The Bio Brief

. How Do Scientists Study Living Cells?

Cells are very small, so most of them cannot be seen with our naked eyes.

Scientists use a microscope to observe and study cells.

 Microscope

A microscope is an instrument that magnifies very small objects, making them appear larger so that we can observe them.

 Stains or Dyes

Cells contain many tiny parts that may be difficult to see clearly.

Scientists use stains (dyes) to give colour to different parts of a cell.

This helps them:

• See the cell more clearly.
• Identify different parts of the cell.
• Study the detailed structure of the cell.

2. Variety in Living Organisms

There are millions of living organisms on Earth.

They differ from one another in:

• Shape
• Size
• Number of cells
• Structure and functions of their organs

For example:

• A tiny organism may consist of only one cell.
• A huge animal like an elephant has billions or trillions of cells.
• A tall tree also contains a very large number of cells.

So, organisms show a great variety in cell number, shape and size.

3. Number of Cells

Have you ever wondered how many cells are present in the human body, a tall tree, or an elephant?

The number can be extremely large—billions or even trillions.

Remember:

1 billion = 1,000 million

1 trillion = 1,000 billion

The human body contains trillions of cells.

These cells are not all the same. They differ in:

• Shape
• Size
• Structure
• Function

Different groups of cells perform different functions in our body.

4. Types of Organisms Based on Number of Cells

Organisms can be divided into two main groups based on the number of cells they have:

A. Unicellular organisms

B. Multicellular organisms

Let’s understand them.

A. Unicellular Organisms

The word unicellular comes from:

• Uni = one
• Cellular = cell

Therefore,

Unicellular organisms are organisms made up of only one cell.

Example:

Amoeba

Although an amoeba consists of only one cell, that single cell performs all the activities necessary for its survival.

Amoeba can:

• Capture food
• Digest food
• Respire
• Excrete wastes
• Grow
• Reproduce

Therefore, one cell is enough for an amoeba to live independently.

Important Point

A unicellular organism is a complete organism, even though it has only one cell.

B. Multicellular Organisms

The word multicellular comes from:

• Multi = many
• Cellular = cell

Therefore,

Multicellular organisms are organisms made up of more than one cell.

Examples:

• Human beings
• Elephants
• Trees
• Dogs
• Cats

Multicellular organisms contain many cells, and different groups of cells perform different functions.

5. Unicellular vs Multicellular Organisms

Unicellular	Multicellular
Made up of one cell	Made up of many cells
Example: Amoeba	Example: Human, elephant, tree
One cell performs all life processes	Different groups of cells perform different functions
Usually simple in organisation	Usually more complex in organisation

 Easy Way to Remember

UNI = ONE

MULTI = MANY

So:

Unicellular → One cell

Multicellular → Many cells

6. How Does a Multicellular Organism Begin Life?

Here is a very interesting fact!

An organism such as a human being may eventually contain trillions of cells, but it does not begin life with trillions of cells.

It begins as one single cell.

This single cell is called the fertilised egg.

Development:

Fertilised egg → Cell division → More cells → Growth and development → Multicellular organism

As the organism develops:

• The fertilised egg multiplies.
• The number of cells increases.
• Different cells become specialised for different functions.

Therefore, even an organism with billions or trillions of cells begins its life as a single cell.

7. Do Smaller Organisms Function Less Efficiently?

Not necessarily.

Smaller organisms may have fewer cells, but this does not affect their ability to perform necessary life functions.

For example, an amoeba has only one cell, but that cell can perform all the essential activities required for life.

So:

The number of cells does not decide whether an organism can perform its life functions successfully.

8. Division of Work in Multicellular Organisms

In multicellular organisms, different cells perform different jobs.

For example, in humans:

• Blood cells have specific functions.
• Muscle cells help in movement.
• Nerve cells help in receiving and transferring messages.

Groups of specialised cells form tissues.

Different tissues combine to form organs.

Simple flowchart:

Cells → Tissues → Organs → Organ systems → Organism

This is called division of labour because different cells and tissues perform different functions.

 9. Shape of Cells

Do all cells have the same shape?

No!

Cells have different shapes, depending mainly on the functions they perform.

Some cells are:

• Round
• Spherical
• Elongated
• Spindle-shaped
• Long
• Branched
• Irregular

10. Shape of Amoeba

Amoeba is a unicellular organism.

It has no definite shape.

Its shape appears irregular because it continuously changes its shape.

Amoeba has temporary projections coming out from its body.

These projections are called pseudopodia.

Pseudopodia

The word pseudopodia means:

• Pseudo = false
• Podia = feet

So, pseudopodia means “false feet.”

They:

• Help amoeba move.
• Help amoeba capture food.
• Appear and disappear.
• May have different lengths.

Therefore, amoeba keeps changing its shape as it moves or feeds.

 Remember:

Amoeba → No definite shape → Pseudopodia → Movement and feeding

11. White Blood Cells Can Also Change Shape

A white blood cell (WBC) in human blood is another example of a cell that can change its shape.

However, there is an important difference:

Amoeba

• It is a single cell.
• It is a complete organism.
• It can live independently.

White Blood Cell

• It is a cell of the human body.
• It is not an independent organism.
• It performs its specific function as part of the human body.

12. Different Cells Have Different Shapes

In multicellular organisms such as humans, different types of cells have different shapes.

Examples include:

 Blood cells

They have shapes suited to their functions.

 Muscle cells

They are generally elongated and help in movement.

 Nerve cells

They are usually long and branched.

The different shapes of cells are related to their specific functions.

 Important Rule:

The shape of a cell is usually related to the function it performs.

13. Common Shapes of Cells

Cells can have many different shapes.

1. Round or spherical cells

Some cells are approximately round or spherical.

2. Elongated cells

Some cells are longer than they are wide.

3. Spindle-shaped cells

Some cells are:

• Long
• Broad in the middle
• Pointed at both ends

Such cells are called spindle-shaped cells.

4. Branched cells

Some cells have branches.

A good example is the nerve cell or neuron.

 14. Nerve Cell or Neuron

A nerve cell is also called a neuron.

It is usually long and branched.

Function of a nerve cell:

The nerve cell:

• Receives messages.
• Transfers messages.
• Helps control different parts of the body.
• Helps coordinate the working of different parts of the body.

Easy memory:

Neuron → Receives + Transfers messages → Control + Coordination

15. What Gives Shape to a Cell?

You may wonder:

What gives a cell its shape?

The components of a cell are enclosed by a cell membrane.

Cell Membrane

The cell membrane:

• Encloses the cell.
• Separates the cell from its surroundings.
• Helps provide shape to plant and animal cells.

Therefore:

The cell membrane helps provide shape to plant and animal cells.

 16. Cell Wall in Plant Cells

Plant cells have an additional covering outside the cell membrane.

This covering is called the cell wall.

Cell wall:

• Is present outside the cell membrane.
• Gives shape to plant cells.
• Provides rigidity and strength.

Bacterial cells

Bacterial cells also have a cell wall.

Easy comparison:

Plant cell: Cell membrane + Cell wall

Animal cell: Cell membrane, but no cell wall

Bacterial cell: Cell wall present

17. Size of Cells

Just as cells have different shapes, they also have different sizes.

The size of cells can vary greatly.

Some cells may be as small as:

One-millionth of a metre

This unit is called a micrometre or micron.

It is written as:

1 micrometre = 1 μm

Some cells can be as large as a few centimetres.

18. Most Cells Are Microscopic

Most cells are microscopic.

Microscopic means:

Too small to be seen clearly with the unaided eye.

Therefore, we need a microscope to observe most cells.

Remember:

Microscopic → Cannot normally be seen with naked eyes → Microscope needed

19. Smallest Cell

The smallest cells mentioned in the chapter are found in bacteria.

Their size is approximately:

0.1 to 0.5 micrometre (μm)

So, bacterial cells can be extremely tiny.

 20. Largest Cell

The egg of an ostrich is considered the largest single cell mentioned in the chapter.

Its size is approximately:

170 mm × 130 mm

That is much larger than most other cells.

 Remember:

Smallest → Bacterial cell → 0.1–0.5 μm

Largest → Ostrich egg → 170 mm × 130 mm

21. Does the Size of an Animal Decide the Size of Its Cells?

No!

This is a very important concept.

The size of a cell is not related to the size of the animal or plant’s body.

For example:

An elephant is much larger than a rat.

But this does not mean that the cells of an elephant must be bigger than the cells of a rat.

22. Elephant and Rat Example

Consider the nerve cells of an elephant and a rat.

Even though an elephant is much larger than a rat:

• Their nerve cells can both be long and branched.
• Their nerve cells perform the same function.
• Both transfer messages.

Therefore:

The size of an organism does not determine the size of its cells.

Instead,

The size and shape of a cell are mainly related to the function it performs.

 Most Important Concepts to Remember

1. Microscope

An instrument used to magnify and observe cells.

2. Stains/Dyes

Used to colour parts of cells so that their structures can be studied clearly.

3. Unicellular organism

An organism made up of one cell.

Example: Amoeba

4. Multicellular organism

An organism made up of many cells.

Examples: Human, elephant, tree

5. Pseudopodia

Temporary projections in amoeba that help in movement and feeding.

Pseudo = false; podia = feet

6. Neuron

A nerve cell that receives and transfers messages and helps in coordination.

7. Cell membrane

A covering around the cell that helps provide shape to plant and animal cells.

8. Cell wall

An additional covering present in plant cells and bacterial cells. It provides shape and rigidity.

9. Micrometre

A very small unit used to measure cells.

1 micrometre = 1 μm

10. Smallest cell

Bacterial cell: 0.1–0.5 μm

11. Largest cell

Ostrich egg: 170 mm × 130 mm

Quick Revision Table

Feature	Important Point
Study of cells	Microscope is used
Stains	Help colour cell parts
1 billion	1,000 million
1 trillion	1,000 billion
Unicellular	Made of one cell
Example	Amoeba
Multicellular	Made of many cells
Examples	Human, elephant, tree
Beginning of multicellular organism	Fertilised egg
Amoeba’s shape	Irregular and constantly changing
Pseudopodia	False feet; help in movement and feeding
WBC	Can change its shape
Neuron	Long and branched
Neuron function	Receives and transfers messages
Cell membrane	Helps provide shape
Cell wall	Gives shape and rigidity
Cell wall found in	Plant and bacterial cells
Most cells	Microscopic
Smallest bacterial cells	0.1–0.5 μm
Largest cell	Ostrich egg
Ostrich egg size	170 mm × 130 mm
Cell size and body size	Not directly related
Cell size and shape	Related to its function