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The Cell - Its Structure and Functions

CBSE Class 8 Science • Chapter 1 • Detailed Master Notes

Chapter Overview

This chapter dives into the fundamental unit of life—the cell. We will explore its historical discovery, the vast variety in its number, shape, and size across living organisms, and its intricate internal structure including the cell membrane, cytoplasm, nucleus, and essential organelles. Finally, we will compare plant and animal cells, and prokaryotic vs. eukaryotic cells.

1.1 Introduction and Discovery of the Cell

What is a Cell?

The cell is defined as the basic structural and functional unit of life in all living organisms. Just as bricks are assembled to make a building, cells are assembled to make the body of every living organism. All vital functions of an organism—such as respiration, digestion, and reproduction—are essentially the outcome of activities occurring within these tiny cells.

The Discovery of the Cell

Cells are microscopic, meaning they cannot be seen with the naked eye. Their discovery became possible only after the invention of magnifying devices (microscopes).

In 1665, an English scientist named Robert Hooke observed thin slices of cork (the bark of a tree) under a self-designed, primitive microscope.
He noticed partitioned boxes or compartments in the cork slice that looked exactly like a honeycomb.
Hooke coined the term "cell" for each box (the word 'cell' in Latin means 'a little room').
Important Note: The cells Robert Hooke observed were actually dead plant cells. He only saw their rigid cell walls.
Later, in 1674, Anton van Leeuwenhoek used an improved microscope to observe and describe living cells (like bacteria and protozoa) in pond water for the first time.

AI Image Prompt: A vibrant, detailed educational illustration split into two halves. On the left side, an antique 17th-century brass microscope resting on a wooden desk with a candle. On the right side, a highly magnified view of a cork slice showing empty honeycomb-like rectangular compartments with thick yellow-brown cell walls, representing Robert Hooke's discovery of dead plant cells.

Topic Practice: Discovery of Cell

Q1. Who discovered the cell in 1665, and what material did he observe?

Ans: Robert Hooke discovered the cell in 1665 by observing a thin slice of cork (tree bark) under his microscope.

Q2. Why is the cell called the 'basic structural unit' of living organisms?

Ans: Because the bodies of all living organisms are made up of cells. Just as bricks are the structural units of a building, cells act as the fundamental building blocks of life.

1.2 Variety in Cell Number, Shape, and Size

There are millions of living organisms in the world. They have different shapes and sizes, and their organs also vary in size. Consequently, the cells that make them up show remarkable variety.

1. Variation in Cell Number

Organisms can be classified based on the number of cells their bodies consist of:

Unicellular Organisms: Organisms made of a single cell. This single cell captures and digests food, respires, excretes, grows, and reproduces.
Examples: Amoeba, Paramecium, Euglena, and Bacteria.
Multicellular Organisms: Organisms made of many cells (ranging from millions to trillions). In these organisms, different groups of cells perform different specialized functions forming tissues and organs.
Examples: Humans, trees, elephants, earthworms.

Levels of Organization in Multicellular Organisms:

Cell → Tissue → Organ → Organ System → Organism

A group of similar cells performing a specific function forms a tissue. Tissues group together to form an organ (e.g., heart, leaf). Organs work together to form an organ system (e.g., digestive system), making up the complete organism.

2. Variation in Cell Shape

Cells have diverse shapes, which are directly related to the specific functions they perform.

Amoeba: Has no definite shape. It constantly changes its shape by projecting out temporary structures called pseudopodia (false feet). These help in movement and in capturing food.
White Blood Cells (WBCs): Present in human blood, WBCs are another example of a single cell that can change its shape (like Amoeba). This flexibility helps them squeeze through blood vessels to engulf and destroy disease-causing germs.
Nerve Cells (Neurons): They are very long and highly branched to receive and transmit electrical messages rapidly over long distances between the brain and other body parts.
Red Blood Cells (RBCs): Usually spherical or disc-shaped (biconcave) so they can squeeze through narrow capillaries to transport oxygen.
Muscle Cells: They are spindle-shaped (long and pointed at both ends) to easily contract and relax, causing body movements.

AI Image Prompt: A stunning, scientifically accurate 3D infographic showing the variety of cell shapes. Highlight three distinct cells on a clean white background: a highly branched blue nerve cell (neuron) with its long axon; an irregularly-shaped Amoeba with stretched pseudopodia capturing food; and several bright red, concave discs representing Red Blood Cells.

3. Variation in Cell Size

The size of cells in living organisms may range from a millionth of a metre (micrometre or micron) to a few centimetres.

The smallest cell is a bacterial cell, measuring about 0.1 to 0.5 micrometre. Mycoplasma is the smallest known cell.
The largest cell is the egg of an ostrich, measuring about 170 mm × 130 mm. It can be seen easily by the naked eye.
The longest cells in the human body are the nerve cells (neurons), which can be up to a metre long!

Important Concept regarding Size:

The size of a cell is related to its function, not to the size of the animal or plant. For example, a nerve cell in an elephant is very similar in size and shape to a nerve cell in a rat—because in both, its function is the same: transmitting messages.

Topic Practice: Cell Variety

Q1. Name a single-celled organism that has no definite shape. What are its false feet called?

Ans: Amoeba has no definite shape. Its temporary finger-like projections are called pseudopodia.

Q2. Why are nerve cells long and branched?

Ans: Nerve cells are long and branched so they can receive and transmit electrical messages rapidly over long distances between the brain and different parts of the body.

Q3. Are the cells in an elephant larger than the cells in a rat?

Ans: No. The size of the cell is related to its function, not the size of the organism. The number of cells in an elephant is vastly greater, but the individual cells (like RBCs or nerve cells) are roughly the same size.

1.3 Basic Parts of the Cell

Although cells vary widely in shape and size, almost all typical cells consist of three basic, universal components: the cell membrane, the cytoplasm, and the nucleus.

1. Cell Membrane (Plasma Membrane)

The basic, thin outer boundary that encloses the cytoplasm and nucleus.

Function: It separates cells from one another and also from the surrounding external medium. It also defines the shape of animal cells.
Selectively Permeable: It is porous and acts as a biological "gatekeeper," allowing only selected, essential materials (like oxygen, water, nutrients) to enter and exit the cell while blocking harmful substances.

The Cell Wall (In Plants Only):

In addition to the cell membrane, plant cells have an extra, thick, rigid outer covering called the Cell Wall (made mainly of cellulose).

Why do plants need it? Plants cannot move (run away or seek shelter). They need protection against extreme temperature variations, high wind speeds, and atmospheric moisture. The rigid cell wall provides this necessary protection, structural support, and shape.

2. Cytoplasm

The thick, jelly-like, semi-fluid substance present between the cell membrane and the nucleus.

It is made up mostly of water, along with various salts and proteins.
Function: It acts as the "arena" of the cell. Most of the chemical reactions necessary to keep the cell alive take place here.
Various specialised, tiny structures called cell organelles remain suspended in the cytoplasm.

3. Nucleus (The Control Centre)

It is generally a spherical structure located in the centre of the cell. It acts as the "Brain of the Cell," governing and directing all cellular activities.

The nucleus consists of the following four main parts:

Nuclear Membrane: A double-layered membrane separating the nucleus from the cytoplasm. It contains nuclear pores that allow the transfer of materials between inside the nucleus and the cytoplasm.
Nucleoplasm: The dense, jelly-like fluid inside the nucleus.
Nucleolus: A smaller, spherical, dense body visible inside the nucleus, involved in the synthesis of ribosomes.
Chromosomes: Thread-like structures present in the nucleoplasm. They contain DNA and carry Genes.
Genes: The unit of inheritance in living organisms. They control the transfer of hereditary characteristics from parents to offspring (e.g., getting brown eyes if your mother has brown eyes). Chromosomes are visible only when the cell divides.

What is Protoplasm?

The entire living substance of the cell is collectively known as protoplasm. It includes the cytoplasm, the nucleus, and all the organelles. It is considered the physical basis of life.

AI Image Prompt: A visually breathtaking, highly detailed 3D microscopic cross-section of a generalized eukaryotic animal cell on a clean light background. In the exact center is a magnificent purple spherical Nucleus cut open to reveal a dense Nucleolus and thread-like DNA chromosomes inside. The surrounding blue jelly-like cytoplasm is filled with floating, intricate organelles like orange mitochondria.

Topic Practice: Parts of the Cell

Q1. Why do plant cells need a cell wall whereas animal cells do not?

Ans: Plants cannot physically move to protect themselves from environmental changes like high wind, extreme temperature, and moisture variations. They need the extra rigid 'cell wall' outside the cell membrane for protection and structural support.

Q2. What are chromosomes? State their main function.

Ans: Chromosomes are thread-like structures present in the nucleus. Their main function is to carry genes, which are responsible for the transfer of hereditary characteristics from parents to their offspring.

1.4 Key Cell Organelles

Besides the nucleus, the cytoplasm contains many tiny, membrane-bound, specialised structures that perform specific functions. These are called cell organelles.

Mitochondria: Often called the "Powerhouse of the Cell". They carry out cellular respiration, breaking down food using oxygen to release energy for all cellular activities. They are rod-shaped or spherical.
Vacuoles: Empty-looking sac-like structures used for the storage of food, water, wastes, or other substances.
→ In Plant Cells: Usually have one exceptionally large, central vacuole taking up most of the space.
→ In Animal Cells: Vacuoles are either absent or very small and numerous.
Plastids: Large organelles found only in plant cells. They are scattered in the cytoplasm and vary in color.
→ Chloroplasts: Green-coloured plastids containing a green pigment called chlorophyll. They act as the "kitchen of the cell," trapping solar energy and making food for the plant via photosynthesis.
Endoplasmic Reticulum (ER) (Extra concept): A network of membranous tubes acting as the internal transport system and framework of the cell.
Golgi Apparatus (Extra concept): Stacked sacs involved in packaging, modifying, and distributing substances manufactured by the cell.
Ribosomes (Extra concept): Tiny dot-like structures attached to ER or floating freely, responsible for manufacturing proteins.

1.5 Prokaryotic vs. Eukaryotic Cells

Cells are broadly classified into two categories based on the organization of their nucleus:

Feature	Prokaryotic Cells	Eukaryotic Cells
Meaning	Pro = primitive, Karyon = nucleus.	Eu = true, Karyon = nucleus.
Nuclear Membrane	Absent. The nuclear material is scattered in the cytoplasm without covering.	Present. They possess a well-organized nucleus bounded by a nuclear membrane.
Cell Organelles	Lack membrane-bound organelles (like mitochondria, chloroplasts).	Possess fully developed, membrane-bound organelles performing specific functions.
Organism Name	Organisms with these cells are called Prokaryotes.	Organisms with these cells are called Eukaryotes.
Examples	Bacteria, Blue-green algae (Cyanobacteria).	All organisms other than bacteria (Amoeba, fungi, plants, animals, humans).

Topic Practice: Organelles & Classifications

Q1. Why are chloroplasts found only in plant cells?

Ans: Chloroplasts contain chlorophyll, which is essential to trap sunlight for photosynthesis. Since only plants make their own food via photosynthesis (animals heterotrophically consume it), chloroplasts are only found in plant cells.

Q2. Which cell organelle is called the powerhouse of the cell?

Ans: The Mitochondria, because they provide the cell with energy through respiration.

Q3. Differentiate between eukaryotes and prokaryotes regarding their nucleus.

Ans: Prokaryotes (e.g., bacteria) have a primitive nucleus without a nuclear membrane, leaving nuclear material scattered. Eukaryotes (e.g., humans, plants) have a true, well-organized nucleus enclosed by a distinct nuclear membrane.

1.6 Comparison: Plant Cell vs. Animal Cell

Even though plant and animal cells are both eukaryotic and share many basic components (membrane, cytoplasm, nucleus, mitochondria), analyzing them under a microscope reveals major structural differences adapted to their distinct modes of life.

Feature / Component	Plant Cell	Animal Cell
Cell Wall	Present. Thick outer layer made of cellulose for rigidity.	Absent. The outer boundary is only the flexible cell membrane.
Plastids (e.g., Chloroplasts)	Present. Needed to trap sunlight and perform photosynthesis.	Absent. Animals do not synthesize their own food.
Vacuoles	Usually have a single, very large central vacuole.	Vacuoles are either absent or very small and numerous.
Nucleus Position	Pushed slightly to one side (peripheral) due to the large central vacuole.	Usually located perfectly in the centre of the cell.
Shape	Generally possess a distinct, regular, rigid shape (often rectangular/box-like).	Generally have an irregular or rounded shape.

AI Image Prompt: An educational, highly detailed comparative 3D diagram showing a Plant Cell on the left and an Animal Cell on the right against a clean white background. Let them be cut-open cross-sections. The Plant cell is rectangular with a thick green rigid cell wall, a massive water-filled central vacuole pushing the purple nucleus to the side, and distinct green oval chloroplasts. The Animal cell is softer, rounded, without a cell wall, has its purple nucleus centered, and contains tiny vesicles/vacuoles. Clear, bold text labels indicating 'Plant Cell' and 'Animal Cell'.