NCERT Solutions For Class 10 Science Chapter 8 How do Organisms Reproduce

NCERT Solutions For Class 10 Science Chapter 8 How do Organisms Reproduce

Question 1.

1. Name the human male reproductive organ that produces sperm and also secretes a hormone. Write the functions of the secreted hormone.
2. Name the parts of the human female reproductive system where:
   1. Fertilisation takes place.
   2. Implantation of the fertilised egg occurs. Explain how the embryo gets nourishment inside the mother’s body.

Answer:

1. Testes-secrete male hormone-testosterone

Functions:

1. Formation of sperms.
2. Development of secondary sexual characters.

2.

1. Fallopian tube/Oviduct.
2. Uterus.

• The placenta is a special disc-like tissue embedded in the mother’s uterine wall and connected to the foetus/embryo.
• The placenta provides a large surface area for glucose and oxygen/nutrients to pass from the mother’s blood to the embryo/foetus.

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Question 2. Identify A, B and C in the given diagram and write their functions.

Mention the role of gamete and zygote in sexually reproducing organisms.

Answer:

1. A Stigma, B→ Pollen tube, C→ Female gamete

Functions:

Stigma: It receives pollen.

Pollen tube: It transfers the male gamete.

Female gamete: After fertilization, a zygote is formed which develops into a new individual.

A gamete is a haploid cell which fuses to form a diploid zygote.

After fertilization, the zygote divides several times to form an embryo within the ovule. The ovule develops a tough coat and is gradually converted into a seed.

Question 3. List five distinguishing features between sexual and asexual types of reproduction in tabular form.
Answer:

Question 4. Explain what happens when:

1. Testosterone is released in males.
2. Pollen grain falls on the stigma of the flower.
3. Egg fuses with sperm cells.
4. Planaria is cut into many pieces.
5. Buds are formed on the notches of the Bryophyllum leaf.

Answer:

1. 1. Formation of sperms, change in appearance.
   2. Thick hair grows on the face and the voice begins to crack.
2. A tube grows out of the pollen grain and travels through the style to reach the ovary.
3. Zygote is formed (Fertilization).
4. Each piece grows into a complete organism.
5. Buds may fall on the soil and develop into new plants.

Question 5. Define the following processes:

1. Fertilization
2. Menstruation
3. Binary fission
4. Vegetative propagation
5. Regeneration

Answer:

1. The fusion of the male gamete with the female gamete is known as fertilization.
2. The cycle which takes place every month when the egg is not fertilized is known as menstruation.
3. It lasts for about two to eight days and during this cycle, the lining of the uterus slowly breaks and comes out through the vagina as blood and mucus.
4. The splitting of the nucleus into two daughter cells which can take place in any plane is known as binary fission. It can be seen in Amoeba.
5. When the vegetative parts of a plant like the root, stem or leaves develop into a new plant under appropriate conditions it is known as vegetative propagation.
6. When the body of an organism cuts into any number of pieces and each piece grows into a complete organism, it is known as regeneration. It can be seen in Hydra and Planaria.

Question 6.

1. Name the parts labelled as A, B, C and D in the diagram given below:
2. What is pollination? State its significance.
3. How does fertilization occur in flowers? Name the parts of the flower that develop into
4. Seed, and
5. Fruit after fertilization.

Answer:

1. Four labelling:
   1. A – Pollen grain
   2. B-Pollen tube
   3. C – Ovary
   4. D-Female germ cell
2. Definition: Transfer of pollen grains from anther to stigma.
   1. Significance: To bring about fertilization.
3. As the pollen lands on the suitable stigma, a pollen tube grows through the style carrying the male germ cell. It reaches the ovary and the male germ cell fuses with the female germ cell and thus fertilization takes place.
   1. Seed → Ovule
   2. Fruit → Ovary

Question 7.

1. Draw a sectional view of the human female reproductive system and label the part where:
   1. Eggs develop.
   2. Fertilization takes place.
   3. The fertilized egg gets implanted.
2. Describe, in brief, the changes the uterus undergoes
   1. To receive the zygote.
   2. If a zygote is not formed.

Answer:

1. Diagram:
   1. Labellings: Ovary (Eggs develop)
   2. Fallopian Tube/Oviduct (Fertilization takes place)
   3. Uterus (Fertilized egg gets implanted)
   1. The inner lining of the uterus thickens and is richly supplied with blood.
   2. The inner lining slowly breaks and comes out through the vagina as blood and mucus (menstruation).

Question 8. Write the two causes of the human population explosion. Explain with the help of suitable examples how this explosion can be checked.
Answer:

1. The declining death rates are due to improved medical facilities.
2. Desire for a male child.
3. Less awareness about birth control methods.
4. Illiteracy/poverty (any two)

Population explosion can be checked by:

1. Using contraceptive methods to avoid pregnancy.
   1. Contraceptive methods are used such as
      1. Mechanical barriers
      2. Drugs (as pills)
      3. Loop or copper-T and
      4. Surgical method.
   2. Pills change the hormonal balance and thus prevent the release of egg, hence fertilization is prevented.
2. Spreading awareness about the importance of a small family. This is because an expanding population makes it harder to improve everybody’s standard of living.

Question 9.

1. Identify the organisms A, and B and the mode of asexual reproduction exhibited by them.
2. How will an organism benefit if it reproduces through spores?
3. Mention the two asexual methods by which Hydra can reproduce. Explain briefly any one such method.

Answer:

1. 1. Bryophyllum-vegetative propagation.
   2. Plasmodium-multiple fission.
2. Spores are covered with thick walls that protect them until they come into contact with a moist surface.
3. Budding and regeneration.

Budding: A bud develops as an outgrowth due to repeated cell division at a specific site, these buds develop into tiny individuals, mature and detach from the parents to become new individuals.

Regeneration: Specialized cells divide to form a large number of cells and undergo changes to become various cell types and tissues.

Question 10. Explain the budding mode of asexual reproduction.
Answer:

Budding: Budding is the forming of offspring from an outgrowth or bud of a parent. The bud may be unicellular or multicellular depending upon the nature of the parent individual.

In coelenterates, tunicates and other multicellular animals it consists of a small number of cells surrounded by an epithelium. Hydra is usually formed in the lower half of the body.

The buds separate from the parent soon after their formation or after some initial growth. In many cases, individuals developed from them do not separate but remain interconnected to form a colony.

Question 11. Describe the structure of a typical flower.
Answer:

Flower is a modified shoot. It is attached to the stem by a stalk called a pedicel. The swollen tip of the pedicel is called the thalamus.

• On the thalamus are arranged four whorls of floral leaves. The outermost whorl is called calyx which is formed of green leaf-like sepals.
• The second whorl is called corolla which is coloured and formed of petals. The third whorl is called androecium which is formed of stamens. A fourth (innermost) whorl is called gynoecium or pistil and is formed of carpels.
• Stamen has two parts-filament and anther which produces grain-like structures called pollens. Stamen is the male reproductive structure of the plant. The pistil has three parts-basal ovary, long style and stigma at the top. The pistil is the female reproductive structure of the plant.

Question 12. Describe asexual reproduction in Amoeba.
Answer:

Single-celled Amoeba reproduces by the asexual method known as binary fission, i.e., during division, Amoeba splits into two halves. The splitting of the two cells during binary fission can take place in any plane.

During binary fission of amoeba, nuclear division starts first, followed by the constriction in the cell membrane. As the nuclear division completes, constriction deepens and divides into two parts, each containing one nucleus. Finally, two independent daughter amoebae are formed.

Question 13. Why are budding, fragmentation and regeneration all considered asexual types of reproduction? With neat diagrams explain the process of regeneration in Planaria.
Answer:

Budding, fragmentation and regeneration are considered asexual types of reproduction because all of them involve only one parent and gametes are not involved in reproduction. No fertilisation and then zygote formation occur in asexual reproduction.

Question 14. Write two points of difference between asexual and sexual types of reproduction. Describe why variations are observed in the offspring formed by sexual reproduction.
Answer:

During sexual reproduction, two types of gametes fuse. Although the gametes contain the same number of chromosomes, their DNA is not identical. This situation generates variations among the offspring.

Question 15. Distinguish between pollination and fertilisation. Mention the site and product of fertilisation in a flower. Draw a neat, labelled diagram of a pistil showing pollen tube growth and its entry into the ovule.
Answer:

1. The process or mechanism of transfer of pollen grains from the anther to the stigma is termed pollination.
2. The fusion of male and female gametes giving rise to a zygote is termed fertilisation.
3. The site of fertilisation is ovule.
4. The product of fertilisation is the zygote.

Question 16. Distinguish between a gamete and a zygote. Explain their roles in sexual reproduction.
Answer:

Gamete represents the sex cell or germ cell in sexual reproduction. There are two types of gametes, male and female.

• A zygote is the product of fertilisation in which a male and a female gamete fuse.
• The two fusing gametes possess characters of their parents in their DNA. Fertilisation brings the characters of both parents into one zygote cell.
• The zygote is the first cell of the next generation. It divides to form an embryo which subsequently grows into a new individual.

Question 17. Draw the diagram of a flower and label the four whorls. Write the names of gamete production organs in the flower.
Answer:

1. Male gamete forming part-anther/stamen
2. Female gamete forming part-pistil/ovary/ovule

Question 18. What is the placenta? Mention its role during pregnancy.
Answer:

The placenta is a disc-like structure in the lining of the uterine wall. It contains villi on the embryo’s side of the tissues and blood spaces on the mother’s side surrounding the villi. The embryo is connected to the placenta by a tube called the umbilical cord.

Role during pregnancy: It is through the placenta that all the requirements of the developing foetus are met.

The villi provide a large surface area that facilitates the passage of nutrition and oxygen to the embryo from the mother through blood.

Wastes produced by the embryo are removed through the placenta into the mother’s blood.

Question 19. What are various ways to avoid pregnancy? Elaborate on any one method.
Answer:

Ways to avoid pregnancy are called contraceptive methods. They include the following:

1. Mechanical barrier e.g., condom (so that sperm does not reach the egg)
2. Drugs (or pills)-change the hormonal balance so that eggs are not released.
3. Loop or copper-T prevents pregnancy.
4. Surgical method-permanent contraception.

Mechanical barrier: Several methods create a barrier between sperm and egg. Some of them are as follows:

Condoms: It is a fine rubber worn over the penis during sexual intercourse. Semen is collected in it and not discharged into the vagina. This method also prevents the spread of STDs and AIDS.

Diaphragms or caps: They can be fitted in the cervix of a woman to prevent semen from reaching the fallopian tube.

Question 20. How does fertilisation take place? Fertilisation occurs once a month. Comment.
Answer:

Fertilisation in human beings: The sperm made in the testis of males are introduced into the vagina of the woman through the penis during copulation or mating.

• The sperms are highly active and mobile and thus move up through the cervix into the uterus. From the uterus, sperm pass into the oviducts. The oviduct contains an ovum released by the ovary during ovulation.
• Millions of sperm are released into the vagina at one time, but only one sperm fuses with the ovum in the oviduct to form a zygote. This is called fertilisation.
• Fertilisation can occur only once a month because the ovary releases just one mature egg every month during a menstrual cycle (around the 14th day).

Question 21. Reproduction is essentially a phenomenon that is not for the survival of an individual but for the stability of a species. Justify.
Answer:

All living organisms need energy for their survival and growth. This energy is obtained from various life processes such as nutrition and respiration. Thus, these phenomena are essential for the living of an individual.

• Compared to these life processes, reproduction may appear to be a waste of energy as it is not essential for the survival of an individual. But it is an important function of a living being as it helps in the production of new individuals of its kind. Continuity of life has been possible through reproduction.
• Genetic material is transferred from one generation to the next by DNA copying as a result of reproduction. DNA copying takes place with high constancy and considerable variations.
• This is advantageous for maintaining features that allow the organism to survive in the changing environment. Therefore, reproduction is related to the stability of a species and is not essential for the survival of an individual.

Question 22. Describe sexually transmitted diseases and mention the ways to prevent them.
Answer:

Sexually Transmitted Diseases (STDs) are infectious diseases that are transmitted during sexual intercourse. These include bacterial infections such as gonorrhoea and syphilis, and viral infections such as warts and HIV-AIDS. To prevent STDs, the following precautions can be taken:

1. Use a protective covering called a condom, over the penis during sex.
2. Avoid multiple sex partners and maintain personal hygiene.