Chapter

Cell - The Basic

2 Unit of Life

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You may recall that all living organisms
O ur earth is a beautiful place where
in different types of organisms
co-exist. From minute mosses to huge
carry out certain basic functions. Can you
list those functions? Different sets of

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conifers, invisible bacteria to huge blue organs perform specific functions. Do you
whales all have a basic unit called ‘Cell’. know, what is the basic structural unit of an

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Let us study about the cell. Before the first
microscope was invented around 350 years
organ? To study about basic structures, a
proper use of microscopes, preparation of
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ago, people were not aware of the living microscopic slides and staining is
world that was not visible to the unaided essential. (You can revise the use of
eye. Thereafter many scientists have been microscope, preparation of microscopic
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observing and describing unknown world slide and staining technique from
with the help of microscopes. Annexure.)
Discovery of the cell
Do you know?
It was in the year 1665 that Robert
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A few of the many scientists

Hooke, a British scientist, observed thin
mentioned are Athanasius Kircher
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(1601–1680), Jan Swammerdam slices of cork (soft bark from Oak tree)
(1637–1680), Antonie van under a simple magnifying device which he
Leeuwenhoek (1632–1723) and Robert had made himself (Fig-1)
Hooke (1635–1702) observed different
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Oil Lamp Eyepiece

things under the microscope Water Flask
Antonie van Leeuwenhoek (1632–
1723) in 1674 was one of the earliest
Barrel
to see living bodies like bacteria, yeast,
protozoa, Red Blood Cell and the
Focusing Screw
streaming life in a drop of water. He
prepared several types of magnifying Objective Lens
glasses, and used these (lenses) to
study about both living and non living Specimen Holder
things. Fig-1 : Robert Hooke’s Microscope
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 He observed that the cork resembled The discovery of ‘cell’ by Robert Hooke
the structure of a honey comb consisting was a milestone in the history of science.
of many empty spaces or empty box like Cells of cork and of match stick are dead
cells. Can we see living cells under the
structures. He thought that the cork was
microscope? If so, how?
made up of very small cavities. Robert Will their structure be the
Hooke called these cavities as “cells”. Cell same as those of dead
is a Latin word for a little room (Fig-2). cells? With the help of the
given activities you will be

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knowing more about cells.

Activity-2

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Observing an onion peel
Peel an onion and cut out a small fleshy

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portion from the bulb [Fig-3(a)]. Break this
piece into two small parts and try to
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Fig-2 : The cells in the thin section of cork
separate them slowly [Fig-3(b)]. You will
notice a thin translucent membrane holding
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appeared like this to Robert Hooke the pieces together. Take out the membrane,
cut a small piece from it and spread it
Now let us try to see what Robert evenly in a drop of water on a slide. While
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Hooke might have observed in the cork. placing the peel on the slide, make sure that
it is not folded. Cover it with a cover slip
Activity-1 and observe it under the microscope. Draw
Observing a match stick the figure of what you have observed.
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In the place of cork, let us try to see a Compare your figure with fig-4.
similar type of structure, as seen by Robert Are there any differences between these
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Hooke, in a section of match stick. two figures? If so. What are they ?
Take a match stick and soak it for half
an hour in water and cut thin slices from it. Fig-3(a)
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Select a thin slice and place it on a slide

with the help of a brush. Put a drop of water
and cover it with a cover slip,without
allowing air bubbles and observe it under
the microscope. Draw the figure of what
you have observed. Fig-3(b)
Compare your figure with Fig-2. Do
you find both of them are similar or
different? Have you noticed the box like
structures? What are they called?
Fig-3 : Extracting the peel from an onion

16 Cell - The Basic Unit of Life

 The onion peel cells that you observed are plant cells. Do animal cells also look similar
to these cells?
Now let us observe cells from our own body (animal cell).

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Fig-4 : Onion peel cells

Activity-3 N
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Observing human cheek cells
You have already prepared a temporary slide of an onion peel. Now prepare a slide of
your own cheek cells. Wash your mouth cleanly.Take a clean wooden or plastic spoon and
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scrape the inner surface of your cheek.

Keep two things in mind. Firstly, wash the spoon thoroughly before using it. Secondly,
do not scrape too hard or else you may hurt yourself. Now take the scrapping that you have
collected, and place it in a drop of water taken on a slide. Cover the slide with a cover slip.
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Observe the slide under the microscope. Draw the figure of what you have observed. The
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cells that you see would be very similar to those shown in Fig-5. Is the outer covering of
both the types of cells similar?
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Fig-5 : Human cheek cell (without stain)

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 Do you know? Activity-4
The observations of a scientist named Observation of the Nucleus in
Robert Brown (1773–1858) have made a onion peel cells
significant contribution to our For this, you need to peel a membrane
understanding of cells. Among different from an onion once again. Now keep this
parts of a cell, the nucleus is the most well membrane on a slide and add 1-2 drops of
known part. In 1831, while observing cells the stain (saffranin, methylene blue or red
in the epidermis of Orchid leaves, Robert

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ink). Cover this with a cover slip and leave
it for about five minutes. Then add water

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drop-wise from one side of the cover slip
while removing the extra water with a filter
paper from the other side. This will help in

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washing away the extra stain. Now observe
this slide under a microscope.

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Cytoplasm

Robert Brown (1773–1858) Cell wall

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Cell
Brown noticed a circular spot that was membrane
slightly more opaque than the surrounding
areas (Fig-6). He noted that similar Fig-7 : Onion cell showing nucleus (stained)

structures were present in other cells as

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The blue or red spot observed within

well. Robert Brown claimed that this the cell is the nucleus.
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structure was an integral part of the cell and Now let us see the nucleus in our own
called it nucleus. cells (animal cells).
Cytoplasm
Acitivity-5
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Stomata Observation of the Nucleus in

Cell wall cheek cells
Cell You could also take cells from the
membrane inner layer of the cheek, stain them with
Nucleus saffranin or methylene blue and try to
Fig-6 : Plant Cell observe the nucleus in them using
In the above figure we can see stomata microscope.
and nucleus. Stomata are the pores through Now let us compare the onion and the
which the leaves exchange the gases. cheek cells.

18 Cell - The Basic Unit of Life

 several functions within the cell and you will
Nucleus study them in class IX. You shall also study
why cells are considered to be, ‘The basic
Cytoplasm
structural and functional units of the
Cell living body’.
Membrane DIVERSITY IN CELLS
In onion peel cells you have seen that
nearly all cells are similar
Fig-8 : Cheek cells showing nucleus (stained)

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in structure and shape. If
• What structures are observed in the you repeat this experiment

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cells? with peels of onions of
• Did you see a tiny dark stained body in different size, what do you
all the cells? think your observations
• Are they located in the centre in both would be? Does bigger

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the cells? onions have bigger cells?
• What is the difference between There are millions of living organisms
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boundary of onion cell and cheek cell?
The outer layer of a cheek cell is the
in nature. They have different shapes, sizes
and vary in the number of cells they contain.
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cell membrane. This gives a shape to the To know more about this, let us observe
cell and selectively allows substances to some more cells.
pass through it, in and out of the cell. About
You will observe permanent slides of
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this you will learn more in higher classes.

Amoeba, Paramoecium, Chlamydomonas
On the other hand, in the cells of the onion
etc. in the chapter on microorganisms. All
peel, the outer covering is clearer than in
these are single celled and are called
cheek cells. It is because there is another
unicellular organisms (Uni-single). In
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layer present over the cell membrane,

these, the single cell is capable of
known as the cell wall. This gives rigidity
performing all the life processes like
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and strength to the cell.

obtaining food, respiration, excretion,
In both the cells you can find a dense growth and reproduction.
round body called nucleus. In cheek cells
Living organisms having more than one
the nucleus is present more or less at the
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cell are called

centre of the cell, whereas in onion cells it
multicellular
is not in the centre, but towards periphery.
organisms. Basic
The jelly like substance between the nucleus
life processes in
and the cell membrane is called cytoplasm.
multicellular
It is a heterogeneous material. Ask your
organisms are
teacher why it is called as heterogeneous. It
carried out by
contains membrane bound structures, called
different types of
cell organelles, as well as more complex Fig-9(a) : cells.
chemicals. Cell organelles help to carry out Chlamydomonas

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 Is your observation similar to the one given
Pseudopodia
in fig-10? You may have seen different types
of cells or groups of cells in the slide?
You can do this experiment with other
Nucleus
leaves as well. It is preferable to choose
thin leaves.

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Fig-9(b) : Amoeba

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C

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(Tridax)
Fig-9(c) : Escherichia coli (Bacteria)
Fig-9 : Unicellular organisms Observe the given diagram of a section
of the stem of Tridax/Spinach. Note the
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different types of cells that you observe in

the section. Fig-11 shows 4 different
groups of cells as described below:
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Group A cells form the outermost

layer of the stem and they give shape to the
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stem as well as protection.

Major portion of stem is made up of
group B cells. In a green stem this portion
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Fig-10 : Cells in a grass blade has special organs that carry out
photosynthesis.
Group C consists of cells that join
Activity-6 together to form long structures that
Observing cells in a leaf conduct food and water in the plant body.
Take a peel of grass leaf on the slide, Group D cells are present in the centre
put a drop of water, cover it with a cover of a young stem and form a hollow structure
slip and observe it under the microscope. in the matured stem.

20 Cell - The Basic Unit of Life

 Thus in Transverse section of Tridax Activity-7
(Gaddichamanthi) /spinach stem you can
see different shapes of cells in one plant. Observe the given figures of different
Think why do the stems contain different kinds of cells in the human body. Observe
types of cells? permanent slides of these cells in your
school laboratory.
Draw the diagram of these and label the
parts that you have learnt so far and collect

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information about the functions of these
cells.

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Smooth Muscle Cell

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Striated Muscle Cell

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Nerve Cell
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Blood Cells

Red Blood Cells White Blood Cells

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Granulocytes Agranulocytes

Monocyte Lymphocyte
Basophil Neutrophil Eosinophil

Fig-12 : Shapes of the different cells in the human body

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 Fill the following table with the help of your teacher or with reference books.
S. No. Name of the Cell Shape of the Cell Parts
observed in it
1 RBC
2 Smooth Muscle Cell
3 Nerve Cell
4 Bone Cell

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5 White blood cell
6 Striated muscle cell

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• Are there any similarities in shape ultimately determined by the specific
of the cells? function of the cells. How do you define
• Do you find nuclei in all the cells? the shape of Amoeba? You may say that the

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shape appear irregular. In fact Amoeba has
• Can you name the longest cell in
no definite shape. It keeps on changing its
all animals?
shape by protruding out of its body. These
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So far you have seen many kinds of are called Pseudopodia (Pseudo: false,
cells. Are all cells similar in shape and size?
Podia: feet). The projections appear and
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The shape and size of cells vary disappear. Pseudopodia help Amoeba in
considerably but all of the cells are feeding & locomotion.
Are the cells in an elephant larger than the cells in a man?
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Fig-13
I may be very tiny
and you may be
large but without me
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no function would
go on. I am the most
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important unit of all

organisms
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Have you listened to the words of the cell? Guess how big a cell is? Is the number
and sizes of cells in both man and elephant the same? Are the cells of an elephant
bigger than that of a man?
22 Cell - The Basic Unit of Life
 The size of the cells in living organism Some of the cells can be seen with
may be as small as the millionth of a meter naked eyes. Human nerve cell is nearly
(micron) or may be as large as a few about 90 to 100 cms. long. The largest cell,
centimeters. Majority of the cells are too measuring nearly 17 cm X 18 cm, is the
small to be seen with unaided eye. They
egg of an Ostrich.
can be seen only through a microscope. The
smallest cell 0.1 to 0.5 micrometers The size of the cell is related to its
(Microns) is found in Bacteria. A human function. For example, nerve cell in both
liver and kidney cell is 20 to 30 in man and elephant are long and branched.

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micrometers in size. They perform the same function of
transferring message in both of them.

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1 Meter = 100 Centimeters (cm)
The size of the organism is depends
1 centimeter = 10 millimeters (mm)
on the number of cells and not on the size
1 millimeter = 1000 micrometers/
of the cell. Cells are of different shapes,
microns (µm)

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sizes, and number.
1 micrometer = 1000 nano meters (nm)

Key words
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Cell, Cell membrane, Cell Wall, Cytoplasm, Nucleus, Unicellular,
Multicellular, Organelles, Pseudopodia, staining, magnification, focusing.
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What we have learnt

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• All living organisms are made of cells.

• Cells were first observed by Robert Hooke in 1665.
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• The cell has 3 main parts- The cell membrane, Cytoplasm, Nucleus.
• Robert Brown discovered nucleus in orchid leaf.
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• Plant cells differ from those of animals in having an additional layer around the cell
membrane termed as cell wall.
• Cell wall gives strength and rigidity to plants.
• Cell exhibits a variety of shapes and sizes and number.
• Single celled Organisms are called unicellular organisms and those with more than one
cell are called multicellular organisms.
• Basic functions in multicellular organisms are carried out by different types of cells.

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 Improve your learning

1. Who discovered the cell for the first time?

What procedure did he follow? (AS 1)
2. Name the factors on which shape of the cells depend? (AS1)
3. Distinguish between unicellular and multi cellular organisms? (AS1)
4. How will you prepare slide without drying quickly? (AS1)

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5. Deekshith said that, “we can’t see cells with unaided eye”. Is the statement true or false?

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Explain. (AS1)
6. Correct the statement and if necessary rewrite. (AS1)
a. Cell wall is essential in plant cells.
b. Nucleus controls cell activity

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c. Unicellular organisms perform all life processes like respiration, excretion, growth,
and reproduction.
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d. To observe nucleus and organelles clearly, staining is not necessary.
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7. Describe the functions of nucleus. (AS1)
8. What is difference between cells in onion peel and cells in spinach? (AS1)
9. Label parts of diagrammes given in page 25, And identify which is plant cell and which
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is animal cell. (AS5)

10. What questions will you pose to know about diversity in cells? (AS2)
11. If you want to know about unicellular and multi cellular organisms, what questions will
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you pose? (AS 2)

12. Get some floating slime from a puddle, pick a very small amount of slime and put it on
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a slide. Separate out one fiber and look at it through the microscope. Draw the digram
of what you observed. (AS 3)
13. Collect different kinds of leaves from your surroundings and observe the shapes of the
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epidermal cells under microscope. Make a table which contains serial number, name of
the leaf, shape of the leaf, shape of the epidermal cells. Do not forget to write specific
findings below the table. (AS 4)
14. Make sketches of animal and plant cells which you observe under microscope. (AS5)
15. Ameer said “Bigger onion has larger cells when compared to the cells of smaller onions”!
Do you agree with his statement or not? Explain why? (AS 2)
16. How do you appreciate the fact that animals, human beings and trees are made of
cells,which are very small and we can look at them through microscope? (AS 6)
17. Deepak said, “A plant can’t stand erect without cell wall ”? Do you support this statement? (AS 7)
24 Cell - The Basic Unit of Life
 C
D

B A
A

C
B

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A ………………… A …………………
B …………………
B …………………
C …………………

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C ………………… D …………………

ANNEXURE N
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Major improvements were made in microscopes from around 400 years ago. In the
early days, there used to be just a single lens in the microscope. This is known as the simple
microscope. Gradually better lenses were made. The compound microscope with a
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combination of more than one lens was also invented. The first compound microscope was
Eye Piece
Draw tube
Coarse adjustment
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Body tube
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Fine adjustment
Revolving Nose Piece
Arm (Limb)
Slide
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Clip
Stage
Condenser

Inclination
Reflector
Base
Foot

Fig-13: Compound Microscope

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made in the year 1595 by the scientist named Jaquarius Janssen. Robert Hooke’s microscope
too was a compound microscope.) With the advent of compound microscopes, more detailed
observations were made possible. Let us see how we can use the microscope.
The correct way to use microscope.
Recall what you have learnt in class VI about microscope. Now let us learn correct way
to use microscope.
1. Check the microscope in the following way (a) remove the lens cap and take out the
lens clean it with soft and clean cloth. (b) If the knob is loose, change the piece of valve

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tube covering it. (c) Mirror of the microscope is always to be kept clean. Adjust it to

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light in such an angle that you see a bright back ground, while looking through the lens.
2. Usually you will find three or four objective lenses on a microscope. They are 4X,
10X, 40X and 100X powers. When coupled with a 10X (most common) eyepiece lens,
we get total magnification of 40X (4X times of 10X), 100X, 400X and 1000X.

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3. Wash the glass slide well and wipe it dry with clean cloth.

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4. You have to move lens up and down till image looks sharp, this is called focussing.
While doing so, make sure that the material doesn’t touch or water in which it is kept
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does not touches the lens, to prevent it cover the material with a cover slip (very thin
glass).
5. Put a drop of water with finger or dropper, put specimen in water, you may use needle
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brush or babul thorn. With the help of needle, cover specimen with cover slip. Dry out
excess water around cover slip with filter paper or blotting paper.
6. Fasten the slide under the clips on microscope by moving the slide sideways so that the
things you want see focus right under the lens, move the lens up and down to focus.
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Now decrease or increase the amount of light by rotating the mirror. Do this until clear
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appearance of object with clear magnification is observed.

Preparation of a microscopic slide

The study material to be viewed under a compound microscope is mounted on a slide.
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For this:
1. Microscopic slide is prepared on a 2mm thick, 3cm X 8cm rectangular strip of clear
and clean glass piece called slide.
2. If the object is thin and flat it can be directly placed on the glass slide towards the centre
in a drop of water with the help of a soft and fine brush. A drop of glycerin is added to
the water if the slide is to be kept for longer time. Glycerin saves the material from
drying (dehydration).
3. If the object is thick, cut it into a thin, nearly 0.5 mm or less thick sections with the

26 Cell - The Basic Unit of Life

 help of a sharp razor. If the object is transparent, it may be stained with iodine, saffranin,
fast green or any other suitable chemical dye, to bring contrast between the kinds of
cells in the material.
4. Place the stained object on a slide. Add a drop of water should cover with cover slip
without forming air bubbles. Remove excess of water on the slide using a blotting or
filter paper.
The Cover slip protects the lense of the microscope from coming in contact with water
and specimen. Now your slide is ready to observe.

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Cover Slip Needle

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Dropper

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Slide

Fig-14: Preparation of microscopic slide

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Staining Techniques
This technique is based on the fact that there are a few coloured substances that get
attached to different parts of a cell. This helps to highlight particular areas in the cell.
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These colouring agents are known as stains and the process is called staining. We can
use this technique to observe several things like microorganisms, different parts of the
cell, etc. For this we need to use stains like saffranin, methylene blue etc. Red ink also
works as an adequate stain. To make saffranin solution, dissolve ¼ tea-spoon of saffranin in
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100 ml. of water.

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