Sunday 4 February 2024

8th Sci All Chp Soln

                             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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