How Do Organisms Reproduce?
Chapter Notes | Class 10 Science
1. Introduction
Reproduction is the biological process by which organisms produce new individuals of their own species. It ensures the continuity of life and maintains the existence of a species over generations.
- Reproduction allows the existence of many individuals of the same species.
- If a species did not reproduce, its population would gradually disappear as existing individuals died.
- New individuals generally resemble their parents because information controlling body design and functions is passed from one generation to the next.
2. Do Organisms Create Exact Copies of Themselves?
Information controlling an organism's body design is present inside its cells. The cell nucleus contains chromosomes, which carry this information in the form of DNA (deoxyribonucleic acid).
DNA and DNA copying
- DNA contains the information required for protein synthesis. Changes in DNA can alter body design and functions.
- A basic event in reproduction is the copying of DNA so that new cells or individuals receive the information needed to build and maintain the organism.
- DNA copying is followed by cell division, which separates the DNA copies into two cells or distributes them to new individuals.
- Small variations may occur during DNA copying because it is a biochemical process.
- Therefore, offspring resemble their parents but are not always perfectly identical.
3. Importance of Variation
- Accurate DNA copying helps preserve an organism's body design and enables it to occupy a particular ecological niche.
- Variation among offspring is important because it allows some individuals to survive when environmental conditions change.
- For example, if the temperature of a waterbody rises, bacteria with heat-resistant variations may survive while other bacteria may perish.
4. Modes of Reproduction Used by Organisms
Reproduction is broadly categorised into asexual reproduction and sexual reproduction.
A. Asexual Reproduction
Asexual reproduction is a mode in which new offspring arise from a single parent. The offspring are genetically very similar to the parent and to one another, so they are often called clones. It occurs in unicellular and multicellular organisms.
Parent cell divides into two or more daughter cells.
Examples: Amoeba, bacteria, Leishmania, Plasmodium
Parent breaks into pieces; each piece grows into a new individual.
Example: Spirogyra
A fragment can regrow missing body parts and develop into an individual in some organisms.
Examples: Hydra, Planaria
A small outgrowth develops on the parent and may detach.
Example: Hydra
New plants develop from roots, stems or leaves rather than seeds.
Examples: Bryophyllum, banana, rose, sugarcane
Protective spores germinate under favourable conditions.
Example: Rhizopus (bread mould)
(a) Fission
Fission is a type of asexual reproduction in which a parent organism divides into two or more parts, and each part develops into a complete individual.
- Binary fission: Many bacteria and some protozoa divide into two approximately equal parts.
- Amoeba: It can divide in different directions because it does not have a fixed shape.
- Leishmania: It divides in a definite plane according to its body shape and whip-like structure.
- Multiple fission: Plasmodium produces many daughter cells at the same time.
(b) Fragmentation
In fragmentation, the parent organism breaks into pieces and each piece develops into a new individual. In Spirogyra, the filament breaks into smaller fragments, and each fragment grows into a new filament.
(c) Regeneration
Regeneration is the process by which a piece of an organism grows into a complete individual after the organism is cut or broken. Special cells multiply rapidly and then differentiate to form the required tissues and organs. This process is called development.
(d) Budding
A small outgrowth called a bud forms on the parent organism and grows into a new individual. When mature, the bud may detach and live independently. Example: Hydra.
(e) Vegetative Propagation
Vegetative propagation is a form of asexual reproduction in plants in which new plants develop from vegetative parts such as the stem, root or leaf, rather than from seeds.
- Methods such as layering, grafting, runners and tubers are commonly used in agriculture and horticulture.
- Plants produced by vegetative propagation often flower and bear fruit sooner than plants grown from seeds.
- It is useful for reproducing seedless or hybrid plants such as bananas, roses and sugarcane while preserving desirable traits.
- In Bryophyllum, buds form at the margins or notches of leaves and grow into new plants.
(f) Spore Formation
- Spores are single cells with thick protective walls. Under favourable conditions, a spore germinates and develops into a new organism.
- In Rhizopus (bread mould), thread-like structures called hyphae bear sporangia that contain spores.
- Sporangia are specialised reproductive structures that produce and release spores; they are not hyphae.
| Mode | Description | Key examples |
|---|---|---|
| Fission | Parent cell divides into two (binary) or many (multiple) daughter cells. | Amoeba, bacteria, Leishmania, Plasmodium |
| Fragmentation | Parent organism breaks into pieces; each piece grows into a full organism. | Spirogyra |
| Regeneration | Broken fragment regrows missing body parts through cell multiplication and development. | Hydra, Planaria |
| Budding | A small outgrowth develops on the parent, matures and detaches. | Hydra |
| Vegetative propagation | New plants grow from roots, stems or leaves rather than seeds. | Bryophyllum, banana, rose, sugarcane |
| Spore formation | Spores enclosed in sporangia germinate under favourable conditions. | Rhizopus (bread mould) |
5. Sexual Reproduction
Sexual reproduction is the process by which offspring are produced by the fusion of male and female gametes (sex cells). Their fusion produces genetically diverse offspring with new combinations of traits.
- In many organisms, gametes come from two parents, increasing variation in offspring.
- Greater variation helps populations adapt to changing environments and improves their chances of long-term survival.
- Gametes contain half the usual number of chromosomes. Fertilisation restores the full chromosome number in the zygote.
- In complex organisms, germ cells are differentiated into small, motile sperm and larger eggs that often contain stored nutrients.
6. Sexual Reproduction in Flowering Plants
In flowering plants, male and female gametes are produced in the reproductive parts of the flower: the stamen and the pistil.
Consists of a filament and anther. The anther contains pollen grains, which carry the male gametes.
Consists of stigma, style and ovary. The ovary contains ovules, and each ovule contains an egg cell.
Pollination and fertilisation
- Pollination: Transfer of pollen from anther to stigma. Agents include wind, water, insects, birds and other animals.
- Self-pollination: Pollen is transferred to the stigma of the same flower or another flower on the same plant.
- Cross-pollination: Pollen is transferred between different plants of the same species.
- Fertilisation: The male gamete fuses with the female gamete (egg) in the ovule to form a zygote.
- The zygote develops into an embryo inside the ovule. The ovule matures into a seed, and the ovary usually develops into a fruit.
- Germination: A seed grows into a seedling when water, oxygen and suitable temperature are available. Stored food nourishes the young embryo initially.
7. Reproduction in Human Beings
Human reproduction is sexual. The reproductive phase involves maturation and production of specialised germ cells, namely sperm and eggs. Adolescence is the period during which sexual maturation occurs. Puberty brings physical changes and the ability to reproduce.
Male reproductive system
The male reproductive system produces and transports sperm and secretes male hormones, notably testosterone.
- Main parts: Testes, scrotum, penis, vas deferens, urethra, seminal vesicles and prostate gland.
- Sperm: Small cells with a head containing genetic material and a tail that helps them move.
- Testes: A pair of testes are contained in the scrotum, outside the abdominal cavity. The scrotum keeps them at a temperature slightly lower than body temperature, necessary for proper sperm production. Testes produce sperm and secrete testosterone.
- Vas deferens: Carries sperm from the testes towards the urethra.
- Urethra: Passage through which semen containing sperm is discharged. Urine also passes through it at other times.
- Associated glands: Seminal vesicles and prostate gland add fluids that nourish sperm and help in their transport. Sperm together with these secretions form semen.
Female reproductive system
The female reproductive system produces eggs, receives sperm, allows fertilisation and supports the development of the embryo and foetus.
- Main parts: A pair of ovaries, oviducts (fallopian tubes), uterus, cervix and vagina. The urethra is a separate passage that carries urine.
- Ovaries: Produce eggs and secrete estrogen and progesterone. At birth, many immature eggs are present; at puberty, some begin to mature. Usually one egg matures and is released each month. This is called ovulation.
- Oviduct: Receives the egg released by the ovary and transports it towards the uterus. Fertilisation normally occurs in the fallopian tube.
- Uterus: A muscular, bag-like organ in which the embryo and later the foetus develop. It opens into the vagina through the cervix.
Fertilisation and development
- The fertilised egg, called a zygote, divides and develops into an embryo. The embryo becomes implanted in the lining of the uterus.
- The embryo obtains nutrition from the mother's blood through a special tissue called the placenta.
- The placenta provides a large surface area for exchange of nutrients, oxygen and waste substances between the mother and the developing embryo.
- The period from fertilisation to birth is called the gestation period. In humans, it is about 9 months.
When the egg is not fertilised: Menstruation
- Each month, the uterus prepares a thick, spongy lining to receive and nourish a fertilised egg.
- If fertilisation does not occur, this lining is not needed. It is shed from the uterus as blood and mucus through the vagina. This process is called menstruation.
- The menstrual cycle is about 28 days in many women, although its length may vary.
8. Reproductive Health
Reproductive health means complete physical, emotional, social and behavioural well-being in all matters related to reproduction.
Sexually Transmitted Diseases (STDs)
- Several infections can be transmitted through sexual contact. Examples include bacterial infections such as gonorrhoea and syphilis, and viral infections such as genital warts and HIV infection, which can lead to AIDS.
- The use of condoms reduces the risk of many sexually transmitted infections, although it does not eliminate every risk entirely.
Contraception
Contraception is the prevention of pregnancy by preventing ovulation, fertilisation or implantation, depending on the method used.
| Category | Mechanism of action | Examples / methods |
|---|---|---|
| Physical barrier methods | Prevent sperm from physically reaching the egg. | Condoms, cervical caps, diaphragms |
| Chemical methods | Oral contraceptive pills alter hormonal balance and may prevent ovulation. | Oral contraceptive pills |
| Intrauterine devices (IUCD) | Placed in the uterus to prevent pregnancy. | Copper-T |
| Surgical methods | Prevent transfer of gametes. | Vasectomy (male); tubectomy / tubal ligation (female) |
9. Female Foeticide and Social Responsibility
- Female foeticide is the practice of terminating a pregnancy because the foetus is female. It is a grave social and ethical problem.
- A healthy society requires a balanced sex ratio. Female foeticide and illegal prenatal sex determination disturb this balance.
- Prenatal sex determination for the purpose of selective abortion is a legal offence in India. It must be discouraged to promote social justice, gender equality and respect for every human life.
10. Quick Revision – Important Points
Reproduction and DNA
- Reproduction ensures continuity of a species.
- DNA carries hereditary information.
- DNA copying can introduce small variations.
- Variation may help organisms survive environmental changes.
Asexual reproduction
Single parent; offspring are genetically very similar. Modes: fission, fragmentation, regeneration, budding, vegetative propagation and spore formation.
Sexual reproduction in plants
Stamen produces pollen; pistil contains ovary and ovules. Pollination transfers pollen to stigma. Fertilisation forms a zygote. Ovule becomes seed and ovary usually becomes fruit.
Human reproduction
Testes produce sperm; ovaries produce eggs. Fertilisation normally occurs in the fallopian tube. The embryo implants in the uterus and receives nourishment through the placenta.
Reproductive health
Includes awareness of STDs and contraception. Methods include physical barriers, chemical methods, IUCDs and surgical methods.
11. Self-Assessment Quiz
Choose one answer for each question and select Check Answers.
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