Class 9 · ScienceNCERT 2026 Revised Syllabus
Chapter 2: Cell: The Building Block of Life (0906CH02)
Exploration · Cell discovery, organelles, osmosis, cell division.
Chapter code: 0906ch02 · Book: Exploration · Class 9 Science
Find complete NCERT solutions for Class 9 Science, Exploration Chapter 2. Every question has a clear, step-by-step answer for chapter code 0906CH02.
Pause and Ponder(7 questions)
1
What argument would you give for the necessity of a cell wall in plants (fixed in one place) versus in animals (moving from place to place)?
School Level Solution
Ans.
Plants are stationary and must withstand turgor pressure, wind, and gravity — the rigid cell wall provides structural support. Animals move freely; a rigid wall would restrict movement. Their flexible cell membrane is sufficient.
2
What consequences would you predict for a plant cell if its cell wall became as flexible as a cell membrane?
School Level Solution
Ans.
A plant cell without a rigid wall would lose mechanical support and would be more vulnerable to changes in water balance. In a hypotonic medium, water entering by osmosis could make the membrane swell and rupture because there is no strong cell wall to resist the pressure.
3
Why is it important to cut potato pieces in equal size and measure their initial weight before placing them in different liquids?
School Level Solution
Ans.
Equal size and initial weight ensure the experiment is controlled — the only variable is the type of liquid. Any observed change in weight can then be attributed solely to osmosis.
4
Do white flowers contain any pigment? Give reasons.
School Level Solution
Ans.
White flowers do not contain coloured pigments like anthocyanins or carotenoids. Their cells contain leucoplasts (colourless plastids). The white colour results from reflection of all wavelengths of visible light.
5
Draw a well-labelled schematic diagram of a plant or animal cell. Clues: nucleus is dark and round; ER spreads like a network; mitochondria and chloroplasts are rod-shaped.
School Level Solution
Ans.
Plant Cell (description): Rectangular boundary = cell wall. Just inside = cell membrane. Large central vacuole. Round dark nucleus with connected ER (network). Rod-shaped chloroplasts and mitochondria. Golgi body (stacked discs). Ribosomes (dots on RER).
6
Instead of many small ones, why does a cell not have a single giant mitochondrion? How does this relate to surface area?
School Level Solution
Ans.
Many small mitochondria have a much larger total surface area-to-volume ratio than one large organelle. Cellular respiration occurs on the inner membrane (cristae) — more surface area means more reaction sites and greater ATP synthesis.
7
If skin cells divided by meiosis instead of mitosis, what would happen to a cut on the skin?
School Level Solution
Ans.
Skin repair requires diploid, genetically identical cells. If skin cells divided by meiosis, the resulting haploid cells (half chromosome number) would be abnormal. The wound would fail to heal correctly.
Activities — How to Do(9 questions)
1
Activity 2.1 (estimate the size of a cell): focus a mm ruler on the microscope stage to measure the field diameter, convert mm to μm, count onion-peel cells along the diameter, and divide. If 25 cells span 5000 μm, what is one cell’s size? How is magnification found?
School Level Solution
Ans.
Focus the mm ruler → field diameter (say 5 mm = 5000 μm) → count cells across (say 25) → size = 5000 ÷ 25 = 200 μm. Magnification = size seen in the eyepiece ÷ real size — e.g., if the 200 μm cell spans 20 mm in view, magnification = 20000 ÷ 200 = 100×.
2
Activity 2.2 (potato osmosis): weigh two potato pieces; keep one in plain water (Beaker A) and one in 20% salt/sugar solution (Beaker B) for an hour; reweigh. What do you observe and infer about weight change?
School Level Solution
Ans.
Beaker A: cells take in water by endosmosis → piece swells, weight increases. Beaker B: concentrated solution draws water out by exosmosis → piece shrinks, weight decreases. Inference: water moves across the membrane from the dilute to the concentrated side.
3
Activity 2.3 (onion and cheek cells): prepare slides of onion/Rhoeo peel (safranin) and cheek cells (methylene blue). Onion cells look box-like and regular, cheek cells irregular — why? What happens on adding 20% sugar solution?
School Level Solution
Ans.
Onion cells look box-like because the cell wall provides rigid support; cheek cells lack a wall and therefore have a more irregular shape. In concentrated sugar solution, water leaves the cells by osmosis. Plant cells can show plasmolysis; animal cheek cells instead shrink/dehydrate, so calling the animal-cell change “plasmolysis” is incorrect.
4
Activity 2.4 (Let us study): observe the diagrams of a bacterial cell, a plant cell and an animal cell (Fig. 2.10) and record structures in Table 2.1. What key differences should the table show?
School Level Solution
Ans.
Table should show: bacterial cell — no true nucleus, no membrane-bound organelles; plant cell — cell wall, chloroplasts, large central vacuole; animal cell — no wall, no chloroplasts, small vacuoles. Common to all: cell membrane, cytoplasm, ribosomes, DNA.
5
Activity 2.5 (onion root-tip mitosis): grow roots, fix in aceto-alcohol, soften with dilute HCl, stain with aceto-carmine, squash and observe. What structural differences do you expect among cells, and why?
School Level Solution
Ans.
Grow roots 5–6 days → fix → HCl softens tissue → aceto-carmine stains chromosomes → squash spreads cells. Cells look different because each is frozen in a different stage: prophase (condensing chromatin), metaphase (equatorial plate), anaphase (separating chromatids), telophase (two nuclei).
6
Think It Over: Where does a cell come from?
School Level Solution
Ans.
All cells arise from pre-existing cells through division (cell theory). New cells are produced by mitosis (growth, repair) or meiosis (gametes); spontaneous generation does not occur.
7
Think It Over: How have technological interventions facilitated new knowledge of the world beyond the naked eye?
School Level Solution
Ans.
The light microscope revealed cells; the electron microscope revealed organelles and viruses; stains (safranin, methylene blue) made transparent structures visible. Each tool extended sight, creating whole sciences — cytology, microbiology, molecular biology.
8
Think It Over: How is the cell the structural and functional unit of life?
School Level Solution
Ans.
Structural: every organism — from bacteria to banyan — is built of cells. Functional: respiration, photosynthesis, protein synthesis and division all occur inside cells. Hence nothing smaller than a cell is independently alive.
9
Think It Over: How does a cell multiply?
School Level Solution
Ans.
Cells multiply by division: mitosis copies chromosomes once and splits once → two identical diploid cells (growth, repair). Meiosis copies once but splits twice → four haploid gametes for reproduction.