Chapter 7: How Do Organisms Reproduce?
Class X Science
Key Concepts
Reproduction is the biological process that ensures continuity of life on earth. It is the process by which living organisms produce new individuals similar to themselves.
Types of Reproduction
Reproduction may be sexual or asexual. Sexual reproduction gives rise to new genetic combinations by fusion of gametes, leads to variation, and promotes evolution.
A. Asexual Reproduction
- It involves only one parent.
- There is no formation and fusion of gametes.
- The young ones formed are almost identical to each other as well as to the parent cell.
- Generally occurs during favourable environmental conditions and when there is an abundance of food.
B. Sexual Reproduction
- Two individuals i.e., one male and one female are needed to give rise to a new individual.
- Gametic fusion takes place.
- Since gametes are derived from two different organisms, it results in a new combination of genes which increases the chances of genetic variations.
- Results in the origin of new species and is adopted by higher organisms.
Modes of Asexual Reproduction
1. Fission
- Binary Fission (e.g., Amoeba): When the amoeba cell reaches its maximum size, the nucleus lengthens and divides into two parts, followed by the division of cytoplasm to form two smaller daughter cells.
- Multiple Fission (e.g., Plasmodium): The nucleus undergoes repeated division to produce a large number of nuclei. Each nucleus along with a little bit of cytoplasm forms a membrane around it. Many daughter cells are produced from the parent cell.
2. Fragmentation
The organism breaks up into smaller pieces; upon maturation, each piece develops into a new individual. Example: Spirogyra.
3. Regeneration
A few organisms may give rise to new individual organisms from their body parts. If the body of Planaria gets cut into a number of pieces, each piece can regenerate into a complete organism. Example: Hydra and Planaria.
[DIAGRAM: Regeneration in Planaria showing a parent Planaria cut into three pieces (head, middle, tail), each growing into a full daughter Planaria]
4. Budding
In some organisms, a bud is formed which develops into a tiny individual. It detaches from the parent body upon maturation and develops into a new individual. Example: Hydra and Yeast.
5. Vegetative Propagation
Method by which plants reproduce by their vegetative parts such as roots, stems, and leaves.
- Natural: Mint (roots), Sugarcane/Jasmine (stems), Bryophyllum (leaves). In Bryophyllum, buds are produced in the notches along the leaf margins and develop into new plants when they fall on the soil.
- Artificial: Tissue culture, layering, grafting.
Importance:
- Plants are genetically almost similar to the parent plant.
- Plants which have lost the ability to produce viable seeds can reproduce (e.g., banana, rose).
- Plants can bear flowers and fruits earlier.
6. Spore Formation
Spores are reproductive cells capable of developing into a new individual. They are covered by a thick wall that protects them from adverse conditions.
Example (Rhizopus/Bread Mould): Grows as filamentous structures called Hyphae. Hyphae give rise to globular structures called Sporangia, which contain spores.
[DIAGRAM: Spore formation in Rhizopus showing Hyphae, Sporangia, and spores dispersing]
Question Bank
Section A: Multiple Choice Questions
- The number of chromosomes in parents and offsprings of a particular species remains constant due to:
Ans: (b) halving of chromosomes during gamete formation - Variations occur as a result of:
Ans: (c) Sexual reproduction - Reproduction is essential for living organisms in order to:
Ans: (d) Continue the species generation after generation - How do spores develop into Rhizopus?
Ans: (a) spores divide and grow into new individual - Cloning is a mode of:
Ans: (b) asexual reproduction - Vegetative propagation refers to formation of new plants from:
Ans: (b) stem, roots, leaves - The ability of a cell to divide into several cells during reproduction in Plasmodium is called:
Ans: (d) multiple fission - A feature of reproduction that is common to Amoeba, Leishmania and Spirogyra is that:
Ans: (a) they reproduce asexually - Identify the organism that can divide in only one plane:
Ans: (b) Leishmania - Bryophyllum can be propagated vegetatively by the:
Ans: (b) leaf - When an animal is cut into pieces and each piece grows into a complex organism. What is the process?
Ans: (d) Regeneration - Hydra usually reproduces by:
Ans: (b) regeneration and budding
Section B: Assertion-Reason Type Questions
Options: (a) Both A & R true, R is correct explanation. (b) Both A & R true, R is not correct explanation. (c) A is true, R is false. (d) A is false, R is true.
- Assertion: Clones of an organism are formed by asexual reproduction.
Reason: Clones have exact copies of genes as parental organism.
Ans: (b) Both A and R are true but R is not the correct explanation of A. - Assertion: Asexual reproduction is a type of reproduction in primitive organisms.
Reason: Asexual reproduction is a simple process involving only one parent.
Ans: (a) Both A and R are true and R is the correct explanation of A. - Assertion: Variations are prominent in sexual reproduction.
Reason: In sexual reproduction the offsprings are exactly similar to parents.
Ans: (c) A is true but R is false. - Assertion: Regeneration is a process of reproduction in certain organisms.
Reason: Spirogyra reproduces by regeneration.
Ans: (c) A is true but R is false (Spirogyra reproduces by fragmentation).
Section C: Short Answer Questions
Q20. Newly formed DNA copies may not be identical at times. Give one reason.
Ans: The process of copying the DNA (replication) leads to some variations each time. These inaccuracies result in DNA copies that are similar but not identical.
Q21. Define reproduction. How does it helps in providing stability to the population of species?
Ans: Reproduction is the production of new organisms by existing ones. It ensures the transfer of specific characters (body design) essential for an individual to live in a particular niche. Variations introduced during reproduction help the species survive changing niches/environments.
Q22. How does Plasmodium reproduce. Is this method sexual or asexual?
Ans: Plasmodium reproduces through multiple fission. This is an asexual method where the parent splits to form many new organisms simultaneously.
Q24. Name the causative agent of the disease “kala-azar” and its mode of asexual reproduction.
Ans: Causative agent: Leishmania. Mode: Binary fission (longitudinally in a definite plane).
Q26. Name an organism which reproduces by spore formation. List three conditions favourable for spores to germinate and grow.
Ans: Organism: Rhizopus. Favourable conditions: Moisture, suitable temperature, and food (nutrition).
Section D: Long Answer Questions
Q27. Name the mode of reproduction of the following organisms:
I. Planaria: Regeneration
II. Hydra: Budding
III. Rhizopus: Spore Formation
(b) List two advantages of vegetative propagation.
Ans: (i) Useful for plants that do not produce viable seeds (ii) Genetically similar plants are produced (iii) Quick method of propagation.
Q28. (A) What is the importance of variation in the survival of individuals?
Ans: Variation helps organisms adapt to environmental changes, makes species resistant to diseases, and aids in the survival of the species during drastic niche alterations.
(C) Differentiate between asexual and sexual reproduction.
Ans: Asexual involves a single parent and little variation. Sexual involves two parents (sexes), gamete fusion, and leads to variation.
Case Study: Vegetative Propagation in Bryophyllum
Context: Preeti observed new plants coming out from Bryophyllum leaves that fell on the ground.
- What does the incidence indicate?
Ans: (d) Both (a) and (b) - Leaves have special buds that germinate; it propagates vegetatively through leaves. - Which of the following plants can propagate vegetatively through leaves like Bryophyllum?
Ans: (b) Begonia. - Which plant is artificially propagated by stem cuttings?
Ans: (c) Rose. - Vegetative propagation is comparatively faster than sexual method. Do you agree?
Ans: Yes, production of new plants through seeds takes more time compared to vegetative propagation.