NCERT Solutions for class 10th Science Chapter 8 How Do Organisms Reproduce

NCERT INTEXT QUESTIONS

Question 1. What is the importance of DNA copying in reproduction?

Sol. Deoxyribonucleic acid (DNA) is a self-replicating material which is present in nearly all living organisms as the main constituent of chromosomes and as a carrier of genetic information. DNA copying (or DNA replication, the process by which DNA makes a copy of itself during cell division) is an essential part of reproduction because additional copies of DNA made during the process of replication is necessary for the formation of new cells formed after the cell division. Without replication, each cell lacks enough genetic material to provide instructions for creating proteins essential for bodily function.

Question 2. Why is variation beneficial to the species but not necessarily for the individual?

Sol. Variations are differences among individuals produced as a result of errors caused during DNA copying at the time of cell division. If a reproducing organism is suitable to a particular niche and niche drastically changed then the population could be wiped out and got adversely affected. The variants may have the ability to survive under adverse condition. The surviving organism may reproduce and develop a kind of population which is suited for their niche. This shows that the species is able to survive but the individual lacking suitable features failed to do so. Thus, variation is beneficial to the species but not necessarily for the individual.

Question 3. How does binary fission differ from multiple fission?

Sol. Difference between binary fission and multiple fission

Question 4. How will an organism be benefited if it reproduces through spores?

Sol. Spore formation is a method of asexual reproduction which is found in non – flowering plants like fungi (Rhizopus) and bacteria, in which the parent plant produces hundreds of tiny spores which turn into a new plant.

An organism can be benefitted in following ways if produces through spores:

  1. Spores are thick walled which protect the organism from unfavourable condition and remain dormant till favourable conditions become available.
  2. Since it is a form of asexual reproduction, so the new organism won’t lose its parental characters.
  3. At a time, several numbers of spores are produced so cannot extinct very easily. Hence, numerous plants can be grown and does not require any medium for dispersal.
  4. Spores are light weighted so they can easily be transfer from one plant to other for germination.

Question 5. Can you think of reasons why more complex organisms cannot give rise to new individuals through regeneration?

Sol. Regeneration is the process carried out by specialised cells which involves the formation of new individual from its body parts. These cells proliferate and make large numbers of cells. From this mass of cells, different cells undergo changes to become various cell types and tissues. Since the simple organisms comprise of only one type of cells which assist that organism to grow and develop like in Hydra and Planaria.
Complex organism have organ – system level of organisation in which all the organs of organ system work as an interconnected unit. They can regenerate their body parts such as skin, muscles etc. but cannot give rise to new individual through regeneration.

Question 6. Why is vegetative propagation practised for growing some types of plants?

Sol. Vegetative propagation is practiced for growing certain types of plants because:
(i) It is the only method of reproduction in those plants which have lost their capacity to produce seeds like banana, orange, rose, jasmine etc.
(ii) Plants raised by vegetative propagation can bear flowers and fruits earlier than those produced from seeds.
(iii) Plants produced by this method are genetically similar enough to the parent plant to have all its characteristics.
(iii) Vegetative propagation is used in methods such as layering or grafting to grow many plants like sugarcane, roses, or grapes for agricultural purposes
Note:Vegetative propagation occurs through vegetative plant structures. In non-vascular plants, the vegetative reproductive structures are spores whereas in vascular plants, the roots, stems, leaves and nodes are the structures that are involved in the propagation

Question 7. Why is DNA copying an essential part of the process of reproduction?

Sol. DNA copying is the biological process of producing two identical copies of DNA from one original DNA molecule. DNA are the carriers of genetic information which transfers the information or characters from parents to offspring. It also helps to generates variations during sexual reproduction and leads to evolution. This explains how DNA copying is an essential part of the process of replication.

Question 8. How is the process of pollination different from fertilisation?
Sol. Difference between pollination and fertilisation

Question 9. What is the role of the seminal vesicles and the prostate gland?

Sol. Seminal vescicle and prostate gland are the organs of male reproductive system.

  • Seminal vesicles are a pair of simple tubular glands located within the pelvis in human male. It secretes a viscous fluid that partly composes the semen for the nourishment of sperms in human male.
  • The prostate gland located between the bladder and the penis in human male. It secretes prostate fluid, one of the components of semen which neutralises the acidity caused due to residual urine.

Both prostate and the seminal vesicles add their secretions so that the sperms are in a fluid which makes their transport easier.

Question 10. What are the changes seen in girls at the time of puberty?

Sol. The changes seen in girls at the time of puberty are as follows:

  • Pelvic region and the overall fat content of the body increases.
  • The waist narrows due to more deposition of fat on the hips under the influence of hormones.
  • Mammary gland starts developing with dark nipples at the tip of breast.
  • Pubic hairs appear in the genital area, underarms, face, hands, and legs.
  • At the time of puberty, menstrual cycle begins.
  • Increase in the size of uterus and ovary.
  • More secretion of oil from the skin, which results in the appearance of pimples.

Note: Puberty is the period between the age of 12 to 16, during which adolescents reach sexual maturity and become capable of reproduction.

Question 11. How does the embryo get nourishment inside the mother’s body?

Sol. The embryo gets nutrition from the mother’s blood with the help of a special tissue called placenta .The placenta is a flattened circular organ in the uterus of pregnant mammals that nourishes and maintains the foetus through the umbilical cord. Through umbilical cord, the placenta provides oxygen and nutrients to the growing baby and removes waste products.

Question 12. If a woman is using a copper-T, will it help in protecting her from sexually transmitted diseases?
Sol.

  • No, copper-T will not help a women to protect her from sexually transmitted disease.
  • The copper-Tis a simple intra-uterine device (IUD) made ofa flexible, “T” shaped piece of plastic wrapped with a thin copper containing wire. It only prevents the implantation of the embryo in the uterus. As copper­ T does not prevent entry of semen so it is not able to protect a woman from sexually transmitted diseases (STDs).

Note: Copper acts as a spermicide within the uterus. The presence of copper increases the levels of copper ions, prostaglandins, and white blood cells within the uterine and tubal fluid. Copper ions prevent pregnancy by inhibiting the movement of sperm as they are toxic to sperm.

Exercise

Question 1. Asexual reproduction takes place through budding in
(a) Amoeba (b)Yeast
(c) Plasmodium (d)Leishmania

Sol. (b) Budding is a type ofasexual reproduction in which a new organisms develops from an outgrowth or bud due to cell division at one particular site. The small bulb like projection coming out from the yeast cell is called a bud.

Question 2. Which of the following is not a part of the female reproductive system in human beings?
(a) Ovary (b) Uterus
(c) Vas deferens (d) Fallopian tube

Sol. (c) Ovary, uterus and fallopian tube are the part of female reproductive system but vas deferens is a part of male reproductive system in human beings. The sperms are delivered through the vas deferens which unites with a tube coming from the urinary bladder.

Question 3. The anther contains
(a) sepals (b) ovules (c) carpel (d) pollen grains

Sol. (d) Stamens are the male reproductive parts of flowers. Astamen consists of an anther (which produces pollen) and a filament. The male germ-cell produced by pollen grain fuses with the female gamete present in the ovule. This fusion of the germ-cells or fertilisation gives us the zygote which is capable of growing into a new plant.

Question 4. What are the advantages of sexual reproduction over asexual reproduction?

Sol. The main advantages of sexual reproduction over asexual reproduction are as follows:

  • Variation -In sexual reproduction, male and female gametes combine which gives rise to variation, resulting in new combination of characters in their offspring. While in asexual reproduction, offspring is genetically identical to their parent because the offspring are clones of the original parent which results in no variation.
  • Survival rate – In sexual reproduction, nutrition of embryo is taken care in the form of stored food materials in egg or through placenta. But in asexual reproduction, the new individuals formed may compete for basic needs and, therefore, only few are able to survive.
  • Evolution – Sexual reproduction is helpful in evolution while asexual reproduction does not help in evolution.

Note: Asexual reproduction generates offspring that are genetically identical to a single parent. in sexual reproduction, two parents contribute genetic information to pro­duce unique offspring.

Question 5. What are the functions performed by the testes in human beings?

Sol. Testes is a part of both male reproductive system and endocrine system. They are located behind the penis in a pouch of skin called the scrotum. The functions performed by the testes in human beings are as follows:

  • It helps in the production of sperms in male which takes part in sexual reproduction.
  • It assists in the secretion of the testosterone hormone, which plays a key role in the development of secondary sexual characters in male.

Question 6. Why does menstruation occur?

Sol. Menstruation, or period, is normal vaginal bleeding that occurs as part of a woman’s monthly cycle. Every month, one egg is released by an ovary and if the egg is not fertilised, it lives for about one day. Since the ovary releases one egg every month, the uterus also prepares itself every month to receive a fertilised egg. Thus its lining becomes thick and spongy. The main purpose of this lining is to provide nourishment to the embryo. Now, however, this lining is not needed any longer. So, the lining slowly breaks and comes out through the vagina as blood and mucous which causes menstruation.

Question 7. Draw a labelled diagram of the longitudinal section of a flower.

Sol. The given diagram represents the longitudinal section of a flower:

Question 8. What are the different methods of contraception?

Sol. The prevention of pregnancy in women by preventing fertilisation is called contraception and the device or chemical (drug) which prevents pregnancy in woman is called contraceptive. The various methods which helps in controlling pregnancy, are given as follows:

  • Mechanical barrier method: It is a method which creates barrier for the sperm and prevents it to reach up to the ovum. e.g., Condoms on the penis or similar coverings worn in the vagina can serve this purpose.
  • Hormonal method: It is a method in which a contraceptives act by changing the hormonal balance of the body so that eggs are not released and fertilisation cannot occur. These drugs commonly need to be taken orally as pills.
  • Surgical method: This method is applicable to both male and female. If the vas deferens in the male is blocked, sperm transfer will be prevented (vasectomy). If the fallopian tube in the female is blocked, the egg will not be able to reach the uterus (tubectomy). In both cases fertilisation will not take place. Surgical methods can be used to create such blocks.
  • Intrauterine devices (IUDs): It is T-shaped device which is placed inside the uterus. It blocks the sperms to enter into the fallopian tube. e.g., Copper-Tis placed in the uterus to prevent pregnancy.

Note: Contraception assist in preventing the unwanted pregnancy in woman but it has many side effects like mood swings and depression, breast pain or tenderness, breast enlargement.fungal infections and cystitis, hormonal imbalance etc.

Question 9. How are the modes for reproduction different in unicellular and multicellular organisms?

Sol. Unicellular organisms are single celled organisms in which all the physiological activities like reproduction, respiration excretion etc. are performed by a single cell. Multicellular organisms are those which consist of more than one cell, in contrast to unicellular organisms. Therefore, modes of reproduction are different in unicellular and multicellular organism as:

  • Unicellular organisms always reproduce by asexual methods like budding, regeneration, spore formation, binary fission, multiple fission, etc. Multicellular organism, on the other hand, uses both sexual and asexual method for reproduction.
  • In unicellular organism, there is only single cell which perform all the activities so there is no specialised cells and tissues that are meant for reproduction. But in multicellular organism, they have well developed reproductive organ which takes part in reproduction.

Question 10.How does reproduction help in providing stability to populations of species?

Sol. Reproduction is a life process which enables an organism to reproduce its own offspring. Thus, they continue their species without extinction.
Reproduction helps in providing stability to populations of species by considering following points:

  • It provides stability to population of species by giving birth to new individuals as the rate of birth must be at par with the rate of death to provide stability to population of a species.
  • The consistency of DNA copying during reproduction is also important for the maintenance of body design features that allow the organism to use that particular niche. Therefore, reproduction is linked to the stability of populations of species.

Question 11. What could be the reasons for adopting contraceptive methods?

Sol. Contraceptive method is any method, medicine, or device used to prevent pregnancy. It includes mechanical barrier method, hormonal method, surgical method, IUDs, etc.
The reasons for adopting contraceptive methods are as follows:

(i) Population control – It helps in controlling the population rate or birth rate.
(ii) Prevents unwanted pregnancies – It helps to maintain a women’s health by preventing unwanted pregnancies and by keeping gap between consecutive pregnancies.
(iii) Prevents from STDs – There are few contraceptive methods like condoms which prevents spread of sexually transmitted diseases and lethal diseases like HIV/AIDS.
(iv) It assists in family planning – Family planning/ contraception reduces the need for abortion, especially unsafe abortion.

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