Introduction: Cell as the Building Block of Life

All living organisms are made up of cells. A cell is the smallest unit of life that can perform basic life processes such as nutrition, respiration, growth, response to stimuli, excretion and reproduction.

Structural unit means the body of an organism is built from cells, just as a building is built from bricks. Functional unit means the activities of life are carried out inside cells.

Discovery of Cell and Cell Theory

The cell was first observed by Robert Hooke in 1665 while studying a thin slice of cork under a microscope. He saw small box-like compartments and called them “cells”. Later, improved microscopes helped scientists observe living cells and their parts.

Scientist	Contribution
Robert Hooke	Observed dead cork cells and introduced the term cell.
Anton van Leeuwenhoek	Observed living cells and microorganisms using improved microscopes.
Schleiden	Proposed that plants are made of cells.
Schwann	Proposed that animals are made of cells.
Virchow	Proposed that new cells arise from pre-existing cells.

Unicellular and Multicellular Organisms

On the basis of number of cells, organisms are classified as unicellular and multicellular.

Feature	Unicellular Organisms	Multicellular Organisms
Number of cells	Made of one cell	Made of many cells
Life processes	One cell performs all functions	Different cells perform specialised functions
Examples	Amoeba, Paramecium, bacteria, yeast	Humans, plants, animals

Prokaryotic and Eukaryotic Cells

Cells are classified into prokaryotic and eukaryotic cells based on the organisation of nuclear material and membrane-bound organelles.

Feature	Prokaryotic Cell	Eukaryotic Cell
Nucleus	No true nucleus	True nucleus present
Nuclear membrane	Absent	Present
Genetic material	Present in nucleoid region	Present inside nucleus as chromosomes
Membrane-bound organelles	Absent	Present
Ribosomes	Present, smaller	Present, larger
Examples	Bacteria, cyanobacteria	Plants, animals, fungi, Amoeba

Cell Membrane / Plasma Membrane

The cell membrane is the thin, living, flexible outer covering of the cell. It separates the internal contents of the cell from the external environment.

Functions of Cell Membrane

• It gives shape to the cell.
• It protects the internal contents of the cell.
• It allows selected substances to enter and leave the cell.
• It helps in exchange of gases, water, ions, nutrients and waste materials.
• It helps cells communicate with the external environment.

Lipid Layer of Cell Membrane

The plasma membrane is mainly made of lipids and proteins. The lipids form a double layer called the phospholipid bilayer. This layer acts as a barrier and helps the membrane remain flexible.

Osmosis and Types of Solutions

Osmosis is the movement of water molecules through a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration.

Solution	Meaning	Effect on Animal Cell	Effect on Plant Cell
Hypotonic	Outside solution is more dilute than cell sap	Cell swells and may burst	Cell becomes turgid
Isotonic	Outside solution has same concentration as cell sap	No net change	No major change
Hypertonic	Outside solution is more concentrated than cell sap	Cell shrinks	Cell becomes plasmolysed

How Do We Study Cells?

Most cells are microscopic, so they are studied using microscopes. A thin section or smear is placed on a glass slide, stained, covered with a cover slip and observed under a microscope.

Common Stains Used to Study Cells

Stain	Use
Safranin	Commonly used to stain plant cells, especially onion peel cells.
Methylene blue	Commonly used to stain animal cells such as cheek cells.
Iodine solution	Used to detect starch and sometimes to improve contrast in plant material.

Cell Wall and Cell Membrane

Plant cells have a rigid cell wall outside the cell membrane. Animal cells do not have a cell wall. The cell wall provides strength, shape and protection to plant cells.

Feature	Cell Wall	Cell Membrane
Nature	Non-living, rigid	Living, flexible
Occurrence	Present in plant cells, fungi and bacteria	Present in all cells
Composition	Cellulose in plants	Lipids and proteins
Permeability	Freely permeable	Selectively permeable
Function	Protection, support, shape	Regulates entry and exit

Nucleus, Nucleolus, DNA, Chromatin and Chromosomes

The nucleus is a large, important cell organelle that controls the activities of the cell. It contains genetic material, which carries instructions for growth, development and reproduction.

Main Parts of Nucleus

• Nuclear membrane: Double membrane surrounding the nucleus.
• Nucleoplasm: Jelly-like fluid inside the nucleus.
• Nucleolus: Dense body involved in ribosome formation.
• Chromatin material: Thread-like DNA-protein material visible in non-dividing cells.
• Chromosomes: Condensed structures formed from chromatin during cell division.

Term	Meaning
DNA	Deoxyribonucleic acid; hereditary material carrying genetic instructions.
Chromatin	Long, thread-like DNA-protein material present in the nucleus during non-dividing stage.
Chromosome	Condensed chromatin visible during cell division.
Gene	Functional segment of DNA that controls a particular character.

Functions of Nucleus

• Controls cell activities.
• Contains hereditary material.
• Transfers characters from parents to offspring.
• Controls cell division.
• Helps in protein synthesis indirectly by controlling instructions sent to ribosomes.

Cytoplasm and Protoplasm

The cytoplasm is the jelly-like substance present between the cell membrane and the nucleus. It contains cell organelles and is the site of many metabolic reactions.

Term	Meaning
Cytoplasm	Jelly-like living substance outside the nucleus but inside the cell membrane.
Protoplasm	Entire living content of the cell: cytoplasm + nucleus.

Cell Organelles: Structure and Functions

Cell organelles are specialised structures present inside the cytoplasm. Each organelle performs a specific function, just like different departments in a factory.

Endoplasmic Reticulum: RER and SER

The endoplasmic reticulum is a network of membrane-bound channels spread in the cytoplasm. It helps in transport of materials inside the cell.

Feature	Rough Endoplasmic Reticulum	Smooth Endoplasmic Reticulum
Surface	Has ribosomes on surface	No ribosomes on surface
Main function	Protein synthesis and transport	Lipid synthesis and detoxification
Appearance	Rough	Smooth

Golgi Apparatus

The Golgi apparatus consists of membrane-bound flattened sacs. It modifies, packages and transports materials produced in the cell. It also helps in the formation of lysosomes.

Mitochondria: Powerhouse of the Cell

Mitochondria are rod-shaped or oval organelles that produce energy in the form of ATP during cellular respiration. ATP is used by the cell to perform various activities.

Lysosomes: Scavengers and Suicide Bags

Lysosomes are small membrane-bound sacs containing digestive enzymes. They digest worn-out organelles, foreign particles and cellular waste.

Ribosomes

Ribosomes are tiny particles responsible for protein synthesis. They may be free in the cytoplasm or attached to rough endoplasmic reticulum.

Vacuoles

Vacuoles are storage sacs present in cells. They store water, food, waste products and other substances.

Feature	Plant Cell Vacuole	Animal Cell Vacuole
Size	Usually one large central vacuole	Small, temporary or few
Function	Stores cell sap and maintains turgidity	Storage and transport
Importance	Helps plant remain firm	Less prominent than in plants

Plastids: Chloroplasts, Chromoplasts and Leucoplasts

Plastids are organelles found mainly in plant cells. They may contain pigments or store food.

Plastid	Pigment/Content	Function	Example
Chloroplast	Chlorophyll	Photosynthesis	Green leaves
Chromoplast	Coloured pigments	Gives colour to flowers and fruits	Tomato, carrot, petals
Leucoplast	No pigment	Stores food	Potato tuber

Cell Division and Mitosis

Cell division is the process by which a parent cell divides to form new cells. It is essential for growth, repair and replacement of damaged cells.

Importance of Mitosis

• Growth of multicellular organisms.
• Repair of damaged tissues.
• Replacement of old cells.
• Asexual reproduction in some organisms.
• Maintains chromosome number.

Difference Between Plant Cell and Animal Cell

Feature	Plant Cell	Animal Cell
Cell wall	Present	Absent
Cell membrane	Present	Present
Plastids	Present	Absent
Vacuole	Large central vacuole	Small or temporary vacuoles
Shape	Usually fixed and regular	Usually irregular or flexible
Centrioles	Usually absent in higher plants	Present

Various Shapes of Cells

Cells have different shapes depending on their functions. Shape helps the cell perform its work efficiently.

Cell Type	Shape	Functional Advantage
Nerve cell	Long and branched	Helps transmit messages over long distances
Muscle cell	Long and contractile	Helps in movement
Red blood cell	Biconcave disc	Increases surface area for oxygen transport
White blood cell	Irregular shape	Helps engulf germs
Sperm cell	Head and tail	Tail helps movement towards egg

Important Advanced and HOTS Concepts

Why do viruses not have a cell membrane?

Viruses are acellular particles. They are not made of cells and do not have cytoplasm, nucleus or cell organelles. Some viruses have an outer envelope taken from the host cell, but this is not a true cellular plasma membrane performing normal cell functions.

Cells without nucleus are called what?

Cells without a true nucleus are called prokaryotic cells. In animals, mature mammalian red blood cells are special enucleated cells, meaning they lose their nucleus during development.

Why do mature human RBCs not have a nucleus?

Mature human red blood cells lose their nucleus to create more space for haemoglobin and to become more flexible. This helps them carry oxygen efficiently and pass through narrow blood capillaries.

Human sperm cells contain lysosomal enzymes

The head of a sperm contains an acrosome, which has enzyme-rich contents that help the sperm penetrate the protective layers around the egg. These enzymes are often compared with lysosomal enzymes because they help digest barriers during fertilisation.

Cell Culture

Cell culture is the technique of growing cells outside the body in a controlled nutrient medium. It is used in research, medicine, vaccine production and cancer studies.

Contact Inhibition

Normal animal cells stop dividing when they come in contact with neighbouring cells. This property is called contact inhibition.

How do cancer cells grow and spread?

Cancer cells lose normal control over cell division. They divide continuously, ignore contact inhibition and may invade nearby tissues. Some cancer cells can enter blood or lymph and spread to other body parts. This spread is called metastasis.

Programmed Cell Death

Programmed cell death, also called apoptosis, is a controlled process in which damaged or unwanted cells destroy themselves safely. It helps in development, tissue maintenance and protection against abnormal cells.