Science · Class 9 · Chapter 2
Cell: The Building Block of Life
Aim: Build a clear, working understanding of this chapter's key ideas, connected to real NCERT examples, worked problems, and everyday situations.
- Osmosis — Water moving across a membrane, dilute to concentrated.
- Cell wall — Rigid outer covering in plant, fungal, bacterial cells.
- Nucleus — Controls the cell; holds genetic material.
- Mitochondria — The cell's powerhouse; site of respiration.
- Prokaryotic / Eukaryotic — No true nucleus / has a true nucleus.
- Mitosis — Division into 2 identical daughter cells.
- Meiosis — Division into 4 gametes, half chromosomes.
- Contact inhibition — Stops cell division once cells touch neighbours.
Chapter 2 · Concept 1 of 9
Studying Cells: Microscopes
- Robert Hooke, 1665 — First observed and named "cells" while examining cork under a self-built microscope.
- Resolution vs magnification — The naked eye's resolution limit is about 0.1 mm. Light microscopes use lenses; electron microscopes reveal structures at the nanometre scale.
Chapter 2 · Concept 2 of 9
The Cell Membrane and Osmosis
- Fluid-mosaic model — A lipid bilayer (water-loving heads out, water-repelling tails in) with embedded proteins that can move — fluid and mosaic-like.
- Osmosis — Water moves across the membrane from a dilute solution to a concentrated one. A potato piece swells in plain water, and shrinks in strong salt solution.
Chapter 2 · Concept 3 of 9
The Cell Wall
- Plant, fungal, and bacterial cells have a rigid cell wall outside the cell membrane — mainly cellulose in plants.
- Why plants don't shrink like animal cells — In strong salt solution, a plant cell's membrane pulls away from its wall — but the rigid wall holds its overall shape. An animal cell (no wall) just shrinks.
Chapter 2 · Concept 4 of 9
Nucleus, Ribosomes, and the ER
- Nucleus — Contains chromatin (DNA + protein); the nucleolus makes ribosomal parts.
- Ribosomes — Sites of protein synthesis — free in the cytoplasm or attached to the ER.
- Rough ER — Has ribosomes attached; makes proteins.
- Smooth ER — No ribosomes; makes lipids and hormones.
Chapter 2 · Concept 5 of 9
Golgi, Lysosomes, and Mitochondria
- Golgi apparatus — Modifies, sorts, and packages proteins — the cell's post office.
- Lysosomes — Enzyme sacs that break down waste — the cell's clean-up crew.
- Mitochondria — Site of cellular respiration, producing ATP — the powerhouse of the cell.
Chapter 2 · Concept 6 of 9
Plastids and Vacuoles
- Plastids — Chloroplasts (green, photosynthesis), chromoplasts (colour in flowers/fruits), and leucoplasts (colourless, store starch/oil).
- Vacuoles — A mature plant cell has one large central vacuole storing cell sap, keeping the cell firm — a wilted plant has lost this water.
Chapter 2 · Concept 7 of 9
Prokaryotic vs Eukaryotic Cells
- | Nucleus | Membrane-bound organelles | Typical size
- Prokaryotic (e.g. bacteria) — No — genetic material in a nucleoid — No — 1–10 µm
- Eukaryotic (plant/animal) — Yes — well-defined — Yes — 10–100 µm
Chapter 2 · Concept 8 of 9
Mitosis and Meiosis
- Why meiosis halves the count — Meiosis happens only in reproductive organs, and halves the chromosome number — so when two gametes fuse at fertilisation, the offspring ends up with the normal full number again.
- 1 cell
- 2 identical daughter cells
- 1 cell
- 4 gametes, half chromosomes
Chapter 2 · Concept 9 of 9
Cell Theory, and When Division Goes Wrong
- Cell Theory — All living things are made of cells; the cell is life's basic unit; and every cell comes from a pre-existing cell.
- When control breaks down — Contact inhibition normally stops animal cells dividing once they touch neighbours. Cancer cells lose this control and divide uncontrollably, forming tumours.
Quick Recap
Check Your Understanding
- 1Differentiate: (i) Cell membrane and cell wall (permeability) (ii) RER and SER (structure) (iii) Chloroplasts and chromoplasts (pigments).
- 2Which pair is correctly placed: present in plant cells, but absent in animal cells? (i) Leucoplast & cell wall (ii) Mitochondria & ribosome (iii) Cell wall & Golgi apparatus (iv) Lysosome & ER.
- 3Renu says all plant parts, even roots, contain plastids. Rohit disagrees, saying roots (underground, no photosynthesis) lack plastids. Who is correct?