How do Organisms Reproduce?

1. Why do organisms reproduce?

Reproduction is not essential to keep an individual alive — unlike nutrition, respiration or excretion. An organism could survive perfectly well without ever reproducing. So why spend energy on it?

The answer is at the level of the species, not the individual. We notice a kind of organism only because there are large numbers of similar individuals. Reproduction is what creates these new individuals that "look very much like themselves", keeping the species going across generations.

2. Reproduction is really DNA copying

Organisms look alike because their body designs are similar; body designs are similar because the blueprints (the instructions) are similar. From Class 9 you know these instructions sit in the chromosomes in the nucleus, in the form of DNA (Deoxyribo Nucleic Acid).

DNA is the information source for making proteins. Change the DNA → different proteins → altered body design. So the most basic event in reproduction is making a DNA copy.

• A reproducing cell uses chemical reactions to build a second copy of its DNA, giving two copies.
• The two copies must separate. Simply pushing one copy out fails — it would have no cellular machinery to live in.
• So DNA copying is accompanied by building an extra cellular apparatus, and then the cell divides into two, each with its own DNA and apparatus.

3. Variation and its importance

No chemical reaction is perfectly reliable, so DNA copying has small variations each time. The new DNA is similar but not always identical to the original.

• A drastic variation may stop the new cell from working with its inherited apparatus — that cell simply dies.
• Milder variations let the cell survive while being subtly different. This built-in tendency for variation is the basis of evolution (next chapter).

Why variation matters for the species (not the individual): populations live in niches (specific places/roles in the ecosystem). If a niche changes drastically — temperature rises, water level drops, a meteorite hits — a uniform population could be wiped out. But if a few individuals carry useful variations, some can survive and grow.

4. Asexual reproduction — overview

Asexual reproduction: new individuals are produced from a single parent, without the fusion of germ-cells. Offspring are genetically almost identical (clones). The mode depends on the body design of the organism. The six NCERT modes are: fission, fragmentation, regeneration, budding, spore formation and vegetative propagation.

5. Fission (in unicellular organisms)

For single-celled organisms, cell division (fission) itself creates new individuals.

• Binary fission = parent splits into two daughter cells. In bacteria and Amoeba this is simple splitting. In Amoeba the plane of splitting can be any direction.
• Some organisms with a more organised body, like Leishmania (causes kala-azar, has a whip-like flagellum), divide in a definite orientation relative to that structure.
• Multiple fission = parent splits into many daughter cells at once. Example: the malarial parasite Plasmodium divides into many daughters simultaneously.

6. Fragmentation

In multicellular organisms with a simple body, the body simply breaks into fragments on maturing, and each fragment grows into a new individual.

• Example: Spirogyra (the green filamentous alga in ponds) breaks up into smaller pieces; each piece grows into a new filament.
• This works because Spirogyra is not a highly organised body — it is essentially a chain of similar cells.

Why complex organisms cannot fragment: their cells are organised into tissues → organs placed at definite positions. Cutting them up randomly would destroy this organisation, so cell-by-cell division is impractical. Complex bodies therefore need specialised reproductive cells/organs.

7. Regeneration

Regeneration: some fully differentiated simple animals, if cut or broken into pieces, can grow each piece back into a complete organism.

• Examples: Hydra and Planaria — cut into many pieces, each piece can regenerate a whole organism.
• It is carried out by specialised cells that proliferate into a mass of cells; these then change into the various cell types and tissues in an organised sequence called development.

8. Budding

Budding: a small outgrowth (a bud) forms on the parent body due to repeated cell division at one specific site; it grows, matures and finally detaches to live independently.

• Example: Hydra uses its regenerative cells to bud. (Yeast also buds — Activity 7.1.)

9. Spore formation

Even some simple multicellular organisms make specific reproductive parts. The bread mould Rhizopus grows thread-like hyphae (not reproductive). On these stand tiny blob-on-a-stick structures called sporangia.

• Sporangia contain hundreds of spores — the actual reproductive units.
• Spores have thick protective walls so they survive until they reach a moist surface, where they germinate into new Rhizopus.

Advantage: spores are produced in large numbers, are light and easily dispersed (by wind), and their thick walls let them survive unfavourable conditions until growth is possible.

10. Vegetative propagation (in plants)

In many plants the vegetative parts — root, stem, leaf — can themselves grow into new plants under suitable conditions. This is vegetative propagation, a form of asexual reproduction unique to plants.

• Natural examples: potato (buds in the "eyes"/notches), and Bryophyllum (buds along the leaf margin fall on soil and grow into new plants).
• Artificial methods: layering and grafting are used to grow sugarcane, roses and grapes.

Advantages of vegetative propagation:

• Plants can flower and fruit earlier than seed-grown plants.
• It lets us grow plants like banana, orange, rose and jasmine that have lost the capacity to make seeds.
• All new plants are genetically similar to the parent, keeping its desirable characteristics.

11. Sexual reproduction — why two parents?

Many organisms need two individuals to reproduce — bulls alone cannot make calves, hens alone cannot make chicks. Why bother with two, when one is "simpler"?

• Asexual DNA copying does create variation, but slowly and only by single accidental errors.
• Each living individual already carries variations accumulated over generations (and proven safe, since the individual is alive).
• Combining DNA from two different individuals creates entirely new combinations of variants — speeding up variation enormously. This is the great advantage of the sexual mode.

The doubling problem & its solution (meiosis): If each new generation simply combined two full DNA sets, the DNA would double every generation. Nature solves this by making special germ-cells (gametes) with half the chromosomes/DNA, by a cell division called meiosis. When two germ-cells fuse, the full chromosome number is restored.

12. Sexual reproduction in flowering plants

The reproductive parts of angiosperms (flowering plants) are in the flower. A flower has four whorls: sepals, petals, stamens, pistil (carpel).

• Stamen = the male reproductive part. It has the anther (makes yellowish pollen grains containing male germ-cells) on a stalk called the filament.
• Pistil/Carpel = the female reproductive part, in the centre. It has three parts: the swollen base ovary (contains ovules, each ovule has an egg cell), the middle style, and the top sticky stigma.
• A unisexual flower has only stamens or only pistil (papaya, watermelon). A bisexual flower has both (Hibiscus, mustard).

Pollination = transfer of pollen from anther to stigma. If pollen lands on the stigma of the same flower it is self-pollination; if carried to another flower it is cross-pollination (by agents like wind, water or animals).

Fertilisation: After pollen lands on a suitable stigma, a pollen tube grows out of the pollen grain, through the style, down to the ovule in the ovary. The male germ-cell fuses with the egg — this fusion is fertilisation, forming the zygote.

After fertilisation: the zygote divides many times to form an embryo inside the ovule. The ovule develops a tough coat and becomes the seed; the ovary ripens into the fruit. Petals, sepals, stamens, style and stigma usually shrivel and fall. The seed holds the future plant, which grows into a seedling by germination.

13. Puberty — sexual maturation in humans

The body grows from childhood, but at early teenage a new set of changes appears — this is puberty, when reproductive tissues mature (as general body growth slows).

• Common to boys & girls: thick hair in armpits and the genital area (and on legs/arms/face), oily skin and pimples, new awareness of one's body.
• In girls: breast size increases with darkening of the nipples; menstruation begins.
• In boys: new thick facial hair, the voice "cracks" (deepens), the penis occasionally becomes enlarged and erect.

These changes happen slowly, at different ages and rates in different people. The visible signs help individuals recognise that another is sexually mature, which matters for mating in animals.

14. Human male reproductive system

It has parts that produce germ-cells and parts that deliver them to the site of fertilisation.

• Testes (testis): make sperms. They lie outside the abdomen in the scrotum, because sperm formation needs a temperature lower than normal body temperature. The testes also secrete the hormone testosterone, which controls sperm formation and brings about the male puberty changes.
• Vas deferens: carries sperms from the testes; it joins a tube from the urinary bladder, so the urethra becomes a common passage for sperms and urine.
• Glands — seminal vesicles & prostate gland: add secretions so sperms travel in a fluid (semen) that makes transport easier and provides nutrition.
• Sperm: a tiny body of mainly genetic material with a long tail to swim towards the female germ-cell.

15. Human female reproductive system

• Ovaries: make the female germ-cells (eggs/ova) and some hormones. At birth a girl's ovaries already hold thousands of immature eggs; at puberty some mature, and one egg is released each month by one ovary.
• Oviduct / Fallopian tube: a thin tube that carries the egg from the ovary towards the uterus; fertilisation occurs here.
• Uterus (womb): the elastic bag where the two oviducts meet and where the embryo develops. Its lining thickens each month to receive an embryo.
• Cervix: the neck where the uterus opens into the vagina.
• Vagina: the passage that receives sperms during intercourse and through which the baby is born.

From fertilisation to birth: Sperms enter through the vagina, swim up and reach the oviduct, where one may fertilise the egg. The fertilised egg (zygote) divides into a ball of cells (embryo), which implants in the thick uterus lining and develops organs to become a foetus.

16. Menstruation

Each month the uterus prepares a thick, spongy, blood-rich lining to nourish a possible embryo. If the released egg is not fertilised (it lives about one day), this lining is no longer needed.

• The lining slowly breaks down and is shed through the vagina as blood and mucus — this is menstruation.
• It occurs roughly every month and usually lasts about 2 to 8 days.

17. Reproductive health & contraception

Sexual maturity does not mean the body or mind is ready for sex or for raising children. Two health concerns matter:

• Sexually Transmitted Diseases (STDs): spread by the intimate sexual act. Bacterial — gonorrhoea, syphilis; viral — warts, HIV-AIDS. A condom (covering for the penis) helps prevent transmission of many of these.
• Unwanted pregnancy: pregnancy makes major demands on a woman's body and mind. Contraceptive methods are used to avoid it.

Categories of contraception:

• Mechanical barriers: condoms (on penis) or coverings worn in the vagina — physically stop sperm reaching the egg. Condoms also reduce STD spread.
• Hormonal (oral pills): change the body's hormonal balance so eggs are not released and fertilisation cannot occur. Can cause side-effects.
• Intra-uterine devices (IUDs): the loop or copper-T placed in the uterus to prevent pregnancy. Can cause side-effects from uterine irritation. (They do not protect against STDs.)
• Surgical methods: blocking the vas deferens in the male (stops sperm) or the fallopian tube in the female (egg can't reach uterus). Safe long-term but surgery can cause infection if done improperly. Surgery is also (mis)used to remove pregnancies.

18. In-text QUESTIONS (Page 114) — answered

1. What is the importance of DNA copying in reproduction? DNA carries the information for body design (it makes the proteins that build the body). Copying DNA passes this blueprint to the offspring, so the new individual has the same body design and belongs to the same species. It also allows small variations that drive evolution.

2. Why is variation beneficial to the species but not necessarily for the individual? An individual is already well-suited to its present niche, so a change may not help it personally. But if the niche changes (e.g. heat, water loss), variation lets some individuals survive, saving the species from extinction. So variation protects the species over time, not every single individual.

19. In-text QUESTIONS (Page 119) — answered

1. How does binary fission differ from multiple fission? In binary fission the parent cell divides into two daughter cells (e.g. Amoeba, Leishmania). In multiple fission the parent divides into many daughter cells simultaneously (e.g. Plasmodium).

2. How will an organism be benefited if it reproduces through spores? Spores are produced in large numbers, are light and easily dispersed (by wind), and have thick protective walls that let them survive unfavourable conditions until they reach a moist surface to germinate.

3. Can you think of reasons why more complex organisms cannot give rise to new individuals through regeneration? Complex organisms have cells highly organised into tissues and organs at definite positions. Reproduction is the job of specialised cells, not every cell, so a random body part cannot rebuild the whole organised body — they need specialised reproductive cells/organs instead.

4. Why is vegetative propagation practised for growing some types of plants? Because it lets plants flower and fruit earlier, lets us grow seedless plants (banana, orange, rose, jasmine), and keeps all new plants genetically identical to the parent (preserving desirable traits). It is also used in tissue culture for disease-free plants.

5. Why is DNA copying an essential part of the process of reproduction? DNA carries the instructions for the body design. Without copying DNA, offspring would lack the blueprint to develop the correct body design of the species, so reproduction could not maintain similar individuals across generations.

20. In-text QUESTIONS (Page 126) — answered

1. How is the process of pollination different from fertilisation? Pollination is the transfer of pollen from the anther to the stigma (no fusion yet; it can be by wind/water/animals). Fertilisation is the fusion of the male germ-cell with the egg inside the ovule to form a zygote. Pollination comes first and is followed by fertilisation.

2. What is the role of the seminal vesicles and the prostate gland? They add secretions to the sperms so that the sperms travel in a fluid (semen). This fluid makes sperm transport easier and provides them nutrition.

3. What are the changes seen in girls at the time of puberty? Thick hair in armpits and genital area, oily skin/pimples; breast size increases with darkening of the nipples; menstruation begins.

4. How does the embryo get nourishment inside the mother's body? Through the placenta, a disc embedded in the uterine wall. Villi on the embryo's side and blood spaces on the mother's side give a large surface for glucose and oxygen to pass from mother to embryo (and wastes back), nourishing it.

5. If a woman is using a copper-T, will it help in protecting her from sexually transmitted diseases? No. The copper-T is an intra-uterine device that only prevents pregnancy; it does not stop the transfer of germs during the sexual act, so it gives no protection against STDs (for that, a condom is needed).

21. EXERCISES (Page 127) — fully answered

Q1. Asexual reproduction takes place through budding in: (a) Amoeba (b) Yeast (c) Plasmodium (d) Leishmania — Answer: (b) Yeast (and Hydra).

Q2. Which of the following is not a part of the female reproductive system in human beings? (a) Ovary (b) Uterus (c) Vas deferens (d) Fallopian tube — Answer: (c) Vas deferens (it is part of the male system).

Q3. The anther contains: (a) sepals (b) ovules (c) pistil (d) pollen grains — Answer: (d) pollen grains.

Q4. What are the advantages of sexual reproduction over asexual reproduction? It combines DNA from two parents, creating more variation (and faster) than asexual reproduction. This variation helps the species adapt and survive environmental change, and is the raw material for evolution.

Q5. What are the functions performed by the testis in human beings? (i) Production of male germ-cells, sperms; (ii) secretion of the male hormone testosterone, which controls sperm formation and brings about the male puberty changes.

Q6. Why does menstruation occur? Each month the uterus builds a thick, blood-rich lining to receive an embryo. If the egg is not fertilised, this lining is not needed; it breaks down and is shed through the vagina as blood and mucus — that shedding is menstruation.

Q7. Draw a labelled diagram of the longitudinal section of a flower. Draw a cut-open flower and label: Stigma, Style, Ovary (these three = Pistil), Anther, Filament (these two = Stamen), Petal and Sepal. (See Figure 7.7 described in §12.)

Q8. What are the different methods of contraception? (i) Mechanical/barrier — condoms, vaginal coverings; (ii) Hormonal — oral pills (stop egg release); (iii) IUDs — loop, copper-T; (iv) Surgical — blocking the vas deferens (male) or fallopian tube (female).

Q9. How are the modes of reproduction different in unicellular and multicellular organisms? Unicellular organisms reproduce by the cell itself dividing — binary or multiple fission, or budding (yeast). Multicellular organisms, having organised tissues/organs, use specialised methods: fragmentation, regeneration, budding (Hydra), spore formation, vegetative propagation, or full sexual reproduction with germ-cells.

Q10. How does reproduction help in providing stability to populations of species? Reproduction makes new individuals to replace those that die, keeping population numbers stable. Accurate DNA copying keeps the body-design features consistent, so the species stays suited to its niche; the variations it also produces let the species cope with change. Thus reproduction links to the stability of populations.

Q11. What could be the reasons for adopting contraceptive methods? (i) To avoid unwanted pregnancy (which strains a woman's health if she is not ready); (ii) to prevent sexually transmitted diseases (condoms); (iii) to space or limit children and help control population growth for a better standard of living.

22. Common mistakes to avoid

• Calling regeneration the same as reproduction — most organisms don't normally need to be cut to reproduce.
• Confusing binary fission (two cells) with multiple fission (many cells).
• Mixing up pollination (pollen transfer) with fertilisation (fusion of gametes).
• Writing that the testes are inside the abdomen — they are in the scrotum, outside, for a lower temperature.
• Thinking the copper-T or pills protect against STDs — only the condom reduces STD spread.
• Saying fertilisation happens in the uterus — in humans it happens in the fallopian tube/oviduct.
• Forgetting that vegetative propagation gives genetically identical plants (no variation), unlike sexual reproduction.

23. Quick revision checklist

• Reproduction = DNA copying + new cellular apparatus; not essential for the individual, vital for the species.
• Asexual (one parent): fission, fragmentation, regeneration, budding, spore formation, vegetative propagation → clones.
• Sexual (two parents): gametes (made by meiosis, half DNA) fuse → variation → evolution.
• Flower: stamen (anther + filament) = male; pistil (stigma + style + ovary) = female. Pollination then fertilisation → seed + fruit.
• Male system: testes (scrotum) → vas deferens → (seminal vesicle, prostate) → urethra → penis; hormone testosterone.
• Female system: ovary → fallopian tube (fertilisation) → uterus (placenta nourishes foetus) → cervix → vagina.
• No fertilisation → menstruation (2–8 days). Contraception: barrier, hormonal, copper-T/loop, surgical.