Cell — Structure and Functions

You already know that everything around us can be divided into two groups - living and non-living things. Living things are different from non-living things because they perform certain basic life functions such as breathing, eating, growing, excreting waste, and reproducing.

To carry out these functions, different organs in our body perform specific jobs, for example, the heart pumps blood, the lungs help us breathe, and the stomach digests food.

But have you ever wondered what makes up these organs?
Every organ is made up of tiny building blocks called cells.

Cells are the basic structural and functional units of life. You can think of them as the bricks that build a house just like bricks come together to make a wall or building, cells come together to form tissues, organs, and ultimately, an entire living organism.

8.1 Discovery of the Cell

Robert Hooke

Cork layer in tree

Discovery by Robert Hooke (1665)

The cell was first discovered by Robert Hooke in 1665. He used a simple microscope to study thin slices of cork, which is part of the bark of a tree. When he looked through the lens, he saw many tiny box-like compartments arranged close together.

Observation

Robert Hooke noticed that these compartments looked like a honeycomb. Each small box was separated from the others by thin walls or partitions. These were the cell walls of dead plant tissue.

Naming the Cell

Hooke called each small box a “cell”, which comes from the Latin word cellula, meaning a small room. He used this word because the cork’s structure reminded him of the tiny rooms in a monastery.

Dead Cells

The cork that Hooke observed was made of dead cells, so he could only see the cell walls, not the living contents inside. Living cells were discovered later when microscopes became more advanced.

Later Developments

For the next 150 years, scientists could not learn much more about cells because microscopes were not powerful enough. With the development of improved microscopes, scientists could observe living cells and study their structure and functions in detail.

Modern Understanding

Today, scientists know that cells are the basic structural and functional units of life. Every living organism, from tiny bacteria to large animals and plants, is made up of cells.

8.2 The Cell

Every living organism is made up of tiny units called cells. Cells are the basic structural and functional units of life, just like bricks are the building blocks of a wall.

Comparison with Bricks

Bricks form buildings.
Cells form living organisms.
Buildings look different though made of the same bricks - similarly, living things look different even though all are made of cells.

Living vs Non-Living

Bricks are non-living.
Cells are living - they can grow, divide, and perform life functions.
Cells take in food, release energy, and remove waste.

Example: Hen’s Egg

The hen’s egg is a single cell.
It’s large enough to be seen with the naked eye.
Most other cells are microscopic and need a microscope to be seen.

Remember

All living beings are made up of cells.
Cells are the building blocks of life.
The cell is the basic structural and functional unit of all living organisms.

8.3 Organisms Show Variety in Cell Number, Shape, and Size

Studying Cells

Scientists study cells using microscopes, which magnify objects too small to see with the naked eye.
To observe the internal structures of cells more clearly, stains or dyes are often applied. These help highlight different parts, such as the nucleus, cytoplasm, and cell membrane.
Using these tools, researchers can understand how cells function, how they reproduce, and how they form tissues and organs.

Number of Cells

Organisms differ in the number of cells they contain.
Unicellular organisms: These are made of a single cell that performs all life functions.
- Examples include Amoeba and Paramecium.
- A single cell in these organisms can capture and digest food, respire, excrete waste, grow, and reproduce independently.
Multicellular organisms: These are made of many cells, often billions or trillions.
- Examples include humans, elephants, trees, and other large animals and plants.
- In multicellular organisms, cells are specialized to perform specific functions. Groups of similar cells form tissues, tissues combine to form organs, and organs work together in organ systems.
Every multicellular organism begins as a single fertilized egg, which divides repeatedly to produce the many cells required for growth and development.

Shape of Cells

The shape of a cell is closely related to its function.
Unicellular organisms like Amoeba have irregular shapes. Their shapes constantly change as they move or feed.
- Amoeba forms pseudopodia (“false feet”) which help it move and capture food.
In multicellular organisms, cell shapes are more specialized.
- Red blood cells are spherical, allowing them to carry oxygen efficiently.
- Muscle cells are spindle-shaped (elongated and pointed at both ends) to help muscles contract and relax.
- Nerve cells or neurons are long and branched, enabling them to receive and transmit messages across the body.
The cell membrane surrounds the contents of all cells, providing structure and shape.
Plant cells have an additional cell wall that provides rigidity and support, allowing plants to stand upright.

Size of Cells

Cells can be microscopic or visible to the naked eye, depending on the type:
- Microscopic cells: Bacterial cells can be as small as 0.1–0.5 micrometers.
- Large cells: The egg of a hen is a single cell visible without a microscope.
Size is not related to organism size:
- For example, nerve cells in elephants and rats are similar in size because they perform the same function.
- Cell size is determined by its role and function, not the overall size of the organism.

Key Ideas

Cells are the basic structural and functional units of life, just like bricks are for buildings.
There is enormous variety in cell number, shape, and size among living organisms.
Unicellular organisms perform all life processes in one cell, while multicellular organisms divide functions among specialized cells.
Understanding cell structure, shape, and function is essential for studying how organisms survive, grow, and reproduce.

8.4 Cell Structure and Function

Organ Systems and Organs: Living organisms have many organs (e.g., digestive organs) that together form organ systems (e.g., the digestive system). Each organ performs a specific function (e.g., digestion, assimilation, absorption).
Specialized Organ Functions: Both animal and plant organs have specialized functions.

Plants: Roots absorb water and minerals; leaves synthesize food (photosynthesis).

Tissues: Each organ is made up of smaller parts called tissues. A tissue is defined as a group of similar cells performing a specific function.
Cells - The Basic Unit: The text concludes that the cell is the basic structural unit in a living organism. The realization is: organ --> tissues --> cells.

8.5 Parts of the Cell

I. Basic Components of a Cell

The three basic components of a cell are the cell membrane, cytoplasm, and nucleus.

1. Cell Membrane (Plasma Membrane)

Location & Function: It is the boundary that encloses the cytoplasm and nucleus. It separates the cell from other cells and the surrounding medium.
Nature: It is porous, allowing materials and substances to move inward and outward.
Role: It gives shape to the cell.

2. Cytoplasm

Nature: A jelly-like substance present between the cell membrane and the nucleus.
Contents: It contains various other components called organelles (like mitochondria, Golgi bodies, ribosomes, etc.).

3. Nucleus

Description: Generally spherical and often located in the center of the cell. It can be easily stained and observed.
Nuclear Membrane: It is separated from the cytoplasm by a porous membrane called the nuclear membrane, which regulates the movement of materials between the nucleus and the cytoplasm.
Contents:
- Nucleolus: A smaller, spherical body inside the nucleus.
- Chromosomes: Thread-like structures that carry genes and help in inheritance (transfer of characteristics from parents to offspring). Chromosomes are visible only when the cell divides.
Gene: The unit of inheritance that controls the transfer of hereditary characteristics.
Role: It acts as the control center of the cell's activities, in addition to its role in inheritance.

II. Protoplasm

The entire content of a living cell, including the cytoplasm and the nucleus, is known as protoplasm.
Protoplasm is called the living substance of the cell.

III. Plant-Specific Structures

1. Cell Wall

Location: An outer thick covering found only in plant cells, surrounding the cell membrane.
Function: It is an additional layer required for protection. Plant cells, being stationary, need protection against variations in temperature, high wind speed, atmospheric moisture, and other environmental factors. A cell wall is absent in animal cells.

2. Vacuoles

Description: Blank-looking structures in the cytoplasm.
Plant Cells: Typically single and big (e.g., in onion cells).
Animal Cells: Usually smaller and much less prominent.

3. Plastids

Location: Small coloured bodies scattered in the cytoplasm of plant leaf cells (e.g., Tradescantia).
Types:
- Chloroplasts: Green-coloured plastids that contain the pigment chlorophyll.
- Role: Chlorophyll in chloroplasts is essential for photosynthesis.

IV. Types of Cells Based on Nucleus Structure

The structure of the nucleus allows scientists to classify cells into two major types:

Eukaryotic Cells

These cells are characterized by having a well-organized nucleus that is separated from the cytoplasm by a membrane called the nuclear membrane.

The term "eukaryotic" comes from eu (true) and karyon (nucleus), essentially meaning "true nucleus."
Examples: Onion cells and cheek cells, and all organisms other than bacteria and blue-green algae.

Prokaryotic Cells

In these cells, the nucleus is not well-organized. The genetic material (nuclear material) exists without a nuclear membrane.

The term "prokaryotic" comes from pro (primitive) and karyon (nucleus), meaning "primitive nucleus."
Examples: Bacteria and blue-green algae.

8.6 Comparison of Plant and Animal Cells

Keywords - Pronounciation and Definition

CELL - sel - The basic building block of all living things; the smallest unit that can carry out life functions.
CELL MEMBRANE / PLASMA MEMBRANE - sel MEM-brayn / PLAZ-muh MEM-brayn - The thin layer that surrounds the cell, protecting it and controlling what goes in and out.
CELL WALL - sel wawl - A rigid outer layer found in plant cells that gives shape and support.
CHLOROPLAST - KLOR-oh-plast - A part of plant cells that makes food using sunlight through photosynthesis.
CHROMOSOME - KROH-muh-sohm - Thread-like structures in the nucleus that carry genes and control inheritance.
CYTOPLASM - SY-tuh-plaz-um - Jelly-like substance inside the cell where most cell activities happen.
EUKARYOTES - yoo-KAR-ee-ohts - Organisms whose cells have a nucleus and other organelles enclosed by membranes.
GENE - jeen - A unit of inheritance; a piece of DNA that controls traits in living organisms.
MULTICELLULAR - MUL-ti-SEL-yuh-ler - Made of many cells; organisms like humans, plants, and animals are multicellular.
NUCLEAR MEMBRANE - NOO-klee-er MEM-brayn - The thin layer that surrounds the nucleus, protecting it and controlling entry and exit of materials.
NUCLEOLUS - noo-KLEE-oh-lus - A small structure inside the nucleus that makes ribosomes.
NUCLEUS - NOO-klee-us - The control center of the cell that stores DNA and regulates cell activities.
ORGAN - OR-gun - A group of tissues working together to perform a specific function, like the heart or leaf.
ORGANELLES - OR-guh-nelz - Tiny structures inside a cell that perform specific jobs, like mitochondria or chloroplasts.
PLASTID - PLAS-tid - A cell structure in plants that stores food, pigments, or helps in photosynthesis.
PROKARYOTES - pro-KAR-ee-ohts - Simple organisms whose cells do not have a nucleus or membrane-bound organelles, like bacteria.
PSEUDOPODIA - SOO-doh-POH-dee-uh - Temporary foot-like extensions of a cell used for movement or capturing food, seen in amoeba.
TISSUE - TISH-oo - A group of similar cells working together to perform a specific function.
UNICELLULAR - YOO-nih-SEL-yuh-ler - Organisms made of a single cell, like amoeba or paramecium.
VACUOLE - VAK-yoo-ohl - A sac-like structure in a cell that stores food, water, or waste.
WHITE BLOOD CELL (WBC) - wyt bluhd sel - Cells in blood that help the body fight infections and protect from diseases.

What You Have Learnt – Cell Structure and Functions

All organisms are made of smaller parts called organs.
Every living being has organs that carry out specific functions necessary for survival.
Organs are made of still smaller parts.
These smaller units are called tissues, which are groups of similar cells working together.
The smallest living part of an organism is a cell.
Cells are the fundamental building blocks of life. They perform all the essential life functions.
Cells were first observed in cork by Robert Hooke in 1665.
He noticed tiny compartments in the cork and coined the term ‘cell’ for these box-like structures.
Cells exhibit a variety of shapes and sizes.
Cells can be spherical, elongated, branched, or irregular in shape depending on their function.
Number of cells also varies from organism to organism.
Some organisms have just one cell, while others, like humans or trees, have billions or trillions of cells.
Some cells are big enough to be seen with the unaided eye.
For example, a hen’s egg represents a single, very large cell that can be observed without a microscope.
Some organisms are single-celled, while others contain a large number of cells.
Unicellular organisms, like amoeba and paramecium, perform all life functions within a single cell. Multicellular organisms, like humans, have specialized cells forming tissues and organs.
The single cell of unicellular organisms performs all the basic functions performed by a variety of cells in multicellular organisms.
It digests food, respires, excretes, grows, and reproduces independently.
The cell has three main parts:
1. Cell membrane: A thin layer that surrounds the cell, controlling the entry and exit of substances.
2. Cytoplasm: A jelly-like substance that holds the organelles and is the site of most cellular activities.
3. Nucleus: The control center of the cell that stores genetic material and regulates activities.
Nucleus is separated from cytoplasm by a nuclear membrane.
This membrane protects the nucleus and controls the movement of substances in and out.
Cells without a well-organised nucleus, i.e., lacking a nuclear membrane, are called prokaryotic cells.
Examples include bacterial cells, which are simpler in structure.
Plant cells differ from animal cells in having an additional layer around the cell membrane called the cell wall.
This layer gives shape and rigidity to plant cells.
Coloured bodies called plastids are found in plant cells only.
Green plastids containing chlorophyll are called chloroplasts, which help in photosynthesis.
Plant cells have a big central vacuole, unlike animal cells which have several small vacuoles.
The vacuole stores water, nutrients, and waste, and helps maintain cell turgor.

Textbook Exercise with Answers

1. Indicate whether the following statements are True (T) or False (F).

a) Unicellular organisms have one-celled body - True (T)

Unicellular organisms like amoeba and paramecium consist of a single cell that performs all life functions.

(b) Muscle cells are branched - False (F)

Muscle cells are generally spindle-shaped or elongated, not branched. Nerve cells, however, are branched.

The basic living unit is the cell, not the organ. Organs are made up of tissues, which in turn are made of cells.

(d) Amoeba has irregular shape - True (T)

Amoeba constantly changes its shape using pseudopodia for movement and capturing food.

2. Make a sketch of the human nerve cell. What function do nerve cells perform?.

Function: Nerve cells (neurons) transmit messages in the form of electrical signals from one part of the body to another, helping in coordination and control of body functions.

3. Write short notes on the following.

(a) Cytoplasm

Cytoplasm is a jelly-like substance inside the cell membrane that surrounds the organelles.
Most of the chemical reactions of the cell take place in the cytoplasm.
It helps in the movement of substances within the cell.

(b) Nucleus of a Cell

The nucleus is the control center of the cell.
It contains chromosomes which carry genetic information.
The nuclear membrane separates it from the cytoplasm and controls the movement of substances in and out.

4. Which part of the cell contains organelles?

Cytoplasm contains all the organelles of a cell, such as mitochondria, endoplasmic reticulum, Golgi apparatus, vacuoles, and plastids (in plants).

5. Make sketches of animal and plant cells. State three differences between them.

6. State the difference between eukaryotes and prokaryotes.

7. Where are chromosomes found in a cell? State their function.

Found in: Nucleus of eukaryotic cells; in nucleoid region of prokaryotes.

Function: Carry genes and genetic information, control heredity and cell activities.

8. ‘Cells are the basic structural units of living organisms’. Explain.

All living organisms are made of cells.
Cells are the smallest units that can perform life processes like nutrition, respiration, excretion, and reproduction.
Multicellular organisms have specialized cells that form tissues and organs for different functions.