Sexual Reproduction in Animals (13.5.2) — "Our Home: Earth, a Unique Life Sustaining Planet"

1. 13 marks

   Name the male and female gametes in animals, and explain what 'external fertilisation' means, using fish or frogs as the example.

   Hint. The chapter names two everyday water animals whose gametes combine outside the body entirely.

   ✦ In animals, gametes are called sperm (male) and eggs (female). External fertilisation means fertilisation happens outside the body — as in fish or frogs, where the male and female eject sperm and eggs, respectively, into the water, where they combine to form the zygote. This is why, in these animals, the development of the zygote into an embryo also takes place in the water, outside either parent's body.

2. 22 marks

   How does fertilisation in birds and mammals differ from fertilisation in fish or frogs?

   Hint. One location is external, in open water; the other is inside a body.

   ✦ In birds and mammals, including humans, sperm are deposited inside the body of the female, and fertilisation takes place when the sperm swim towards the egg produced by the female — this is internal fertilisation, since it happens inside the body, unlike the external fertilisation in water seen in fish and frogs.

3. 34 marks

   Compare how birds and mammals nourish their developing embryo after fertilisation, and explain what 'strategy' each represents.

   Hint. One strategy packs nutrition into the egg itself before it leaves the body; the other keeps supplying nutrition continuously from inside.

   ✦ In birds, the fertilised egg (zygote) is laid by the female, and the embryo develops into hatching afterwards, outside the mother's body — this requires the female parent to put enough 'food' into each egg to last the developing embryo until it hatches, since no more nutrition can be added once the egg is laid. In most mammals, by contrast, the development of the zygote into an embryo takes place inside the body of the female, whose body provides all the food and oxygen the baby needs to grow, continuously, until it is born — a fundamentally different nutrition strategy from the one-time provisioning birds rely on.

4. 43 marks

   The chapter asks: what are the advantages and disadvantages of giving birth to live young versus laying eggs? Give one of each.

   Hint. Think about protection during development versus how many offspring a parent can support at once.

   ✦ One advantage of live birth (as in most mammals) is that the developing embryo is continuously protected and nourished inside the mother's body, likely giving it a higher chance of survival than an egg left exposed to the environment. One advantage of egg-laying (as in birds) is that a parent can produce and lay many eggs relatively quickly since it doesn't need to carry and nourish each developing embryo inside its own body for an extended time, though this comes with the disadvantage that eggs are more exposed to predators and environmental risk than an embryo developing inside a parent's body.

5. 53 marks

   Why do human babies not look exactly like their mother or their father, and why can siblings even look different from each other?

   Hint. Trace this back to how gametes are formed and how they combine, the same mechanism explained earlier for plants and animals generally.

   ✦ Every baby gets a mix of genetic information from both parents through gametes, and each gamete carries a different set of instructions for things like eye colour and hair type. These instructions mix in different ways each time sperm and egg join to form a baby, so each child is unique — which is why one child may inherit a nose similar to their mother's while another inherits eyes similar to their father's, and why even brothers and sisters from the same two parents can look noticeably different from each other.

6. 62 marks

   Are gametes only found in plants and animals? What does the chapter say about microorganisms?

   Hint. The chapter extends the two-parent idea to organisms that don't have distinct male and female sexes in the usual sense.

   ✦ No — the chapter states that some microorganisms like bacteria and yeast also have two 'mating types' that act like the two parents in sexual reproduction, which means the basic two-parent, gamete-combining pattern of sexual reproduction extends even to single-celled organisms, not just plants and animals.

Solutions written by the tuition.in editorial team and checked against NCERT Curiosity — Textbook of Science for Grade 8, Chapter 13 (hecu113.pdf), Reprint 2026-27, pages 210-228. Questions are referenced from the NCERT textbook for identification.