NCERT Solutions for Class 9th Science Chapter 2 Cell: The Building Block of Life
Updated on 2026-09-19
About this chapter
The unaided eye resolves about 0.1 mm at its near point of 25 cm. Most plant and animal cells are 10–100 µm across, so a light microscope (typically 10X eyepiece × 10X objective = 100X ) is needed; an electron microscope reaches the nanometre scale. The cell membrane is 7–10 nm thick, built of a lipid bilayer with proteins embedded in it — the fluid-mosaic model . It is selectively permeable : water crosses freely, dissolved salt and sugar do not. Diffusion is the net movement of particles down a concentration gradient, with or without a membrane. Osmosis is the diffusion of water across a selectively permeable membrane — the reason a potato piece gains weight in plain water and loses it in 20 per cent salt solution. Plant, fungal and bacterial cells have a rigid, freely permeable cell wal
- The four questions the chapter sets out to answer
- How to Study Cells?
- Cell membrane
- Cell wall
- The Cell Interior
- Threads of Curiosity
- Lysosomes
- Plastids
- Plastids and Vacuoles
- Why do eukaryotic cells need these organelles?
Quick revision
| Term | What it means | Where you meet it in Chapter 2 | Key detail to remember |
|---|---|---|---|
| Limit of resolution | The smallest separation at which two points are still seen as two | Section 2.1, the two dots on paper | 0.1 mm for the eye at a near point of 25 cm |
| Magnification | How many times larger the image looks than the object | Activity 2.1 | Total = eyepiece × objective; 10X × 10X = 100X |
| Selectively permeable | Lets some substances through and blocks others | Cell membrane, Section 2.2.1 | Water passes; salt and sugar molecules do not |
| Diffusion | Net movement of particles from higher to lower concentration | Recalled from Grade 8, Chapter 7 | Needs no membrane at all |
| Osmosis | Diffusion of water across a selectively permeable membrane | Activity 2.2, the potato pieces | Water moves towards the more concentrated solution |
| Hypotonic / isotonic / hypertonic | Outside solute concentration less than / equal to / greater than inside | Fig. 2.6, ‘What if…’ box on page 12 | A cell swells / stays the same / shrinks |
| Fluid-mosaic model | Lipid bilayer with proteins set in it like tiles, all able to move sideways | Fig. 2.7, page 12 | Membrane is only 7–10 nm thick |
| Cell wall | Rigid, freely permeable cellulose covering outside the membrane | Section 2.2.2 | Present in plants, fungi and bacteria; absent in animals |
| Prokaryotic cell | Cell with no membrane-bound nucleus or organelles | Table 2.2, page 15 | 1–10 µm; DNA lies free in the nucleoid |
| Eukaryotic cell | Cell with a true nucleus and membrane-bound organelles | Table 2.2, page 15 | 10–100 µm; may be unicellular or multicellular |
| Plastids | Plant organelles for making and storing food | Section 2.3.1, page 18–19 | Chloroplast (green), chromoplast (coloured), leucoplast (colourless) |
| Mitosis | Division giving two genetically identical daughter cells | Section 2.4.1, Fig. 2.18 | Growth, repair, maintenance, asexual reproduction |
| Meiosis | Two-step division giving four cells with half the chromosomes | Section 2.4.1, Fig. 2.19 | Only in testes, ovaries, anthers; creates variation |
| Contact inhibition | Animal cells stop dividing when they touch neighbours | Section 2.5.1, page 23 | Lost in cancer cells, so tumours form |
Exercises
- The four questions the chapter sets out to answer — Think It Over Page 8
- In-text Questions — How to Study Cells? Page 9
- In-text Question — How to Study Cells? Page 10
- Activity 2.1: Let us estimate the size of a cell — How to Study Cells? Page 10
- In-text Question — Cell membrane Page 11
- Activity 2.2: Let us experiment — Cell membrane Page 11
- What if… — Cell membrane Page 12
- In-text Question — Cell wall Page 13
- Activity 2.3: Let us investigate — Cell wall Page 13
- Pause and Ponder — Cell wall Page 14
- Activity 2.4: Let us study — The Cell Interior Page 14
- In-text Question — The Cell Interior Page 15
- Threads of Curiosity — In-text Question Page 16
- Lysosomes — In-text Question Page 17
- Plastids — In-text Question Page 18
- Plastids and Vacuoles — In-text Questions Page 19
- Pause and Ponder — Why do eukaryotic cells need these organelles? Page 19
- In-text Question — How do Normal Cells Grow and Divide? Page 20
- Activity 2.5: Let us enhance our skills — How do Normal Cells Grow and Divide? Page 20
- Pause and Ponder — Cell division Page 22
- Threads of Curiosity — In-text Question Page 23
- Ready to Go Beyond — Do cells grow and reproduce forever? Page 23
- End-of-chapter questions — Revise, Reflect, Refine Page 24
- Project work — The Journey Beyond Page 27
- Questions to carry forward — The Quest Continues … Page 27