NCERT Solutions for CBSE Class 10 Biology chapter 8 - How do Organisms Reproduce?

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Chapter 8 - How do Organisms Reproduce? Excercise 128

Solution 1
DNA copying is essential to produce organisms which are similar to their parents. As copying of DNA brings some variations each time, the surviving cells are similar to parent cell but subtly different from each other. This tendency for variation during reproduction brings variations among the individuals of the same species.

Concept insight: Recall composition of nucleus and role of DNA.
Solution 2
Niches are well-defined places in the ecosystem where population of organisms lives in. If population of reproducing organisms is suited to a particular niche and if that niche is drastically altered or destroyed, because of uncontrollable reasons like temperature changes, water level changes etc, the population can be wiped out. However, if some variations are present in a few individuals in these populations, there will be chances for their survival. The surviving individual can further reproduce and develop a population as per the changed niche. In this way, variation is beneficial to the species but not necessarily for the individual.

Concept insight: Importance of variation.

Chapter 8 - How do Organisms Reproduce? Excercise 133

Solution 1
                    Binary Fission
i. Nucleus divides into two daughter nuclei.
ii. Constriction is formed in the cell after nuclear division.
iii. Only two daughter cells are produced.
iv. It takes place in favourable conditions.
                                                Multiple fission
i. Nucleus divides into many daughter nuclei.
ii. Division of cytoplasm occurs very late thus many cells are produced simultaneously.
iii. Many new daughter cells are produced.
iv. It takes place in adverse conditions.

 

Concept insight:

(i) Differences should always be written in tabular form.

(ii) Differences should be written in terms of their significance.

(iii)Write only those many numbers of differences as stated in the question.  

(iv) Give example wherever possible.

Solution 2
Spores have a hard outer covering that is able to withstand adverse conditions of high temperature, shortage of water and food.  
So, if an organism reproduces through spore formation it can easily withstand these adverse conditions.  

Concept insight: Recall the structure of spores.
Solution 3

Higher complex organisms cannot give rise to new individuals through regeneration because:  

i. Higher organisms have very complex body designs i.e. they show the following hierarchy of organization cell-tissue-organ-organ system.

ii. Higher organisms do not have special calls which help in regeneration of lost part.  

Concept insight: Recall the hierarchy of organization in multicellular organism.

Solution 4
Vegetative propagation is used to grow some plants such as banana, orange, rose etc because these plants have lost the capacity to produce viable seeds.  

Concept insight: List the advantages of vegetative propagation.
Solution 5
DNA copying is essential to produce organisms which are similar to their parents. As copying of DNA brings some variations each time, the surviving cells are similar to parent cell but subtly different from each other. This tendency for variation during reproduction brings variations among the individuals of the same species.

Concept insight: Recall composition of nucleus and role of DNA.

Chapter 8 - How do Organisms Reproduce? Excercise 140

Solution 1
Pollination Fertilization
i. It is the process of transfer of pollen grains from anther to stigma. i. It is the process of fusion of male and female nucleus.
ii. Through this process, pollen grains carrying male nucleus reaches at the tip of stigma (part of female reproductive system). ii. Through this process, diploid zygote is formed which develops into a seed.

 Concept insight:

(i) Differences should always be written in tabular form.

(ii) Differences should be written in terms of their significance.

(iii) Write only those many numbers of differences as stated in the question.  

(iv) Give example wherever possible.

Solution 2
Secretions of seminal vesicle and prostate gland provides nutrition and motility to the sperms. This secretion also helps in the transport of sperms.  

Concept insight: Recall functions of parts of male reproductive system.
Solution 3

Following changes are seen in girls at the time of puberty:

i. Increase in the breast size of the girls.

ii. Growth of hair under the arm-pits and pubic hair starts.

iii. Menstruation cycle commences.

iv. Skin becomes oily.  

Concept insight: Recall the changes occurring in boys and girls during adolescence.

Solution 4

Embryo in mother's body get nutrition through a nutritive connection called placenta.

Concept insight: Recall the importance of placenta.
Solution 5
No, because copper-T will not prevent contact with body fluids. Thus, it will not protect her from sexually transmitted diseases.  

Concept insight: Recall the various methods of contraception.

Chapter 8 - How do Organisms Reproduce? Excercise 141

Solution 1
(b) yeast  

Concept insight: List the various methods of asexual reproduction.
Solution 2
(c ) vas deferens  

Concept insight: List the various parts of female reproductive system.
Solution 3
(d) pollen grains  

Concept insight: Recall the various parts of a flower.
Solution 4

Following are the advantages of sexual reproduction over asexual reproduction:

i. In sexual reproduction, cells with different genetic material fuse together. Thus, it leads to mixing of different characters and emergence of new characters.

ii. Accumulation of variations leads to formation of new species.

iii. Sexual reproduction has speeded up the process of evolution.

Concept insight: List the advantages of sexual reproduction over asexual reproduction.
Solution 5

Testes are the primary reproductive organs in males. The function of testes is to make male sex cells called sperms and also to make male sex hormone called testosterone. 

Concept insight: Recall the functions of male reproductive system.

Solution 6

The removal of the inner, thick and soft lining of the uterus along with its blood vessels as well as blood in the form of vaginal bleeding is called menstruation.   In humans, after a girl attains puberty, ovaries start producing mature ovum every month and also uterus lining gets thickened to receive zygote.   When the ovum does not get fertilized and uterus does not receive any zygote then menstruation occurs to excrete lining of uterus and degenerating ovum.

Concept insight: Recall the sexual cycle in females.

Solution 7


 

Concept insight: (i) Diagram should be neat and correctly drawn. (ii) Label the parts according to significance.
Solution 8

The different methods of controlling child birth are:

(i) Hormonal methods: Various kinds of pills containing hormones which prevent the release of egg from the ovary, without interfering with other phases of menstrual cycle, are taken orally.  

(ii) Barrier method: These are the physical barriers which prevent the sperms meeting the egg. Physical devices such as condoms, diaphragms and cervical caps are used.  

(iii) Intrauterine Devices: Commonly called as I.U.Ds, they are the devices made of plastics and come in different shapes. The most commonly used is copper-'T'. These devices are placed inside the uterine cavity and prevents the implantation in the uterus.  

(iv) Surgical methods: The surgical methods are safe and permanent. Vasectomy occurs in males and tubectomy occurs in females.

a. Vasectomy - In this procedure, a small piece of vas deferens is cut and removed and the two ends of the cut vas deferens are tied.

b. Tubectomy - In this procedure, fallopian tubes are cut and tied  which prevents the passage of eggs.  

Concept insight: List the various methods of contraception.

Solution 9

Unicellular organisms contain only one cell so they reproduce by asexual reproduction.  
Example: budding, binary and multiple-fission are some of the asexual methods.  

Multicellular organisms which have complex body design and have specialised reproductive organs. So, they reproduce by sexual means. Example - Humans, animals, plants reproduce sexually.  

Concept insight: Recall the difference between asexual and sexual reproduction.

Solution 10
Reproduction is the process through which an organism produces new organism of its own kind. It is necessary because in a population, organisms die due to old age or disease thus, it keeps the number of organisms in a population constant and provides stability to a population.  

Concept insight: Recall the importance of reproduction.
Solution 11

Following are some reasons for adopting contraceptive methods:

(i) For population control.

(ii) To prevent spread of STDs (sexually transmitted diseases).  

Concept insight: List the importance of contraception.