallopian tubes/oviducts (where fertilisation occurs), uterus (where embryo implants and develops), and vagina (birth canal). Fertilisation occurs in the fallopian tube when a sperm fuses with an egg to form a zygote. The zygote divides repeatedly as it moves towards the uterus and implants in the uterine wall (implantation). The embryo gets nutrition from the mother through the placenta — a disc-like tissue with finger-like villi that provide a large surface area for exchange of glucose, oxygen, and waste between maternal and foetal blood (without mixing). After about 9 months, the baby is born.

The menstrual cycle is approximately 28 days long and is controlled by hormones. Each month, one ovary releases a mature egg (ovulation, around day 14). Simultaneously, the uterus prepares for pregnancy by thickening its lining with blood vessels. If the egg is fertilised, it implants in this lining and pregnancy begins. If the egg is not fertilised, the thickened uterine lining breaks down and is shed along with blood and the unfertilised egg — this is menstruation (lasts about 3-5 days). Contraception methods prevent pregnancy: (1) Barrier methods — condoms (male/female), diaphragm; physically block sperm. (2) Chemical methods — oral pills (hormones that prevent ovulation), copper-T/IUD (placed in uterus, prevents implantation). (3) Surgical methods — vasectomy (vas deferens cut in males), tubectomy (fallopian tubes cut in females). Only condoms protect against both pregnancy and sexually transmitted diseases (STDs) like HIV/AIDS.