NCERT Exemplar Class 10 Science Solutions
Chapter 08-How do Organisms Reproduce?-Short Answers
Short Answer Type Questions
Q.28. In a bisexual flower in spite of the young stamens being removed artificially, the flower produces fruit. Provide a suitable explanation for the above situation.
Answer. A bisexual flower has both male and female reproductive organs. Removal of young stamens to avoid self-pollination does not rule out the possibility of cross pollination. Transfer of pollen grains from the anther of another flower to the stigma of emasculated flower causes cross-pollination and fertilized flower forms fruits.
Q.29. Can you consider cell division as a type of reproduction in unicellular organism? Give one reason.
Answer. Unicellular organisms reproduce by asexual reproduction which is simple division of parent cell into two daughter cells. Now, these two daughter cells grow into mature organism. Hence, cell division can be considered as mode of reproduction in single celled organisms. Eg: Amoeba.
Q.30. What is a clone? Why do offspring formed by asexual reproduction exhibit remarkable similarity?
Answer. Clones are the group of cells or individuals produced by asexual reproduction. Since asexual reproduction does not involve gamete formation by meiosis and fertilization of male and female gametes, the offspring are genetically similar to the single parent. Meiosis and fertilization are the two main sources of genetic variations in sexually reproducing plants.
Q.31. Explain how, offspring and parents of organisms reproducing sexually have the same number of chromosomes?
Answer. Sexual reproduction includes gamete formation through meiosis and fertilization. Meiosis reduces the number of chromosomes to half in male and female gametes. This reduced chromosome number is then restored to normal during fertilization of male and female gametes. This is how; constant chromosome number is maintained in sexually reproducing organisms.
Q.32. Colonies of yeast fail to multiply in water, but multiply in sugar solution. Give one reason for this.
Answer. Water does not serve as source of all required nutrients necessary for growth and reproduction of yeast cells. On the other hand, sugar solution serve as source of organic compounds and cellular respiration of these compounds release energy needed for reproduction. That’s why yeast colonies do not multiply in water but do so in sugar solution.
Q.33. Why does bread mould grow profusely on a moist slice of bread rather than on a dry slice of bread?
Answer. Wet dump substratum with supply of nutrients is favored by Rhizopus for spore germination. Spores remain dormant till they come in contact with moist surface. Hence spores of bread mold germinate on moist bread serving as source of moisture and nutrients.
On the other hand, dry bread does not provide moisture needed for spore germination and growth of hyphae.
Q.34. Give two reasons for the appearance of variations among the progeny formed by sexual reproduction.
Answer. Production of genetic variation during sexual reproduction is result of:
(1) Meiosis during gamete formation; crossing over and independent assortment of genes produce genetic variations.
(2) Random fusion of genetically diverse gametes male and female gametes further aid in genetic diversity of offspring.
Q.35. Would a Planaria cut vertically into two halves regenerate into two individuals? Complete the given figure D and E by indicating the regenerated regions.
Answer. Planaria exhibits regeneration. If its body is cut into two vertical halves, each piece of the body of Planaria grows into a complete individual.
Q.36. From the internet gather information about the chromosome numbers of five animals and five plants. Correlate the number with the size of organism and answer the following questions.
(a) Do larger organisms have more number of chromosomes/cells?
(b) Can organism with fewer chromosomes reproduce more easily than organisms with more number of chromosomes?
(c) More the number of chromosomes/cells greater is the DNA content. Justify.
Answer. Number of chromosomes in somatic cells of some animals and plants
(a) No, there is no correlation between size of organism and its chromosome number. Even smaller organisms may have more chromosomes than the larger ones.
(b) No, process of reproduction is independent of the chromosome number and depends on a environmental factors like nutrients availability, water source, suitable mate etc.
(c) Yes, Genes are the stretch of DNA. Genes are linearly arranged on chromosomes. Hence, more the number of chromosomes, greater is the amount of DNA.
Q.37. In tobacco plant, the mate gametes have twenty four chromosomes. What is the number of chromosomes in the female gamete? What is the number of chromosomes in the zygote?
Answer. Male and female gametes of a species have same number of chromosomes. Hence, number of chromosome in the female gamete of tobacco plants is 24. Zygote is formed by fusion of male and female gamete, it will have 48 chromosomes.
Q.38. Why cannot fertilization take place in flowers if pollination does not occur?
Pollination is transfer of pollen from one flower to the stigma of same/other flower of same or another plant. It takes place after gamete formation. Only after arrival of pollen grains on stigma and entry of pollen tube into ovary, male gamete can fuse with female gamete (ovum). Hence, pollination is prerequisite for fertilization.
Q.39. Is the chromosome number of zygote, embryonal cells and adult of a particular organism always constant? How is the constancy maintained in these three stages?
Answer. Yes, zygote, embryonic cell and adult of a particular organism always constant have constant chromosome number.
Sexual reproduction includes gamete formation through meiosis and fertilization. Meiosis reduces the number of chromosomes to half in male and female gametes. This reduced chromosome number is then restored to normal during fertilization of male and female gametes. This is how; constant chromosome number is maintained in sexually reproducing organisms. Growth and development of zygote into embryonic cell and then into adult one takes place by mitosis (equatorial cell division) which produce the daughter cells carrying same chromosome number as that of parent cell.
Q.40. Where is the zygote located in the flower after fertilization?
Answer. Pollination is transfer of pollen from one flower to the stigma of same/other flower of same or another plant. It takes place after gamete formation. Only after arrival of pollen grains on stigma and entry of pollen tube (carrying the male gametes) into ovary, male gamete can fuse with female gamete (ovum). Ovum is present in ovule and ovule is present in ovary.
Therefore, the zygote is present inside the ovule.
Q.41. Reproduction is linked to stability of population of a species. Justify the statement.
Answer. Each species faces many forces that reduce the number of individuals constantly. Some of them are namely struggle for survival, competition for natural resources, predation, natural cycle of ageing and death, any natural calamity etc. All these natural forces reduce the number of individuals per species.
Reproduction is the process of production of own kind. It includes production of offspring having both similarities and variations among themselves and from parents. Further, the process of DNA replication and its inheritance to offspring ensure production of own kind only. Therefore, reproduction not only restore the number of individuals removed from the species by above mentioned natural forces but also maintain heredity of genetic characters and introduction of variations, as needed for continuity of a species.
Q.42. How are general growth and sexual maturation different from each other?
Answer. General growth includes increase in height, weight gain, changes in shape and size of the body. Also called as somatic phase, the reproductive organs are less active during somatic growth.
Sexual maturation is a cumulative process that includes all the changes needed to prepare the body for sexual reproduction. It ensures onset of puberty and reproductive phase of an individual.
Q.43. Trace the path of sperm during ejaculation and mention the gland and their functions associated with the male reproductive system.
Answer. Path of sperm during ejaculation is Testis -> vas deferens à urethra.
Glands associated with male reproductive system are
(a) Testis Secrete the androgens (testosterone).
(b) Prostate gland serves to provide the force needed for ejaculation. It secretes thin, milky alkaline fluid that adjusts the pH of seminal fluid and serves in motility of sperms.
(c) Cowper glands Their secretion lubricates the end of penis during sexual intercourse and may contain few sperms.
(d) Seminal vesicle adds alkaline pH (to counteract acidic pH of vagina), fructose (a nutrient), mucus and coagulating and local acting enzymes.
Q.44. What changes are observed in the uterus if fertilization does not occur?
Answer. If fertilization does not occur, thickened endometrium (inner lining of uterus) is disintegrated and sloughed off during menstruation phase (day1-5). Endometrium is needed to support implantation and nutrition of embryo only under pregnancy and is otherwise disintegrated as it does not have any function in non pregnant woman.
Q.45. What changes are observed in the uterus subsequent to implantation of young embryo?
Answer. The uterine lining is thickened every month support the growing embryo. Followed by fertilization, placenta is developed which serve to provide nutrition and oxygen to developing embryo from the mother’s blood. It is a disc embedded in the uterine wall and contains villi on the embryo’s side of the tissue while blood spaces surrounding the villi are present on mother’s side. Waste products of developing embryo are removed by transferring them into the mother’s blood through the placenta.
Q.46. What are the benefits of using mechanical barriers during sexual act?
Answer. (i) It serves a barrier to prevent arrival of sperm to egg and thus avoids unwanted pregnancy.
(ii) It also protects a person from sexually transmitted diseases.
Q.47. In the given figure label the parts and mention their functions
(a) Production of egg
(b) Site of fertilization
(c) Site of implantation
(d) Entry of the sperms
(a) Ovary (production of egg)
(b) Oviduct (site of fertilization)
(c) Uterus (site of implantation)
(d) Vagina (passage for entry of the sperms)
Q.48. What would be the ratio of chromosome number between an egg and zygote? How is the sperm genetically different from the egg?
Answer. The ratio of chromosome number between egg and its zygote is 1 : 2. Egg is product of meiosis and hence contains half the chromosome number of its parent cell. Zygote is product of fertilization of male and female gamete and therefore, contains double the chromosome number of egg cell.
Sperms and eggs differ from each other in type of sex chromosome. The sperm contain either X or Y-chromosome whereas an egg will always have an X-chromosome.