How do Organisms Reproduce? — NCERT Solutions

Correct Answer: (b) Yeast.

Justification: Yeast reproduces asexually by budding, in which a small outgrowth (bud) develops on the parent cell, grows, and eventually separates to form a new individual. Amoeba and Leishmania reproduce by binary fission, while Plasmodium reproduces by multiple fission (sporulation).

Correct Answer: (c) Vas deferens.

Justification: Vas deferens (also called the sperm duct) is a part of the male reproductive system. It carries sperms from the testes to the urethra. The ovary, uterus, and fallopian tube are all parts of the female reproductive system.

Correct Answer: (d) Pollen grains.

Justification: The anther is the terminal part of the stamen (male reproductive organ of a flower). It produces and contains pollen grains, which carry the male gametes. Ovules are found in the ovary (female part), sepals are the outermost whorl of the flower, and the pistil is the entire female reproductive organ.

Advantages of Sexual Reproduction over Asexual Reproduction:

1. Greater genetic variation: Sexual reproduction involves the fusion of gametes from two parents. This results in new combinations of genes, producing offspring that are genetically different from both parents. This variation is the raw material for evolution.

2. Better adaptability: Because of genetic variation, some offspring may be better suited to survive in changing environments, giving the species a better chance of survival.

3. Elimination of harmful mutations: During sexual reproduction, harmful mutations from one parent may be masked by the normal gene from the other parent (due to dominance), reducing their harmful effects.

4. Basis of evolution: The variations generated through sexual reproduction allow natural selection to act, driving the evolution of species over generations.

5. Healthier offspring: The combination of genetic material from two individuals can result in offspring with improved traits (hybrid vigour).

In contrast, asexual reproduction produces genetically identical offspring (clones), which means all individuals are equally vulnerable to the same diseases or environmental changes.

Given: The testes are the primary male reproductive organs located in the scrotum outside the abdominal cavity.

Functions of the Testis:

1. Production of sperms (Spermatogenesis): The testes produce male gametes called spermatozoa (sperms) in the seminiferous tubules. The testes are located outside the body in the scrotum because sperm production requires a temperature slightly lower than the normal body temperature ($37°C$).

2. Secretion of male sex hormone — Testosterone: The testes contain Leydig cells (interstitial cells) that secrete the hormone testosterone. This hormone:

• Regulates the production of sperms.
• Is responsible for the development of secondary sexual characteristics in males at puberty (e.g., growth of facial hair, deepening of voice, increase in muscle mass, broadening of shoulders).

Conclusion: The testis performs both exocrine (sperm production) and endocrine (testosterone secretion) functions.

Concept: Menstruation is the periodic shedding of the uterine lining.

Explanation:

• Every month, one of the ovaries releases a mature egg (ovum) — this is called ovulation.
• In preparation for a possible pregnancy, the inner lining of the uterus (endometrium) thickens and becomes richly supplied with blood vessels to receive and nourish a fertilised egg.
• If fertilisation does NOT occur (i.e., the egg is not fertilised by a sperm), the egg degenerates.
• Since there is no embryo to implant, the thickened uterine lining is no longer needed.
• The levels of hormones (oestrogen and progesterone) drop, causing the uterine lining to break down and shed along with blood and mucus through the vagina.
• This process is called menstruation and lasts approximately 3 to 5 days.
• The entire cycle repeats approximately every 28 days.

Conclusion: Menstruation occurs because the prepared uterine lining is shed when fertilisation does not take place.

Labelled Diagram of the Longitudinal Section of a Flower:

A complete flower has four whorls. The key parts to label are:

Description of the diagram (to be drawn):

Draw a flower cut along its length showing the following labelled parts:

1. Petal (Corolla) — brightly coloured leaf-like structures that attract pollinators.
2. Sepal (Calyx) — green leaf-like structures at the base that protect the flower bud.
3. Stamen (Male reproductive organ):

• Anther — contains pollen grains (male gametes).
• Filament — stalk supporting the anther.

4. Pistil / Carpel (Female reproductive organ):

• Stigma — sticky top surface that receives pollen grains.
• Style — tube connecting stigma to the ovary.
• Ovary — swollen base containing ovules (which contain the female gamete/egg cell).

5. Receptacle — the base of the flower on which all parts are attached.
6. Peduncle (Flower stalk)
7. Ovule — inside the ovary; contains the egg cell.

$$\text{[Draw the diagram with all the above parts clearly labelled with arrows]}$$

Note: The stamen (anther + filament) is the male part and the pistil (stigma + style + ovary) is the female part of the flower.

Contraception refers to methods used to prevent unwanted pregnancy.

Different Methods of Contraception:

1. Barrier Methods: These physically prevent the sperm from reaching the egg.

• Condom (male and female) — also protects against STDs.
• Diaphragm — a dome-shaped device placed over the cervix.

2. Chemical Methods (Oral Contraceptives / Pills):

• Oral contraceptive pills contain hormones (oestrogen and progesterone) that prevent ovulation.
• They change the hormonal balance so that eggs are not released.

3. Intra-Uterine Devices (IUDs):

• Copper-T — a copper device placed inside the uterus by a doctor.
• It prevents fertilisation or implantation of the fertilised egg.

4. Surgical Methods (Permanent):

• Vasectomy (in males) — the vas deferens is cut and tied, preventing sperm from being released.
• Tubectomy (in females) — the fallopian tubes are cut and tied, preventing the egg from reaching the uterus.

5. Natural Methods:

• Abstinence — avoiding sexual intercourse.
• Safe period / Calendar method — avoiding intercourse during the fertile period.

Note: Condoms are the only contraceptive method that also provides protection against sexually transmitted diseases (STDs) including HIV/AIDS.

Difference in Modes of Reproduction in Unicellular and Multicellular Organisms:

Key Point:

• In unicellular organisms, the entire organism is one cell, so reproduction simply involves cell division (fission or budding).
• In multicellular organisms, the body design is complex, so specialised reproductive organs and cells (gametes) have evolved. Simple cell division of the whole body is not possible; instead, specific cells or organs are involved in reproduction.

Reproduction and Population Stability:

Given: Reproduction is the biological process by which organisms produce offspring of their own kind.

How it provides stability:

1. Maintains population size: Individuals in a population die due to natural causes, predation, disease, and accidents. Reproduction replaces the individuals that die, maintaining the population size and preventing extinction.

2. Compensates for death rate: The rate of reproduction balances the death rate, keeping the population stable over time.

3. Genetic variation through sexual reproduction: Sexual reproduction introduces genetic variations in the population. This means that when the environment changes (e.g., new disease, climate change), some individuals with favourable variations will survive and reproduce, ensuring the survival of the species.

4. Adaptation and evolution: Variations generated during reproduction allow natural selection to act, enabling species to adapt to changing environments, thus providing long-term stability.

5. Prevents inbreeding depression: In sexually reproducing populations, mixing of genes from different individuals maintains genetic health and vigour.

Conclusion: Reproduction ensures that the number of individuals in a population remains stable and that the species can adapt to environmental changes, thereby providing stability to the population over time.

Reasons for Adopting Contraceptive Methods:

1. To prevent unwanted pregnancy: The primary reason is to avoid unplanned or unwanted pregnancies, especially when a couple is not ready to have a child.

2. To plan the family (Family planning): Couples use contraception to decide the number of children they want and the spacing between births, ensuring better care and resources for each child.

3. To protect the health of the mother: Frequent pregnancies can be harmful to a woman's physical health. Contraception allows adequate recovery time between pregnancies.

4. To prevent the spread of Sexually Transmitted Diseases (STDs): Methods like condoms protect against STDs including HIV/AIDS, gonorrhoea, and syphilis.

5. Economic reasons: Raising children requires significant financial resources. Contraception helps families limit the number of children to what they can afford.

6. Social and educational reasons: Preventing unwanted pregnancies allows women to pursue education and careers, contributing to their social and economic empowerment.

7. Population control: At a national level, widespread use of contraception helps control the overall population growth rate, reducing pressure on resources.

8. Medical reasons: In some cases, pregnancy may be medically inadvisable due to the mother's health conditions, making contraception necessary.

Conclusion: Contraceptive methods are adopted for personal, health, social, economic, and national reasons to ensure the well-being of individuals, families, and society.