SELF LEARNING MATERIAL

NAME-SHOBHA KUMARI

STUDENT ID-14162066

PROGRAMME -B. Ed 2014-15

BOARD-CBSE

CLASS-8

CHAPTER-CELL: STRUCTURE AND FUNCTION

CELL: STRUCTURE AND FUNCTION

To enable the learners to arouse prior knowledge of different sets of organs.

To enable the learners to classify cell: variation in number, shape and size.

To enable the learner to explore structure of a generalized cell, plant and animal

cell.

To enable to learners to devise cell division and growth.

As we already know about all living things carry out certain basic functions with

help of different sets of organs. Look at these pictures .

Cell are called structural unit of an organs. These may be compared to the bricks

of wall. As bricks are assembled to make a wall, similarly cells are assembled to

form an organism.

Let us find out more about cells.

CELLS: VARIATION IN NUMBER , SHAPE AND SIZE

The cell is the basic structural and functional unit of life; all organisms are

composed of cells. So we can say that “The smallest structural unit of an organism

is called a cell. Cells of organisms show variation in terms of their number, shape

and size. Let us discuss each of these variations in detail.

Variation in Number

Bodies of organism consist of one or many cells. Organisms whose body consists

of a single cell are called unicellular organisms. Examples of unicellular organisms

are Amoeba, Paramoecium, Euglena and Bacteria .Thus in a unicellular organisms,

a single cell performs all vital activities like feeding, movement, respiration, and

reproduction. Organisms whose body consist of many cells are called multicellular

organisms. Most plants and animals human beings are multicellular organisms

Variation in Shape

Cells exist in different shapes. They can be disc-shaped, polygonal, rectangular,

branched, or even irregular. The shape of a cell depends on the specific function it

performs. Here are a few examples that illustrate this.

Nerve cell Muscle cell

Nerve cell receives and transfers messages Muscle cell helping to movement

From brain to body thereby helping to of body.

Control and coordinate the working of

Different Parts of body.

Muscle cell

Skin cell

Variation in Size

Most cells can be seen by microscope and cannot be seen by naked eye. Cell size

may vary from a micrometer to a few centimeters. The smallest cells are bacteria,

which generally range in size from 0.1 to 0.5 micrometer. The largest cell is the

egg of an ostrich, which is 170 millimeter in diameter. Human nerve cells are

believed to be the longest cells.

Test your knowledge:

Match the following:

A B

Unicellular organisms Human being

Amoeba

Multicellular organisms Banyan Tree

Paramoecium

Bacteria

Activity:

Aim: To observe a single cell(hen egg)

Material needed: A hen’s egg, a boiling pan, water

Method: Boiled the egg and remove the shell. Cut the boiled egg into two halves.

Observations: The boiled egg has a yellow part and a white part surrounding it.

The part is called albumin while the yellow part is called yolk. In an unboiled egg,

the albumin is a jelly-like transparent liquid. The yolk looks like a thicker yellow

jelly.

STRUCTURE OF A GENERALIZED CELL

As we know, cells may differ in shape and size ,all of them have the same basic

structure, referred to as the generalized cell. A cell has both living and non- living

parts. The living parts of the cell that have a definite shape, structure and function

are called organelles.

The main parts of a generalized cell are cell membrane, cytoplasm, and nucleus.

Cell membrane: It is the outer covering of a cell. It is a porous membrane through

which selected substances can enter or leave the cell. The cell membrane is called

Plasma membrane.

Cytoplasm: It is a jelly- like fluid that fills up the part of the cell between the cell

membrane and nucleus. Several organelles are embedded in the cytoplasm.

Nucleus: It is considered to be brain of the cell.Nucleus is surrounded by a

membrane called the nuclear membrane and is filled with semi-solid substances

called nucleoplasm.Chromosomes carry genes, which transfer the characteristics

of a cell to the new cells that are formed during cell division.

Also present inside the nucleus is a round granule called nucleolus, which contains

a fibrous materials called chromatin fibres.

Here, this figure shows a generalized cell. It is important to note some of

organelles shown in this figure may be present only in plant cells and not in

animal cells and vice versa.

Structure of generalized cell

Let us learn about the cytoplasmic organelles shown in figure.

Golgi apparatus(or golgy body):It is made up of tubules(long tube-like

structures),vesicles(small vessels) and vacuoles and is responsible for the

secretion of chemical substances like enzymes,harmones, and proteins.

Vacuole: It is a fluid –filled space enclosed in a membrane. Vacuoles store excess

water, useful minerals, pigments, and many other substances.

Lysosomes: Lysosomes contain chemical substances called enzymes that are

capable of digestive cells and a variety of intra –and extra-cellular materials. In

terms of emergency, lysosomes burst and destroy the cell. Hence, they are also

called suicide bags of cell.

Endoplasmic reticulum: It is network of tubules and channels and is involved in

the synthesis, storage, and transport of cell products.

Ribosomes: These are small granular structures scattered in the cytoplasm that

act as sites of protein synthesis.

Centrosome: This structure is present only animal cells. The main function of

centrosomes is to initiate and regulate cell division.

Plastides: These are present in plant cells. Plastids contain certain pigments that

have a specific role to play in the functioning of the plant. Depending of the

pigment colour, plastids are of three kinds: chromoplasts ,chloroplasts, and

leucoplasts.

Choroplasts contain the green pigment chlorophyll, which helps plants in

making food .

Chromoplasts contain non green pigments that give colour to flower and

fruits.

Leucoplasts are colorless and store food in the form carbohydrates, fats,

and proteins.

Mitochondria(singular mitocondrian): These are tiny spherical or rod-shaped

bodies .They acts as sites of energy production and are, therefore, called the

powerhouse of the cell.

Test your knowledge:

Fill in the blanks with correct words.

1.The nucleus is filled with a semi-solid substances

called……………………(nucleoplasm/cytoplasm).

2.Chromosomes carry……………….(jeans/genes).

3………………..(Ribosomes/Lysosomes are called suicide bags of the cell.

4…………………(Chloroplasts/Chromoplasts) contain the green pigment chlorophyll.

5………………….(Vacuoles/ Mitochondria )are called the powerhouse the cell.

Let us now examine the differences between plant and animal cells.

PLANT AND ANIMALS CELLS:

While plant and animals cells have the same basic structure, there are certain

differences between them.

Structure of Plant cell

Differences between a plant cell and an animal cell:

Plant cell Animal cell

1.Cell wall is present 1. Cell wall is absent. 2. Cytoplasm is not as dense as in an animal cell.

2. Cytoplasm is dense.

3 .A large vacuole is present. 3. Vacuoles are generally absent, they are small in size.

4 . Plastids are usually present. 4. Plastids are absent. 5. Centrosome is absent. 5. Centrosome is present.

Activity :

Aim: To prepare a temporary slide of an onion peel and the cells under a

microscope.

Material needed: Onion, blade, forceps, glass slides, coverslip, glycerine, iodine

solution, dropper, blotting paper, and microscope.

Method:

1. Cut the onion into four halves.

2. Separate the flashy scales with the help of a pair forceps and carefully peel

out a piece of the onion scale.

3. Place the peeled scale on a glass slide and add a drop of dilute iodine

solution to colour the onion peel.

4. Take another glass slide and put a drop of glycerin at its centre.Place the

coloured onion peel on the glycerine drop and carefully cover it with a

coverslip. Wipe any extra glycerine on the side of coverslip using a blotting

paper.

5. View the slide under microscope.

Observation: You should be able to see the large vacuoles and thick cell walls

peculiar to plant cells.

Prokaryotic and Eukaryotic Cells:

Cells that lack a well- defined nucleus surrounded by a nuclear membrane are

called prokaryotic cells. Organisms that have such cells are prokaryotes that

means pro, primitive, nucleus. Examples of prokaryotes are bacteria and blue -

green algae.

Cells that have well- defined nucleus are called eukaryotic cells. All organisms

apart from bacteria, blue green algae, and viruses are eukaryotes.

CELL DIVISION AND GROWTH:

The food that we eat leads to an increase in the size of cells. After reaching a

certain size, a cell divides into two by a process known as cell division. Increase in

the number of cells in the body as a result of cell division is responsible for growth

in organisms .Cell division also replaces the dead or damaged cells with new ones

and is responsible for healing wounds.

Test your knowledge:

1. Define prokaryotic and eukaryotic cells?

2 . What is the function of cell division?

SUMMARY:

Cells of organisms show variation in terms of the number, shape, size of

cells.

Most cells can be viewed with the help of compound microscope.

Though cells may differ in size, all of them have the same basic structure,

referred to generalized cell structure or generalized cell.

Although plant and animal cells have the same basic structure, there are

certain differences between them.

Cell division is responsible for growth and healing of wounds in organisms.

EXERCISES:

A. Objective type questions:

I. Choose the correct option.

1.The lowest level of organization in a multicellular organism is the

(a.) cell (b) organ (c) tissue (d)organ system

2. Skin cells cover a large area and are , therefore,

(a) Elongated (b) thin and long (c) flat (d) circular

3. The outer covering of a cell is the

(a) Nuclear membrane (b) enplasmic reticulum (c) cell membrane (d) chloroplast

4. Which of these is present only in animal cells?

(a) Cell wall (b) Centrosome (c) ribosomes (d)cytoplasm

5. Which of the following is responsible for growth?

(a) Healing wounds (b) cell division (c) cell maturity (d) differences in shape

II. Write T for true statement and F for the false one. Correct the false statements.

1. A multicellular organism is made up of many cells.

2. Cells exist in different shapes.

3. Cell wall is present in the animal cell.

4. Chloroplasts give colour to flowers and fruits.

5. Bacteria and blue-green algae are examples of eukaryotes.

B. Very short answer questions:

I Write one word for the following.

1. Organisms whose body consists of a single cell. …………………………..

2. Organisms whose body consists of many cells. ……………………………..

3. Cells that have well-defined nucleus with nuclear membrane. …………………………

4. Cells that lack a well- defined nucleus. …………………………….

5 The powerhouses of the cell. ……………………………….

II. Write two examples for each of the following.

1. Kinds of plastids.

2. Form in which food is stored in leucoplasts.

3. Prokaryotes

4. Eukaryotes

5. Unicellular organisms

C. Short answer questions

1. What is cell? What variations do the cells of organisms show?

2. Differentiate between unicellular and multicellular organisms?

3. With the help of examples, show how the shape of a cell depends on the

function it performs?

4 . What are cell organelles?

5. Name the different kinds of plastids?

6. Differentiate between eukaryotic and prokaryotic cells?

7. How cell division responsible for growth?

D. Long answer questions

1. With the help of examples, describe how cell vary in terms of shape, size,

and number?

2. With the help of a well-labeled diagram describe the different parts of a

generalized cell?.

3. What are the functions of different organelles in a cell?

4. Differentiate between plant cell and animal cell?

5. Describe stepwise how you would make a slide of a onion peel and observe

it under a microscope?

References:

www.ncert.nic.in

http://www.excellup.com/ClassEight/scienceight/celeight.aspx