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| REPRODUCTION IN ORGANISMS |
HOW DO ORGANISMS REPRODUCE?
NCERT TEXTBOOK QUESTIONS
Q: What is the importance of
DNA copying in reproduction?
Answer:
(i) The chromosomes
in the nucleus of a cell contain information for the inheritance of features
from the parents to the next generation in the form of DNA (Deoxyribo Nucleic Acid)
molecules. So, the first importance of DNA copying is that the characteristics
of the parent organism are transmitted to its offsprings.
(ii) When the DNA
already present in the nucleus of a parent cell is copied by making more of DNA
by certain biochemical reactions, then slight variations come in the two copies
formed. These slight variations in the copying of DNA molecules lead to slight
variations in the offspring produced. Thus, another importance of DNA copying
is that some variations are produced in the offsprings during reproduction
which form the basis for evolution.
Q: Why is variation beneficial
to the species but not necessarily for the individual?
Answer: Variation
is useful for the survival of a species even in adverse environmental
conditions. This happens as follows: There may be some drastic changes like
excessive heat or cold or shortage of water (drought), etc., in the habitat of
a species of organisms. Now, if all the organisms of a population living in
that habitat are exactly identical, then there is the danger that all of them may
die and no one would survive under those conditions. This will eliminate the
species from that habitat completely. However, if some variations are present
in some individual organisms to tolerate excessive heat or cold or survive on
meagre water supply, then there is a chance for them to survive and flourish
even in an adverse environment. For example, if there is a population of certain
bacteria living in temperate water (which is neither very hot nor very cold)
and the temperature of water increases too much due to global warming, then
most of these bacteria will not be able to tolerate excessive heat and hence
die. But some bacteria which had variations to resist heat would survive and
grow further.
Q: How does binary fission
differ from multiple fission?
Answer: In binary fission, the parent organism splits (or divides) to
form two new organisms. On the other hand, in multiple fission, the parent
organism splits (or divides) to form many new organisms simultaneously. Amoeba
reproduces by the process of binary fission whereas the malarial parasite
Plasmodium reproduces by the process of multiple fission.
Q: How will an organism be benefitted
if it reproduces through spores?
Answer: The
reproduction by spores takes place in plants. Spores are covered by hard
protective coat which enables them to survive under unfavourable conditions
like lack of food, lack of water and extreme temperatures. But when the
conditions become favourable (food and water are available, and the temperature is
suitable), then the spores can grow to produce new plants. Thus, the
reproduction by spores benefits the plants because by surviving under adverse
conditions, the spores make these plants live forever.
Q: Can you think of reasons
why more complex organisms cannot give rise to new individuals through
regeneration?
Answer: In
complex multicellular organisms, specialised cells make up tissues; tissues
make up organs; organs make up organ systems, and finally, organ systems make up
organisms. Since complex multicellular organisms have a very high degree of
organisation in their body, they cannot be reproduced from their cut body parts
by the process of regeneration. For example, a dog is a complex multicellular
organism which cannot be regenerated from its cut body part say, a cut tail.
This is because the cells present in the cut tail of a dog cannot produce dog’s
organs like heart, brain, lungs, stomach, intestines and limbs, etc., needed
for the making of a complete dog. The complex multicellular organisms need more
complex ways of reproduction like sexual reproduction.
Q: Why is vegetative
propagation practised for growing some types of plants?
Answer: Vegetative
propagation is practised for growing some type of plants because it has the
following advantages:
(i) All the plants
produced by vegetative propagation are genetically similar enough to the parent
plant to have all its characteristics
(ii) The fruit
trees grown from seeds may take many years before they start to bear fruits.
But the fruit trees grown by vegetative propagation methods like cuttings or by
grafting start to bear fruits much earlier (only after a few growing seasons).
(iii) The plants
grown by vegetative propagation usually need less attention in their early
years than the plants grown from seeds.
(iv) Many plants
can be grown from just one parent plant by artificial propagation.
(v) Vegetative
propagation makes possible the propagation of plants such as banana, rose,
jasmine and orange that have lost the capacity to produce viable seeds.
Q: Why is DNA copying an
essential part of the process of reproduction?
Answer: DNA
contains information for the inheritance of characteristics from the parents to
the next generation. DNA copying is an essential part of the process of
reproduction because it makes possible the transmission of characteristics of
the parents to its offsprings in the next generation.
Q: How is the process of
pollination different from fertilisation?
Answer: Pollination is the transfer of pollen grains from the anther of the stamen of a flower to the stigma of a carpel in the same flower or another
flower of the same species. On the other hand, fertilisation occurs when the
male gamete present in the pollen grain joins with the female gamete (or egg)
present in ovule to form a zygote.
Q: What is the role of seminal
vesicles and prostrate gland?
Answer: Seminal
vesicles and prostrate gland occur in male reproductive system. The seminal
vesicles and prostrate gland add their secretions to the vas deferens which
carries sperms from the testes. The secretions of seminal vesicles and
prostrate gland provide nutrition to the sperms and also make their further
transport easier.
Q: What are the changes seen
in girls at the time of puberty?
Answer: The
various changes which occur in girls at puberty are: Hair grows under armpits
and pubic region. Mammary glands (or breasts) develop and enlarge. The hips
broaden. Extra fat is deposited in various parts of the body like hips and
thighs. Fallopian tubes, uterus and vagina enlarge. Ovaries start to release
eggs. Menstruation (monthly periods) starts. Feelings and sexual drives
associated with adulthood begin to develop.
Q: How does the embryo get
nourishment inside the mother’s body?
Answer: The embryo gets nutrition from the mother’s blood with the help of a special tissue
called the placenta. The placenta is a disc-shaped tissue which is embedded in the
uterus wall (uterine wall). It has villi on the embryo side of the tissue. On
the mother’s side are blood spaces which surround the villi. Placenta provides
a large surface area for glucose and oxygen to pass from the mother to the
embryo. The developing embryo also produces waste substances which can be
removed by transferring them into the mother’s blood through the placenta.
Q: If a woman is using a
copper-T, will it help in protecting her from sexually transmitted diseases?
Answer: No, the use of copper-T for contraception will not protect a
woman from sexually transmitted diseases.
#How Do Organisms reproduce?
NCERT EXERCISE
Q: Asexual reproduction takes
place through budding in:
(a) amoeba (b) yeast (c) plasmodium (d) leishmania
Answer: (b) yeast
Q: 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
Q.3. The anther contains:
(a) sepals (b) ovules (c) carpel (d)
pollen grains
Answer: (d) pollen grains
Q: What are the advantages of
sexual reproduction over asexual reproduction?
Answer:
(i) Sexual
reproduction combines DNA from two individuals (male and female) due to which
the offspring has a lot of variations. On the other hand, in asexual
reproduction, only the DNA of one individual is copied due to which the
variations in the offspring are extremely small.
(ii) Due to a lot of
variations sexual reproduction allows species to change to more advanced forms
from one generation to the next and speed up evolution. On the other hand,
asexual reproduction does not allow a species to change much from one
generation to the next and hence evolution becomes very, very slow.
Q: What are the functions
performed by testes in human beings?
Answer: The function of testes is to make male sex cells (or male gametes) called sperms
and also to make the male sex hormone called testosterone. The testosterone
hormone brings about changes seen in the appearance of boys at the time of
puberty such as deeper voice, beard, moustache, and more body hair (than
girls).
Q: Why does menstruation
occur?
Answer: Since the ovary of a woman releases one egg (or ovum) every
month, therefore, the uterus also prepares itself every month to receive a
fertilised egg (if formed). In this process, the inner lining of the uterus
becomes thick and soft with lots of blood capillaries (blood vessels) in it.
This preparation in the uterus is necessary because in case the egg is fertilised
by a sperm, then the uterus has to keep this fertilised egg and nourish it to
develop it into a baby. If, however, the egg released by the ovary is not
fertilised, then the thick lining of the uterus is not needed. So, the uterus
lining breaks down and comes out through the vagina in the form of blood and
mucous. This is called menstruation.
Q: Draw a
labelled diagram of the longitudinal section of a flower.
Answer:
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| LONGITUDINAL SECTION A FLOWER |
Q: What are the different
methods of contraception?
Answer: The
various methods of contraception (preventing pregnancy in a woman) are Barrier
methods, Chemical methods, use of Loop or Copper–T, and Surgical methods.
(i) In the barrier
methods of preventing pregnancy, physical devices such as condoms and
diaphragm (or cap) are used. Condoms are used by males (by putting them as a
covering on the penis). Diaphragm (or cap) is used by females (by putting it in
the vagina to cover the cervix). Condom, as well as the diaphragm, prevent the sperms
from meeting the ovum (or egg) by acting as a barrier between them.
(iii) In the
chemical methods of preventing pregnancy, the females use oral pills. The oral
pills contain hormones which stop the ovaries from releasing ovum (or eggs)
into the oviduct.
(iii) The loop or
copper–T is also very effective in preventing pregnancy. A loop or copper–T is
placed inside the uterus by a doctor or a trained nurse. The loop or copper–T
prevents the implantation of a fertilised egg in the uterus. Loop and copper–T
are called intra-uterine contraceptive devices (IUCD).
(iv) Surgical
methods of birth control are available for males as well as females. In males,
a small portion of the sperm duct (or vas deferens) is removed by surgical
operation and both the cut ends are ligated (or tied) properly. This prevents
the sperms from coming out. The surgical procedure carried out in males is
called ‘vasectomy’. In females, a small portion of the oviducts is removed by
surgical operation and the cut ends are ligated (or tied). This prevents the
ovum (or egg) from entering into the oviducts. The surgical procedure carried
out in females is called tubectomy.
Q: How are the modes of
reproduction different in unicellular and multicellular organisms?
Answer:
(a)
Most of the unicellular organisms (such as protozoa and bacteria) reproduce by
the asexual process of ‘fission’. In this process, mere cell division leads to
the creation of new individuals.
(b) In simple
multicellular organisms, reproduction occurs by asexual methods such as
budding, spore formation, fragmentation and regeneration, etc. But in complex
multicellular organisms, reproduction takes place by sexual methods involving
gametes (sex cells) from two parents — a male and a female.
Q: How does reproduction help
in providing stability to populations of species?
Answer: The
process of reproduction introduces some variations in the individual organisms
of a species. The variations introduced in some individual organisms may enable
them to survive even in adverse environmental conditions such as excessive heat
or cold or shortage of water, etc. (when most other members of the species will
die). In this way, the introduction of variations during reproduction provides
stability to the populations of various species by preventing some of their
individuals from getting wiped out during adverse environmental conditions.
Q: What could be the reasons
for adopting contraceptive methods?
Answer:
(i) The use of
contraceptive methods helps in family planning (birth control). By adopting
contraceptive methods, a couple can avoid unwanted pregnancy. They can choose
how many children to have and when to have them. A couple can also space the
birth of children properly by using contraceptive methods.
(ii)
Some of the contraceptive methods (like the use of a condom) also provide
protection to a person from sexually transmitted diseases.
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| TYPES OF REPRODUCTION |
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| GERMINATION AND SEED STRUCTURE |
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| POLLINATION AND ITS TYPE |
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| VEGETATIVE REPRODUCTION |
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| TYPES OF ASEXUAL REPRODUCTION |
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| FERTILIZATION |
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| ARTIFICIAL VEGETATIVE PROPAGATION |
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| SEXUAL REPRODUCTION |
