the living world-translation

7/29/2019 The Living World-translation

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The living world

The students have gained familiarity with this aspect in class 6th and 7th through their

science textbooks. Therefore you should provide more and more opportunities to class

8th students to minutely explore the living world and learn about its finer details.

The following four chapters are covered in the unit:

1. conservation of plants and animals

2. cell structure and function

3. reproduction in animals

4. reaching the age of adolescence

You can see that the chapters are linked to one another. For example, an

understanding of the cell is necessary for understanding reproduction in animals.

Similarly, understanding of the cell and of reproduction will help in building an

understanding of conservation of plants and animals.

Conservation is an interesting topic in itself. You can see several examples ofconservation around you and the students too are familiar with them. This chapter will

play a crucial role in helping the students develop an interest in their environment and

establish a relationship with it.

The chapter on cell requires both you and the students to patiently observe different

types of cells. There is a considerable overlap between the chapters on reproduction

and adolescence. In their previous classes, the students have already studied how a

species reproduces through the example of plants. The chapter will highlight different

modes of reproduction by looking at humans and other animals. The students will be

able to relate to the changes going on in their bodies through the chapter on

adolescence and see how these prepare them for reproduction in later life.

1. Cell

Why should we study the cell?

You all will agree that cell is an important concept in biology. This is made clear by

the cell theory. But the question of why we should study the cell remains and more

importantly how do we motivate the students to study this topic. Several phenomenon

are studied because they are present in our environment and others because they are

necessary to explain them. But the cell is different because without a microscope we

will not be able to see it. Further, most of the properties of living beings can beexplained satisfactorily without referring to the cell. For example, the process of

digestion can be explained as follows. We eat food, this goes into the stomach and

intestines where it gets digested and the digested products are absorbed and waste

products are excreted. This explanation shows that different organs are present in the

body and they are linked to each other but it does not require a cell. However, there

are also several properties of living beings especially sexual reproduction in animals

that cannot be explained without cells.

Although several processes can be explained without the cell, its existence makes it

necessary for us to reexamine our theories.

For example, understanding of reproduction, especially in multicellular organisms,

depends on the knowledge of cell. This is because cell is the starting unit of an animal


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after fertilization. This cell does not show any property of the adult animal and these

appear slowly. This indicates the presence of a single unit.

Reproduction and genetics is an interesting aspect of biology and the discovery of cell

and cell science or cytology has enabled better understanding of reproduction and

heredity.

What do we need to observe the cell? A good microscope and lots of patience

The microscope is useful for both teachers and students. For proper use, it is

necessary that we know the various parts of a microscope and their functions.

Microscopes are of two types simple and compound. A compound microscope is

required to observe the cell. You should help students become familiar with

microscope by encouraging them to observe various minute things through it.

How to observe a cell?

The textbook has discussed different types of cells in great detail. You should try to

arrange the observation of several cells. Some material will be required for this: a

good microscope, sample from your environment that can be studied such as waterfrom a pond or from a sewage site.

Put one or two drops of this water on a slide. Along with the water, you can put some

algae (?) on the slide as well. Put a cover slip on the slide and see it under the

microscope. Observe all parts of the drop.

A lot of patience is required in this activity. Were you and your students able to see

some microorganisms moving around under the microscope? Ask the students to drawany cell or microorganism that they see. Ask them to describe its movements. Usually

we have a picture of microorganisms in our mind as we have seen in textbooks. But in

reality when we see them under a microscope they are very different. The pictures in

textbooks are generalizations/standardizations.

Now ask the students to observe the samples mentioned in the textbooks under the

microscope. The students will enjoy this activity and this will also help them

understand the foundations of biology. Additionally, they will also develop the skill of

using the microscope which will help them in later classes.

Motivate the students to observe take thin cuttings of the tender stems of young plants

and observe them under the microscope. The thinner their cuttings, the better they will

be able to observe. They can also observe the pollen of various plants and try andunderstand the similarities and differences in them at the cellular level.

Shape of different cells

While the students are observing the cells, you can also ask them to note down their

shapes. The cells shown in textbooks are actually representations. It is difficult to

guess the shape of cells from these pictures. And therefore this aspect of study of cells

often remains untouched. So you must discuss that cells are of various shapes. For

example, cells of the stems of plants are cylindrical while cells observed in an onion

peel are flat. Similarly when students are observing plant stem cells, draw their

attention towards their different shapes and dimensions.

Usually, plant cells are easier to get if we want to observe cells using a microscope.

And they are also easier to observe. Why do you think this is so?


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By now the students will know that cells are extremely minute. But how small are

they exactly? Some bacteria are as small as 0.2 micrometer. If we divide 1 meter into

10 lakh equal divisions, then 1 division is equal to 1 micrometer. Or if we divide 1 cm

into 10,000 divisions, then also one division is equal to 1 micrometer. An average

animal cell is 20 micrometers. However, we must not forget that some animal cells

are very big. For example, the cell of the egg of an ostrich is about 7 cm in diameter.Here the egg of the ostrich refers to the yellow yolk within the egg.

Activity

When observing the cells, ask the students to estimate their size. They can think of the

size of the cells in an onion peel.

Take a graph paper which has mm divisions. Put a drop of oil on it so that it becomes

translucent. Now observe this under a microscope at low resolution. Without blinking

your eyes, try and see how many mm squares you can observe. Now see the onion cell

slide at the same resolution. How many onion cell can you see? This activity will

broadly give an idea about how big a cell is.

Note: Your role in all these activities and their preparation is extremely crucial. It will

be helpful if you try out all these experiments yourself first.


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What is inside the cell?

You must have heard that cell is the structural and functional unit of life. All living

things are made of cells. Cell has a dual identity. It is an independent unit and also the

building block of an organism. For example, cells together form tissues which

combine to form an organ. And an organism has several such organs.

But is there anything inside the cell? What is the cell made of?

A cell has several components such as cytoplasm, nucleus, mitochondria, golgi

bodies, ribosomes etc. Although the students will study these in detail in higher

classes some of them can be observed by a little extra effort. The nucleus in an onionpeel cell can be seen easily.

A cell is surrounded by a membrane that defines its shape and boundary. It also

protects the cell. Many substances reach inside the cell by passing through the cell

membrane. One characteristic of the cell membrane is that it does not allow

everything to pass into or from the cell. The substances that can pass through it are

chosen carefully. That is why a cell membrane is said to be selective permeable.

Dont present all this information to the student. Rather, if you feel that students are

inclined towards asking such questions then only include these in discussions.

How to observe the nucleus?

You can observe the nucleus in onion peel cells. Put a few drops of blue dye solutionon the peel section on the slide using a dropper. You will now have to gently warm

the slide. For this, take a burning matchstick and move it 6-8 times under the slide.

Keep the slide aside for 10 minutes. After this, use a dropper to put water over the

slide till all the dye is washed away. Now put a cover slip on this coloured onion peel

and observe it under a microscope.

Can you see a dark dot within the cell? This is the cell nucleus.

Comparison between plant and animal cells

Historical Background

Leeuvenhoek observed different microorganisms under a microscope he had made

himself. He called these microorganisms animalcules. His observations were so

good that he managed to observe bacteria by 1683. You can show students those

samples that were possibly observed by Leeuvenhoek.Cork cells

To observe cork sections, soak it for an hour in water. Now cut a thin section of this

cork, put it on the slide in a drop of water and put a cover slip on it. If you dont

have cork, you can use a section of a match stick instead. Now observe it under a

microscope.

Were your students able to see what Leeuvenhoek had seen first? Discuss with your

students that knowing the history of an area makes it easier to understand it deeply.

What we know about cells now was not known before Leeuvenhoek. We should

also understand the importance of the discovery of the microscope in discovery of

the cell.


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Ask the students to observe the edges of the cell in the slide prepared above. Can they

see thick lines outside the cell? These thick lines between adjoining cells are actually

cell walls surrounding each cell.

In animal cell, the outer region of the cell is a membrane only. But in plants, apart

from this membrane there is also a strong layer made of cellulose called the cell wall.

This is one of the major differences between plant and animal cells.

2. Reproduction in animals

Ask the students if all animals give birth to young ones similar to themselves? Tell

them that this is an important characteristic of living things. The chapter can be

started by asking the students questions such as:

1. Is there any plant or animal that does not reproduce?

2. can a parrot or hen come out from a pigeons egg?

Discuss with the students that it is necessary that a new animal or plant can come only

from a similar existing plant or animal. Also discuss that you get mango seeds from

mango tree and not jamun or orange. Similarly, a cow gives birth to a calf and not agoat or puppy.

Several social issues are linked to the topic of reproduction. Many superstitions are

prevalent regarding reproduction in human beings. This chapter has several themes

revolving around reproduction including the science involved. Some times talking

about reproduction leads to discomfort and teachers hesitate in discussing this topic.

Discussion on this theme with students is necessary because currently they are unable

to do so at home. Lack of proper dialogue can lead to their moving in the wrongdirection with the little information that they have. You should try and tell them that

reproduction is a relevant aspect of human life and talking about its social and

scientific importance is necessary.

When looking at chapter 9 (reproduction) and chapter 10, you try and create an

atmosphere in the classroom that encourages the students to talk about reproduction

and reproductive organs. Let them ask questions related to this aspect. Try to plan

your class in such a manner that it gives space to the children to ask questions and

discuss problems.

Children have already studied reproduction in plants in class 7 th and you can start thischapter by going back to and recalling it. Give them examples about the animals they

see around each other. They may have observed how birds, cats, dogs, butterflies etc

are born. Try to make them realize that animals can reproduce by giving birth to their

young or by laying eggs. They can make a table where list the animals around them in

these two categories.

Sexual reproduction

When you discuss reproduction, children can see a lot of examples around them of

animals who reproduce sexually. They will probably be able to distinguish the male

and females in the animals around them. For example, cow and bull, cock and hen etc.

The process where plants and animals produce young ones of their kind and thus

increase their population is known as reproduction.


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Ask them to list animals where they find it difficult to tell the male from the female or

it is not possible for them.

Two types of gametes

Having two types of gametes male and female - is the first condition for sexual

reproduction. This chapter discusses sexual reproduction by taking the example ofhuman beings. The textbook has diagrams showing the male and female reproductive

organs. The students will understand that male gametes and reproductive organs are

seen in men and female gametes in women.

Before starting this chapter, students must have read and understood the chapter on

cells. So the students can be told that male gametes or sperms and female gametes or

ova (eggs) are a type of cells. They are called reproductive cells. The fusion between

male and female gametes to form a zygote is another critical condition for sexual

reproduction.

Fertilization

You can talk with the students about the fertilization between the sperm and the egg.

You can discuss the two types of fertilization external or outside the body and

internal or inside the body. You can give examples of both types of fertilization. For

example in humans and other breast feeding animals, fertilization happens inside the

female. And in frogs it happens outside the body in water. In external fertilization, the

male releases sperms and the females lays eggs which then combine to form the

zygote.

Frogs are a good example of external fertilization. Ask the students to recall the noise

made by frogs in the rainy season. This is their reproduction season. The male frogtries to attract the female by making loud croaking noises. Sometimes you can see the

male and female frogs stuck to each other. Male frogs release sperms in lakhs in water

sources such as ponds. You can ask the children to observe the eggs of a frog and the

hatched tadpoles in water bodies during the rainy season.

Similarly you can attract the students attention towards reproduction. They can

observe sparrows in their homes. They collect twigs and build their nest in a secure

place. Then the female sparrow lays eggs in the nest and also takes care of the young

chicks when they are hatched. You can ask students to observe butterflies, caterpillars

and the eggs laid by many other animals, birds in the rainy season.

Some students from rural backgrounds might have seen the birthing process in farm

animals. They will know that only the female can give birth. You can ask the

students the role of the male in reproduction.

Some students may have birds as pets. Ask them to tell or find out if a female bird

kept alone in a cage lays eggs or not.

Take the students to a nearby poultry farm if possible. Ask them to look around and

ask questions. They will find that a cock is not kept with hens that lay eggs for

human consumption. No chicks can hatch from these eggs.

The students can find out what is done in the poultry farm when they want chickens

from eggs.


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The students can ask why if only one sperm fertilizes the egg, then so many sperms

are formed. You can discuss that reproduction is very important and the large number

of sperms is to ensure definite fertilization of the egg under all circumstances. You

can explain that a frog lays hundreds of eggs but not all of them produce young frogs.

Many of these eggs are eaten by other animals. Sometimes the young tadpole is eaten

by other animals. Therefore, nature ensures that a species can continue by producingeggs and sperms in large quantities. The students can watch programmes on National

geographic and discovery channels that are entertaining and informative.

You can tell the students that in humans and in all cases of sexual reproduction some

characteristics in the young one are from the father and some from the mother. They

can look at themselves and their friends and see which characteristics they got from

their father and which from their mother.

By class 8th students will be aware that there are some families that cannot have

children. Very often, the female is held responsible. You can discuss with the class

whether the male also can be responsible for not having children. Compel them tothink on these issues. Many times non-scientific methods are used by families in the

hope of getting children. Students can discuss whether it is right to do so.

Test tube baby

Test tube babies are no longer a new technique. The children especially in cities may

have heard of test tube babies. This certainly does not mean that the baby develops in

a test tube. You can discuss how under certain circumstances, fertilization is not

possible in the mothers body. Then external fertilization is tried by doctors and the

fertilized embryo is placed inside the mothers body.

The development of the embryo

Embryo development is one important aspect of reproduction. Students may be

fascinated to learn how the embryo develops within the mothers body. At what stage

which organs are formed? Some people believe that the sex of the child, that is

whether it will be a boy or a girl, is determined during the development of the

embryo. You must clarify that sex determination occurs during fertilization itself.

Therefore any efforts to get a boy after pregnancy are generally unsuccessful. Here

you should also logically discuss that it is matter of chance whether a boy or a girl

will be born. And it may be relevant here to discuss that girls are in no way inferior to

boys.

You can also discuss the laws regarding sex determination during pregnancy. It is

now illegal and punishable to determine the sex of the child at any clinic.

Activity

You can ask the students to observe the eggs of different egg laying creatures. They

can first examine the external characteristics of the egg like the hardness of its shell

and the different colours of different eggs.

Picture: Development stages of the embryo.


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Similarly they can also observe insects. If you look at the lower surface of leaves

during the rainy season, youll be able to observe very small (minute) eggs. You may

need a hand lens to see such eggs. If you place these eggs carefully in a box along

with some leaves you may see some caterpillars coming out after some time.

You can also tell the children that the leeks in their hair are the eggs laid by lice. They

can also observe these.

Note: Ask the students, not to break the eggs during observation because these eggs

are necessary to continue the species.

Asexual Reproduction

Asexual reproduction has been discussed at the end of the chapter. The number of

examples of asexual reproduction seen in plants is way more than that in animals.

Actually, it is more common in microorganisms. You can discuss this through a ready

made hydra slide.

You can also talk about the differences in sexual and asexual reproduction. The

characteristics in a young one formed during sexual reproduction are a mixture of

both its parents but in asexual reproduction only the characteristics of one parent get

passed on.

Cloning has been discussed in the textbook. But this is also an example of asexual

reproduction. Cloning does not require male and female gametes. When the sheepclone called dolly was made, it resembled its mother in all respects. In English

language, the word clone is used for an identical image or form. Therefore, clone

implies an identical offspring.

Table

No.

Name ofanimal

Eggs oryoungones

Metamorphosis

Or not

Wholooksafterthe

youngoneafterbirth

1 Frog2 Fly3 butterfly45

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In metamorphosis, the young one coming out of the egg is very different from itsparents. For example, the tadpole is completely vegetarian while the frog is

totally carnivorous. Similarly, the tadpole cannot survive outside water while a

frog can. You can also discuss metamorphosis in insects. Also discuss the

advantages of metamorphosis for the animal.


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Project

As project work, you can ask the students to collect information about all animals

seen around them and list their properties. They can identify animals that lay eggs,

that give birth to young ones. They can also see where there is external fertilizationand where there is internal fertilization. What is done to raise the young one and is

there a special season when an animal reproduces. You can add more such points. The

children can also make charts and display them in the classroom.

3. Reaching the age of adolescence

This topic is important at several levels. Children of class 8 th are themselves passing

through this phase and therefore it is important for them to be aware of the changes in

their body. Secondly, adolescence is the preparatory stage for reproduction. Everyone

passes through this phase. In humans it generally starts at 11 years and continues till

18-19 years.

You can start the chapter by asking certain questions that are often also asked by the

children. For example:

o Why dont girls have beards?

o Why do boys have beards?

o Why do girls menstruate?

These are some questions linked to adolescence. There can be other questions that

motivate students to think about the characteristics of adolescence. This is the time

when a girl or a boy are undergoing several physical changes but do not get an

opportunity to talk about them with anyone. In these circumstances, your role as a

teacher becomes very important.

The changes due to adolescence are more visible in girls but it is still not considered

appropriate to talk about physical changes, menstrual cycle etc. Lack of discussion

can lead to misconceptions and mental stress.

Since this chapter is very similar to reproduction, you can teach it in continuation.

During adolescence, girls and boys undergo several physical changes. Shoes and

clothes brought one month, dont fit them after some time. At the age of around 10-

11, boys suddenly start growing taller rapidly. Their bodies also start changing and

their sweat and oil glands become active. Hair growth is seen in the underarm area, ontheir arms and legs and around the reproductive organs. Their voice starts changing.

Some facial hair and chest hair also starts appearing.

Activity Ask the students to make a detailed list of the changes occurring during

adolescence. If both girls and boys are in the class, you can divide them into twogroups and ask them to write down the changes that they can see or feel. Ask

When we start discussions about adolescence, we should not ignore gender. There are

several beliefs in the society that girls are weaker and boys are strong. There are

many beliefs regarding menstruation. Where ever possible, discuss these issues while

teaching the chapter.


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them to write down only the changes that they can see in themselves. You will

have to be very careful while discussing this list so that it takes place properly.

Changes seen in girls:

S.No. Physical Mental

Changes seen in boys:

S.No. Physical Mental

The students can also analyse their observations.

Ask the students to try out activity 10.1 from their textbook among themselves.

This will give the estimated height of each child at the end of the growth period.You can get a measuring tape and the children can measure their heights and

calculate it when they are 18. This can prove to be an interesting activity for

children.

It can be often seen in the class that one child could be quite tall and another is very

short. The second child is often called names like pidda, shorty etc by his

classmates and sometimes even adults make fun of him. Some parents also get

concerned if their child is not gaining height and try several ways to make their

child grow tall. They give him tonics or medicines with hormones.

But here it is more important for us to find out the factors that affect height.


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Try to attract the students attention to the fact that the changes seen during

adolescence are mental as well as physical. Make them aware that this is an age

where the capacity of the brain to learn is quite high.

Factors behind the changes seen in adolescence

By now, the students must have formed some understanding of the changes seen

during adolescence. Now is the time to discuss the factors in our body that are

responsible for these changes. You can make this topic interesting by presenting

it in terms of a clock or system within our body that keeps track of the changeswith time.

Actually, hormones are responsible for these changes. Discusswith the students that many chemicals are produced in our bodythat control various functions. These chemicals are calledhormones and are formed in glands. No special pipe or duct isrequired to transport these hormones to different parts of thebody. Therefore these glands are also called ductless glands.Hormones enter the blood straight from these glands. These arealso called endocrine glands.

It is possible that hormones are being discussed for the firsttime. Therefore it is necessary that this concept becomes very

Discuss with the students that the height of any person is very similar to some other

family member. This is because height depends on a gene that a child gets from

parents. However, nutrition also plays a role in the growing years.

Students sometimes worry whether they will grow or not. They start feeing lowly.

Sometimes fun is made of changes happening during adolescence. If you talk to

students about this, it will change their thinking. Make such discussions a part ofnormal classroom process.


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clear for the students. Especially clarity on their role in terms ofadolescence can lead to relevant discussions. Ask the children torecall why they are advised to eat iodized salt. This can open thewindow to discussions on hormone production. Iodine is requiredfor proper functioning of the thyroid gland.

The hormones responsible for onset of adolescence have beendiscussed in the textbook. Remember that the names ofhormones should not be mentioned simply for the purpose ofmemorization. Rather, they should connect it with the detailsand complexities of adolescence. Whether, boy or a girl, thepurpose of this chapter is to try and give them a proper pictureof life processes.

Major Hormones

Where Hormone

Is Produced

Hormone Function

Pituitary gland Antidiuretic hormone

(vasopressin)

Causes kidneys to retain water and, along with aldosterone,

helps control blood pressure

Corticotropin (ACTH) Controls the production and secretion of hormones by the

adrenal glands

Growth hormone Controls growth and development; promotes protein

production

Luteinizing hormone and

follicle-stimulating hormone

Control reproductive functions, including the production of

sperm and semen, egg maturation, and menstrual cycles;

control male and female sexual characteristics (including hair

distribution, muscle formation, skin texture and thickness,

voice, and perhaps even personality traits)

Oxytocin Causes muscles of the uterus and milk ducts in the breast to

contract

Prolactin Starts and maintains milk production in the ductal glands of

the breast (mammary glands)

Thyroid-stimulating hormone Stimulates the production and secretion of hormones by the

thyroid gland

Ovaries Estrogen Controls the development of female sex characteristics and

the reproductive system

Progesterone Prepares the lining of the uterus for implantation of a fertilized

egg and readies the mammary glands to secrete milk

Testes Testosterone Controls the development of male sex characteristics and the

reproductive system

This chapter also discusses the menstrual cycle in girls. Usually

this occurs in the 12 to 50 years age in women. It happensapproximately once a month in this time span apart from the


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time the female is pregnant. A woman can reproduce from thetime she starts menstruating till the time of the last menses.Menstruation is the process where a womans body preparesitself to have a child. This cycle start when the girl entersadolescence which can start anywhere between the age of 9 to

16 depending on the individual.

Several thoughts are prevalent in society regarding this process.You can list these after talking to your students especially girlsand then discuss.

There is flow of blood during menstruation which can last from 4to 5 days. These days are very painful for many women andstomach aches, back aches and pain in legs are frequentcomplaints. Some women experience mild discomfort, othermore and some other extreme pain. Some feel it just beforemenstruation starts and it stops once the bleeding starts andmany feel pain during the flow of blood.

You can discuss in the class that the uterus lining is shed duringthis time. To remove this lining from the body, the uterus needto contract and this breaking and contraction leads to pain.Menstruation can also affect mood and make a person tense.The exact reason for this is not very clear but the endocrineglands play an important role. More or less hormones aresecreted by these glands to maintain some sort of balance in the

body. Some discomfort is caused by this increase and decrease.

Menstruation does not occur in some girls. There could be manyreasons for this. The onset of the menstrual cycle is an indicatorof development of the reproductive system and depends on theendocrine glands. If there is some problem or disease in thesetwo, the girl may not experience menstruation or the cycle maystart and then stop.

Stopping of menstruation during adolescence could be due toblood deficiency or malnutrition. We need more nutritious food inthis period. There could be loss of blood during menstruationwhich has to be produced again. You can take iron tablets or ironrich food for blood deficiency. Food like dal, green leafyvegetables, amla, sprouts should be part of the diet.

Sex determination

Our attitude and views regarding menstruation, the behavior ofpeople around us, our food, health and lack of blood,malnutrition and being ill for a long time can all effect themenstrual cycle.


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Sex determination during pregnancy is one related aspect.Although, in the chapter on reproduction the students readabout the formation of sperms and eggs and by now they havesome knowledge of chromosomes also.

Recall that embryo is formed by the fertilization of the egg by asperm. Discuss in detail that the sperms and eggs have thecharacteristics of father and mother as chromosomes and theirgenes. Therefore chromosomes are called the carriers ofheredity.

You can start by telling the children that the number ofchromosome in each organism is fixed. Humans have 23 paired(or 46) chromosomes. But this number is half in sperms and ova.The chromosomes that determine the male sex are different.The chromosome for determining boys or males is Y. This meansthat the sex chromosome in males is XY and in females it is XX.So the gametes formed in reproductive organs of males are oftwo types ones that have X chromosome and others that haveY. But all female gametes or eggs are of the same types with Xchromosome. Sex is determined by the type of sperm thatfertilizes the egg. When the Y type gamete of the male fertilizesthe egg, males are produced and when the X type fertilizes girlsare born. So whether a girl is born or a boy, depends on thesperm and it is only chance that determines the type of spermthat will fertilize the egg.

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