While examining a thin slice of cork, Robert

Hooke saw that the cork resembled the

structure of a honeycomb consisting of many

little compartments. Cork is a substance

which comes from the bark of a tree. This

was in the year 1665 when Hooke made this

chance observation through a self-designed

microscope. Robert Hooke called these boxes

cells. Cell is a Latin word for ‘a little room’.

This may seem to be a very small and

insignificant incident but it is very important

in the history of science. This was the very first

time that someone had observed that living

things appear to consist of separate units. The

use of the word ‘cell’ to describe these units is

being used till this day in biology.

Let us find out about cells.

5.1 What are Living Organisms

Made Up of?

Activity ______________ 5.1

• Let us take a small piece from an onion

bulb. With the help of a pair of forceps,

we can peel off the skin (called

epidermis) from the concave side (inner

layer) of the onion. This layer can be

put immediately in a watch-glass

containing water. This will prevent the

peel from getting folded or getting dry.

What do we do with this peel?

• Let us take a glass slide, put a drop of

water on it and transfer a small piece

of the peel from the watch glass to the

slide. Make sure that the peel is

perfectly flat on the slide. A thin camel

hair paintbrush might be necessary to

help transfer the peel. Now we put a

drop of safranin solution on this piece

followed by a cover slip. Take care to

avoid air bubbles while putting the

cover slip with the help of a mounting

needle. Ask your teacher for help. We

have prepared a temporary mount of

onion peel. We can observe this slide

under low power followed by high

powers of a compound microscope.

Fig. 5.1: Compound microscope

What do we observe as we look through

the lens? Can we draw the structures that

we are able to see through the microscope,

on an observation sheet? Does it look like

Fig. 5.2?

Eyepiece

Coarse adjustment

Fine adjustment

Arm

Objective lens

Stage

Swivel

Mirror

Base

Body tube

Clip

Microscope slide

Condenser

Fig. 5.2: Cells of an onion peel

HEHE

F F

UNDAMENTALUNDAMENTAL

UNDAMENTAL

U U

NITNIT

NIT

OFOF

L L

IFEIFE

IFE

hapter

Nucleus

Cells

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SCIENCE50

Chlamydomonas, Paramoecium and bacteria.

These organisms are called unicellular

organisms (uni = single). On the other hand,

many cells group together in a single body

and assume different functions in it to form

various body parts in multicellular organisms

(multi = many) such as some fungi, plants

and animals. Can we find out names of some

more unicellular organisms?

Every multi-cellular organism has come

from a single cell. How? Cells divide to

produce cells of their own kind. All cells thus

come from pre-existing cells.

Activity ______________ 5.2

• We can try preparing temporary

mounts of leaf peels, tip of roots of

onion or even peels of onions of different

sizes.

• After performing the above activity, let

us see what the answers to the following

questions would be:

(a) Do all cells look alike in terms of

shape and size?

(b) Do all cells look alike in structure?

cells from different parts of a

plant body?

(d) What similarities could we find?

Some organisms can also have cells of

different kinds. Look at the following picture.

It depicts some cells from the human body.

Sperm

Bone

cell

Smooth

muscle

cell

Blood

cells

Ovum

Fig. 5.3: Various cells from the human body

More to know

We can try preparing temporary mounts

of peels of onions of different sizes. What do

we observe? Do we see similar structures or

different structures?

What are these structures?

These structures look similar to each other.

Together they form a big structure like an

onion bulb! We find from this activity that

onion bulbs of different sizes have similar

small structures visible under a microscope.

The cells of the onion peel will all look the

same, regardless of the size of the onion they

came from.

These small structures that we see are

the basic building units of the onion bulb.

These structures are called cells. Not only

onions, but all organisms that we observe

around are made up of cells. However, there

are also single cells that live on their own.

Cells were first discovered by

Robert Hooke in 1665. He observed

the cells in a cork slice with the help

of a primitive microscope.

Leeuwenhoek (1674), with the

improved microscope, discovered the

free living cells in pond water for the

first time. It was Robert Brown in

1831 who discovered the nucleus in

the cell. Purkinje in 1839 coined the

term ‘protoplasm’ for the fluid

substance of the cell. The cell theory,

that all the plants and animals are

composed of cells and that the cell is

the basic unit of life, was presented

by two biologists, Schleiden (1838)

and Schwann (1839). The cell theory

was further expanded by Virchow

(1855) by suggesting that all cells

arise from pre-existing cells. With the

discovery of the electron microscope

in 1940, it was possible to observe and

understand the complex structure of

the cell and its various organelles.

The invention of magnifying lenses led to

the discovery of the microscopic world. It is

now known that a single cell may constitute

a whole organism as in Amoeba,

Nerve Cell

Fat cell

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THE FUNDAMENTAL UNIT OF LIFE 51

The shape and size of cells are related to

the specific function they perform. Some cells

like Amoeba have changing shapes. In some

cases the cell shape could be more or less

fixed and peculiar for a particular type of cell;

for example, nerve cells have a typical shape.

Each living cell has the capacity to

perform certain basic functions that are

characteristic of all living forms. How does

a living cell perform these basic functions?

We know that there is a division of labour in

multicellular organisms such as human

beings. This means that different parts of

the human body perform different functions.

The human body has a heart to pump blood,

a stomach to digest food and so on. Similarly,

division of labour is also seen within a single

cell. In fact, each such cell has got certain

specific components within it known as cell

organelles. Each kind of cell organelle

performs a special function, such as making

new material in the cell, clearing up the

waste material from the cell and so on. A

cell is able to live and perform all its

functions because of these organelles. These

organelles together constitute the basic unit

called the cell. It is interesting that all cells

are found to have the same organelles, no

matter what their function is or what

organism they are found in.

uestions

1. Who discovered cells, and how?

2. Why is the cell called the

structural and functional unit of

life?

5.2 What is a Cell Made Up of?

What is the Structural

Organisation of a Cell?

We saw above that the cell has special

components called organelles. How is a cell

organised?

If we study a cell under a microscope, we

would come across three features in almost

every cell; plasma membrane, nucleus and

cytoplasm. All activities inside the cell and

interactions of the cell with its environment

are possible due to these features. Let us see

how.

5.2.1 PLASMA MEMBRANE OR CELL

MEMBRANE

This is the outermost covering of the cell that

separates the contents of the cell from its

external environment. The plasma membrane

allows or permits the entry and exit of some

materials in and out of the cell. It also

prevents movement of some other materials.

The cell membrane, therefore, is called a

selectively permeable membrane.

How does the movement of substances

take place into the cell? How do substances

move out of the cell?

Some substances like carbon dioxide or

oxygen can move across the cell membrane

by a process called diffusion. We have studied

the process of diffusion in earlier chapters.

We saw that there is spontaneous movement

of a substance from a region of high

concentration to a region where its

concentration is low.

Something similar to this happens in cells

when, for example, some substance like CO

(which is cellular waste and requires to be

excreted out by the cell) accumulates in high

concentrations inside the cell. In the cell’s

external environment, the concentration of

is low as compared to that inside the

cell. As soon as there is a difference of

concentration of CO

inside and outside a cell,

moves out of the cell, from a region of

high concentration, to a region of low

concentration outside the cell by the process

of diffusion. Similarly, O

enters the cell by

the process of diffusion when the level or

concentration of O

inside the cell decreases.

Thus, diffusion plays an important role in

gaseous exchange between the cells as well

as the cell and its external environment.

Water also obeys the law of diffusion. The

movement of water molecules through such a

selectively permeable membrane is called osmosis.

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SCIENCE52

The movement of water across the plasma

membrane is also affected by the

amount of substance dissolved in water. Thus,

osmosis is the net diffusion of water across a

selectively permeable membrane toward a

higher solute concentration.

What will happen if we put an animal cell

or a plant cell into a solution of sugar or salt

in water?

One of the following three things could

happen:

1. If the medium surrounding the cell has

a higher water concentration than the

cell, meaning that the outside solution

is very dilute, the cell will gain water by

osmosis. Such a solution is known as

a hypotonic solution.

Water molecules are free to pass

across the cell membrane in both

directions, but more water will come

into the cell than will leave. The net

(overall) result is that water enters the

cell. The cell is likely to swell up.

2. If the medium has exactly the same

water concentration as the cell, there

will be no net movement of water across

the cell membrane. Such a solution is

known as an isotonic solution.

Water crosses the cell membrane in

both directions, but the amount going

in is the same as the amount going out,

so there is no overall movement of

water. The cell will stay the same size.

3. If the medium has a lower

concentration of water than the cell,

meaning that it is a very concentrated

solution, the cell will lose water by

osmosis. Such a solution is known as

a hypertonic solution.

Again, water crosses the cell

membrane in both directions, but this

time more water leaves the cell than

enters it. Therefore the cell will shrink.

Thus, osmosis is a special case of diffusion

through a selectively permeable membrane.

Now let us try out the following activity:

Activity ______________ 5.3

Osmosis with an egg

(a) Remove the shell of an egg by dissolving

it in dilute hydrochloric acid. The shell

is mostly calcium carbonate. A thin

outer skin now encloses the egg. Put

the egg in pure water and observe after

5 minutes. What do we observe?

The egg swells because water passes

into it by osmosis.

(b) Place a similar de-shelled egg in a

concentrated salt solution and observe

for 5 minutes. The egg shrinks. Why?

Water passes out of the egg solution

into the salt solution because the salt

solution is more concentrated.

We can also try a similar activity with dried

raisins or apricots.

Activity ______________ 5.4

• Put dried raisins or apricots in plain

water and leave them for some time.

Then place them into a concentrated

solution of sugar or salt. You will

observe the following:

(a) Each gains water and swells

when placed in water.

(b) However, when placed in the

concentrated solution it loses

water, and consequently shrinks.

Unicellular freshwater organisms and

most plant cells tend to gain water through

osmosis. Absorption of water by plant roots

is also an example of osmosis.

Thus, diffusion is important in exhange

of gases and water in the life of a cell. In

additions to this, the cell also obtains

nutrition from its environment. Different

molecules move in and out of the cell

through a type of transport requiring use

of energy.

The plasma membrane is flexible and is

made up of organic molecules called lipids

and proteins. However, we can observe the

structure of the plasma membrane only

through an electron microscope.

The flexibility of the cell membrane also

enables the cell to engulf in food and other

material from its external environment. Such

processes are known as endocytosis. Amoeba

acquires its food through such processes.

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THE FUNDAMENTAL UNIT OF LIFE 53

Activity ______________ 5.5

• Find out about electron microscopes

from resources in the school library or

through the internet. Discuss it with

your teacher.

uestions

1. How do substances like CO

and

water move in and out of the cell?

Discuss.

2. Why is the plasma membrane

called a selectively permeable

membrane?

5.2.2 CELL WALL

Plant cells, in addition to the plasma

membrane, have another rigid outer covering

called the cell wall. The cell wall lies outside

the plasma membrane. The plant cell wall is

mainly composed of cellulose. Cellulose is a

complex substance and provides structural

strength to plants.

When a living plant cell loses water

through osmosis there is shrinkage or

contraction of the contents of the cell away

from the cell wall. This phenomenon is known

as plasmolysis. We can observe this

phenomenon by performing the following

activity:

Activity ______________ 5.6

• Mount the peel of a Rhoeo leaf in water

on a slide and examine cells under

the high power of a microscope. Note

the small green granules, called

chloroplasts. They contain a green

substance called chlorophyll. Put a

strong solution of sugar or salt on the

mounted leaf on the slide. Wait for a

minute and observe under a

microscope. What do we see?

• Now place some Rhoeo leaves in boiling

water for a few minutes. This kills the

cells. Then mount one leaf on a slide

and observe it under a microscope. Put

a strong solution of sugar or salt on

the mounted leaf on the slide. Wait for

a minute and observe it again. What

do we find? Did plasmolysis occur now?

What do we infer from this activity? It

appears that only living cells, and not dead

cells, are able to absorb water by osmosis.

Cell walls permit the cells of plants, fungi

and bacteria to withstand very dilute

(hypotonic) external media without bursting.

In such media the cells tend to take up water

by osmosis. The cell swells, building up

pressure against the cell wall. The wall exerts

an equal pressure against the swollen cell.

Because of their walls, such cells can

withstand much greater changes in the

surrounding medium than animal cells.

5.2.3 NUCLEUS

Remember the temporary mount of onion peel

we prepared? We had put iodine solution on

the peel. Why? What would we see if we tried

observing the peel without putting the iodine

solution? Try it and see what the difference

is. Further, when we put iodine solution on

the peel, did each cell get evenly coloured?

According to their chemical composition

different regions of cells get coloured

differentially. Some regions appear darker

than other regions. Apart from iodine solution

we could also use safranin solution or

methylene blue solution to stain the cells.

We have observed cells from an onion; let

us now observe cells from our own body.

Activity ______________ 5.7

• Let us take a glass slide with a drop of

water on it. Using an ice-cream spoon

gently scrape the inside surface of the

cheek. Does any material get stuck on

the spoon? With the help of a needle

we can transfer this material and

spread it evenly on the glass slide kept

ready for this. To colour the material

we can put a drop of methylene blue

solution on it. Now the material is ready

for observation under microscope. Do

not forget to put a cover-slip on it!

• What do we observe? What is the

shape of the cells we see? Draw it on

the observation sheet.

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SCIENCE54

• Was there a darkly coloured, spherical

or oval, dot-like structure near the

centre of each cell? This structure is

called nucleus. Were there similar

structures in onion peel cells?

The nucleus has a double layered covering

called nuclear membrane. The nuclear

membrane has pores which allow the transfer

of material from inside the nucleus to its

outside, that is, to the cytoplasm (which we

will talk about in section 5.2.4).

The nucleus contains chromosomes,

which are visible as rod-shaped structures

only when the cell is about to divide.

Chromosomes contain information for

inheritance of characters from parents to next

generation in the form of DNA (Deoxyribo

Nucleic Acid) molecules. Chromosomes are

composed of DNA and protein. DNA molecules

contain the information necessary for

constructing and organising cells. Functional

segments of DNA are called genes. In a cell

which is not dividing, this DNA is present as

part of chromatin material. Chromatin

material is visible as entangled mass of thread

like structures. Whenever the cell is about to

divide, the chromatin material gets organised

into chromosomes.

The nucleus plays a central role in cellular

reproduction, the process by which a single

cell divides and forms two new cells. It also

plays a crucial part, along with the

environment, in determining the way the cell

will develop and what form it will exhibit at

maturity, by directing the chemical activities

of the cell.

In some organisms like bacteria, the

nuclear region of the cell may be poorly

defined due to the absence of a nuclear

membrane. Such an undefined nuclear region

containing only nucleic acids is called a

nucleoid. Such organisms, whose cells lack

a nuclear membrane, are called pr

okaryotes

(Pro = primitive or primary; karyote ≈ karyon

= nucleus). Organisms with cells having a

nuclear membrane are called eukaryotes.

Prokaryotic cells (see Fig. 5.4) also lack

most of the other cytoplasmic organelles

present in eukaryotic cells. Many of the

functions of such organelles are also

performed by poorly organised parts of the

cytoplasm (see section 5.2.4). The chlorophyll

in photosynthetic prokaryotic bacteria is

associated with membranous vesicles (bag

like structures) but not with plastids as in

eukaryotic cells (see section 5.2.5).

Ribosomes

Plasma

membrane

Cell wall

Nucleoid

Fig. 5.4: Prokaryotic cell

5.2.4 CYTOPLASM

When we look at the temporary mounts of

onion peel as well as human cheek cells, we

can see a large region of each cell enclosed

by the cell membrane. This region takes up

very little stain. It is called the cytoplasm.

The cytoplasm is the fluid content inside the

plasma membrane. It also contains many

specialised cell organelles. Each of these

organelles performs a specific function for the

cell.

Cell organelles are enclosed by

membranes. In prokaryotes, beside the

absence of a defined nuclear region, the

membrane-bound cell organelles are also

absent. On the other hand, the eukaryotic

cells have nuclear membrane as well as

membrane-enclosed organelles.

The significance of membranes can be

illustrated with the example of viruses.

Viruses lack any membranes and hence do

not show characteristics of life until they enter

a living body and use its cell machinery to

multiply.

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THE FUNDAMENTAL UNIT OF LIFE 55

uestion

1. Fill in the gaps in the following

table illustrating differences

between prokaryotic and

eukaryotic cells.

Prokaryotic Cell Eukaryotic Cell

1. Size : generally 1. Size: generally

small ( 1-10 µm) large ( 5-100 µm)

1 µm = 10

–6

2. Nuclear region: 2. Nuclear region:

_______________ well defined and

_______________ surrounded by a

and known as__ nuclear membrane

3. Chromosome: 3. More than one

single chromosome

4. Membrane-bound 4. _______________

cell organelles _______________

absent _______________

5.2.5 CELL ORGANELLES

Every cell has a membrane around it to keep

its own contents separate from the external

environment. Large and complex cells,

including cells from multicellular organisms,

need a lot of chemical activities to support

their complicated structure and function. To

keep these activities of different kinds

separate from each other, these cells use

membrane-bound little structures (or

‘organelles’) within themselves. This is one of

the features of the eukaryotic cells that

distinguish them from prokaryotic cells. Some

of these organelles are visible only with an

electron microscope.

We have talked about the nucleus in a

previous section. Some important examples

of cell organelles which we will discuss now

are: endoplasmic reticulum, Golgi apparatus,

lysosomes, mitochondria and plastids. They

are important because they carry out some

very crucial functions in cells.

5.2.5 (i) ENDOPLASMIC RETICULUM (ER)

The endoplasmic reticulum (ER) is a large

network of membrane-bound tubes and

sheets. It looks like long tubules or round or

oblong bags (vesicles). The ER membrane is

similar in structure to the plasma membrane.

There are two types of ER– rough endoplasmic

reticulum (RER) and smooth endoplasmic

reticulum (SER). RER looks rough under a

microscope because it has particles called

ribosomes attached to its surface. The

ribosomes, which are present in all active

cells, are the sites of protein manufacture.

The manufactured proteins are then sent to

various places in the cell depending on need,

using the ER. The SER helps in the

manufacture of fat molecules, or lipids,

important for cell function. Some of these

proteins and lipids help in building the cell

membrane. This process is known as

membrane biogenesis. Some other proteins

and lipids function as enzymes and

hormones. Although the ER varies greatly in

appearance in different cells, it always forms a

network system.

Fig. 5.5: Animal cell

Thus, one function of the ER is to serve as

channels for the transport of materials

(especially proteins) between various regions

of the cytoplasm or between the cytoplasm

and the nucleus. The ER also functions as a

cytoplasmic framework providing a surface

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SCIENCE56

Camillo Golgi was born at

Corteno near Brescia in

1843. He studied

medicine at the University

of Pavia. After graduating

in 1865, he continued to

work in Pavia at the

Hospital of St. Matteo. At

that time most of his

investigations were

concerned with the nervous system, In 1872

he accepted the post of Chief Medical Officer

at the Hospital for the Chronically Sick at

Abbiategrasso. He first started his

investigations into the nervous system in a

little kitchen of this hospital, which he had

converted into a laboratory. However, the

work of greatest importance, which Golgi

carried out was a revolutionary method of

staining individual nerve and cell structures.

This method is referred to as the ‘black

reaction’. This method uses a weak solution

of silver nitrate and is particularly valuable

in tracing the processes and most delicate

ramifications of cells. All through his life,

he continued to work on these lines,

modifying and improving this technique.

Golgi received the highest honours and

awards in recognition of his work. He shared

the Nobel prize in 1906 with Santiago

Ramony Cajal for their work on the structure

of the nervous system.

5.2.5 (iii) LYSOSOMES

Structurally, lysosomes are membrane-bound

sacs filled with digestive enzymes. These

enzymes are made by RER. Lysosomes are a

kind of waste disposal system of the cell. These

help to keep the cell clean by digesting any

foreign material as well as worn-out cell

organelles. Foreign materials entering the cell,

such as bacteria or food, as well as old

organelles end up in the lysosomes, which

break complex substances into simpler

substances. Lysosomes are able to do this

because they contain powerful digestive

enzymes capable of breaking down all organic

material. During the disturbance in cellular

metabolism, for example, when the cell gets

Fig. 5.6: Plant cell

for some of the biochemical activities of the

cell. In the liver cells of the group of animals

called vertebrates (see Chapter 7), SER plays

a crucial role in detoxifying many poisons

and drugs.

5.2.5 (ii) GOLGI APPARATUS

The Golgi apparatus, first described by

Camillo Golgi, consists of a system of

membrane-bound vesicles (flattened sacs)

arranged approximately parallel to each other

in stacks called cisterns. These membranes

often have connections with the membranes

of ER and therefore constitute another portion

of a complex cellular membrane system.

The material synthesised near the ER is

packaged and dispatched to various targets

inside and outside the cell through the Golgi

apparatus. Its functions include the storage,

modification and packaging of products in

vesicles. In some cases, complex sugars may

be made from simple sugars in the Golgi

apparatus. The Golgi apparatus is also

involved in the formation of lysosomes [see

5.2.5 (iii)].

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THE FUNDAMENTAL UNIT OF LIFE 57

damaged, lysosomes may burst and the

enzymes digest their own cell. Therefore,

lysosomes are also known as the ‘suicide bags’

of a cell.

5.2.5 (iv) MITOCHONDRIA

Mitochondria are known as the powerhouses

of the cell. Mitochondria have two membrane

coverings. The outer membrane is porous

while the inner membrane is deeply folded.

These folds increase surface area for ATP-

generating chemical reactions. The energy

required for various chemical activities needed

for life is released by mitochondria in the form

of ATP (Adenosine triphopshate) molecules.

ATP is known as the energy currency of the

cell. The body uses energy stored in ATP for

making new chemical compounds and for

mechanical work.

Mitochondria are strange organelles in the

sense that they have their own DNA and

ribosomes. Therefore, mitochondria are able

to make some of their own proteins.

5.2.5 (V) PLASTIDS

Plastids are present only in plant cells. There

are two types of plastids – chromoplasts

(coloured plastids) and leucoplasts (white or

colourless plastids). Chromoplasts containing

the pigment chlorophyll are known as

chloroplasts. Chloroplasts are important for

photosynthesis in plants. Chloroplasts also

contain various yellow or orange pigments in

addition to chlorophyll. Leucoplasts are

primarily organelles in which materials such

as starch, oils and protein granules are stored.

The internal organisation of the Chloroplast

consists of numerous membrane layers

embedded in a material called the stroma. These

are similar to mitochondria in external

structure. Like the mitochondria, plastids also

have their own DNA and ribosomes.

5.2.5 (vi) VACUOLES

Vacuoles are storage sacs for solid or liquid

contents. Vacuoles are small sized in animal

cells while plant cells have very large vacuoles.

The central vacuole of some plant cells may

occupy 50-90% of the cell volume.

In plant cells vacuoles are full of cell sap

and provide turgidity and rigidity to the cell.

Many substances of importance in the life of

the plant cell are stored in vacuoles. These

include amino acids, sugars, various organic

acids and some proteins. In single-celled

organisms like Amoeba, the food vacuole

contains the food items that the Amoeba has

consumed. In some unicellular organisms,

specialised vacuoles also play important roles

in expelling excess water and some wastes

from the cell.

uestions

1. Can you name the two organelles

we have studied that contain

their own genetic material?

2. If the organisation of a cell is

destroyed due to some physical

or chemical influence, what will

happen?

3. Why are lysosomes known as

suicide bags?

4. Where are proteins synthesised

inside the cell?

Each cell thus acquires its structure and

ability to function because of the organisation

of its membrane and organelles in specific

ways. The cell thus has a basic structural

organisation. This helps the cells to perform

functions like respiration, obtaining nutrition,

and clearing of waste material, or forming

new proteins.

Thus, the cell is the fundamental structural

unit of living organisms. It is also the basic

functional unit of life.

Cell Division

New cells are formed in organisms in order to

grow, to replace old, dead and injured cells,

and to form gametes required for

reproduction. The process by which new cells

are made is called cell division. There are two

main types of cell division: mitosis

and meiosis.

The process of cell division by which most

of the cells divide for growth is called mitosis.

In this process, each cell called mother cell

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SCIENCE58

What

you have

learnt

• The fundamental organisational unit of life is the cell.

• Cells are enclosed by a plasma membrane composed of lipids

and proteins.

• The cell membrane is an active part of the cell. It regulates

the movement of materials between the ordered interior of

the cell and the outer environment.

• In plant cells, a cell wall composed mainly of cellulose is

located outside the cell membrane.

• The presence of the cell wall enables the cells of plants,

fungi and bacteria to exist in hypotonic media without

bursting.

• The nucleus in eukaryotes is separated from the cytoplasm

by double-layered membrane and it directs the life processes

of the cell.

• The ER functions both as a passageway for intracellular

transport and as a manufacturing surface.

• The Golgi apparatus consists of stacks of membrane-bound

vesicles that function in the storage, modification and

packaging of substances manufactured in the cell.

• Most plant cells have large membranous organelles called

plastids, which are of two types—chromoplasts and

leucoplasts.

Fig. 5.7: Mitosis Fig. 5.8: Meiosis

divides to form two identical daughter cells

(Fig. 5.7). The daughter cells have the same

number of chromosomes as mother cell. It

helps in growth and repair of tissues

in organisms.

Specific cells of reproductive organs or tissues

in animals and plants divide to form gametes,

which after fertilisation give rise to offspring.

They divide by a different process called

meiosis which involves two consecutive

divisions. When a cell divides by meiosis it

produces four new cells instead of just two (Fig.

5.8). The new cells only have half the number

of chromosomes than that of the mother cells.

Can you think as to why the chromosome

number has reduced to half in daughter cells?

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THE FUNDAMENTAL UNIT OF LIFE 59

• Chromoplasts that contain chlorophyll are called chloroplasts

and they perform photosynthesis.

• The primary function of leucoplasts is storage.

• Most mature plant cells have a large central vacuole that

helps to maintain the turgidity of the cell and stores

important substances including wastes.

• Prokaryotic cells have no membrane-bound organelles, their

chromosomes are composed of only nucleic acid, and they

have only very small ribosomes as organelles.

• Cells in organisms divide for growth of body, for repalcing

dead cells, and for forming gametes for reproduction.

Exercises

1. Make a comparison and write down ways in which plant

cells are different from animal cells.

2. How is a prokaryotic cell different from a eukaryotic cell?

3. What would happen if the plasma membrane ruptures or

breaks down?

4. What would happen to the life of a cell if there was no Golgi

apparatus?

5. Which organelle is known as the powerhouse of the cell? Why?

6. Where do the lipids and proteins constituting the cell

membrane get synthesised?

7. How does an Amoeba obtain its food?

8. What is osmosis?

9. Carry out the following osmosis experiment:

Take four peeled potato halves and scoos each one out to

make potato cups. One of these potato cups should be

made from a boiled potato. Put each potato cup in a trough

containing water. Now,

•

(a) Keep cup A empty

(b) Put one teaspoon sugar in cup B

(d) Put one teaspoon sugar in the boiled potato cup D.

Keep these for two hours. Then observe the four potato

cups and answer the following:

(i) Explain why water gathers in the hollowed portion of

B and C.

(ii) Why is potato A necessary for this experiment?

(iii) Explain why water does not gather in the hollowed

out portions of A and D.

10. Which type of cell division is required for growth and repair

of body and which type is involved in formation of gametes?

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From the last chapter, we recall that all living

organisms are made of cells. In unicellular

organisms, a single cell performs all basic

functions. For example, in Amoeba, a single

cell carries out movement, intake of food,

gaseous exchange and excretion. But in multi-

cellular organisms there are millions of cells.

Most of these cells are specialised to carry out

specific functions. Each specialised function

is taken up by a different group of cells. Since

these cells carry out only a particular function,

they do it very efficiently. In human beings,

muscle cells contract and relax to cause

movement, nerve cells carry messages, blood

flows to transport oxygen, food, hormones and

waste material and so on. In plants, vascular

tissues conduct food and water from one part

of the plant to other parts. So, multi-cellular

organisms show division of labour. Cells

specialising in one function are often grouped

together in the body. This means that a

particular function is carried out by a cluster

of cells at a definite place in the body. This

cluster of cells, called a tissue, is arranged and

designed so as to give the highest possible

efficiency of function. Blood, phloem and

muscle are all examples of tissues.

A group of cells that are similar in structure

and/or work together to achieve a particular

function forms a tissue.

6.1 Are Plants and Animals Made

of Same Types of Tissues?

Let us compare their structure and functions.

Do plants and animals have the same

structure? Do they both perform similar

functions?

There are noticeable differences between

the two. Plants are stationary or fixed – they

don’t move. Since they have to be upright, they

have a large quantity of supportive tissue. The

supportive tissue generally has dead cells.

Animals on the other hand move around

in search of food, mates and shelter. They

consume more energy as compared to plants.

Most of the tissues they contain are living.

Another difference between animals and

plants is in the pattern of growth. The growth

in plants is limited to certain regions, while this

is not so in animals. There are some tissues in

plants that divide throughout their life. These

tissues are localised in certain regions. Based

on the dividing capacity of the tissues, various

plant tissues can be classified as growing or

meristematic tissue and permanent tissue. Cell

growth in animals is more uniform. So, there

is no such demarcation of dividing and non-

dividing regions in animals.

The structural organisation of organs and

organ systems is far more specialised and

localised in complex animals than even in very

complex plants. This fundamental difference

reflects the different modes of life pursued by

these two major groups of organisms,

particularly in their different feeding methods.

Also, they are differently adapted for a

sedentary existence on one hand (plants) and

active locomotion on the other (animals),

contributing to this difference in organ system

design.

It is with reference to these complex animal

and plant bodies that we will now talk about

the concept of tissues in some detail.

ISSUESISSUES

ISSUES

hapter

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