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NCERT Solutions for Class 12 Biology Chapter 1 Reproduction in Organisms
Why is reproduction essential for organisms?
Reproduction is a fundamental feature of all living organisms. It is a biological process through which living organisms produce off springs (young ones) similar to them. Reproduction ensures the continuance of various species on the Earth. In the absence of reproduction, the species will not be able to exist for a long time and may soon get extinct.
Which is a better mode of reproduction sexual or asexual? Why?
Sexual reproduction is a better mode of reproduction. It allows the formation of new variants by the combination of the DNA from two different individuals, typically one of each sex. It involves the fusion of the male and the female gamete to produce variants, which are not identical to their parents and to themselves. This variation allows the individual to adapt constantly changing and challenging environment. Also, it leads to the evolution of better suited organisms which ensures greater survival of a species. On the contrary, asexual reproduction allows very little or no variation at all. As a result, the individuals produced are exact copies of their parents and themselves.
Why is the offspring formed by asexual reproduction referred to as clone?
A clone is a group of morphological and genetically identical individuals. In the process of asexual reproduction, only one parent is involved and there is no fusion of the male and the female gamete. As a result, the off springs so produced are morphological and genetically similar to their parents and are thus, called clones.
Offspring formed due to sexual reproduction have better chances of survival. Why? Is this statement always true?
Sexual reproduction involves the fusion of the male and the female gamete. This fusion allows the formation of new variants by the combination of the DNA from two (usually) different members of the species. The variations allow the individuals to adapt under varied environmental conditions for better chances of survival.
However, it is not always necessary that the offspring produced due to sexual reproduction has better chances of survival. Under some circumstances, asexual reproduction is more advantageous for certain organisms. For example, some individuals who do not move from one place to another and are well settled in their environment. Also, asexual reproduction is a fast and’ a quick mode of reproduction which does not consume much time and energy as compared to sexual reproduction.
How does the progeny formed from asexual reproduction differ from those formed by sexual reproduction?
|Progeny formed from asexual reproduction||Progeny formed from sexual reproduction|
|1.||Asexual reproduction does not involve the fusion of the male and the female gamete. Organisms undergoing this kind of reproduction produce offsprings that are morphologically and genetically identical to them.||Sexual reproduction involves the fusion of the male and the female gamete of two individuals, typically one of each sex. Organisms undergoing this kind of reproduction produce offsprings that are not identical to them.|
|2.||Offsprings thus produced do not show variations and are called clones.||Offsprings thus produced show variations from each other and their parents.|
Distinguish between asexual and sexual reproduction. Why is vegetative reproduction also considered as a type of asexual reproduction?
|Sexual reproduction||Asexual reproduction|
|1.||It involves the fusion of the male and the female gamete.||It does not involves the fusion of the male and the female gamete.|
|2.||It requires two (usually) different individuals.||It requires only one individual|
|3.||The individuals produced are not identical to their parents and show variations from each other and also, from their parents.||The individuals produced are identical to the parent and are hence, called clones.|
|4.||Most animals reproduce sexually. Both sexual and asexual modes of reproduction are found in plants.||Asexual modes of reproduction are common in organisms having simple organisations such as algae and fungi.|
|5.||It is a slow process.||It is a fast process.|
Vegetative reproduction is a process in which new plants are obtained without the production of seeds or spores. It involves the propagation of plants through certain vegetative parts such as the rhizome, sucker, tuber, bulb, etc. It does not involve the fusion of the male and the female gamete and requires only one parent. Hence, vegetative reproduction is considered as a type of asexual reproduction.
What is vegetative propagation? Give two suitable examples.
Vegetative propagation is a mode of asexual reproduction in which new plants are obtained from the vegetative parts of plants. It does not involve the production of seeds or spores for the propagation of new plants. Vegetative parts of plants such as runners, rhizomes, suckers, tubers, etc. can be used as propagules for raising new plants.
Examples of vegetative reproduction are given below :
1. Eyes of potato : The surface of a potato has several buds called eyes. Each of these buds when buried in soil develops into a new plant, which is identical to the parent plant.
2. Leaf buds of Bryophyllum : The leaves of Bryophyllum plants bear several adventitious buds on their margins. These leaf buds have the ability to grow and develop into tiny plants when the leaves get detached from the plant and come in contact with moist soil.
- Juvenile phase,
- Reproductive phase,
- Senescent phase.
- Juvenile Phase : All organisms have to reach a certain stage of growth and maturity in their life, before they can be reproduce sexually. This phase of growth is called the juvenile phase or vegetative phase in plants.
- Reproductive Phase : When the juvenile phase is over the organisms enter the period of reproductive phase or sexual maturity. It is indicated by showing various morphological and physiological changes such as development of secondary sexual characters in animals and by flowering in plants. This is the actual period of the life span of any organism when it is capable of producing offspring’s. This phase is of variable duration in different organisms.
- Senescent Phase : This is the final and third stage of growth cycle. It can be considered as the end of reproductive phase. It is accompanied by reduction in functional capacity and increase in cellular break down and metabolic failures. It ultimately leads to death.
Higher organisms have resorted to sexual reproduction in spite of its complexity. Why?
Although sexual reproduction involves more time and energy, higher organisms have resorted to sexual reproduction in spite of its complexity. This is because this mode of reproduction helps introduce new variations in progenies through the combination of the DNA from two (usually) different individuals. These variations allow the individual to cope with various environmental conditions and thus, make the organism better suited for the environment. Variations also lead to the evolution of better organisms and therefore, provide better chances of survival. On the other hand, asexual reproduction does not provide genetic differences in the individuals produced.
Explain why meiosis and gametogenesis are always interlinked?
Meiosis is a process of reductional division in which the amount of genetic material is reduced. Gametogenesis is the process of the formation of gametes. Gametes produced by organisms are haploids (containing only one set of chromosomes), while the body of an organism is diploid. Therefore, for producing haploid gametes (gametogenesis), the germ cells of an organism undergo meiosis. During the process, the meiocytes of an organism undergo two successive nuclear and cell divisions with a single cycle of DNA replication to form the haploid gametes.
Identify each part in a flowering plant and write whether it is haploid (n) or diploid (2n).
- Ovary ……………………………….
- Anther ……………………………….
- Egg ……………………………….
- Pollen ……………………………….
- Male gamete ……………………………….
- Zygote ……………………………….
- diploid (2n)
- diploid (2n)
- haploid (n)
- haploid (n)
- haploid (n)
- diploid (2n)
Define external fertilisation. Mention its disadvantages.
External fertilisation is the process in which the fusion of the male and the female gamete takes place outside the female body in an external medium, generally water. Fish, frog, starfish are some organisms that exhibit external fertilisation.
Disadvantages of external fertilisation In external fertilisation, eggs have less chances of fertilisation. This can lead to the wastage of a large number of eggs produced during the process. Further, there is an absence of proper parental care to the offspring, which results in a low rate of survival in the progenies.
Differentiate between a zoospore and a zygote.
|1.||A zoospore is a motile asexual spore that utilises the flagella for movement.||A zygote is a non-motile diploid cell formed as a result of fertilisation.|
|2.||It is an asexual reproductive structure.||It is formed as a result of sexual eproduction|
|3.||Zoospores are formed in simple plants like, algane or fungi.||Zygote is formed in complex organism.|
Differentiate between gametogenesis from embryogenesis.
|1.||It is the process of the formation of haploid male and female gametes from diploid meiocytes (gamete mother cell) through the process of meiosis.||It is the process of the development of the embryo from the repeated mitotic divisions of the diploid zygote.|
|2.||Gametes may be either homogametes or heterogametes.||Animals may be either oviparous or viviparous.|
Describe the post-fertilisation changes in a flower.
As a result of double fertilisation in flowering plants, zygote (2n) and the primary endosperm nucleus (3n) is produced. The calyx, corolla, stamens and style wither away. The calyx may persist or even show growth in certain cases. The post fertilisation changes which take place are (i) Endosperm formation (ii) Embryo formation (iii) Seed formation and (iv) Fruit formation.
The primary endosperm nucleus becomes active and forms a nutritive vegetative tissue. The endosperm at the expense of food present in the nucellus, Endosperm may be completely used up by the developing embryo (non-endospermic seeds e.g., pea) or may persist in the seed (endospermic seed e.g., castor). The zygote, waits for sometime till the formation of endosperm and then develops into embryo, by withdrawing nutrition from the endosperm. Ultimately the ovules are transformed into seeds and the ovary becomes a fruit. The formation of fruit helps in the nourishment and protection to the developing seeds and later helps in seed dispersal. Under favourable conditions the seeds germinate to form new plants.
What is a bisexual flower? Collect five bisexual flowers from your neighbourhood and with the help of your teacher find out their common and scientific names.
A flower that contains both the male and female reproductive structure (stamen and pistil) is called a bisexual flower.
Examples of plants bearing bisexual flowers are:
- Water lily (Nymphaea odorata)
- Rose (Rosa multiflora)
- Hibiscus (Hibiscus Rosa-sinensis)
- Mustard (Brassica nigra)
- Petunia (Petunia hybrida)
Examine a few flowers of any cucurbit plant and try to identify the staminate and pistillate flowers. Do you know any other plant that hears unisexual flowers?
Cucurbit plant bears unisexual flowers as these flowers have either the stamen or the pistil. The staminate flowers bear bright, yellow coloured petals along with stamens that represent the male reproductive structure. On the other hand, the pistillate flowers bear only the pistil that represents the female reproductive structure.
Other examples of plants that bear unisexual flowers are corn, papaya, cucumber, etc.
Why are offspring of oviparous animals at a greater risk as compared to offspring of viviparous animals?
Oviparous animals lay eggs outside their body. As a result, the eggs of these animals are under continuous threat from various environmental factors. On the other hand, in viviparous animals, the development of the egg takes place inside the body of the female. Hence, the offspring of an egg-laying or oviparous animal is at greater risk as compared to the offspring of a viviparous animal, which gives birth to its young ones.
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