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CBSE · Class 9 · Science

Chapter 11: Reproduction: How Life Continues

Asexual and sexual reproduction, vegetative propagation, flowers, pollination, fertilisation, animal reproduction and the human reproductive system.

Notes

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Reproduction and its two types
  • Reproduction is the biological process by which living organisms produce new individuals of their own kind. It ensures continuity of life and transfer of genetic information.
  • Asexual: a single parent produces offspring that are almost exact copies. Sexual: offspring inherit a mix of characters from two parents.
  • Differences between parents and offspring, accumulated over generations, help living beings adapt and sometimes give rise to new kinds of organisms.
Vegetative propagation in plants
  • New plants arise from vegetative parts (stems, roots, leaves) of a plant. Examples: potato and ginger (fleshy underground stems), money plant and sugarcane stem cuttings, Bryophyllum leaves sprouting plantlets.
  • Only one parent is involved, so the offspring are genetically identical.
  • Cutting: take shoots in the morning, remove leaves from the lower half, insert about half the length into soil with compost at 45–60°, and water regularly.
  • Grafting: a stem piece of Plant B is fitted into a slit in a rooted Plant A; the wound is wrapped with cloth or film to keep pests out until it heals.
  • Layering: bury the middle of a flexible twig (such as lemon) in soil; roots form in 10–15 days; then cut it from the parent.
  • Tissue culture: mass-produces healthy plantlets from the shoot tip (apical meristem). It helps remove virus-infected plants and gives high yield, e.g. banana.
  • These methods help farmers grow desirable crops on a large scale.
Asexual reproduction in other organisms
  • Budding: in yeast and hydra, repeated cell division at one site makes a small outgrowth (bud) that enlarges and separates to live independently. A hydra can have many buds at once.
  • Spore formation: fungi such as Rhizopus and Aspergillus form spores in a sac or on a swollen vesicle at the tip of hyphae. Spores are lightweight, usually single-celled, produced in millions and float in air until they find moisture and nutrients.
  • Moulds grow on moist bread in a warm place (25–35 °C); refrigeration slows or stops their reproduction.
  • Louis Pasteur proved that new life always comes from pre-existing life, strengthened the cell theory and led to food and instrument sterilisation.
  • The central process in asexual reproduction is mitosis: two daughter cells with the same number of identical chromosomes. Offspring are genetically identical clones. The method is fast and increases population quickly in favourable conditions.
Sexual reproduction and meiosis
  • Each species has a fixed chromosome number. Humans have 23 pairs (46 chromosomes), one of each pair from each parent.
  • Meiosis is the special cell division that forms gametes, reducing the chromosome number from diploid to half (haploid). If gametes carried the full set, the number would double in every generation.
  • Gametes: sperm (male) and egg (female) in animals; in plants the pollen grain carries the male gametes and the ovule contains the female gamete (egg).
  • Chromosomes of each pair separate randomly. With just three pairs of characters, 2 × 2 × 2 = 8 combinations are possible; with 23 pairs the number is huge. This mixing gives variation, important for survival and evolution.
Flower structure
  • A complete flower has four parts: sepals (outermost whorl, protect the bud), petals (coloured, attract pollinators), stamens (male) and pistil (female).
  • Stamen = filament + anther (produces pollen grains containing male gametes).
  • Pistil = stigma (tip, sticky or flat), style (long tube), ovary (swollen base containing ovules; each ovule has an egg cell).
Pollination
  • Pollination: transfer of pollen grains from the anther to the stigma. Experiment on pea: removing stamens from a bud stops fruit formation, so pollen transfer is necessary.
  • Self-pollination: pollen reaches the stigma of the same flower or another flower of the same plant.
  • Cross-pollination: pollen goes to the stigma of a flower of another plant of the same type.
  • Pollinators: wind, water, insects, birds. Wind (wheat, maize, rice): light, small pollen in huge numbers, long feathery stigma. Water (Vallisneria, Hydrilla). Insects (sunflower, hibiscus, marigold): bright flowers, nectar, fragrance, large sticky or spiny pollen and sticky stigma. Birds (coral tree, hibiscus): white-eyes and sunbirds.
  • Wind-pollinated grasses release 5,00,000–10,00,000 pollen grains and form 50–200 seeds; insect-pollinated sunflower releases 20,000–40,000 and forms 800–1,000 seeds. Huge pollen numbers compensate for the low chance of each grain reaching a stigma.
Fertilisation and seed formation
  • A pollen grain on a compatible stigma grows a pollen tube through the style to the ovary. The male gamete moves down and fuses with the egg in the ovule: fertilisation.
  • The fertilised egg is the zygote, which develops into an embryo.
  • The ovary becomes the fruit and the ovules become seeds. Seeds are dispersed by wind, water and animals (madar and dandelion seeds are light and hairy for wind dispersal) and germinate when water, air and temperature are favourable.
  • Order: pollination → pollen germination on the stigma → pollen tube growth → fertilisation → zygote → seed and fruit.
Reproduction in animals
  • External fertilisation (fish, frogs): eggs and sperm released into water; many eggs are destroyed or eaten, so the number of eggs is large (fish hundreds to thousands; frog 5,000–50,000) and survival of young is low.
  • Internal fertilisation (reptiles, birds, mammals): inside the female body; fewer eggs (lizard 2–20, bird 1–15) and better survival.
  • Fish, amphibians and insects: yolk is enough only for a larva, which feeds and grows; then a transformation gives the adult (butterfly: egg → larva → pupa → adult). Reptiles and birds: each egg has enough yolk to develop a young one. Mammals: the zygote grows inside the female body, and the young are fed on milk.
Human reproductive systems
  • Male: two testes in the scrotum (keeps them slightly cooler) produce sperm and a hormone that controls sperm production and puberty changes. Sperm travel through the vas deferens to the urethra (common passage for urine and sperm). Seminal vesicles and the prostate gland add nourishing fluids. A sperm has a head with genetic material and a long tail.
  • Female: a pair of ovaries (produce eggs and hormones), oviducts (fallopian tubes), the uterus (where the foetus develops), the cervix (narrow passage) and the vagina.
  • Gametogenesis is the formation of gametes by meiosis: sperm and egg have 23 chromosomes (body cells have 46), so the zygote has 46.
  • Sperm: very small, millions, no stored nutrients, actively motile. Egg: large, few, nutrients stored, non-motile.
Fertilisation, menstrual cycle and pregnancy
  • From puberty usually one mature egg is released every month from one ovary: ovulation. The uterine lining thickens beforehand. The egg moves into the oviduct; if a sperm fuses with it, a zygote forms (fertilisation in the oviduct).
  • The zygote divides by mitosis while travelling to the uterus and then implants in the lining; implantation marks the start of pregnancy.
  • If the egg is not fertilised it degenerates after about a day, and the lining sheds with some blood: menstruation (3–7 days). The cycle repeats every 21–35 days (often about 28), from puberty (10–14 years) till menopause (about 50).
  • 28-day cycle: days 1–5 menstruation; 6–14 lining rebuilds and egg matures; day 14 ovulation; days 15–28 lining thickens, and breaks down about day 28 if there is no fertilisation.
  • Sex determination: females XX, males XY. The mother always gives X; the father gives X or Y, so the father's chromosome decides the baby's sex.
  • Pregnancy lasts about nine months in three trimesters: embryo in the first two months (foetus from about the ninth week), growth in the second, readiness for birth in the third. Contractions of uterine muscles push the foetus out.
  • Care: breastfeeding, vaccination, balanced diet and rest for the mother, check-ups, avoiding smoking, alcohol and medicines without advice. Menstrual hygiene: clean pads or cups, change in 4–6 hours, wash hands, dispose of wrapped (do not flush).
Sexual maturity, contraception and STIs
  • Sexual maturity happens gradually in adolescence, but emotional maturity takes longer; responsible decisions prevent unplanned pregnancy and infections.
  • Sexually transmitted infections (STIs): gonorrhoea, herpes, syphilis, genital warts, HIV (can lead to AIDS). Some are not curable. Condoms help prevent them and also prevent pregnancy.
  • Contraception: barriers (condoms, vaginal covers), oral pills (change release of eggs by altering hormones), IUD such as copper-T placed in the uterus, and surgical methods (blocking the vas deferens or fallopian tubes).
  • Abortion: removal of an unwanted pregnancy by surgery, generally within the first trimester. Prenatal sex determination is strictly prohibited by law in India to prevent an imbalance in the sex ratio.