How Do Organisms Reproduce Class 10: Notes and Questions

How Do Organisms Reproduce? is the Class 10 chapter that explains how living things make new individuals, from a single cell splitting in two to flowering plants and humans. This guide covers DNA copying and variation, every type of asexual reproduction, sexual reproduction in plants and humans, and reproductive health, followed by important questions with model answers.

DNA copying and variation

The nucleus contains chromosomes, which carry inheritance information as DNA (Deoxyribo Nucleic Acid). DNA is the source of information for making proteins; if the information changes, different proteins are made and the body design changes. So a basic event in reproduction is making a copy of DNA.

When a cell reproduces, it makes two copies of its DNA. But a DNA copy alone cannot keep life going, so the cell also makes extra cellular apparatus before it divides into two.

No biochemical reaction is 100% accurate, so DNA copies are similar but not identical. These small differences are variations. Drastic variations may kill the new cell, but minor ones build up over generations. This built-in tendency to vary is the basis of evolution.

Why variation helps a species survive

Organisms fit particular niches (places in the ecosystem) because of their body designs. If the environment changes drastically, through global warming, changing water levels or meteorite hits, a population perfectly suited to the old niche could be wiped out.

Variation is a safety net. Imagine bacteria living in temperate water. If the temperature rises, most may die, but a few heat-resistant variants survive and multiply. The species continues even though not every individual benefits.

Asexual reproduction: fission and fragmentation

In asexual reproduction, a single parent produces offspring without germ cells fusing.

Amoeba can split in any plane. Leishmania, which has a whip-like structure at one end, splits in a particular orientation.

Fragmentation: simple multicellular organisms like Spirogyra break into pieces on maturing, and each piece grows into a new individual. This works because their body is not a complex arrangement of specialised tissues. Humans cannot reproduce this way, because our cells form tissues and organs at definite positions that a random piece cannot rebuild. Complex organisms rely instead on specialised cells that can multiply and develop into different tissues.

Regeneration and budding

Regeneration: if Planaria or Hydra is cut into pieces, each piece can grow into a whole organism. Specialised cells multiply into a mass of cells, which then turn into different tissues in an organised sequence called development. Regeneration is not the same as reproduction: most organisms do not depend on being cut up to produce offspring.

Budding in Hydra:

1. Outgrowth: repeated cell division at one spot makes a small bud.
2. Development: the bud grows tentacles and a mouth while still attached.
3. Detachment: when mature, it pinches off.
4. Independence: it lives as a new Hydra.

Yeast also reproduces by putting out small buds.

Vegetative propagation and spores

Vegetative propagation means new plants grow from roots, stems or leaves.

• Farmers use layering or grafting for sugarcane, roses and grapes.
• It lets plants that have lost the ability to make seeds, such as banana and jasmine, reproduce.
• Offspring are genetically identical to the parent and often flower and fruit earlier than plants grown from seed.
• Bryophyllum forms buds along its leaf margins that grow into new plants.
• A potato's "eyes" (buds) give new shoots. Pieces without eyes simply rot.
• Tissue culture: cells from a plant's growing tip are placed in an artificial medium to grow many disease-free plantlets.

Spore formation: Rhizopus (bread mould) produces sporangia, blob-on-a-stick structures that hold spores. Spores have thick protective walls, survive bad conditions and spread through the air. On a moist surface they germinate into new hyphae.

The logic of sexual reproduction

Asexual reproduction makes very little variation. Sexual reproduction combines DNA from two individuals, creating new combinations and speeding up variation.

But if two ordinary cells fused, the offspring would have double the DNA, which would disturb cell control. The solution is germ cells (gametes), made by a special division called meiosis. Gametes have half the chromosomes and half the DNA of body cells. When two gametes fuse in fertilisation, the zygote gets back the normal amount.

In complex organisms, the male gamete (sperm) is small and motile, while the female gamete (egg) is large and stores food for the early embryo.

Sexual reproduction in flowering plants

A flower has sepals, petals, stamens (male part) and a pistil (female part). Stamens make yellowish pollen grains with male germ cells. The pistil has a sticky stigma, a style (tube) and an ovary with ovules; each ovule has an egg cell.

Pollination is the transfer of pollen from stamen to stigma: within the same flower it is self-pollination; to another flower by wind, water or animals it is cross-pollination.

The path to fertilisation:

1. Pollen from the anther lands on the stigma.
2. A pollen tube grows down the style.
3. The tube enters the ovary and reaches the ovule.
4. The male germ cell fuses with the egg, which is fertilisation.

After fertilisation, the zygote becomes an embryo, the ovule becomes a seed with a tough coat, and the ovary ripens into a fruit. Petals and stamens usually fall off. The seed germinates under favourable conditions.

Human reproduction: puberty and the male system

Puberty is the period in adolescence when reproductive tissues mature. Common changes include hair in the armpits and genital area and oily skin. Girls show breast enlargement and the start of menstruation; boys show facial hair, voice cracking and erection of the penis.

Male system:

• Testes make sperms and the hormone testosterone, which regulates sperm production and secondary sexual characteristics.
• Testes sit in the scrotum, outside the abdomen, because sperm formation needs a temperature lower than body temperature.
• Sperms travel through the vas deferens to the urethra. The prostate gland and seminal vesicles add fluids that nourish sperms and ease transport; the mixture is semen.
• A sperm is a tiny cell with genetic material and a long tail for movement.

The female system and pregnancy

The female system has ovaries, fallopian tubes (oviducts), the uterus and the vagina. An ovary releases one mature egg every month, and it travels through the fallopian tube, where fertilisation may happen.

The zygote divides into an embryo, which implants in the thick, blood-rich lining of the uterus. The placenta, a disc-like tissue, forms between mother and embryo. It has villi on the embryo's side and blood spaces on the mother's side, giving a large surface for glucose and oxygen to pass to the embryo and wastes to pass back. Development takes about nine months.

If the egg is not fertilised, it lives for about a day. The thick lining is not needed, so it breaks down and leaves through the vagina as blood and mucus. This monthly menstruation lasts 2 to 8 days.

Reproductive health

Sexual contact can spread diseases: bacterial (syphilis, gonorrhoea) and viral (warts, HIV-AIDS). Condoms help prevent many of these.

Illegal sex-selective abortion (female foeticide) has lowered the female-male sex ratio in some areas, and laws prohibit prenatal sex determination. Managing population size also matters for standards of living.

Remember this

• DNA copying is never perfect, which creates variation.
• Binary fission: Amoeba, Leishmania. Multiple fission: Plasmodium. Fragmentation: Spirogyra. Budding: Hydra, yeast. Spores: Rhizopus.
• Gametes have half the DNA; fertilisation restores the full amount.
• Ovule → seed, ovary → fruit.
• Copper-T does not protect against STDs.

Important questions with answers

1. Why is DNA copying essential for reproduction? It passes on the information for body design. Without it, new cells would lack instructions to make the proteins that shape the organism.

2. Why are the two cells from division similar but not identical? They get the same DNA blueprint, but copying reactions are not perfectly accurate, so small variations appear.

3. How does variation help a species survive? Some individuals may have traits, like heat resistance, that let them survive a drastic change, so the whole species is not wiped out.

4. Why can humans not reproduce by fragmentation? Human cells form specialised tissues and organs at fixed positions, which a random piece of the body cannot rebuild.

5. Give two advantages of vegetative propagation. It lets seedless plants such as banana reproduce, and the offspring are identical to the parent, keeping desirable traits.

6. How does sexual reproduction avoid doubling DNA each generation? Meiosis makes gametes with half the DNA; their fusion restores the normal amount.

7. Distinguish between pollination and fertilisation. Pollination is the transfer of pollen from anther to stigma. Fertilisation is the fusion of the male germ cell with the egg in the ovule.

8. Why are the testes in the scrotum? Sperm formation needs a temperature lower than body temperature.

9. What does the placenta do? It passes glucose and oxygen from the mother to the embryo and removes the embryo's wastes.

10. Will a Copper-T protect against STDs? No. It prevents pregnancy but is not a barrier against infections.

Common mistakes to avoid

• Calling regeneration a normal way of reproducing.
• Mixing up pollination and fertilisation.
• Saying the ovule becomes the fruit; the ovary becomes the fruit.
• Thinking all contraceptives protect against STDs.

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