CHAPTER 2 — THE CELL
THE BUILDING BLOCK OF LIFE
Class 9 Science • Crisp + easy-to-revise notes
1. What is a Cell?
A cell is the basic structural and functional unit of life. In simple words, it is the smallest part
of a living organism that can carry out life processes.
Think of it like this: A building is made of bricks; similarly, living organisms are made of
cells. A single cell can itself be a complete organism, as in many unicellular organisms.
2. Discovery of the Cell
Scientist Key contribution
Robert Hooke (1665) Observed cork under a microscope and saw tiny box-like
compartments. He named them cells.
Antonie van Leeuwenhoek Observed living cells and microorganisms with improved
microscopes.
Robert Brown Discovered the nucleus in plant cells.
Schleiden & Schwann Concluded that plants and animals are made of cells.
Rudolf Virchow Proposed that new cells arise from pre-existing cells.
3. Cell Theory
1. All living organisms are made up of cells.
2. The cell is the basic structural and functional unit of life.
3. All cells arise from pre-existing cells.
4. Main Parts of a Cell
Cell membrane: Thin outer covering. It is selectively permeable, so it controls the movement
of substances into and out of the cell.
Cytoplasm: Jelly-like substance between the cell membrane and nucleus. Many chemical
reactions occur here.
Nucleus: Contains genetic material (DNA) and controls many activities of the cell.
Cell wall: Rigid outer layer in plant cells. Gives shape, strength and protection.
⚙️ 5. Cell Organelles — Super Easy
Mitochondria → release energy from food; called the “powerhouse” of the cell.
Ribosomes → make proteins.
Class 9 Science • Chapter 2 • The Cell 1
THE BUILDING BLOCK OF LIFE
Class 9 Science • Crisp + easy-to-revise notes
1. What is a Cell?
A cell is the basic structural and functional unit of life. In simple words, it is the smallest part
of a living organism that can carry out life processes.
Think of it like this: A building is made of bricks; similarly, living organisms are made of
cells. A single cell can itself be a complete organism, as in many unicellular organisms.
2. Discovery of the Cell
Scientist Key contribution
Robert Hooke (1665) Observed cork under a microscope and saw tiny box-like
compartments. He named them cells.
Antonie van Leeuwenhoek Observed living cells and microorganisms with improved
microscopes.
Robert Brown Discovered the nucleus in plant cells.
Schleiden & Schwann Concluded that plants and animals are made of cells.
Rudolf Virchow Proposed that new cells arise from pre-existing cells.
3. Cell Theory
1. All living organisms are made up of cells.
2. The cell is the basic structural and functional unit of life.
3. All cells arise from pre-existing cells.
4. Main Parts of a Cell
Cell membrane: Thin outer covering. It is selectively permeable, so it controls the movement
of substances into and out of the cell.
Cytoplasm: Jelly-like substance between the cell membrane and nucleus. Many chemical
reactions occur here.
Nucleus: Contains genetic material (DNA) and controls many activities of the cell.
Cell wall: Rigid outer layer in plant cells. Gives shape, strength and protection.
⚙️ 5. Cell Organelles — Super Easy
Mitochondria → release energy from food; called the “powerhouse” of the cell.
Ribosomes → make proteins.
Class 9 Science • Chapter 2 • The Cell 1
