Reproduction in Plants and Humans — NEET Biology
Reproduction is one of the largest scoring blocks in NEET Biology, worth 8–10 questions a year across the plant and human chapters. It rewards exact structural recall — the parts of the flower and the embryo sac, the events of double fertilisation, the ploidy of every cell in the life cycle, the phases of the menstrual cycle and the hormones that drive them. This chapter builds flowering-plant reproduction first, then the human system, in the ordered, value-rich form the exam quotes almost verbatim.
Part A — Sexual Reproduction in Flowering Plants
1. The flower and its whorls
The flower is the reproductive shoot. It has four whorls on the thalamus:
- Calyx (sepals — protect the bud), corolla (petals — attract pollinators) — the accessory whorls.
- Androecium (stamens — the male whorl; each stamen = anther + filament) and gynoecium (carpels/pistil — the female whorl; each = stigma + style + ovary) — the essential whorls.
A bisexual flower has both androecium and gynoecium; a unisexual flower has only one.
2. The stamen and microsporogenesis — making pollen
The anther is typically tetrasporangiate (four pollen sacs). Inside, diploid microspore mother cells undergo meiosis to form haploid microspores in tetrads — this is microsporogenesis.
Each microspore matures into a pollen grain, whose wall has a tough outer exine (of sporopollenin, the most resistant biological material) and an inner intine. At shedding a pollen grain is usually 2-celled — a vegetative cell and a generative cell; the generative cell later divides to give two male gametes (so the mature male gametophyte is 3-celled).
The innermost anther wall layer, the tapetum, nourishes the developing pollen.
Worked example 2.1. How many pollen grains and how many male gametes arise from one microspore mother cell? One microspore mother cell undergoes meiosis to give four microspores → four pollen grains. Each pollen grain's generative cell divides to form two male gametes, so one mother cell ultimately yields 4 pollen grains carrying 8 male gametes in total.
3. The pistil and megasporogenesis — making the embryo sac
Inside the ovary, ovules (megasporangia) develop. In each ovule a diploid megaspore mother cell undergoes meiosis to form four megaspores; usually three degenerate and one functional megaspore survives — this is megasporogenesis.
The functional megaspore develops into the female gametophyte (embryo sac) by three free-nuclear mitotic divisions, giving the typical 7-celled, 8-nucleate embryo sac:
- Egg apparatus at the micropylar end: one egg cell + two synergids (with the filiform apparatus that guides the pollen tube).
- Three antipodal cells at the chalazal end.
- One central cell with two polar nuclei (which fuse to form the diploid secondary nucleus).
Worked example 3.1. A typical embryo sac is described as 7-celled and 8-nucleate. Reconcile these numbers. The embryo sac has 8 nuclei but only 7 cells because the large central cell contains two (polar) nuclei. Counting cells: 1 egg + 2 synergids + 3 antipodals + 1 central cell = 7 cells; counting nuclei: those six single-nucleus cells (6) + the two polar nuclei of the central cell (2) = 8 nuclei.
4. Pollination
Pollination is the transfer of pollen from anther to stigma.
- Self-pollination (autogamy): pollen to the stigma of the same flower.
- Cross-pollination (xenogamy): pollen to the stigma of a different plant — brings genetic variation.
Agents: wind (anemophily), water (hydrophily), insects/animals (entomophily/zoophily). Plants have outbreeding devices (self-incompatibility, unisexuality, different maturation times of anther and stigma) to promote cross-pollination.
5. Double fertilisation — unique to angiosperms
The pollen grain germinates on the stigma; the pollen tube grows through the style, enters the ovule (usually through the micropyle) and discharges two male gametes into the embryo sac. Then, in the event unique to flowering plants, double fertilisation occurs:
- Syngamy: one male gamete (n) + egg (n) → zygote (2n) → embryo.
- Triple fusion: the other male gamete (n) + the two polar nuclei / secondary nucleus (2n) → primary endosperm nucleus (3n) → endosperm (nourishes the embryo).
Worked example 5.1. Why is fertilisation in angiosperms called "double fertilisation", and what is the ploidy of the two products? Because two fusion events occur with the two male gametes: syngamy gives the diploid (2n) zygote, and triple fusion gives the triploid (3n) primary endosperm nucleus. So the embryo is 2n and the endosperm is 3n — a defining feature of angiosperms.
6. Seed and fruit — post-fertilisation
After fertilisation:
- Ovule → seed (integuments → seed coat; zygote → embryo; PEN → endosperm).
- Ovary → fruit (ovary wall → pericarp).
A fruit formed from the ovary alone is true; one involving other floral parts (e.g. apple) is false. A fruit formed without fertilisation is parthenocarpic (e.g. banana — seedless). Apomixis is seed formation without fertilisation; polyembryony is more than one embryo in a seed.
Part B — Human Reproduction
7. The male reproductive system
- Testes (in the scrotum, ~35°C, below body temperature) contain seminiferous tubules where sperm form, and Leydig (interstitial) cells that secrete testosterone. Sertoli cells nourish developing sperm.
- Ducts: epididymis → vas deferens → ejaculatory duct → urethra.
- Glands: seminal vesicles, prostate, bulbourethral (Cowper's) — secretions + sperm = semen.
A sperm has a head (nucleus + acrosome with enzymes to penetrate the egg), a middle piece (packed with mitochondria for energy) and a tail (flagellum for motility).
8. The female reproductive system
- Ovaries produce eggs (ova) and secrete oestrogen and progesterone.
- Fallopian tubes (oviducts) — carry the egg; fertilisation normally occurs in the ampulla (the wider part).
- Uterus (womb) — its inner lining, the endometrium, is where the embryo implants; cervix opens into the vagina.
9. Gametogenesis — spermatogenesis and oogenesis
Spermatogenesis (in seminiferous tubules): diploid spermatogonia → (mitosis) → primary spermatocytes (2n) → (meiosis I) → secondary spermatocytes (n) → (meiosis II) → spermatids (n) → (spermiogenesis) → sperms. One primary spermatocyte → 4 sperms; a continuous process from puberty.
Oogenesis (in the ovary): oogonia → primary oocytes (2n) — these arrest in prophase I before birth. At each cycle one completes meiosis I to give a secondary oocyte (n) + a polar body; meiosis II completes only if fertilised. One primary oocyte → 1 ovum + polar bodies (unequal division conserves cytoplasm).
Worked example 9.1. Compare the number of functional gametes from one primary spermatocyte versus one primary oocyte. One primary spermatocyte produces four functional sperms (equal meiotic divisions). One primary oocyte produces only one functional ovum (plus 2–3 polar bodies that degenerate), because the cytoplasm is divided unequally to give the egg a large food store.
10. The menstrual cycle
The human menstrual cycle averages 28 days (menarche at puberty to menopause ~45–50 yr). Four phases:
- Menstrual phase (days 1–5): the endometrium sheds (bleeding) if no fertilisation.
- Follicular / proliferative phase (days 6–13): FSH grows a follicle; rising oestrogen rebuilds the endometrium.
- Ovulation (~day 14): a surge of LH ruptures the follicle and releases the secondary oocyte.
- Luteal / secretory phase (days 15–28): the corpus luteum secretes progesterone, maintaining the endometrium. If no pregnancy, it degenerates → progesterone falls → menstruation.
Worked example 10.1. Which hormone surge triggers ovulation, and from which gland? A sharp surge of LH (luteinising hormone) from the anterior pituitary, around day 14, triggers ovulation — the rupture of the mature Graafian follicle and release of the secondary oocyte. FSH grows the follicle; the LH surge releases the egg.
11. Fertilisation, implantation and pregnancy
- Fertilisation (in the ampulla of the oviduct): a sperm penetrates the secondary oocyte → completes meiosis II → zygote (2n).
- Cleavage produces a morula, then a blastocyst, which implants in the endometrium (~day 7).
- The placenta forms — the exchange organ between mother and foetus (nutrients, O₂, wastes) and an endocrine organ (secretes hCG, hPL, oestrogen, progesterone). hCG maintains the corpus luteum early on (basis of pregnancy tests).
- Gestation in humans is about 9 months (~280 days). Parturition (birth) is triggered by oxytocin (from the posterior pituitary); prolactin and oxytocin drive lactation — the first milk, colostrum, is rich in antibodies.
Worked example 11.1. Where does fertilisation normally occur, and what hormone underlies the common pregnancy test? Fertilisation normally occurs in the ampulla of the fallopian tube. The pregnancy test detects hCG (human chorionic gonadotropin) — secreted by the developing placenta — in the mother's urine; hCG also sustains the corpus luteum so progesterone continues early in pregnancy.
12. Common traps NEET sets here
- Microspore mother cell → 4 microspores → 4 pollen grains; each pollen → 2 male gametes.
- Megaspore mother cell → 4 megaspores, 3 degenerate, 1 functional.
- Embryo sac: 7-celled, 8-nucleate (central cell has 2 polar nuclei).
- Double fertilisation: syngamy (zygote 2n) + triple fusion (endosperm 3n).
- Sporopollenin (exine) is the most resistant biological material; tapetum nourishes pollen.
- One primary spermatocyte → 4 sperms; one primary oocyte → 1 ovum + polar bodies.
- Primary oocytes arrest in prophase I before birth; meiosis II completes only on fertilisation.
- FSH grows follicle; LH surge → ovulation (~day 14); corpus luteum → progesterone.
- Fertilisation in the ampulla; hCG maintains corpus luteum (pregnancy test).
- Oxytocin → parturition; prolactin → milk; colostrum has antibodies.
13. Memory aids
- "MMC makes 4, each pollen makes 2" — microspore mother cell → 4 pollen, each → 2 gametes.
- "3 die, 1 survives" — the four megaspores.
- "7 cells, 8 nuclei — the central cell counts twice" — the embryo sac.
- "2n zygote + 3n endosperm = double fertilisation."
- "Sperm 4-for-1, Egg 1-for-1" — gamete yields.
- "FSH Fills the follicle, LH Lets the egg go."
- "hCG holds the corpus luteum" — early-pregnancy hormone.
14. Exam protocol
- Flower whorls; anther (microsporogenesis → 4 pollen, each → 2 gametes); pollen wall (exine/sporopollenin, intine).
- Ovule (megasporogenesis → 1 functional megaspore); embryo sac (7-celled, 8-nucleate; egg apparatus, antipodals, central cell).
- Pollination types/agents; double fertilisation (syngamy 2n + triple fusion 3n).
- Post-fertilisation: ovule → seed, ovary → fruit; parthenocarpy, apomixis, polyembryony.
- Male system (testes/seminiferous tubules, Leydig → testosterone, sperm structure); female system (ovary, oviduct ampulla, uterus/endometrium).
- Spermatogenesis (1 → 4) vs oogenesis (1 → 1); menstrual cycle phases + hormones (FSH, LH surge, progesterone).
- Fertilisation (ampulla), implantation, placenta/hCG, gestation ~9 months, parturition (oxytocin), lactation (prolactin, colostrum).
