13.5 Reproduction Continues Life
Earth’s conditions allow life — but what keeps life from simply dying out? The answer is reproduction: each kind of organism makes more of its kind, carrying life forward generation after generation.
Genes: Life’s Instruction Manual Cows have calves, cats have kittens — offspring resemble their parents because parents pass on instructions for how to grow from a single cell. These instructions, the genetic material or genes, sit inside every cell (Chapter 2). Think of genes as a detailed instruction manual: some pages say how to make blood, others bones, muscles or skin — together turning a calf into a cow.
Continuity and Change Reproduction does two things at once. It keeps each kind continuing — and it allows small changes in the instructions passed down. Sometimes a change helps an organism survive better: camels evolved humps to store fat for deserts; some bacteria became antibiotic-resistant (Chapter 3). Over many generations, such changes can add up to new features — or whole new kinds of life. So reproduction both preserves life and lets it adapt.
Two ways to reproduce
Asexual Reproduction In asexual reproduction, a single parent produces new individuals that are exact copies of itself (same instructions). In plants, this often happens when a part — a leaf, stem or root — grows into a new plant, called vegetative propagation (Activity 13.3: try a money-plant cutting, a potato’s “eyes”, or a piece of ginger). Microbes and simple animals do it too: bacteria and Amoeba split into two; Hydra grows buds that break off; and Planaria (a flatworm) can regrow from a small fragment of its body!
Sexual Reproduction In sexual reproduction, two parents (male and female) combine their instructions, so the offspring are not exactly like either parent — they mix traits from both. To avoid doubling the genes each generation, each parent makes special reproductive cells, gametes, carrying only half the genetic material. When a male and female gamete join, they form a new cell with a complete set — half from each parent. That mixing is why siblings differ, and why each child is unique.
Sort these examples by the type of reproduction.
How sexual reproduction happens
In Plants Flowers carry both gametes: pollen (in the anther) is the male gamete; ovules (deep inside) are the female. Pollination carries pollen to a flower (by wind, insects or animals); when the gametes combine — fertilisation — a zygote forms and becomes a seed, while the flesh around the ovule becomes a fruit. Animals that eat the fruit drop the seeds far away, helping plants spread — a banyan seed may sprout in a wall crack after the rains.
In Animals: Eggs vs Live Birth In animals, the gametes are sperm (male) and eggs (female). In fish and frogs, both are released into water, where fertilisation and the zygote’s development happen. In birds, the female lays the fertilised egg, and the embryo develops after laying — so each egg must hold enough food to last until hatching. In most mammals (including humans), the embryo develops inside the mother’s body, which supplies all its food and oxygen until birth. (So could a dog or cow lay eggs like a hen? No — mammals are built to nourish their young inside the body.)