Why do most complex animals and flowering plants use sexual reproduction, while many simple organisms like yeast and hydra mainly reproduce asexually?
Key idea
Two Modes of Reproduction
1
Asexual
One parent produces offspring that are near-exact genetic copies of itself — seen in bacteria, yeast, hydra, and many plants.
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2
Sexual
Two parents each contribute genetic material — offspring inherit a mix of traits, creating variation between generations.
Key idea
Vegetative Propagation in Plants
New plants sprout directly from a parent's stem, root, or leaf — no seeds involved, and only one parent.
Everyday examplesPotato and ginger sprout from fleshy underground stems; sugarcane and money plant grow from stem cuttings; Bryophyllum leaves sprout tiny plantlets along their edges.
Key idea
Cutting, Grafting, Layering, and Tissue Culture
Method | How it works
Cutting
A stem piece is planted directly in soil, and grows roots of its own
Grafting
A stem piece from one plant is fitted into a cut/slit on a rooted plant of another variety
Layering
A twig is bent and buried while still attached to the parent; once it roots, it's cut free
Tissue culture
Shoot-tip cells are grown on nutrient media to mass-produce healthy, virus-free plantlets
Key idea
Budding and Spore Formation
1
Budding
In yeast and hydra, a small outgrowth (bud) forms on the parent, enlarges, and eventually separates to live independently.
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2
Spore formation
Fungi (like the mould on bread) release millions of lightweight spores into the air, which germinate into new individuals wherever moisture and nutrients allow.
Key idea
Mitosis: The Basis of Asexual Reproduction
Every method in this section
vegetative propagation, budding, spore formation — relies on the same cell-division process underneath.
Mitosis produces identical daughter cells
Since offspring arise from a single parent through mitosis, they carry the exact same chromosome number and genetic makeup — they are clones of the parent.
Key idea
Sexual Reproduction Needs Meiosis
If every generation inherited a full chromosome set from both parents, the number would double each time. Meiosis solves this.
Diploid → haploid gametesMeiosis halves the chromosome number, forming gametes (sperm and egg in animals; pollen and ovule in plants) — so when two gametes fuse, the normal number is restored.
Key idea
Meiosis Creates Genetic Variation
As chromosome pairs separate during meiosis, each gamete randomly receives one chromosome from each pair.
Millions of possible combinations
With just 3 pairs of traits, 8 combinations are possible. With 23 human chromosome pairs, the number of possible combinations is enormous — making each person genetically unique, even among siblings.
Key idea
Structure of a Flower
Part | Role
SepalProtects the flower while it's still a bud
PetalAttracts pollinators with colour and fragrance
Pistil (female)Stigma + style + ovary — the ovary holds ovules with the egg cell
Key idea
Self-Pollination, Cross-Pollination, and Pollinators
1
Self vs. cross
Self-pollination: pollen moves to the stigma of the same or another flower on the same plant. Cross-pollination: pollen travels to a flower on a different plant of the same species.
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2
Agents of pollination
Wind (wheat, maize), water (Vallisneria, Hydrilla), insects (sunflower, hibiscus), or birds (coral tree) — each shapes the flower's colour, scent, and pollen.
Key idea
Fertilisation and Seed Formation
A pollen tube grows from the stigma, down through the style, to the ovule — carrying the male gamete to fuse with the egg cell.
Zygote → embryo, ovule → seed, ovary → fruitThis fusion of gametes is fertilisation. The fertilised egg (zygote) develops into an embryo, the ovule becomes the seed, and the ovary swells into the fruit around it.
Key idea
Two Pollination Strategies Compared
Strategy | Pollen released per flower | Average seeds formed
Wind-pollinated (maize, wheat)
5,00,000 – 10,00,000 — 50 – 200
Insect-pollinated (sunflower)
20,000 – 40,000 — 800 – 1,000
Key idea
External and Internal Fertilisation in Animals
1
External fertilisation
Eggs and sperm are released into water (frogs, most fish). Many eggs are laid, but survival is low — currents and predators destroy most of them.
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2
Internal fertilisation
Occurs inside the female's body (reptiles, birds, mammals). Fewer eggs are produced, but the embryo is better protected, so survival is generally higher.
Key idea
Reproductive Strategies Across Animals
Animal | Fertilisation | Eggs produced | Survival of young
Fish
External — 100s – 1000s at a time — Low
Frog
External — 5,000 – 50,000 at a time — Low
Lizard
Internal — 2 – 20 at a time — Moderate
Bird
Internal — 1 – 15 at a time — Moderate to High
Key idea
The Male Reproductive System
Organ | Function
Testes (in the scrotum)Produce sperm and male hormones; kept cooler than body temperature
Vas deferensCarries sperm from the testes to the urethra
Seminal vesicles & prostate glandAdd fluid that nourishes sperm and helps them move
UrethraCommon passage for sperm and urine, through the penis
Key idea
The Female Reproductive System
Organ | Function
Ovaries
Produce eggs (female gametes) and hormones
Oviducts (fallopian tubes)
Connect each ovary to the uterus; where fertilisation occurs
Uterus
The bag-like organ where a fertilised egg implants and a foetus develops
Cervix & vagina
Narrow passage connecting the uterus to the outside of the body
Key idea
Gametogenesis: Sperm vs. Egg
Feature | Sperm | Egg
SizeVery small — Large
Number producedMillions — Few (one per month, from puberty)
Stored nutrientsAbsent — Present
MotilityActively motile — Non-motile
Key idea
What Happens When a Sperm Meets an Egg
After ovulation, a released egg travels into the oviduct. Sperm entering the reproductive tract swim to meet it.
Fusion, division, implantation
If a sperm fuses with the egg, a zygote forms. It divides repeatedly while travelling to the uterus, then implants into the thickened uterine lining — marking the start of pregnancy.
Key idea
The Menstrual Cycle
Days | What happens
1 – 5Menstruation: the thickened uterine lining sheds, if no fertilisation occurred
6 – 14The uterine lining gradually rebuilds; an egg matures in the ovary
~14Ovulation: the ovary releases a mature egg
15 – 28The lining thickens further, rich with blood vessels; if unfertilised, it breaks down and the cycle repeats
Key idea
Pregnancy and Childbirth
Trimester | What happens
First
The embryo forms and major organs begin developing; called a foetus from about week 9
Second
The foetus grows bigger and stronger; movements usually become noticeable
Third
Rapid growth as the baby prepares for life outside the womb
Key idea
Sexual Maturity, STIs, and Contraception
Physical maturity ≠ emotional readinessThe body becomes capable of reproduction gradually during adolescence, but emotional and social maturity — making thoughtful, responsible decisions — takes longer to develop.
Preventing infection and unwanted pregnancyCondoms are a barrier method that reduces the spread of Sexually Transmitted Infections (STIs) and also prevents pregnancy. Other methods include contraceptive pills and IUDs (like Copper-T).
Chapter · Key terms to remember
Key Terms
Vegetative propagation
New plants grow from stems, roots, or leaves — no seeds.
Budding / Spore formation
Asexual reproduction in yeast, hydra, and fungi.
Meiosis
Cell division that halves the chromosome number to form gametes.
Chapter · Key terms to remember
More Key Terms
Pollination
Transfer of pollen from anther to stigma.
Fertilisation
Fusion of male and female gametes, forming a zygote.
External / Internal fertilisation
Gametes meet outside / inside the female's body.
Chapter · Key terms to remember
More Key Terms
Ovulation
Release of a mature egg from the ovary.
Menstruation
Monthly shedding of the uterine lining, if no fertilisation occurs.
Questions for your notebook
Write these down, then discuss
1
Arrange the stages of sexual reproduction in plants in the correct order: (i) Pollen germination on stigma (ii) Fertilisation (iii) Pollination (iv) Formation of zygote
2
Assertion (A): The zygote formed after fertilisation immediately attaches to the uterus wall. Reason (R): The uterus wall is always prepared to receive the zygote. Choose the correct option.
3
Why does asexual reproduction produce offspring that are genetically identical to the parent?
Project as-is — students copy the questions, then the class discusses answers together.