NCERT Exemplar Solutions
Class 10 - Science - CHAPTER 8: How do Organisms Reproduce?

Long Answer Questions

Budding, fragmentation and regeneration are considered asexual types of reproduction because each of them involves only one parent and gametes are not involved. The offspring produced are genetically identical to the parent because they arise from mitotic cell division.

Regeneration in Planaria

Planaria has a remarkable ability to regenerate. When the organism is cut vertically or horizontally, each piece can grow and develop into a complete individual. This is possible because specialized cells present in the organism divide rapidly to form large numbers of cells, which then differentiate into various tissues and organs.

Steps of Regeneration

1. Specialized cells present in Planaria proliferate at the site of the cut.
2. These cells undergo repeated mitotic divisions.
3. The new cells differentiate to form tissues, organs and organ systems.
4. Gradually, a complete organism is regenerated from the cut sections.

Diagram Description: The regenerated regions in pieces D and E show the formation of missing body parts such as the head or tail, depending on the fragment.

Differences between Asexual and Sexual Reproduction

Reason for Variations in Sexual Reproduction

During sexual reproduction, male and female gametes fuse. Although the gametes have the same number of chromosomes, their DNA is not identical. The recombination and assortment of genes during meiosis lead to genetic variations. When these gametes fuse during fertilisation, the resulting offspring inherits a unique combination of genes, resulting in variations.

Difference between Pollination and Fertilisation

• Pollination: The process of transfer of pollen grains from the anther to the stigma of a flower.
• Fertilisation: The fusion of male and female gametes to form a zygote.

Site of fertilisation: Ovule

Product of fertilisation: Zygote

Pistil Diagram Description: The pistil consists of stigma, style and ovary. After pollination, the pollen grain germinates on the stigma and forms a pollen tube. The pollen tube grows through the style and reaches the ovary, finally entering the ovule where fertilisation takes place.

Difference between Gamete and Zygote

• Gamete: A sex cell (male or female) involved in sexual reproduction. It contains half the number of chromosomes (haploid).
• Zygote: The first cell formed after the fusion of male and female gametes. It is diploid and develops into an embryo.

Roles in Sexual Reproduction

Gametes carry genetic material from parents. When the male and female gametes fuse during fertilisation, the zygote is formed. The zygote undergoes repeated mitotic divisions to develop into an embryo, eventually forming a new individual. The zygote inherits the characteristics of both parents.

Four Whorls of a Flower: Calyx, Corolla, Androecium, Gynoecium.

Gamete-producing organs:

• Male gamete producing organ: Anther (part of stamen/androecium)
• Female gamete producing organ: Ovary (part of pistil/gynoecium)

Diagram Description: The flower shows clearly labelled calyx, corolla, androecium and gynoecium with reproductive organs including anther and ovary.

The placenta is a special tissue connection between the embryo and the uterine wall. It contains villi that increase the surface area for exchange.

Role of Placenta

• Facilitates passage of nutrition and oxygen from the mother to the embryo.
• Removes waste substances produced by the embryo into the mother's blood.
• Provides hormones required for maintaining pregnancy.

Various contraceptive methods include mechanical methods, chemical methods, and surgical methods.

Example: Mechanical Method

Mechanical barriers such as condoms prevent the sperm from entering the female reproductive tract. This avoids fertilisation because the sperm cannot reach the egg. Condoms also prevent the transmission of sexually transmitted infections.

Process of Fertilisation:

• During sexual intercourse, sperm enters the female reproductive tract.
• The sperm moves through the vagina, uterus and enters the oviduct.
• The egg released from the ovary reaches the oviduct.
• Fertilisation occurs when the sperm encounters and fuses with the egg in the oviduct.

Why fertilisation occurs once a month:

The ovary releases one egg approximately every month. If the egg is not fertilised, it degenerates and menstruation occurs. Therefore, fertilisation can occur only during the period when the egg is available.

Reproduction ensures the continuity of a species by producing new individuals. Although an individual organism may not need reproduction for its own survival, the species as a whole depends on it.

Justification:

• Reproduction passes genetic information from one generation to the next.
• DNA copying occurs with consistency but introduces minor variations, helping species adapt to environmental changes.
• These variations contribute to stability and survival of the species over long periods of time.

Sexually transmitted diseases (STDs) are infectious diseases spread through sexual contact. They may be caused by bacteria or viruses.

Common STDs: Syphilis, Gonorrhoea, HIV-AIDS

Prevention:

• Use of mechanical barriers such as condoms to prevent transmission.
• Maintaining good sexual hygiene.
• Avoiding sexual contact with infected persons.