Complete notes for Chapter 2 from the NCERT Exploration textbook for Class 9. Topics covered: Discovery of cell, Cell Theory,
Shape & size of cells, Prokaryotic vs Eukaryotic cells, Cell structure,
Cell Membrane, Cell Wall, Cell organelles, and Nucleus. Aligned with CBSE syllabus 2026-27.
The body of all organisms is made up of tiny microscopic units called cells.
The cell is a fundamental, structural and functional unit of living organisms and basic unit of life.
All living organisms are made up of cells. Cells make tissues, tissues together make organ, organs make organ system and organ systems make body of organisms. So cell is called structural unit of life.
All the basic functions of the body like respiration, excretion etc. are carried out by cell through its cell organelles, so cell is called functional unit of life.
Cell biology is the study of cells in all aspects of structure and functions.
💡 Important Concept
Hierarchy of Organisation: Cell → Tissue → Organ → Organ System → Organism
Example: Nasal pores + Nasal cavity + Trachea + Lungs = Respiratory System
Cell was first discovered by Robert Hooke in 1665. He observed the cell in a cork slice with the help of a primitive microscope.
Anton Von Leeuwenhoek (1674) was the first to observe free cells, like bacteria, protozoa, red blood cells and sperms in his home made microscope.
🔬 Did You Know?
Robert Hooke's microscope could magnify objects only 200-300 times, but that was enough to see the
box-like compartments in cork which he named "cells" (from Latin cella meaning small room).
The cell theory, that all the plants and animals are composed of cells and that the cell is basic unit of life, was presented by two biologists, M. Schleiden (1838) and T. Schwann (1839).
The cell theory was further expanded by a German physiologist, Rudolf Virchow (1855). He gave the phrase Omnis cellula-e-cellula, i.e., all cells arise from pre-existing cells.
Modified cell theory is termed as cell principle or modern cell theory which postulates that:
All living organisms are composed of cells or cell products.
All living cells arise from pre-existing cells.
All cells are basically alike in chemical composition and metabolic processes.
The functions of an organism as a whole is the result of the activities and interaction of the constituent cells.
⭐ Three Postulates of Cell Theory
All living organisms are made up of one or more cells
The cell is the basic unit of structure and function
Plasma membrane permits the entry and exit of some materials in the cells. It also prevents movement of some other material. Therefore, the plasma membrane is called a selectively permeable membrane.
"Diffusion is the spontaneous movement of molecules from a region of its higher concentration to a region of its lower concentration."
Example: Movement of carbon dioxide and oxygen during respiration in organisms.
Osmosis
The movement of water molecules through a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration is called osmosis.
In plants, water from the soil enters root cells by this process.
What happens if you put a cell into solution of sugar or salt prepared in water?
In plant cells, there occurs a rigid cell wall which lies outside the plasma membrane.
Cell wall is thick, non-living and permeable covering made up of cellulose. Cellulose is a kind of carbohydrate (polysaccharide) and it provides structural strength to the plant.
Functions:
Cell wall protects cell membrane and the internal structures of the cell.
It provides rigidity and determines the shape of the plant cell.
It prevents drying of the cell and helps it to bear unfavourable conditions.
It provides mechanical strength to the plant cells.
🌿 Why do plants need cell walls?
Plants cannot move from place to place, so they need rigidity to withstand environmental stresses
like wind and rain. The cell wall provides this support. Animal cells don't need cell walls because
they can move and their flexible cells support tissue movement.
All cells contain living substance called protoplasm. It is jelly-like, viscous, colourless semi-fluid substance in which various cell organelles and inclusion remains in colloidal form.
Protoplasm can be distinguished in two forms:
Cytoplasm: it is that part of protoplasm which surrounds the nucleus.
Nucleoplasm: it is that part of protoplasm which is located inside the nucleus.
Protoplasm = Cytoplasm + Nucleoplasm
Cytoplasm
The part of the cell which occurs between the plasma membrane and nucleus envelope is called the cytoplasm
Cytoplasm consists of an aqueous ground substance, the cytosol, containing a variety of cell organelles and other inclusions such as insoluble waste and storage product (starch, lipid, etc.).
Chemically cytoplasm contains about 90% water, 7% proteins, 2% carbohydrates and lipids and 1% inorganic minerals, vitamins, etc.
Cell organelles
A cell has to perform different functions with the help of its various membrane-bound cell organelles.
Cell organelles are "small organs" of the cell and are found embedded in the cytosol. They form living part of the cell and each of them has a definite shape, structure and function.
Cell Organelles — Summary Table
Organelle
Nickname
Function
Nucleus
Control centre
Controls all cell activities; stores genetic information (DNA)
Ribosomes
Protein factory
Site of protein synthesis
Endoplasmic Reticulum (ER)
Manufacturing factory
Synthesis and transport of proteins (RER) and lipids (SER)
Golgi Apparatus
Post office
Modifies, sorts, and packages proteins for transport
Mitochondria
Powerhouse
Produces energy (ATP) through cellular respiration
Lysosomes
Suicide bags / Cleaners
Digest waste and damaged organelles
Chloroplasts
Food factory
Photosynthesis (convert sunlight to glucose)
Vacuoles
Storage tanks
Store water, minerals, waste; maintain turgor pressure
The nucleus is a major, centrally located spherical cellular component. It is the centre from where all cellular activities are controlled. It is the carrier of hereditary material in the cell.
It is bounded by two membranes, both forming a nuclear envelope. The nuclear envelope contains many pores known as nuclear pores and encloses the liquid ground substance, the nucleoplasm.
The nuclear pores allow transfer of materials between the nucleoplasm and the cytoplasm. Within nucleoplasm are embedded two types of nuclear structures — nucleolus and chromatin material.
Nucleolus:
The nucleolus may be one or more in number and is not bounded by any membrane.
Nucleolus consists of DNA, RNA and protein.
It is the site of ribosome formation. Nucleolus is known as factory of ribosomes.
Chromatin material:
The chromatin is a thin, thread-like intermingled mass of chromosome material and composed of the genetic substance DNA (deoxyribonucleic acid) and proteins (i.e., histones).
DNA stores all the information necessary for the cell to function (metabolism), to grow and to reproduce further cells of the next generation.
The chromatin is condensed into two or more thick ribbon-like chromosomes during the division of cell (see mitosis and meiosis).
Functional segments of DNA are called genes.
Genes carry information for protein synthesis, which in turn control the development of characters. Since genes are present on chromosomes and chromosomes pass from one generation to the next, genes are called hereditary units.
Functions of nucleus:
The nucleus controls all metabolic activities of the cell. If the nucleus is removed from the cell, the protoplasm ultimately dries up and dies.
It regulates the cell cycle.
It is the storehouse of genes which are concerned with the development and manifestation of all the body characters.
Nucleus helps in the formation of ribosomes and RNA.
It helps in the transmission of characters from one generation to the next.
