8TH
SCIENCE SOLUTION
CHAPTER 11 CELL - STRUCTURE AND FUNCTIONS
1.
Indicate whether the following statements are True (T) or False (F).
(a)
Unicellular organisms have a one-celled body.
(b)
Muscle cells are branched.
(c)
The basic living unit of an organism is an organ.
(d)
Amoeba has an irregular shape.
Ans:
(a)
True
(b)
True
(c)
False
(d) True
2.
Make a sketch of the human nerve cell. What function do nerve cells perform?
Ans:
The
function of the nerve cell is to receive and transfer messages.
3.
Write short notes on the following.
(a)
Cytoplasm
(b)
Nucleus of a cell
Ans:
(a)
Cytoplasm:
★
The jelly-like substance found between
the nucleus and the cell membrane is called cytoplasm.
★
organelles, like mitochondria, Golgi
bodies, ribosomes, etc., of cells are present in the cytoplasm.
(b)
Nucleus of a cell:
★
Nucleus of a cell is an important
component of the living cell.
★
It is located at the centre of the
cell.
★ It contains genetic material.
4.
Which part of the cell contains organelles?
Ans:
Cytoplasm
5.
Make sketches of animal and plant cells. State three differences between them.
Ans:
Plant cells
Animal cells
(i)
cell wall present
(i) cell wall present absent
(ii)
Plastids are present in plant cells. (ii) Plastids are absent in animal
cells.
(iii)
Large vacuoles present (iii) No or small vacuoles are present.
(iv)
It lacks centrosomes and lysosomes. (iv) They have centrosomes and
lysosome
6.
State the difference between eukaryotes and prokaryotes.
Ans:
Eukaryotes Prokaryotes
(i)
membrane-bound organelles present. (i) lack membrane-bound organelles.
(ii)
Nucleus of the cell has nuclear membrane. (ii) Nucleus is not bounded by
membrane.
Example: higher plants and animals. Example: bacteria and blue-green algae.
7.
Where are chromosomes found in a cell? State their function.
Ans:
★
Chromosomes are present in the nucleus.
★ transfer the character from parents to the next generation.
8.
‘Cells are the basic structural units of living organisms.’ Explain.
Ans:
Different cells combine to form tissues and tissues combine to form organs. Similarly, organs combine to form body. Thus they are termed as the basic structural unit of every living organism.
9.
Explain why chloroplast are found only in plant cells?
Ans:
Chloroplasts are plastids required for the food making process, called photosynthesis, and thus they are only present in plant cells.
10.
Complete the crossword with the help of clues given below.
Across
1.
This is necessary for photosynthesis.
3.
Term for component present in the cytoplasm.
6.
The living substance in the cell.
8.
Units of inheritance present on the chromosomes.
Down
1.
Green plastids.
2.
Formed by collection of tissues.
4.
It separates the contents of the cell from the surrounding medium.
5.
Empty structure in the cytoplasm.
7.
A group of cells.
Ans:
CHAPTER 12 REPRODUCTION IN ANIMALS
1.
Explain the importance of reproduction in organisms.
Ans:
Reproduction
is essential for existence and continuity of life.
2.
Describe the process of fertilisation in human beings.
Ans:
Fertilisation
is a fusion of male gamete and female gamete.
sperms
travel through the fallopian tubes, where they meet the eggs.
fertilisation
takes place in the fallopian tube.
The
male gamete cell (sperm) and female gamete cell (egg) fuse together to form a
zygote.
The
zygote which becomes the embryo.
3.
Choose the most appropriate answer.
(a)
Internal fertilisation occurs
(i)
in female body. (ii) outside
female body.
(iii)
in male body. (iv) outside
male body.
Ans: (i) in female body.
(b)
A tadpole develops into an adult frog by the process of:
(i)
fertilization (ii) metamorphosis (iii) embedding (iv) budding
Ans: (ii) metamorphosis
(c)
The number of nuclei present in a zygote is:
(i)
none (ii) one (iii) two (iv) four
Ans: (ii) one
4.
Indicate whether the following statements are True (T) or False (F).
1. Oviparous animals
give birth to young ones.
2. Each sperm is a
single cell.
3. External
fertilisation takes place in frog.
4. A new human
individual develops from a cell called gamete.
5. Egg laid after
fertilisation is made up of a single cell.
6. Amoeba reproduces by
budding.
7. Fertilisation is
necessary even in asexual reproduction.
8. Binary fission is a
method of asexual reproduction.
9. A zygote is formed as
a result of fertilisation.
10. An embryo is made up
of a single cell.
Ans:
1. False
2. True
3. True
4. False
5. True
6. False
7. False
8. True
9. True
10.
False
5.
Give two differences between a zygote and a foetus.
Ans:
Zygote
Foetus
(i)
It is a single celled, (i)
It is multicelled,
(ii)
It is formed by the fusion of male gamete (ii) It is formed by the repeated division
of the zygote.
or sperm and female gamete or ova (egg).
6.
Define asexual reproduction. Describe two methods of asexual reproduction in
animals.
Ans: The mode of
reproduction in which only a single parent is involved is called asexual
reproduction.
There
are mainly two methods of asexual reproduction:
(i)
Binary fission: In binary fission, a single parent cell is divided into two
equal individual cells as in Amoeba.
(ii)
Budding: In budding, the organism develops a bulge called bud which further
develops into an adult organism in Hydra.
7.
In which female reproductive organ does the embryo get embedded?
Ans:
Uterus
8.
What is metamorphosis? Give examples.
Ans:
The
transformation of larva into an adult is called metamorphosis. Example, frog
and butterfly.
9.
Differentiate between internal fertilisation and external fertilisation.
Ans:
|
Internal
Fertilisation |
External
Fertilisation |
|
(i) occurs inside the
body of a female partner, such as human beings, birds, and mammal. |
(i) it takes place
outside the body of a female partner, such as in frog, fish and starfish. |
|
(ii) The female partner
lays either fertilised eggs or a fully grown young one. |
(ii) The female partner
discharges unfertilised eggs. |
|
(iii) Offsprings have a
high chance of survival. |
(iii) Offsprings have a
low chance of survival. |
10. Complete
the crossword puzzle using the hints given below.
Across
1. The
process of the fusion of the gametes.
6. The type
of fertilisation in a hen.
7. The term
used for bulges observed on the sides of the body of Hydra.
8. Eggs are
produced here.
Down
2. Sperms are
produced in these male reproductive organs
3. Another
term for the fertilised egg.
4. These
animals lay eggs.
5. A type of
fission in Amoeba.
CHAPTER 13
REACHING THE AGE OF ADOLESCENCE .
1. What
is the term used for secretions of endocrine glands responsible for changes
taking place in the body?
Ans: Hormones
2. Define
adolescence.
Ans: The period of life, when the
body undergoes changes, leading to reproductive maturity, is called
adolescence.
3. What
is menstruation? Explain.
Ans:
★
In females the ovary produces one egg every
month and the uterus prepares to receive the fertilised egg.
★
Its wall becomes thick with blood vessels.
★
If fertilisation does not take place then the
uterus wall breaks and comes out of the vagina as blood and mucous.
★
This cycle takes place once every month and
is called menstruation.
4. List
changes in the body that take place at puberty.
Ans:
★
Increase in height.
★
Change in body shape.
★
Voice change.
★
Increased activity of sweat and sebaceous
glands
★
Development of sex organs.
★
Reaching mental, intellectual and emotional
maturity.
5.
Prepare a table having two columns depicting names of endocrine glands and
hormones secreted by them.
Ans:
Endocrine
glands Hormones
1.
Pituitary gland (i) Growth hormones
2.
Ovaries (ii) Estrogen
3. Testes (iii) Testosterone
4.
Thyroid (iv) Thyroxine
5.
Pancreas (v) Insulin
6. What
are sex hormones? Why are they named so? State their function.
Ans:
★ The
hormones which help and control the formation of secondary sexual character.rs
are called sex hormones.
★ They are
named so because they control the sexual activities
★ Male sex
hormone: It is also called testosterone. It is secreted by testes and causes
changes in secondary sexual character of boys.
★ Female
sex hormone: It is also called estrogen. It is secreted by ovaries and controls
the secondary sexual characters in females.
7. Choose
the correct option.
(a)
Adolescents should be careful about what they eat, because:
(i)
proper diet develops their brains.
(ii)
proper diet is needed for the rapid growth taking place in their body.
(iii)
adolescents feel hungry all the time.
(iv)
taste buds are well developed in teenagers.
Ans: (ii)
proper diet is needed for the rapid growth taking place in their body.
(b)
Reproductive age in women starts when their:
(i)
menstruation starts.
(ii)
breasts start developing.
(iii)
body weight increases.
(iv)
height increases.
Ans: (i)
menstruation starts.
(c) The
right meal for adolescents consists of:
(i)
chips, noodles, coke.
(ii)
chapati, dal, vegetables.
(iii)
rice, noodles and burger.
(iv)
vegetable cutlets, chips and lemon drink.
Ans: (ii)
chapati, dal, vegetables.
8. Write
notes on:
(a)
Adam’s apple
(b)
Secondary sexual characters.
(c) Sex
determination in the unborn baby.
Ans:
(a)
Adam’s apple: In boys, the voice box tends to protrude out on the upper part of
the neck, below the chin and is often referred to as Adam’s apple.
(b)
Secondary sexual characters: Those characters which are visible more apparently
and help in distinguishing a male from a female are called secondary sexual
characters. For example, in girls, breasts and in boys, facial hair, i.e.,
moustache and beard.
(c) Sex
determination in the unborn baby: A newborn will get one of the X chromosomes
from their mother and either X or the Y chromosome from their father. Thus, a
child which gets the Y chromosome will be a boy and the one who gets the X
chromosome will be a girl. Therefore, the sex chromosomes of the father
determine the sex of an unborn baby.
9. Word
game: Use the clues to work out the words.
Across:
3.
Protruding voice box in boys
4. Glands
without ducts
7.
Endocrine gland attached to brain
8.
Secretion of endocrine glands
9.
Pancreatic hormone
10.
Female hormone
Down:
1. Male
hormone
2.
Secretes thyroxine
3.
Another term for teenage
5.
Hormone reaches here through blood stream
6. Voice
box
7. Term
for changes at adolescence
Ans:
10. The
table below shows the data on likely heights of boys and girls as they grow in
age. Draw graphs showing height and age for both boys and girls on the same
graph paper. What conclusions can be drawn from these graphs?
Age(Years)
Height (cm)
Boys Girls
0 53 53
4 96 92
8 114 110
12 129 133
16 150 150
20 173 165
Ans: From the
above graph, we conclude that initially boys grow faster than girls from 0-8
years, and girls grow faster than boys from 8-16 years. But both reach their
maximum height at the age of 20. Here boys show a greater increase in height
than girls.
CHAPTER 14 CHEMICAL
EFFECTS OF ELECTRIC CURRENT .
1. Fill
in the blanks.
(a) Most
liquids that conduct electricity are solutions of ______ , ______ and ______
(b) The
passage of an electric current through a solution causes _______ effects.
(c) If
you pass current through copper sulphate solution, copper gets deposited on the
plate connected to the ________ terminal of the battery.
(d) The
process of depositing a layer of any desired metal on another material by means
of electricity is called _______
Ans:
(a)
acids, bases, salts
(b)
chemical
(c)
negative
(d)
electroplating
2. When
the free ends of a tester are dipped into a solution, the magnetic needle shows
deflection. Can you explain the reason?
Ans: the
solution conducts electricity, i.e., it is a good conductor.
3. Name
three liquids, which when tested in the manner shown in Fig. 14.5. may cause
the magnetic needle to deflect.
Ans:
Ans: Vinegar,
lemon juice and tap water.
4. The
bulb does not glow in the setup shown in Fig. 14.6. List the possible reasons.
Explain your answer.
Ans:
The bulb
may not glow because of the following reasons:
(i) The
wires in the circuit may be loosely connected.
(ii) The
bulb may be fused.
(iii) The
cells may be used up.
(iv) The
liquid may be an insulator, i.e., a poor conductor of electricity.
5. A
tester is used to check the conduction of electricity through two liquids,
labelled A and B. It is found that the bulb of the tester glows brightly for
liquid A while it glows very dimly for liquid B. You would conclude that
(i)
liquid A is a better conductor than liquid B.
(ii)
liquid B is a better conductor than liquid A.
(iii)
both liquids are equally conducting.
(iv)
conducting properties of liquid cannot be compared in this manner.
Ans:
(i)
liquid A is a better conductor than liquid B.
6. Does
pure water conduct electricity? If not, what can we do to make it conducting?
Ans: No, pure
water doesn’t conduct electricity. But when salt is dissolved in pure water, it
conducts electricity.
7. In
case of a fire, before the firemen use the water hoses, they shut off the main
electrical supply for the area. Explain why they do this.
Ans: The water
used in the water hoses is not pure water and is a good conductor of
electricity.
8. A
child staying in a coastal region test the drinking water and also the seawater
with his tester. He finds that the compass needle deflects more in the case of
seawater. Can you explain the reason?
Ans: The
seawater contains a huge amount of salts in comparison to drinking water, hence
the seawater is a better conductor of electricity.
9. Is it
safe for the electrician to carry out electrical repairs outdoors during heavy
downpour? Explain.
Ans: No, it is
highly dangerous to carry out the electrical repairs outdoors during the heavy
downpour. It can cause electrocution, as water is a good conductor of
electricity.
10.
Paheli had heard that rainwater is as good as distilled water. So she collected
some rainwater in a clean glass tumbler and tested it using a tester. To her
surprise, she found that the compass needle showed deflection. What could be
the reasons?
Ans: Rainwater
is pure water which is an insulator but it gets mixed with air pollutants like
sulphur dioxide and nitrogen oxides and form acidic solution, which is a good
conductor of electricity. So, the compass needle showed deflection.
11.
Prepare a list of objects around you that are electroplated.
Ans: Objects
that are electroplated are door handles, taps, rims of cycles, showers, the
handlebar of cycles and bikes, gas burner, tin cans, metallic almirahs, buckles
of belts, etc.
CHAPTER 15 SOME NATURAL
PHENOMENA .
Select
the correct option in Questions 1 and 2.
1. Which
of the following cannot be changed easily by friction?
(a) A
plastic scale (b) A copper rod (c) An inflated balloon (d) A woollen cloth
Ans: (b) A
copper rod
2. When a
glass rod is rubbed with a piece of silk cloth the rod
(a) and
the cloth both acquire a positive charge.
(b)
becomes positively charged while the cloth has a negative charge.
(c) and
the cloth both acquire a negative charge.
(d)
becomes negatively charged while the cloth has a positive charge
Ans: (b)
becomes positively charged while the cloth has a negative charge.
3. Write
T against true and F against false in the following statements.
(a) Like
charges attract each other.
(b) A
charged glass rod attracts a charged plastic straw.
(c)
Lightning conductor cannot protect a building from lightning.
(d)
Earthquakes can be predicted in advance.
Ans:
(a) False (b) True
(c) False (d) False
4. Sometimes,
a crackling sound is heard while taking off a sweater during winters. Explain.
Ans:
The
electric discharge takes place between the body and the sweater due to
friction. In this case energy is released in the form of crackling sound.
5. Explain
why a charged body loses its charge if we touch it with our hand.
Ans: Human
body is a conductor of electricity. When a charged body is touched with hand,
our body conducts its charges to the earth. Hence, by this way charged body
loses its charge.
6. Name
the scale on which the destructive energy of an earthquake is measured. An
earthquake measures 3 on this scale. Would it be recorded by a seismograph? Is
it likely to cause much damage?
Ans:
Richter
Scale.
Yes, it
would be recorded by seismograph because this scale has the readings from 1 to
10. No it is not likely to cause much damage as earthquakes of magnitude higher
than 5 is considered destructive in nature.
7.
Suggest three measures to protect ourselves from lightning.
Ans:
(i) Do
not use TV or cable phone during lightning.
(ii) Stay
indoor or under covered area.
(iii)
Don’t take bath during lightning.
8.
Explain why a charged balloon is repelled by another charged balloon whereas an
uncharged balloon is attracted by another charged balloon?
Ans:
★ A charged
balloon is repelled by another charged balloon because both carry same type of
charges.
★ an
uncharged balloon is attracted by another charged balloon as they have opposite
charges.
8.
Explain why a charged balloon is repelled by another charged balloon whereas an
uncharged balloon is attracted by another charged balloon?
Ans:
An
electroscope can be used to detect whether a body is charged or not. The
following figure shows a simple electroscope.
in
Electroscope, two metal foils are hanged with the help of a conductor in a
glass jar with a lid. When a charged body touches this conductor, its charged
gets transferred to the metal foil and they move apart due to like charges.
This shows that an electroscope detects charges.
10. List
three states in India where earthquakes are more likely to strike.
Ans:
Kashmir,
Rajasthan and Gujarat.
11. Suppose
you are outside your home and an earthquake strikes. What precaution would you
take to protect yourself?
Ans:
Some of
the precautions are:
→ Try to
move to an open space.
→ We
should not take shelter under trees, buildings or overhead power lines.
→ If
travelling in a bus or a car, then do not come out when an earthquake strikes.
Ask the driver to drive in an open field
12. The
weather department has predicted that a thunderstorm is likely to occur on a
certain day. Suppose you have to go out on that day. Would you carry an
umbrella? Explain.
Ans:
No, it is
not advisable to carry an umbrella at all. Its metallic objects are more prone
to lightning stroke. So an umbrella increases the risk.
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