Cell - Class 7 CBSE Science Notes - Complete Study Guide 2026

Introduction to Cell - The Building Block of Life

The cell is the smallest structural and functional unit of life in all living organisms. Every living thing, from microscopic bacteria to giant blue whales, is made up of cells. The word "cell" originates from the Latin term "cellula," meaning "a small room" or compartment.

Fact: Cells are often called the "building blocks of life" because all living organisms are composed of one or more cells.

Historical Discovery of Cell

Robert Hooke, an English scientist (1635-1703), is credited with discovering the cell in 1665. His groundbreaking observation revolutionized biology and our understanding of life itself.

How Robert Hooke Discovered Cells

1. Material Used: Hooke examined thin slices of cork (bark of a tree) under his microscope
2. Observation: He noticed small, box-like compartments arranged in a pattern
3. Comparison: These compartments resembled a honeycomb structure
4. Naming: He coined the term "cell" from the Greek word meaning "compartment"

Important Note: What Robert Hooke actually observed were dead cell walls of plant cells, not living cells. The living cell was later discovered by Anton van Leeuwenhoek.

Cell Theory - Foundation of Modern Biology

The Cell Theory is one of the fundamental principles of biology, establishing the relationship between cells and living organisms.

Development of Cell Theory

Three Main Postulates of Cell Theory

1. All living organisms are made up of cells - Whether unicellular or multicellular, life exists in cellular form
2. Cell is the structural and functional unit of life - Cells perform all life processes independently
3. All cells arise from pre-existing cells - New cells are formed through cell division, not spontaneous generation

Modern Additions to Cell Theory:

• Energy flow occurs within cells through metabolism
• Hereditary information (DNA) is passed from cell to cell
• All cells have the same basic chemical composition

The Microscope - Window to the Cellular World

Cells are extremely small and invisible to the naked eye. The microscope made it possible to observe and study cells.

Evolution of Microscope Technology

1. Simple Microscope (First Microscope)

• Inventor: Antony van Leeuwenhoek
• Design: Single biconvex lens
• Magnification: Up to 200 times
• Achievement: First to observe living cells, bacteria, and microorganisms

2. Compound Microscope

• Inventor: Robert Hooke
• Design: System of 3 lenses working together
• Magnification: 2,000 to 4,000 times
• Advantage: Better clarity and higher magnification

3. Electron Microscope

• Inventors: Max Knoll and Ernst Ruska (1932)
• Technology: Uses electron beams instead of light
• Magnification: 100,000 to 300,000 times
• Advantage: Can observe ultra-fine cellular structures and viruses

Practical Tip: In your CBSE Class 7 lab, you'll use a compound microscope to observe onion peel cells and cheek cells.

Cell Size - From Microscopic to Visible

Cells vary dramatically in size depending on their type and function.

Size Range of Cells

Smallest Cells

• Bacteria Mycoplasma gallisepticum: 0.1 micrometers (µm)
• These are among the tiniest living cells known

Largest Cells

• Ostrich egg: 170 mm × 135 mm (largest single cell)
• Human nerve cell (neuron): Up to 90 cm long
• Acetabularia (unicellular algae): Up to 10 cm long

Common Cell Sizes

• Red Blood Cells (RBCs): 7-8 µm diameter
• White Blood Cells (WBCs): 10-12 µm diameter
• Typical Plant Cells: 10-100 µm
• Typical Animal Cells: 10-30 µm

Measurement Units:

• 1 micrometer (µm) = 1/1,000,000 meter = 0.001 millimeter
• 1 millimeter (mm) = 1,000 micrometers

Cell Shape - Form Follows Function

Cells acquire different shapes based on the specific functions they perform in the body.

Factors Influencing Cell Shape

1. Surface tension and viscosity of protoplasm
2. Mutual pressure from surrounding cells
3. Rigidity of the cell membrane
4. Internal environment and cellular functions

Common Cell Shapes with Examples

Interesting Fact: Human nerve cells can be extremely long (up to 1 meter) to transmit signals from the brain to distant body parts like toes.

Structure of a Cell - Components and Functions

A typical cell consists of three main parts: Cell membrane, Cytoplasm, and Nucleus. Plant cells have an additional outer covering called the cell wall.

Major Cell Components

1. Cell Wall (Plant Cells Only)

Structure: Rigid, non-living outer covering outside the plasma membrane

Composition: Primarily made of cellulose (a complex polysaccharide)

Functions:

• Provides structural support and rigidity to plant cells
• Protects the cell from mechanical damage
• Prevents over-expansion when water enters the cell
• Gives definite shape to plant cells
• Acts as a barrier against pathogens

Why don't animal cells have a cell wall?

Animals have internal or external skeletons for support and can move. Plants are stationary and need rigid cell walls for structural support.

2. Plasma Membrane (Cell Membrane)

Nature: Living, thin, flexible unit membrane present in all cells

Structure: Semi-permeable membrane composed of lipids and proteins

Functions:

• Selective permeability: Controls what enters and exits the cell
• Regulates movement of water, nutrients, and waste products
• Protects internal cell contents
• Maintains cell shape in animal cells
• Facilitates cell communication and recognition

Main Concept - Selective Permeability: The plasma membrane allows certain substances to pass through while blocking others, like a selective security guard.

3. Protoplasm - The Living Substance

Definition: The entire living content of a cell, including cytoplasm and nucleus

Components: Water (75-85%), organic compounds (proteins, carbohydrates, lipids), and inorganic salts

Characteristics:

• Semi-transparent, jelly-like substance
• Site of all metabolic activities
• Considered the "physical basis of life"

4. Cytoplasm - The Cellular Factory

Location: Gel-like substance between the plasma membrane and nucleus

Composition:

• Water: 70-90%
• Organic substances: Proteins, carbohydrates, lipids
• Inorganic substances: Mineral salts, ions
• Cell organelles: Embedded structures performing specific functions

Functions:

• Houses all cell organelles
• Site of most cellular activities and chemical reactions
• Provides medium for transport of materials
• Stores nutrients and metabolic products

5. Nucleus - The Control Center

Discovery: Robert Brown discovered the nucleus in 1833

Location: Usually centrally located in the cell (spherical or oval structure)

Structure:

a) Nuclear Envelope (Nuclear Membrane)

• Double-layered membrane surrounding the nucleus
• Contains nuclear pores for material exchange
• Selectively permeable like the plasma membrane

b) Nucleoplasm

• Jelly-like fluid inside the nucleus
• Contains dissolved nutrients and ions

c) Chromatin Fibers

• Thread-like structures in the nucleoplasm
• Made of DNA and proteins
• Condense to form chromosomes during cell division

d) Nucleolus

• Small, dense spherical body inside the nucleus
• Involved in ribosome formation

Functions of Nucleus:

1. Controls all cellular activities - Acts as the cell's "brain"
2. Contains genetic information (DNA)
3. Regulates protein synthesis
4. Essential for cell division and reproduction
5. Transmits hereditary characteristics from parents to offspring

Important: Mature human red blood cells lack a nucleus, which allows more space for hemoglobin to carry oxygen.

Cell Organelles - Specialized Cellular Structures

Organelles are specialized structures within cells that perform specific functions, much like organs in our body.

1. Mitochondria - The Powerhouse of the Cell

Term coined by: C. Benda

Structure:

• Double membrane: Outer smooth membrane and inner folded membrane
• Cristae: Folds of the inner membrane (increase surface area)
• Matrix: Fluid inside containing enzymes

Functions:

• Cellular respiration: Breaks down glucose to release energy
• ATP production: Generates ATP (adenosine triphosphate), the cell's energy currency
• Contains own DNA: Can self-replicate

Why "Powerhouse"? Mitochondria produce most of the cell's energy supply through the oxidation of food molecules, just like a power plant generates electricity.

Interesting Fact: Cells that require lots of energy (like muscle cells) contain thousands of mitochondria, while less active cells have fewer.

2. Endoplasmic Reticulum (ER) - The Transport Network

Structure: Network of interconnected membrane-bound channels and tubules

Connectivity:

• One end connects to the nuclear membrane
• Other end connects to the plasma membrane

Types:

a) Rough Endoplasmic Reticulum (RER)

• Appearance: Studded with ribosomes on the surface
• Function: Protein synthesis and modification

b) Smooth Endoplasmic Reticulum (SER)

• Appearance: No ribosomes attached
• Functions:
  • Lipid and steroid synthesis
  • Detoxification of harmful substances
  • Calcium ion storage

Main Function: Acts as a transport system, distributing materials from one part of the cell to another.

3. Golgi Bodies (Golgi Apparatus) - The Packaging Unit

Discovery: Camillo Golgi discovered this organelle

Structure: Stack of flattened, membrane-bound sacs (cisternae)

Location: Present as granules in the cytoplasm

Functions:

1. Processing and packaging of proteins and lipids
2. Secretion of enzymes and hormones
3. Formation of lysosomes
4. Synthesis of carbohydrates, proteins, and enzymes
5. Modification of molecules from ER

Analogy: Like a post office that receives, packages, labels, and ships products to their destinations.

4. Lysosomes - The Digestive Bags

Structure: Small, spherical vesicles bounded by a single membrane

Content: Powerful hydrolytic (digestive) enzymes

Functions:

1. Intracellular digestion: Breaks down food molecules
2. Waste removal: Digests old and worn-out cell organelles
3. Defense: Destroys foreign particles and bacteria
4. Cell death: Causes cell death when necessary

Special Names:

• "Suicidal bags" - Can digest their own cell during cell death
• "Digestive bags" - Contain digestive enzymes

Safety Mechanism: The membrane keeps digestive enzymes contained. If it ruptures, the enzymes can digest the entire cell.

5. Vacuoles - Storage Compartments

Structure: Membrane-bound sacs filled with cell sap

Presence:

• Plant cells: Large central vacuole (occupies 50-90% of cell volume)
• Animal cells: Small, numerous vacuoles (if present)

Functions:

1. Storage: Water, nutrients, minerals, sugars, proteins
2. Waste storage: Isolates harmful waste products
3. Maintains turgor pressure in plant cells (keeps plants rigid)
4. Provides mechanical support to plants
5. Pigment storage: Contains colored pigments in flower petals

Why are plant cell vacuoles larger?

Plants cannot move to find water, so they store large amounts of water in vacuoles. This storage also provides structural support.

6. Plastids - The Pigment Makers (Plant Cells Only)

Size: Largest organelles in plant cells

Presence: Found ONLY in plant cells and some protists

Types of Plastids:

a) Chloroplasts (Green Plastids)

• Pigment: Chlorophyll (green pigment)
• Function: Photosynthesis - converts light energy into chemical energy
• Location: Abundant in leaves and green parts of plants

b) Chromoplasts (Colored Plastids)

• Pigments: Carotenoids (yellow, orange, red)
• Function: Provides color to flowers and fruits
• Example: Red tomatoes, yellow flowers, orange carrots

c) Leucoplasts (Colorless Plastids)

• Pigment: None (colorless)
• Function: Storage of starch, oils, and proteins
• Location: Non-green parts like roots, seeds, tubers

Photosynthesis Importance: Chloroplasts make food for the entire plant and release oxygen for all living organisms - making plants "food factories" of nature.

7. Centrioles - Division Controllers (Animal Cells Only)

Structure: Pair of cylindrical structures near the nucleus

Presence: Found in animal cells, absent in most plant cells

Function:

• Cell division: Help in the formation of spindle fibers
• Spindle fibers: Pull chromosomes apart during cell division

Organization: Usually found in pairs positioned at right angles to each other

8. Ribosomes - The Protein Factories

Structure: Tiny, spherical organelles (not membrane-bound)

Composition: Made of RNA and proteins

Location:

• Free in cytoplasm
• Attached to Rough ER

Function:

• Protein synthesis: Assembles amino acids into proteins
• Essential for cell growth and repair

Size: Smallest cell organelles, visible only under electron microscope

Specialized Structures for Movement

Some unicellular organisms have projections that help them move through their aquatic environment.

Types of Movement Structures

1. Cilia (Singular: Cilium)

• Structure: Short, hair-like projections
• Number: Numerous (hundreds to thousands)
• Movement: Beat in coordinated waves
• Example:Paramecium - covered with cilia for swimming

2. Flagella (Singular: Flagellum)

• Structure: Long, whip-like projections
• Number: One or few (typically 1-8)
• Movement: Whip-like motion for propulsion
• Example:Euglena - has one or two flagella for movement

Function: These structures help organisms move toward food, light, or away from danger.

Plant Cell vs Animal Cell - Key Differences

Understanding the differences between plant and animal cells is crucial for CBSE Class 7 exams.

Comprehensive Comparison Table

Similarities Between Plant and Animal Cells

Despite differences, both cell types share fundamental features:

1. Plasma membrane - Present in both
2. Cytoplasm - Contains organelles
3. Nucleus - Controls cell activities
4. Mitochondria - Generates energy
5. Endoplasmic Reticulum - Transport network
6. Golgi apparatus - Packaging unit
7. Ribosomes - Protein synthesis
8. DNA - Genetic material

Cell Life Cycle - Birth to Death

Cells, like all living things, have a lifespan - they are born, they live, and they die.

Lifespan of Different Cells

What Happens to Dead Cells?

1. Skin cells: Flake off as we wash or move
2. Internal organ cells: Pass through the body with waste products
3. Blood cells: Broken down by the spleen and liver
4. Recycling: Components are reused to build new cells

Cell Division - Creating New Cells

Main Principle: Every cell comes from a pre-existing cell through cell division

Process:

• One cell divides to form two cells
• Two cells divide to form four cells
• Four cells divide to form eight cells... and so on

Human Development:

• Humans start life as a single fertilized egg cell (zygote)
• Through repeated cell division: 1 → 2 → 4 → 8 → 16 → ... → trillions of cells
• Adult human body contains approximately 37.2 trillion cells

Interesting Facts:

• Your body produces about 25 million new cells every second
• You completely replace your skin every 2-4 weeks
• Cell division allows growth, repair, and reproduction

Quick Reference: Formulas and Concepts

Important Terms and Definitions

Cell: The smallest structural and functional unit of life capable of performing all life processes independently.

Cell Theory: Fundamental biological principle stating all organisms are made of cells, cells are the basic unit of life, and all cells arise from pre-existing cells.

Protoplasm: The living substance of a cell, including cytoplasm and nucleus.

Cytoplasm: Jelly-like substance inside the cell membrane containing organelles and dissolved nutrients.

Nucleus: Control center of the cell containing genetic material (DNA) and directing all cellular activities.

Organelles: Specialized structures within cells that perform specific functions (like organs in a body).

Prokaryotic Cell: Simple cell without a membrane-bound nucleus (bacteria, blue-green algae).

Eukaryotic Cell: Complex cell with a true membrane-bound nucleus (plants, animals, fungi).

Selectively Permeable: Property of cell membrane allowing only certain substances to pass through.

Chromosome: Condensed form of chromatin fibers carrying genetic information, visible during cell division.

Photosynthesis: Process by which green plants make food using sunlight, carbon dioxide, and water in chloroplasts.

Cellular Respiration: Process of breaking down glucose in mitochondria to release energy (ATP).

CBSE Exam Tips and Important Questions

Most Important Topics for Exams

1. Discovery of cell by Robert Hooke
2. Cell Theory (three postulates)
3. Differences between plant and animal cells
4. Functions of cell organelles
5. Structure and function of nucleus
6. Types of plastids

Expected Question Types

Very Short Answer (1 Mark):

• Who discovered the cell?
• Define cell theory
• Name the powerhouse of the cell
• What are plastids?

Short Answer (2-3 Marks):

• Write three postulates of cell theory
• Differentiate between RER and SER
• What are the functions of nucleus?
• Why are lysosomes called suicidal bags?

Long Answer (5 Marks):

• Draw a neat labeled diagram of plant/animal cell
• Explain the structure and function of mitochondria
• Describe the differences between plant and animal cells
• Explain various cell organelles and their functions

Diagram-Based Questions:

• Label the parts of a cell
• Draw and label: nucleus, mitochondria, chloroplast
• Identify organelles from given diagrams

Diagram Practice

Make sure you can draw and label:

1. Plant Cell (with all organelles)
2. Animal Cell (with all organelles)
3. Nucleus (showing nuclear membrane, chromatin, nucleolus)
4. Mitochondria (showing cristae)
5. Chloroplast

Learning Resources and Study Tips

For Better Understanding

1. Use Microscope: If available in school lab, observe onion peel and cheek cells
2. 3D Models: Create cell models using clay or thermocol balls
3. Animations: Watch educational videos showing cell structure
4. Mnemonics: Create memory tricks for organelle functions
5. Flash Cards: Make cards for each organelle with functions

Mnemonic for Plant vs Animal Cells

"Plants Can Vanish, Animals Cannot"

• Plastids - Plants have them
• Cell wall - Plants have them
• Vacuole (large) - Plants have them
• Animals
• Centrioles - Animals have them
• Cannot - Animals cannot make food

Common Mistakes to Avoid

1. Confusing cell wall with cell membrane
2. Stating plastids are present in animal cells
3. Saying all cells are visible to naked eye
4. Mixing up functions of different organelles
5. Forgetting that Robert Brown discovered nucleus (not cell)
6. Writing rough diagrams without labels

Practice Questions

Section A: Multiple Choice Questions

1. Who discovered the cell?
   • a) Robert Brown
   • b) Robert Hooke
   • c) Anton van Leeuwenhoek
   • d) Rudolf Virchow
2. Which organelle is known as the powerhouse of the cell?
   • a) Nucleus
   • b) Ribosome
   • c) Mitochondria
   • d) Golgi body
3. Plastids are found in:
   • a) Animal cells only
   • b) Plant cells only
   • c) Both plant and animal cells
   • d) Neither plant nor animal cells

Section B: Short Answer Questions

1. State the three postulates of cell theory.
2. Why is plasma membrane called selectively permeable?
3. What is the function of lysosomes?
4. Write three differences between plant and animal cells.

Section C: Long Answer Questions

1. Describe the structure and functions of nucleus.
2. Draw a labeled diagram of a plant cell.
3. Explain the various cell organelles present in an animal cell and their functions.

Additional Resources

NCERT References

• Class 7 Science Textbook: Chapter 8 - Cell Structure and Functions
• NCERT Solutions: Available on official NCERT website

Further Reading

• Cells and tissues
• Cell division (Mitosis and Meiosis) - Class 8/9
• Genetics and heredity - Class 9/10

Summary Points

• Cell is the basic structural and functional unit of life
• Robert Hooke discovered the cell in 1665
• Cell theory has three main postulates
• All cells have plasma membrane, cytoplasm, and genetic material
• Plant cells have cell wall, chloroplasts, and large vacuoles
• Animal cells have centrioles and small/no vacuoles
• Mitochondria produce energy (ATP)
• Nucleus controls all cell activities
• Chloroplasts perform photosynthesis
• Cells vary in size, shape, and lifespan
• New cells arise from pre-existing cells through cell division