Download Reproduction in Organisms

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Reproduction in Organisms
BASIC CONCEPTS
1. The time period for which an organism is functional from birth to natural death is called
its lifespan.
2. During its lifespan an organism grows, develops, attains maturity and gives rise to new
individuals of the same type.
3. All the reproductive methods of living organisms are broadly categorised into two types –
Asexual reproduction and sexual reproduction.
4. Asexual reproduction is common in unicellular or multicellular simple organisms such as
algae, fungi, protozoans and some invertebrates.
5. The types of asexual reproduction are – Fission, budding, spore formation and vegetative
propagation.
6. Most of the prokaryotes (like bacteria) and unicellular protists reproduce by cell division
or binary fission of the parent cell.
7. Budding is common in yeast (a unicellular fungi).
8. Spore formation is the most common means of asexual reproduction in fungi and algae.
The common types of spores are – zoospores, sporangiospores, chlamydospores, oidia and
conidia.
9.Plants are unique because the parts other than seeds (viz. root, stem, leaves, etc.)
are capable of regeneration into whole new plants. This type of reproduction is called
vegetative reproduction. It may occur naturally or can be induced artificially.
10. Artificial methods of vegetative propagation have been developed by plant growers and
horticulturists for commercial production of economically important plant. These methods
include – cutting, layering, grafting and micropropagation.
11. Vegetative propagation is the only method of reproduction in plants which have lost their
capacity to produce seeds.
12. The technique of grafting is used to improve the varieties of many plants of horticultural
importance.
13. Sexual reproduction involves meiosis and fusion of gametes.
14. There are three events of sexual reproduction – Pre-fertilization (includes gametogenesis
and gamete transfer), Syngamy (or fertilization) and Post-fertilization (include formation
of zygote and embryogenesis).
BIOLOGY—XII
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NCERT TEXTBOOK EXERCISES
(Pages 17–19)
Q. 1. Why is reproduction essential for organisms?
Ans. Reproduction is essential because it enables the continuity of the species generation
after generation.
Q. 2. Which is a better mode of reproduction: sexual or asexual? Why?
Ans. Sexual reproduction is considered a better mode as it gives rise to genetic variation.
(i)This genetic variation may confer some survival advantages on the offspring,
under stressful environmental conditions.
(ii) It also contributes to evolution.
Q. 3. Why is the offspring formed by asexual reproduction referred to as clone?
Ans. A sexual reproduction results in the production of offsprings which are morphologically
and genetically similar. Such individuals are called clones.
Q. 4. Offspring formed due to sexual reproduction have better chances of survival.
Why? Is this statement always true?
Ans. Offsprings formed through sexual reproduction inherits genetic material from two
genetically different parents and that too, with certain recombination.
This genetic variation may confer survival advantages on the offspring. This statement
is true in majority of cases provided embryonic safety and care is ensured.
Q. 5. How does the progeny formed from asexual reproduction differ from those
formed by sexual reproduction?
Ans. Since asexual reproduction does not involve meiosis and fusion of gametes, the progeny
formed from asexual reproduction are genetically similar to parents and they do not
show variation. The individuals produced as a result of meiosis and gametic fusion
exhibit genetic variation and difference from either of the two parents as well as among
themselves.
Q. 6. Distinguish between asexual and sexual reproduction. Why is vegetative
reproduction also considered as a type of asexual reproduction?
Ans. Differences between sexual and asexual reproduction:
Sexual Reproduction:
(i) Two parents of opposite sex are involved in the process.
(ii) Gamete formation and fertilisation always take place.
(iii) Offsprings show genetic and morphologic variation from the parents.
(iv) The process involves both meiotic and mitotic cell division.
Asexual Reproduction:
(i) Only one parent is involved.
(ii) Generally gamete formation and fertilisation not required.
(iii) Offsprings are genetically and morphologically same as the parents.
(iv) Only mitotic cell division takes place. Vegetative reproduction is also considered as
a form of asexual reproduction since the formation of the offspring from vegetative
propagules does not involve—
(a) two parents.
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(b) formation or fusion of gametes.
(c) genetic variation.
Q. 7. What is vegetative propagation? Give two suitable examples.
Ans. The vegetative propagation involves production of new individuals from some vegetative
plan propagules such as buds, tubers, rhizomes, etc.
Examples
(i) Eyes (buds) on the potato tuber.
(ii)Adventitious buds on the leaves of Bryophyll are capable of giving rise to new
plants.
Q. 8. Define
(a) Juvenile phase (b) Reproductive phase (c) Senescent phase
Ans. Juvenile phase is the period of growth between the birth of an individual and
reproductive maturity.
Reproductive phase starts after juvenile phase and remains up to the stage when an
organism is capable of reproduction.
Senescent phase is the phase of ageing when an organism loses its capacity of
reproduction. In plants, it is characterised by yellowing and leaf fall.
Q. 9. Higher organisms have resorted to sexual reproduction in spite of its
complexity. Why?
Ans.In spite of its complexity, higher organisms have resorted to sexual reproduction
because it enables these organisms to survive during unfavourable conditions. It
contributes to evolution of the species by introducing variation in a population much
more rapidly than asexual reproduction.
Q. 10. Explain why meiosis and gametogenesis are always interlinked?
Ans. Gametogenesis is the process of the formation of haploid gametes inside the gametangia.
The normal body of organism is usually diploid but the gametes are haploid. Thus,
gametes are formed as a result of meiosis so that their chromosome number becomes
haploid.
Q. 11. Identify each part in a flowering plant and write whether it is haploid (n) or
diploid (2n).
(a) Ovary
(b) Anther
(c) Egg
(d) Pollen
(e) Male gamete
(f) Zygote
Ans. (a)Ovary
– Diploid
(b)Anther
– Diploid
(c)Egg
– Haploid
(d)Pollen
– Haploid
(e) Male gamete – Haploid
(f)Zygote
– Diploid
Q. 12. Define external fertilisation. Mention its disadvantages.
Ans. External fertilisation is defined as the type of syngamy that occurs outside the
female body, in external medium, e.g., water. The major disadvantages of this type of
fertilisation are:
(i) the fertilisation of egg is not always sure,
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(ii) the offsprings are not protected from the predators, and
(iii) a large number of gametes are wasted.
Q. 13. Differentiate between a zoospore and a zygote.
Ans. A zoospore is usually flagellated, haploid or diploid spore formed inside a zoosporangium.
The zygote, on the other hand, is always diploid and formed as a result of fusion of
gametes. The zygote is usually nonflagellated.
Q. 14. Differentiate between gametogenesis and embryogenesis.
Ans. Gametogenesis occurs inside the gametangia where meiosis occurs resulting in the
formation of gametes.
Embryogenesis is the development of zygote into an embryo. During this process, the
zygote divided by mitotic divisions.
Q. 15. Describe the post-fertilisation changes in a flower.
Ans. Post-fertilisation changes in a flower are as follows:
(i) The sepal, petal and stamen of the flower withers and fall off, but pistil remains
attached to the plant.
(ii) The zygote develops into the embryo and ovules develop into the seeds.
(iii) Ovary develops into the fruit and a thick protective wall called pericarp, covers
the fruit.
Q. 16. What is a bisexual flower? Collect five bisexual flowers from your
neighbourhood and with the help of your teacher find out their common and
scientific names.
Ans. A bisexual flower possesses both stamens (male) and pistil (female) parts.
Scientific Name
Common Name
Cucumis sativus
Cucumber
Cocos nucifera
Coconut
Rosa alba
Rose
Lathyrus odoratus
Sweet pea
Hibiscus rosa-sinensis
Gurhal, China flower
Q. 17. Examine a few flowers of any cucurbit plant and try to identify the staminate
and pistillate flowers. Do you know any other plant that bears unisexual
flowers?
Ans. The staminate flowers bear bright coloured petal and stamens. They do not develop
fruits. The pistillate flowers develop fruits. The other unisexual plant is papaya. Date
palm is also unisexual and bear unisexual flowers.
Q. 18. Why are offspring of oviparous animals at a greater risk as compared to
offspring of viviparous animals?
Ans. The offsprings of oviparous animals are at a greater risk because they develop outside
the female body and are exposed to varying environmental conditions and predator
threat.
Whereas in viviparous animals, young ones develop inside the female body, which
ensures proper embryonic care and protection.
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Important Questions
Q. 1. Mention the site where syngamy occurs in amphibians and reptiles.
Ans. In amphibians, syngamy occurs in water outside the body of the organism (external
fertilization and external development). In reptiles, it occurs inside the body of female
(internal fertilization and external development).
Q. 2. Mention a characteristic feature and a function of zoospores in some algae.
Ans. Zoospores in algae are motile cells helping in asexual reproduction.
Q. 3. Name the phase all organisms have to pass through before they can reproduce
sexually.
Ans. Juvenile phase.
Q. 4. Name the phenomenon and one bird where the female gamete directly
develops into a new organism.
Ans. This phenomenon is basically known as parthenogenesis. Example of one bird is turkey.
Q. 5. Coconut palm is monoecious while date palm is dioecious. Why are they
called so?
Ans. In coconut palm, both male and female flowers develop on the same individual. This
condition is called monoecious. In date palm, the male and female flowers develop on
separate individuals. This condition is called dioecious.
Q. 6. What do the following parts of the flower develop into after fertilisation?
(a) Ovary
(b)Ovules
Ans. (a) Fruit (b) Seeds.
Q. 7. Write the modes of asexual reproduction in the following organisms:
Bryophyllum, Potato, Yeast, Rhizopus, Penicillium.
Ans. The modes of reproduction are listed below:
(i) Bryophyllum: Reproduce asexually by formation of marginal adventitious buds
in mature leaves.
(ii) Potato: Buds in the eyes in potato tuber grow to form aerial shoots.
(iii) Yeast: Budding.
(iv) Rhizopus: Sporangiospores formed inside sporangia.
(v) Penicillium: Conidia formed on conidiophares.
Q. 8. Suggest a possible explanation why the seeds in a pea pod are arranged in a
row. Whereas, those in tomato are scattered in the juicy pulp.
Ans. In pea ovary, the ovules are arranged on one margin of it and carpel is monocarpellary
but in tomatoes the gynoecium is tricarpellary with axile placentation.
Important Figure
Q. 1. Draw the sketches of a zoospore and a conidium.
Ans. Zoopore
Conidium
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BIOLOGY—XII
Multiple Choice Questions
Q. 1. In which of the following organisms self fertilization is seen?
(a)Fish
(b)Roundworm
(c)Earthworm
(d) Liver fluke
Ans. (d) Liver fluke
Q. 2. Spermatids are transformed into spermatozoa by
(a)spermiation
(b)spermatogenesis
(c)meiosis
(d)spermatosis
(e)spermiogenesis
Ans. (e) spermiogenesis
Q. 3. Study of pollen grain is called
(a)Ethmology
(b)Polynology
(c)Poleobotany
(d)Co-taxonomy
Ans. (b) Polynology
Q. 4. External fertilization occurs in majority of
(a)algae
(b)fungi
(c)liverwarts
(d)mosses
Ans. (a) algae
Q. 5. Which of the following is a hermaphrodite?
(a)Ants
(b)Aphids
(c)Earthworms
(d)Cockroaches
Ans. (c) Earthworms
Q. 6. Banana is vegetatively propagated by
(a)tubers
(b)rhizomes
(c)bulbs
(d)suckers
Ans. (b) rhizomes
Q. 7. Which part would be the most suitable for raising virus-free plants for micropropagation?
(a)Bark
(b) Vascular tissue
(c)Meristem
(d)Node
Ans. (c) Meristem
Q. 8. Example of corm is
(a)ginger
(b)colocasia
(c)onion
(d)potato
Ans. (b) colocasia