Reproduction in Organisms and Sexual Reproduction in Flowering Plants

Why This Chapter Matters

Reproduction is the first two chapters of Class 12 Biology and carries 12-15 marks in CBSE boards. Human reproduction and reproductive health are also included. All are high-scoring with straightforward direct questions.

Core Concepts

1. Reproduction in Organisms

Asexual reproduction: Single parent, no gametes, offspring genetically identical (clone).

Types: Binary fission (Amoeba, Paramecium), Budding (Hydra, Yeast), Fragmentation (Spirogyra), Regeneration (Planaria, Hydra), Vegetative propagation (plants).

Sexual reproduction: Two parents (or one — self-fertilisation), gametes involved, genetic variation.

Gametogenesis → Gamete transfer → Fertilisation → Zygote → Embryo → Organism.

Juvenile/Vegetative phase → Reproductive phase → Senescent phase.

In plants: reproductive phase = flowering.

Annual plants: once in lifetime. Perennial: multiple seasons.

2. Sexual Reproduction in Flowering Plants

Flower structure:

Sepals (calyx), Petals (corolla), Stamens (androecium — male), Carpels (gynoecium — female).

Bisexual (perfect): both stamens and carpels. Unisexual: either stamens or carpels.

Stamen structure:

Filament + Anther. Anther: 4 microsporangia (bilobed). Each microsporangium → pollen grains.

Pollen grain: 2-celled (generative cell + vegetative cell) OR 3-celled (2 male gametes + tube nucleus).

Carpel structure:

Stigma + Style + Ovary (with one or more ovules inside).

Ovule: nucellus + integuments + embryo sac (megagametophyte).

Embryo sac (8-nucleated, 7-celled): Egg cell + 2 synergids + 2 polar nuclei (central cell) + 3 antipodal cells.

Pollination: Transfer of pollen from anther to stigma.

Self-pollination (autogamy): same flower or same plant.

Cross-pollination (allogamy): different plant.

Agents: wind (anemophily — light, non-sticky pollen), water (hydrophily), insects (entomophily — sticky, scented pollen), birds (ornithophily), bats.

Double Fertilisation (unique to angiosperms):

Two male gametes from pollen tube:

Total: two fertilisations → double fertilisation.

Fruit and Seed:

Ovary → Fruit (ovary wall becomes pericarp).

Ovule → Seed (integuments become seed coat/testa).

Seed: embryo (plumule, radicle, cotyledons) + endosperm (nutrition) + seed coat.

Monocot seeds: 1 cotyledon (scutellum). Dicot seeds: 2 cotyledons.

3. Human Reproduction (Brief)

Male reproductive system: Testes (sperm + testosterone), epididymis, vas deferens, seminal vesicles, prostate, urethra, penis.

Spermatogenesis: In testes. Spermatogonia → primary spermatocyte → secondary spermatocyte → spermatids → spermatozoa.

Female reproductive system: Ovaries, fallopian tubes (oviducts), uterus, cervix, vagina.

Oogenesis: Oogonia → primary oocyte (birth) → secondary oocyte (puberty, released during ovulation) → ovum (after fertilisation).

Menstrual cycle (28 days):

Menstrual phase (1-5): shedding of endometrium.

Follicular phase (1-13): FSH stimulates follicle development, oestrogen rises.

Ovulation (14): LH surge → release of secondary oocyte.

Luteal phase (15-28): Corpus luteum → progesterone → prepare endometrium.

If no fertilisation: corpus luteum degenerates → cycle repeats.

Fertilisation and implantation:

Fertilisation: in fallopian tube (ampullary-isthmic junction). Zygote → blastocyst.

Implantation: blastocyst embeds in endometrium (~7 days after fertilisation).

HCG (human chorionic gonadotropin): secreted by trophoblast → maintains corpus luteum → basis of pregnancy test.

PYQs (CBSE)

CBSE 2023: Explain double fertilisation. Why is it significant?

Two male gametes travel through pollen tube. One fuses with egg → zygote (2n) → embryo. Second fuses with two polar nuclei → PEN (3n) → endosperm (triploid) which provides nutrition to embryo.

Significance: Only in angiosperms, ensures nutritive tissue (endosperm) forms simultaneously with embryo.

CBSE 2022: Write the components of an embryo sac. How many cells and nuclei does it have?

7 cells, 8 nuclei. Cells: 1 egg cell + 2 synergids + 1 central cell (with 2 polar nuclei) + 3 antipodal cells = 7 cells, 8 nuclei.

Revision Notes

ASEXUAL REPRODUCTION: 1 parent, no gametes, identical offspring
Binary fission (Amoeba) | Budding (Hydra, Yeast) | Fragmentation | Vegetative propagation

FLOWER:
Stamen: filament + anther (microsporangia → pollen grains)
Carpel: stigma + style + ovary (with ovules)
Embryo sac: 7 cells, 8 nuclei (1 egg + 2 synergids + 1 central cell + 3 antipodals)

DOUBLE FERTILISATION (angiosperms only):
Male gamete 1 + egg → zygote (2n) → embryo
Male gamete 2 + 2 polar nuclei → PEN (3n) → endosperm

POLLINATION: self (autogamy) or cross (allogamy)
Agents: wind, water, insects, birds, bats

MENSTRUAL CYCLE: 28 days
Follicular(FSH) → Ovulation(LH surge, day14) → Luteal(progesterone)
HCG: basis of pregnancy test (secreted by trophoblast)