How do Organisms Reproduce?
CBSE Class 10 · Science · Notes and practice questions
This chapter explains how living organisms make copies of themselves, from asexual reproduction in simple organisms and vegetative propagation in plants to sexual reproduction in flowers and humans. It also covers the menstrual cycle, reproductive health and contraceptive methods.
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Reproduction is how organisms produce new individuals of their own kind, and this chapter begins with why living things need it. When a cell divides, its DNA is copied and passed to the daughter cells. Copying is not always perfect, so small variations appear, giving each generation new traits. The chapter then groups reproduction into asexual forms, where one parent gives rise to offspring, and sexual forms, where male and female germ cells fuse. This classification runs through the whole chapter.
Asexual reproduction is quick and produces genetically identical offspring. Amoeba splits by binary fission, while Plasmodium can undergo multiple fission. Spirogyra reproduces by fragmentation when its filaments break into pieces. Hydra and yeast produce buds, and organisms such as Planaria show regeneration. Plants can grow from roots, stems and leaves through vegetative propagation, and Rhizopus spreads by releasing spores. All these modes have one common feature: a single parent is enough, and no gametes fuse.
In flowering plants, the flower is the reproductive organ. The stamen makes pollen grains, and the carpel contains the stigma, style and ovary with ovules. Pollination is the transfer of pollen to the stigma, carried out by wind, insects or other agents. A pollen grain then grows a pollen tube down the style, and a male germ cell fuses with the egg cell inside an ovule. After fertilisation, ovules become seeds and the ovary becomes a fruit, completing the life cycle of the plant.
Human reproduction involves separate male and female systems. The testes produce sperms and the male hormone, while the ovaries release eggs and female hormones. Sperms meet the egg in the oviduct, and the embryo gets embedded in the uterus. At puberty, hormonal changes bring sexual maturity, and the menstrual cycle prepares the uterus every month. The chapter closes with reproductive health, including contraceptive methods that help prevent unwanted pregnancy and reduce the spread of sexually transmitted infections.
Key terms
- Fission
- A mode of asexual reproduction in which a parent cell splits to form new individuals. In binary fission, the nucleus divides first and then the cytoplasm splits into two daughter cells, as in Amoeba. In multiple fission, the nucleus divides repeatedly and many daughter cells are formed at once, as in Plasmodium.
- Fragmentation
- A type of asexual reproduction in which the parent body breaks into two or more fragments, and each mature fragment grows into a complete new organism. The filamentous alga Spirogyra reproduces in this way.
- Regeneration
- The ability of an organism to regrow a whole body from a detached piece, or to replace a lost part, using specialised dividing cells. For example, when a Planaria is cut into pieces, each piece can regenerate into a new worm; Hydra can also regenerate.
- Budding
- An asexual method in which a small outgrowth called a bud forms on the parent body. The bud grows by repeated cell division and separates when mature to live as an independent organism. Budding is seen in Hydra and yeast.
- Vegetative propagation
- Reproduction in plants from vegetative parts such as stems, roots or leaves, rather than from seeds. New individuals develop from buds present on these parts, so the offspring are genetically identical to the parent. Examples include potato tubers, ginger rhizomes, Bryophyllum leaves and sugarcane stem cuttings.
- Spore formation
- A mode of reproduction in which organisms such as the bread mould Rhizopus produce tiny spores inside sacs called sporangia. Each spore has a thick wall that helps it survive unfavourable conditions, and it germinates to form new hyphae when conditions improve.
- Pollination
- The transfer of pollen grains from the anther of a flower to the stigma. It brings the male germ cells close to the female gametes; pollen may be carried by wind, water, insects or birds. Self-pollination and cross-pollination are the two main types.
- Fertilisation in flowering plants
- The fusion of a male germ cell with an egg cell inside the ovule after pollination. The pollen grain forms a pollen tube that grows down the style to deliver the male gamete. The fertilised egg develops into a zygote, the ovule becomes a seed, and the ovary becomes a fruit.
- Menstrual cycle
- The reproductive cycle in human females, usually about 28 days long, during which an egg matures and the uterus lining thickens. If the egg is not fertilised, the lining and blood break down and are passed out of the body as menstruation, and a new cycle begins.
Practice questions with answers
1. What is the difference between self-pollination and cross-pollination?
In self-pollination, pollen grains from an anther fall on the stigma of the same flower or on another flower of the same plant. In cross-pollination, pollen is transferred to the stigma of a flower on a different plant of the same species. Cross-pollination needs an external agent such as wind or insects.
2. State the function of the pollen tube in flowering plants.
After a pollen grain lands on the stigma, a pollen tube grows down through the style towards the ovule. It carries the male germ cell and releases it into the ovule, making fertilisation possible.
3. Why is vegetative propagation used to grow sugarcane?
Sugarcane usually does not produce viable seeds, and vegetative propagation from stem cuttings produces mature plants quickly. This method is also useful because the new plants are genetically identical to the parent, so desirable qualities such as high sugar content remain unchanged.
4. After fertilisation, what do the ovule and the ovary of a flower develop into?
The ovule develops into a seed, and the ovary develops into a fruit. The other floral parts such as sepals, petals, stamens and style usually dry up and fall off after fertilisation.
5. How does binary fission in Amoeba differ from multiple fission in Plasmodium?
In binary fission, the nucleus of Amoeba divides into two, followed by division of the cytoplasm, producing two daughter cells. In multiple fission, the nucleus divides repeatedly to form many nuclei, and the cytoplasm then splits into numerous small cells, as in Plasmodium.
6. A Planaria is cut into small pieces and each piece is kept in water. What will happen, and why?
Each piece can regenerate into a complete Planaria because Planaria has specialised dividing cells. This process is called regeneration, and it can restore a whole organism from a detached body part without the involvement of gametes.
7. What are spores, and how do they help Rhizopus survive?
Spores are tiny reproductive bodies produced in the sporangia of Rhizopus. They are covered by a thick wall that protects them from heat, cold and dryness. When conditions become favourable, a spore germinates into a new Rhizopus, so spores also help the fungus spread widely.
8. What happens in the uterus when a fertilised egg reaches it, and what happens if the egg is not fertilised?
When a fertilised egg reaches the uterus, it gets embedded in the thickened uterine lining and continues to develop. If the egg is not fertilised, the uterine lining breaks down and is shed as menstruation, after which a new cycle begins.
9. What is contraception? List two methods and state the advantage of using barrier methods.
Contraception is the deliberate prevention of pregnancy. Barrier methods such as condoms physically stop the sperms from reaching the egg. Intrauterine devices such as the Copper-T are placed inside the uterus by a doctor and prevent pregnancy. An important advantage of barrier methods, especially condoms, is that they also give protection against sexually transmitted infections.
10. Where are sperms produced in human males, and which hormone controls the development of male secondary sexual characters?
Sperms are produced in the testes, which are located in the scrotum outside the abdominal cavity. The testes also release testosterone, the hormone responsible for male secondary sexual characters such as deepening of the voice and growth of facial hair.
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