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NCERT Exemplar · Class 10 Science How do Organisms Reproduce?

58 questions · 58 still being checked

Long Answer Questions 49–58 (part 6 of 6)

  1. Exercise 49

    Why are budding, fragmentation and regeneration all considered as asexual types of reproduction? With neat diagrams explain the process of regeneration in Planaria.

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    NCERT’s answer
    Budding, fragmentation and regeneration are considered as asexual types of reproduction because all of them involve only one parent and gametes are not involved in reproduction NCERT_Solution_Class10_Science_Exemplar_Ch8_Q49_ncert
    Budding, fragmentation and regeneration are asexual because each involves a single parent, no fusion of male and female gametes, and the offspring are genetically identical to the parent.In Planaria, a cut or injured body regenerates because specialised cells proliferate rapidly at the wound, produce a mass of cells that differentiate into all the missing tissues and organs.Each fragment regrows into a complete worm; several worms can thus arise from one parent.Answer: Clonal, single-parent origin makes all three asexual; Planaria regenerates lost parts from proliferating specialised cells at the cut surface, each fragment forming a complete organism.

  2. Exercise 50

    Write two points of difference between asexual and sexual types of reproduction. Describe why variations are observed in the offspring formed by sexual reproduction.

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    Asexual reproductionSexual reproduction
    (a) Involves only one parent(i) Often involves two parents
    (b) Gametes are not produced(ii) Gametes are produced
    (c) No fertilisation and zygote formation(iii) Fertilisaton and zygote formation is observed.
    (d) Meiosis does not occur at anytime during reproduction(iv) Meiosis occurs at the time of gamete formation
    During sexual reproduction two types of gametes fuse. Although the gametes contain the same number of chromosomes, their DNA is not identical. This situation generates variations among the offsprings.
    Asexual reproduction involves a single parent and no gamete fusion, giving offspring genetically identical to the parent; sexual reproduction involves two parents and fusion of male and female gametes, giving offspring that differ from both parents.Variations arise because each gamete carries half the parental DNA, and small copying inaccuracies occur during gamete formation. Fusion of two such gametes combines DNA from both parents in a new combination, so the zygote differs from either parent.Answer: Asexual: one parent, no gamete fusion, identical offspring. Sexual: two parents, gamete fusion, variable offspring from new DNA combinations and copying differences.

  3. Exercise 51

    Distinguish between pollination and fertilisation. Mention the site and product of fertilisation in a flower. Draw a neat, labelled diagram of a pistil showing pollen tube growth and its entry into the ovule.

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    NCERT’s answer
    The process or mechanism of transfer of pollen grains from the anther to the stigma is termed pollination. The fusion of male and female gaemtes giving rise to zygote is termed fertilisation The site of fertilisation is ovule. The product of fertilisation is zygote. NCERT_Solution_Class10_Science_Exemplar_Ch8_Q51_ncert
    Pollination is the physical transfer of pollen grains from the anther to the stigma (self- or cross-pollination); no fusion of cells occurs. Fertilisation is the fusion of a male gamete carried by the pollen tube with the egg cell inside the ovule, forming a zygote.On the stigma a pollen grain germinates and its tube grows down through the style, enters the ovule through the micropyle, and releases male gametes into the embryo sac.Site: the ovule, inside the ovary. Product: the zygote (diploid), which later develops into the embryo.Answer: Pollination = pollen transfer to the stigma; fertilisation = gamete fusion inside the ovule, producing a zygote.

  4. Exercise 52

    Distinguish between a gamete and zygote. Explain their roles in sexual reproduction.

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    NCERT’s answer
    Gamete represents the sex cell or germ cell in sexual reproduction. There are two types of gametes, male and female. Zygote is the product of fertilisation in which a male and a female gamete fuse with each other. The two fusing gametes possess characters of their parents in their DNA. Fertilisation brings characters of both parents into one zygote cell. Zygote is the first cell of the next generation. It divides to form an embryo which subsequently grows into a new individual.
    A gamete is a haploid reproductive cell (sperm or egg) produced by a parent through meiosis; it cannot develop into a new organism by itself. A zygote is a diploid cell formed when a male and a female gamete fuse; it is the first cell of the new organism.Male and female gametes each contribute half the chromosome number, so their fusion restores the full diploid number and combines DNA from both parents, creating variation. The zygote then divides repeatedly and differentiates into an embryo, which develops into the offspring.Answer: Gamete = haploid sex cell that must fuse to develop further; zygote = diploid fusion product that itself develops into the new organism.

  5. Exercise 53

    Draw the diagram of a flower and label the four whorls. Write the names of gamete producing organs in the flower.

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    NCERT’s answer
    Male gamete forming part – anther/stamen Female gamete forming part – pistil/ovary/ovule NCERT_Solution_Class10_Science_Exemplar_Ch8_Q53_ncert
    Four whorls, outer to inner: calyx, corolla, androecium, gynoecium.Gamete-producing organs: the stamen (anther), whose pollen grains carry the male gametes, and the pistil (ovary), whose ovule holds the egg cell, the female gamete.Answer: Whorls: calyx, corolla, androecium, gynoecium; gametes arise in the stamen (male, via pollen grains) and the pistil (female, egg cell in the ovule).

  6. Exercise 54

    What is placenta? Mention its role during pregnancy?

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    NCERT’s answer
    Hints—
    (a)
    Special tissue connection between embryo and uterine wall
    (b)
    Possesses villi that increases the surface area.
    (c)
    Facilitate passage of nutrition and oxygen to embryo from mother through blood.
    (d)
    Waste substances produced by embryo are removed through placenta into mother's blood.
    The placenta is a disc-shaped tissue that embeds in the uterine wall and links the embryo to the mother via the umbilical cord.Finger-like villi on its embryonic side project into maternal blood spaces, giving a large surface for exchange, while the embryonic and maternal blood stay in separate vessels there. Across this surface the embryo draws glucose, oxygen and other nutrients from the mother's blood, and passes \(\displaystyle \mathrm{CO_2}\) and other wastes back into it.Answer: The placenta is the embryo–mother exchange organ, embedded in the uterine wall and joined to the embryo by the umbilical cord, supplying nutrients and oxygen and removing wastes throughout pregnancy.

  7. Exercise 55

    What are various ways to avoid pregnancy? Elaborate any one method.

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    NCERT’s answer
    Hints—
    (a)
    Contraceptive methods are used such as (i) mechanical (ii) drugs (as pills) (iii) loop or copper T and (iv) surgical method.
    (b)
    Pills change the hormonal balance and thus prevent the release of egg, hence fertilisation is prevented.
    Contraceptive methods include barrier devices (condom, diaphragm) that block sperm from reaching the egg; chemical methods (oral pills, vaginal creams/foams) that alter hormones or kill sperm; intra-uterine devices (Copper-T, loop) that stop implantation; surgical methods (vasectomy, tubectomy) that block the sperm or egg ducts; and periodic abstinence around the fertile period.Oral contraceptive pills, elaborated: taken daily, they alter the woman's hormone levels so that ovulation is suppressed.\[\text{daily pill} \Rightarrow \text{ovulation suppressed} \Rightarrow \text{no egg to fertilise} \]Irregular doses can let ovulation resume, so the pill must be taken daily; it gives no protection against sexually transmitted infections.Answer: Barrier, chemical, IUD, surgical and abstinence methods exist; oral pills work by hormonally suppressing ovulation.

  8. Exercise 56

    How does fertilisation take place? Fertilisation occurs once in a month. Comment.

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    NCERT’s answer
    Hints—
    (a)
    Sperm enters through the vaginal passage during sexual intercourse and moves upwards.
    (b)
    Egg released from the ovary reaches the oviduct.
    (c)
    Sperm encounters egg in the oviduct and fertilization takes place.
    (d)
    Egg is released once every month by ovary.
    During mating, semen carrying sperm enters the vagina; the sperm swim up through the uterus into the oviduct (fallopian tube), the site of fusion.There a sperm nucleus fuses with the egg nucleus:\[\text{sperm}\ (n) + \text{egg}\ (n) \rightarrow \text{zygote}\ (2n) \]The zygote then divides while moving down to implant in the uterine wall.In each roughly $\displaystyle 28$-day menstrual cycle only one egg is released (ovulation), and it stays fertilisable for about a day, so fertilisation is possible only around that single release.Answer: Sperm fuses with the egg in the oviduct, forming a diploid zygote; since one egg is ovulated per ~$\displaystyle 28$-day cycle, fertilisation occurs at most once a month.

  9. Exercise 57

    Reproduction is essentially a phenomenon that is not for survival of an individual but for the stability of a species. Justify.

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    NCERT’s answer
    Hints—
    (a)
    Organisms need energy for survival which they obtain from life processes such as nutrition and respiration.
    (b)
    Reproduction needs a lot of energy.
    (c)
    Genetic material is transferred from one generation to the next as a result of reproduction through DNA copying.
    (d)
    DNA copying takes place with high constancy and considerable variations, that is, advantages to the species for stability in the changing environment.
    An individual stays alive through life processes such as nutrition and respiration, not through reproduction, which costs energy and often weakens the parent.Reproduction serves the species:\[\text{offspring} \Rightarrow \text{losses replaced} \Rightarrow \text{population continues} \]\[\text{DNA copy, high constancy} \Rightarrow \text{body design kept} \]\[\text{small errors} \Rightarrow \text{variation} \Rightarrow \text{survivors if conditions change} \]Answer: Reproduction does not keep the individual alive; it replaces dying members and copies DNA with high constancy plus small variations, so the species stays stable and can adapt.

  10. Exercise 58

    Describe sexually transmitted diseases and mention the ways to prevent them.

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    NCERT’s answer
    Hints—
    (a)
    These are infectious diseases transmitted during sexual contact.
    (b)
    They may be bacterial like or viral like.
    (c)
    Use of mechanical barrier like condom prevents transmission of infection.
    Sexually transmitted diseases (STDs) spread through sexual contact with an infected partner. Bacterial ones include gonorrhoea and syphilis; viral ones include genital warts, hepatitis B and HIV-AIDS, which disables the immune system and has no cure.Prevention: a condom blocks both the pathogen and sperm; avoiding multiple or unknown partners, and having them screened, lowers exposure; hepatitis B is preventable by vaccination; and early symptoms (discharge, sores, itching) should prompt testing and treatment, since an untreated STD can also cause infertility.Answer: STDs (gonorrhoea, syphilis, genital warts, hepatitis B, HIV-AIDS) pass through sexual contact; condom use, partner screening, avoiding multiple partners, hepatitis B vaccination and prompt treatment prevent them.