Saturday 23 November 2013

CBSE Class 10th Science | Chapter 8 : How do Organisms Reproduce ? | Solved Exercises

Exercises | Chapter 8 : How do Organisms Reproduce? | Science |Class 10th | CBSE
Question 1. Asexual reproduction takes place through budding in
(a) amoeba.
(b) yeast.
(c) plasmodium.
(d) leishmania.

Answer. (b) yeast

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

Answer. (c) Vas deferens

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

Answer. (d) pollen grains.

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

  1. Sexual reproduction in different modes ensures diversity in the ecosystem
  2. In sexual reproduction, inbuilt tendency for genetic variation , plays an important role in the origin of new species and forms the the basis for evolution
  3. Variation resulting from sexual reproduction, is useful for the survival of species over time
  4. Sexual reproduction is also linked to stability of populations of species

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

Answer. The testis is the main organ of male reproductive system in human beings. The formation of germ- cells or sperms takes place in the testes. These are located outside the abdominal cavity in scrotum because sperm formation requires a lower temperature than the normal body temperature. The testes produces germ cells or sperm. They secrete a hormone called testosterone, which, regulate the formation of germ cells or sperms and also helps in development of secondary sexual characteristics as seen in males at the time of puberty. The sperms so formed are tiny bodies that consist of mainly genetic material and a long tail that helps them to move towards the female germ-cell.

Question 6. Why does menstruation occur?

Answer. In Human–female reproductive system, the ovary releases one egg every month, during the same period, the uterus also prepares itself to receive a fertilised egg. Thus its lining becomes thick and spongy. This would be required for nourishing the embryo if fertilisation had taken place.

If the egg is not fertilised, it lives for about one day and then it is lost. The lining earlier formed to receive the fertilised egg, is not needed any longer. So, the lining slowly breaks and comes out through the vagina as blood and mucous. This cycle takes place roughly every month and is known as menstruation. It usually lasts for about two to eight days

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

Longitudinal Section of a Flower | Chapter 8 : How do Organisms Reproduce? | CBSE Class 10th Science

Question 8. What are the different methods of contraception?

Answer. The different methods of contraception are :
  1. Mechanical barrier : Creation of a mechanical barrier so that sperm does not reach the egg. This can be achieved with a condom which is worn around the penis or a similar device that is placed in the vagina. Intra uterine devices such as copper-T can also be used, however they may cause side effect in the form of uterus irritation.
  2. Chemicals : Contraceptives pills containing special chemicals, when taken orally, acts by changing the hormonal balance of the body so that eggs are not released and fertilisation cannot occur. However, since they change hormonal balances, they can cause side-effects too.
  3. Surgical : If the vas deferens in the male is blocked through Surgical methods like vasectomy, sperm transfer will be prevented. Similarly, if the fallopian tube in the female is blocked through Surgical methods like tubectomy, the egg will not be able to reach the uterus. In both cases fertilisation will not take place.

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

Answer. Unicellular organisms have just one cell. There are no specialised tissue for reproduction. So for reproduction they make use of asexual methods such as binary fission, budding, multiple fission, etc. Multicellular organisms,on the other hand, as they are made up of simple cells as well as of complex tissues and organs with specialised function, use both asexual and sexual methods for reproduction.Simple organisms employ the asexual methods of reproduction; where as most of the multicellular organisms utilise sexual methods of reproduction.

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

Answer. In an ecosystem, populations of species, use their ability of reproduction to occupy a well-defined places, or niches. The consistency of DNA copying during reproduction is important for the maintenance of body design features that allow the organism to successfully adopt to that particular niche for survival. Every species and its member for its survival, struggle for food and place of habitat. Natural predators which form the echo chain and extremities of nature keep on depleting large chunk of population of species on continuous basis. Apart from these, death at the end of the natural life cycle of a species consumes its population on continuous basis. To over come this continuous loss of populations in diverse species, reproduction is very important and essential. Thus, it can be said that reproduction is linked to the stability of population of a species.

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

Answer. Since the beginning of human age on this planet, the human population, has increased manifolds. In recent times, due to unplanned human reproduction, this ever increasing size of the human population, has resulted in creating pressure on life supporting resources such as food, air, water, fuel and habitat which are limited and are are facing continuous depletion or contamination. Due to this living standards of human beings are likely to degrade further. Now the population growth is a cause for concern at individual level as well as, at community level. At community level, adopting different contraceptive methods, may check population growth. Which will result in better basic health care, education and economic conditions for the community members. Similarly, at personal level, adopting contraceptive methods will not only will limit the number of children but also help in maintaining the gap between children births. This will ensure improved over all living standards for the family and good up bringing of children.

Intext Questions | Page 128 | Chapter 8 : How do Organisms Reproduce? | CBSE Class 10th Science
Question 1. What is the importance of DNA copying in reproduction?

Answer. The creation of a DNA copy is a basic and an important event in reproduction. Organisms appear to be same because of similarity in their inherited body designs. DNA in the cell nucleus, contains the blueprint of body design and information for inheritance of characteristic features from parents to next generation. During Reproduction, as the cell divides, a copy of DNA along with additional cellular apparatus , is transferred from the parent cell to its offspring. DNA copying mechanisms supports life sustaining process with replicating encoded genetic instructions in new cells. Some minor changes during DNA copying result in bringing variations which are useful for ensuring the survival of the species in long run.

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

Answer. The process of copying the DNA will have some variations each time. As a result, the DNA copies generated will be similar, but may not be identical to the original. The surviving cells are similar to, but subtly different from each other. Sometimes these variations are so drastic that the new DNA copy cannot work with the cellular apparatus it inherits. Such a newborn cell will simply die. On the other hand, there could still be many other variations in the DNA copies that would not lead to such a drastic outcome.This inbuilt tendency for variation during reproduction is the basis for evolution of new species. variation beneficial to the species but not necessarily for the individual

Intext Questions | Page 133 | Chapter 8 : How do Organisms Reproduce? | CBSE Class 10th Science
Question 1. How does binary fission differ from multiple fission?

Answer. Binary fission : Some unicellular organisms under favourable conditions simply split into two equal halves during cell division. This is called binary fission. Examples are different bacteria and protozoa such as Amoeba
Multiple Fission : Some organisms like the malarial parasite, Plasmodium, divide into many daughter cells simultaneously. This process division is called multiple fission.

Question 2. How will an organism be benefited if it reproduces through spores?

Answer. The spores are covered by thick walls of tiny blob-on-a-stick like structures, which are not reproductive parts, but protect the spores from unfavorable conditions, until they come into contact with another moist surface and can begin to grow. Such spores are produced in large numbers and can spread through air, water or animals in contact. They germinate when favorable conditions return Thus, organism like Rhizopus is benefited greatly when it is produced through spores

Question 3. Can you think of reasons why more complex organisms cannot give rise to new individuals through regeneration?

Answer. More complex organisms cannot give rise to new individuals through regeneration because :
Complex organism are made up of Multi-cellular organisms. In complex organisms, specialised cells are organised as tissues, and tissues are organised into organs, which are strategically placed to perform different specialized functions. The tissues in complex organisms cannot regenerate a new individual as cell-by-cell division would be impractical. For example, human hair cannot regenerate into a new individual as it is a highly differentiated tissue performing a special function. In plants which do not produce viable seeds it is the only method of reproduction. b. Vegetative propagation is quicker and mature plants can be raised very quickly by this method. c. Genetically similar plants can be produced. This is of great importance in agriculture and gardening.

Question 4. Why is vegetative propagation practised for growing some types of plants?

Answer. Some plants like banana, orange, rose and jasmine, can not be propagated through seeds as they do not produce viable seeds. In such plants, the propagation of plants is done through vegetative propagation, in which new plants are produced under ideal conditions, using their parts like the root, stem and leaves . Plants raised by vegetative propagation can bear flowers and fruits earlier than those produced from seeds. Another advantage of vegetative propagation is that all plants produced are genetically similar in characteristics to the parent plant.

Question 5. Why is DNA copying an essential part of the process of reproduction?

Answer. DNA copying is a basic and an essential event in reproduction. Organisms appear to be same because of similarity in their inherited body designs. DNA in the cell nucleus, contains the blueprint of body design and information for inheritance of characteristic features from parents to next generation . During process of reproduction, as the cell undergo division, they use chemical reactions to build copies of their DNA. In the absence of a cellular apparatus, creation of DNA copies alone can not sustain the life process further. Therefor, DNA copying is accompanied by the creation of an additional cellular apparatus, and then the DNA copies separate, each with its own cellular apparatus. In this way, a cell divides in to two cells, each of which are capable of supporting life process further. DNA copying mechanisms creates variations which are useful for ensuring the survival of the species in the long run.

Intext Questions | Page 140 | Chapter 8 : How do Organisms Reproduce? | CBSE Class 10th Science
Question 1. How is the process of pollination different from fertilisation?

Answer. Pollination : The process of pollination involves transfer of a pollen grain from an anther to a stigma in flowering plants (Angiosperms type) or to the area of the ovule in more primitive, non-flowering plants such as ferns ( Gymnosperms Type ). This transfer of pollen grain may be with in the same flower, flowers of same plant or different plants. Air, water and animal movements facilitate the process of pollination. In this process, no fusion of two cells takes place.
Fertilisation : In this process, actual fusion of two cells in the form of male gamete and the female gamete takes place and result in the formation of zygote.

Question 2. What is the role of the seminal vesicles and the prostate gland?

Answer.Seminal vesicles and the prostate gland form the part of the accessory glands, which are associated with the male reproductive system. Seminal vesicles and prostrate gland secrete a fluid which makes the transport of sperms easier along the path of urethra and at the same time provides nutrition to the sperms. This secreted fluid together with the sperms is called semen. -

Question 3. What are the changes seen in girls at the time of puberty?

Answer.At the time of Puberty, which generally sets in between 8 to 13 years of age, the following changes can be seen in the girls :
  1. Enlargement of breasts
  2. Hair growth in the genital region and under the arms.
  3. Change in Voice, body weight and shape
  4. Change in emotions
  5. Onset of menstruation cycle .

Question 4. How does the embryo get nourishment inside the mother’s body?

Answer. During the fertilisation process, the zygote is formed and gets implanted in the lining of the uterus, and starts dividing to form the embryo. The growth of embryo is supported with thickening of uterus lining and increased supply of blood.
The embryo gets nutrition from the mother’s blood with the help of a special tissue called placenta. This is a disc which is embedded in the uterine wall. It contains villi on the embryo’s side of the tissue. On the mother’s side are blood spaces, which surround the villi. This provides a large surface area for glucose and oxygen to pass from the mother to the embryo. In this way, the embryo get nourishment inside the mother’s body
In addition to this, the placenta also removes waste substances generated by the developing embryo by transferring them into the mother’s blood.

Question 5. If a woman is using a copper-T, will it help in protecting her from sexually transmitted diseases?

Answer. No, copper-T is just a method of birth control. It will not help in protecting a woman from getting sexually transmitted diseases which include bacterial infections such as gonorrhoea and syphilis, and viral infections such as warts and HIV-AIDS. Due to nature of act, possibility of disease infection, through contact or fluid exchange, will always be there. For protection use of condom is strongly advisable.

Activity 8.1 | Page 129 | Chapter 8 : How do Organisms Reproduce? | CBSE Class 10th Science
  • Dissolve about 10 gm of sugar in 100 mL of water.
  • Take 20 mL of this solution in a test tube and add a pinch of yeast granules to it.
  • Put a cotton plug on the mouth of the test tube and keep it in a warm place.
  • After 1 or 2 hours, put a small drop of yeast culture from the test tube on a slide and cover it with a cover slip.
  • Observe the slide under a microscope.
  • Observation : Formation of yeast cells can be seen. Some of them, may show chain budding

Activity 8.2 | Page 129 | Chapter 8 : How do Organisms Reproduce? | CBSE Class 10th Science
  • Wet a slice of bread, and keep it in a cool, moist and dark place.
  • Observe the surface of the slice with a magnifying glass.
  • Record your observations for a week.
  • Observation : During the week, on observing the surface of the slice with a magnifying glass, a layer of white cotton like  mass is seen covering the surface of slice. Area of white mass coverage found to be increasing on subsequent days.
    After a weak, layer of white cotton like mass turns black showing formation of sporangia or spores.

Activity 8.3 | Page 129 | Chapter 8 : How do Organisms Reproduce? | CBSE Class 10th Science

  • Observe a permanent slide of Amoeba under a microscope.
  • Similarly observe another permanent slide of Amoeba showing binary fission.
  • Now, compare the observations of both the slides
  • Observation : In the permanent slide of amoeba, an amoeba cell is seen containing normal cytoplasm and nucleus. In permanent slide showing binary fission, nucleus seen to be dividing and the constriction is also seen in cytoplasm, suggesting formation of two daughter' nuclei as shown in the figure.
Binary fission in Amoeba | Chapter 8 : How do Organisms Reproduce? | CBSE Class 10th Science

Activity 8.4 | Page 130 | Chapter 8 : How do Organisms Reproduce? | CBSE Class 10th Science
  • Collect water from a lake or pond that appears dark green and contains filamentous structures.
  • Put one or two filaments on a slide.
  • Put a drop of glycerine on these filaments and cover it with a coverslip.
  • Observe the slide under a microscope.
  • Can you identify different tissues in the Spirogyra filaments?
  • Observation : Spirogyra filament consists of many cells. These cells are attached in linear fashion to form a filament.

Activity 8.5 | Page 132 | Chapter 8 : How do Organisms Reproduce? | CBSE Class 10th Science
  • Take a potato and observe its surface. Can notches be seen?
  • Cut the potato into small pieces such that some pieces contain a notch or bud and some do not.
  • Spread some cotton on a tray and wet it. Place the potato pieces on this cotton. Note where the pieces with the buds are placed.
  • Observe changes taking place in these potato pieces over the next few days. Make sure that the cotton is kept moistened.
  • Question. Which are the potato pieces that give rise to fresh green shoots and roots?
  • Observation : The potato pieces which were having buds in notches shown growth of young shoot and roots. The pieces which were not having eye buds did not show any growth of shoot or root.

Activity 8.6 | Page 132 | Chapter 8 : How do Organisms Reproduce? | CBSE Class 10th Science
  • Select a money-plant.
  • Cut some pieces such that they contain at least one leaf.
  • Cut out some other portions between two leaves.
  • Dip one end of all the pieces in water and observe over the next few days.
  • Question. Which ones grow and give rise to fresh leaves?
  • Answer : The portion of money-plant having leaves at the node show formation of fresh leaves. It also shows formation of branch from axillary buds present in the exile of the leaf.
  • Question. What can you conclude from your observations?
  • Answer : Green leaves can sysnthesise food and have the ability to grow into a plant, through vegetative propagation.
Leaf of Bryophyllum with buds | Chapter 8 : How do Organisms Reproduce? | CBSE Class 10th Science

Activity 8.7 | Page 135 | Chapter 8 : How do Organisms Reproduce? | CBSE Class 10th Science
  • Soak a few seeds of Bengal gram (chana) and keep them overnight.
  • Drain the excess water and cover the seeds with a wet cloth and leave them for a day. Make sure that the seeds do not become dry.
  • Cut open the seeds carefully and observe the different parts.
  • Compare your observations with the Fig. and see if you can identify all the parts.
  • Observation : On the basis of observation, we can say that all the parts can be identified by us. These parts are similar as shown in figure
Germination - Chapter 8 : How do Organisms Reproduce? | CBSE Class 10th Science