Chapter 2 — Cell: The Building Block of Life

Chapter 2 of the Class 9 Science NCERT textbook, "The Cell", explains that the cell is the basic structural and functional unit of all living organisms, that all living things are composed of cells, and that all cells arise from pre-existing cells.

• Cell as the Unit of Life — The cell is the basic structural and functional unit of all living organisms. Unicellular organisms like bacteria are a single cell, while multicellular organisms like humans have millions of cells working together.
• Prokaryotic vs Eukaryotic — Cells fall into two broad types: prokaryotic cells like bacteria lack a well-defined nucleus and membrane-bound organelles, while eukaryotic plant and animal cells have a true nucleus and membrane-bound organelles.
• Membrane and Cell Wall — The selectively permeable cell membrane follows the fluid-mosaic model, controlling transport. Plant, fungal, and bacterial cells add a rigid outer cell wall for support, which animal cells lack.
• Organelles and Division — Organelles divide labour: mitochondria produce ATP and chloroplasts run photosynthesis. Cells reproduce by division, mitosis giving identical cells for growth and repair, meiosis giving gametes for reproduction.

1. 01The cell is the basic structural and functional unit of all living organisms; unicellular organisms like bacteria consist of a single cell, while multicellular organisms like humans have millions of cells working together.
2. 02Prokaryotic cells (e.g., bacteria) lack a well-defined nucleus and membrane-bound organelles; eukaryotic cells (plants and animals) have a true nucleus and membrane-bound organelles.
3. 03The cell membrane is selectively permeable and follows the fluid-mosaic model — a lipid bilayer with embedded proteins that act as gatekeepers for substance transport.
4. 04Plant cells have a rigid cell wall made primarily of cellulose outside the cell membrane, which provides structural support; animal cells lack a cell wall and can change shape more freely.
5. 05Mitochondria are the powerhouses of the cell, producing ATP through cellular respiration; chloroplasts in plant cells carry out photosynthesis using chlorophyll to absorb sunlight.
6. 06Mitosis produces two genetically identical daughter cells for growth and repair, while meiosis produces four daughter cells with half the chromosome number, used in sexual reproduction.