CBSE Class 9 Science Revision Notes Chapter 2: Cell: The Building Block of Life

The cell is the basic structural and functional unit of every living organism studied in CBSE Class 9 Science. Chapter 2 explains how cells are observed, organised, supplied with materials and multiplied through cell division.

All living organisms consist of cells. Bacteria and yeast may have one cell, while plants and animals contain millions of cells. Similar cells form tissues, tissues form organs and organs work together as organ systems.

Use these CBSE Class 9 Science Revision Notes Chapter 2 for the 2026–27 session. Revise microscopes, cell boundaries, organelles, mitosis, meiosis and Cell Theory through compact explanations and tables.

Key Takeaways

• Cell: It is the basic structural and functional unit of life.
• Cell membrane: It is selectively permeable and controls the movement of substances.
• Eukaryotic cells: They contain a well-defined nucleus and membrane-bound organelles.
• Cell division: Mitosis forms two identical cells, while meiosis forms four gametes.

Access Class 9 Science Chapter 2 Cell: The Building Block of Life Notes in 30 Minutes

Revise the chapter in three parts:

• First 10 minutes: Cell as the basic unit, microscopes, cell membrane, diffusion and osmosis
• Next 10 minutes: Cell wall, prokaryotic and eukaryotic cells, nucleus and major cell organelles
• Final 10 minutes: Plant and animal cell differences, mitosis, meiosis and Cell Theory

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Cell: The Basic Unit of Life

A cell is the smallest unit capable of carrying out the basic functions of life. It interacts with its surroundings, uses energy and produces new cells.

Organisms may be unicellular or multicellular.

• Unicellular organisms: They consist of one cell. Bacteria and yeast are examples.
• Multicellular organisms: They consist of many cells. Plants, fish, birds and humans are examples.

From Cells to Tissues and Organ Systems

The levels of organisation in a multicellular organism are:

Cells → Tissues → Organs → Organ systems → Organism

A group of similar cells performing similar functions forms a tissue. Different tissues form an organ.

Several organs work together in an organ system. The nasal pores, nasal cavity, trachea and lungs form parts of the respiratory system.

How Cells Are Studied Using Microscopes

Most cells are too small to be observed with the unaided eye. Microscopes magnify cells and reveal their structures.

Robert Hooke first observed cells in 1665. He used a self-designed microscope to examine a thin slice of cork and saw small box-like compartments.

Limit of Resolution

The limit of resolution is the minimum distance at which two nearby points appear separate.

The limit of resolution of the human eye is about 0.1 mm when an object is viewed from nearly 25 cm.

Objects smaller than this limit require microscopes.

Light and Electron Microscopes

A light microscope uses visible light and lenses. School microscopes usually contain an eyepiece and objective lenses.

An electron microscope uses a beam of electrons. It reveals much finer cell structures at the nanometre scale.

The three important features of microscopes are:

• Magnification: How much larger the object appears
• Resolution: The clarity with which nearby details appear separate
• Contrast: The difference in brightness between different parts

Estimating Cell Size and Magnification

The approximate size of a cell can be calculated using the microscope’s field of view.

Estimated cell size = Diameter of field of view ÷ Number of cells across the diameter

Unit conversion:

1 mm = 1000 µm

Suppose the field diameter is 5 mm.

5 mm = 5000 µm

If 25 cells fit across the diameter:

Estimated cell size = 5000 µm ÷ 25
Estimated cell size = 200 µm

Total microscope magnification depends on the eyepiece and objective lens.

Total magnification = Eyepiece magnification × Objective magnification

For a 10X eyepiece and 10X objective:

Total magnification = 10 × 10 = 100X

Cell Membrane, Diffusion and Osmosis

The cell membrane forms the boundary between the cell and its surroundings. It allows the cell to exchange materials with the external environment.

Structure and Functions of the Cell Membrane

The cell membrane is also called the plasma membrane. It is thin and protects the contents of the cell.

It is selectively permeable. This means it permits certain substances to cross while restricting others.

The membrane is about 7 to 10 nm thick. It mainly contains lipids and proteins.

The fluid-mosaic model describes its structure:

• It has a lipid bilayer.
• Proteins remain embedded in the lipid layers.
• Molecules can move sideways, rotate or flip.
• Membrane proteins help substances cross the membrane.
• Its arrangement resembles tiles in a mosaic.

Diffusion and Osmosis

Diffusion is the net movement of particles from higher concentration to lower concentration.

It can occur without a membrane. The spreading of fragrance through air is an example.

Osmosis is the movement of water across a selectively permeable membrane.

Water moves from an area containing more water and less solute towards an area containing less water and more solute.

Plant roots absorb water from the soil through osmosis.

Isotonic, Hypotonic and Hypertonic Solutions

A potato piece placed in plain water swells because water enters its cells.

A potato piece placed in concentrated salt or sugar solution shrinks because water moves out.

Cell Wall and Its Role in Plant Cells

Plant cells have a cell wall outside the cell membrane. Fungi and bacteria also have cell walls.

The plant cell wall is mainly made of cellulose. It is rigid but permeable.

Its major functions are:

• Protecting the cell
• Maintaining cell shape
• Providing structural support
• Helping plants remain upright
• Allowing water and some minerals to pass through

When a plant cell loses water in concentrated sugar solution, its inner contents shrink. The cell membrane pulls away from the cell wall.

The rigid cell wall still maintains the outer shape of the cell.

Animal cells lack a cell wall. Therefore, they shrink considerably when they lose water.

Cell Membrane and Cell Wall

Prokaryotic and Eukaryotic Cells

Cells are classified according to the organisation of their nucleus and organelles.

A prokaryotic cell lacks a well-defined nucleus. Its genetic material lies in a region called the nucleoid.

A eukaryotic cell contains a well-defined nucleus. It also has several membrane-bound organelles.

The cytoplasm of eukaryotic cells also contains a network called the cytoskeleton. It supports cell shape, movement and internal transport.

Cell Organelles and Their Functions

Most cells have a cell membrane, cytoplasm and genetic material. Eukaryotic cells also contain specialised structures called organelles.

Each organelle performs a particular function.

Nucleus, Chromatin, Chromosomes and Genes

The nucleus controls cell activities and stores genetic information.

It has a double-layered nuclear membrane. Nuclear pores allow materials to move between the nucleus and cytoplasm.

The nucleolus is a dense structure inside the nucleus. It helps form ribosomal subunits.

The nucleus contains chromatin material. Before cell division, chromatin becomes organised into chromosomes.

Chromosomes contain DNA and associated proteins. Functional sections of DNA are called genes.

The sequence is:

Cell → Nucleus → Chromosome → DNA → Gene

Prokaryotic cells lack a membrane-bound nucleus. Their genetic material remains in the nucleoid.

Ribosomes and Endoplasmic Reticulum

Ribosomes are the sites of protein synthesis.

They may remain free in the cytoplasm or attached to the rough endoplasmic reticulum.

The endoplasmic reticulum, or ER, forms a network inside the cell. It is continuous with the outer nuclear membrane.

There are two types of ER:

Golgi Apparatus and Lysosomes

The Golgi apparatus consists of flattened, sac-like structures.

It receives substances from the ER. It modifies, sorts and packages proteins or lipids into vesicles.

These materials may be transported, secreted or used to form lysosomes.

Lysosomes are membrane-bound sacs containing digestive enzymes. They break down:

• Unwanted proteins
• Carbohydrates
• Fats
• Foreign materials
• Damaged cell parts

The useful products may return to the cytoplasm for reuse.

Mitochondria and ATP

Mitochondria supply energy for cell activities. Therefore, they are called the powerhouses of the cell.

Each mitochondrion has two membranes:

• A smooth outer membrane
• A folded inner membrane

The folds of the inner membrane are called cristae. They increase the surface area available for chemical reactions.

Cellular respiration occurs in mitochondria. It releases energy from glucose and other molecules.

The released energy is stored as ATP.

ATP stands for Adenosine Triphosphate. It acts as the energy currency of the cell.

Mitochondria also contain their own DNA and ribosomes.

Plastids in Plant Cells

Plastids occur in plant cells. They help in food synthesis, storage and colour formation.

There are three important types:

Chloroplasts are double-membrane-bound organelles. Their internal fluid is called the stroma.

Chlorophyll-containing membrane structures absorb sunlight during photosynthesis.

Like mitochondria, plastids contain their own DNA and ribosomes.

Vacuoles and Cell Support

A mature plant cell usually has one large central vacuole. Its membrane is selectively permeable.

The vacuole contains a watery liquid called cell sap.

It stores:

• Water
• Minerals
• Sugars
• Waste materials

Water stored inside the vacuole creates pressure. This pressure keeps the cell firm.

When a plant loses water, its vacuoles lose water too. The cells become less firm and the plant wilts.

Animal cells may have smaller vacuoles for temporary storage.

Plant Cell and Animal Cell Difference

Plant and animal cells are eukaryotic. Both contain a nucleus, cytoplasm, cell membrane, mitochondria, ribosomes, ER and Golgi apparatus.

Their structures still differ in several ways.

Quick Functions of Cell Organelles

Cell Division: Mitosis and Meiosis

Cell division is the process through which new cells arise from pre-existing cells.

It supports growth, repair, maintenance and reproduction.

Mitosis

Mitosis produces two genetically identical daughter cells from one parent cell.

Each daughter cell receives the same genetic information and chromosome number as the parent cell.

Mitosis is important for:

• Body growth
• Repair of damaged tissues
• Replacement of old cells
• Maintenance
• Asexual reproduction

Skin cells divide through mitosis to replace damaged or lost cells.

Meiosis

Meiosis produces gametes for sexual reproduction.

In animals, it occurs in reproductive organs. It produces sperm in males and eggs in females.

In plants, meiosis occurs in reproductive structures that form pollen grains and egg cells.

The parent cell divides twice. Four daughter cells are formed.

Each daughter cell has half the chromosome number of the parent cell.

During fertilisation, two gametes combine and restore the original chromosome number.

Mitosis and Meiosis Difference

Errors in mitosis may cause uncontrolled cell division. Errors in meiosis may affect chromosome number, fertility or development.

Cell Theory Class 9 Notes

Cell Theory connects the structure, function and continuity of all living organisms.

Matthias Schleiden reported that plants consist of cells. Theodor Schwann found that animals also consist of cells.

Rudolf Virchow later stated that new cells arise from pre-existing cells.

The classical Cell Theory states:

• All living organisms consist of one or more cells.
• The cell is the basic structural and functional unit of life.
• All cells arise from pre-existing cells.

Controlled Cell Growth and Contact Inhibition

Normal cells grow, divide and die in a controlled manner.

They remain in the correct place and perform their assigned functions. Old or damaged cells are replaced by new cells.

Many animal cells stop dividing when they touch neighbouring cells. This control is called contact inhibition.

Cancer cells may lose this control. They continue dividing and can form tumours.

Plant cells have rigid cell walls and follow a different growth pattern.

Class 9 Science Chapter 2 Quick Revision

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