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Class 9 Science | Chapter 2: The Building Block of Life – Q&A
CHAPTER 2

Cell: The Building Block of Life

📖 Pages 28–47 | Exploration | Grade 9 Science

💭 Think It Over p. 28

Think It Over
Where does a cell come from? How have technological interventions facilitated the creation of new knowledge in understanding the world beyond the naked eye? How is the cell structural and functional unit of life? How does a cell multiply?
✅ Answers
Cell origin: Cells come from pre-existing cells (cell theory).
Technology: Microscopes (light and electron) allowed visualization of cells and organelles.
Structural & functional unit: All life processes occur within cells; cells make up tissues and organs.
Multiplication: Cells divide by mitosis (growth, repair) or meiosis (gamete formation).

🧠 Pause and Ponder pp. 34, 39, 42

Q1 (p. 34)
What argument would you give for the necessity of a cell wall in plants usually fixed in one place versus in animals usually moving from one place to the other?
✅ Answer
Plants need rigid cell walls for structural support, to withstand wind and gravity, and to maintain shape. Animals need flexibility for movement, so they lack cell walls.
Q2 (p. 34)
What consequences would you predict for a plant cell if its cell wall were to become as flexible as a cell membrane?
✅ Answer
The plant would lose rigidity, unable to stand upright; cells might burst in hypotonic conditions; overall structure would collapse.
Q3 (p. 34)
Why is it important to cut the two potato pieces in roughly equal size and measure their initial weight before placing them in different liquids?
✅ Answer
To ensure a fair experiment – any change in weight can be attributed solely to osmosis, not to initial size differences.
Q4 (p. 39)
Do white flowers contain any pigment? Give reasons.
✅ Answer
White flowers lack pigments (chromoplasts) but may reflect all wavelengths of light; they have leucoplasts or no pigments.
Q5 (p. 39)
Draw a well-labelled schematic diagram of a plant or animal cell using clues: nucleus appears dark, ER spreads like network, mitochondria and chloroplasts rod-shaped.
✅ Answer
(Diagram description) Central nucleus with dark appearance; ER continuous with nuclear envelope; mitochondria rod-shaped; chloroplasts (in plants) rod-shaped with grana.
Q6 (p. 42)
Instead of many small ones, why does a cell not have a single giant mitochondrion? How does this relate to surface area?
✅ Answer
Many small mitochondria increase surface area for ATP production (more cristae). A single giant one would have lower surface-to-volume ratio, reducing efficiency.
Q7 (p. 42)
If skin cells started dividing by meiosis instead of mitosis, what would happen to a cut on the skin?
✅ Answer
Meiosis produces haploid gametes, not identical diploid cells. Wound healing would fail because cells would have half the chromosomes and cannot function properly for repair.

📘 Revise, Reflect, Refine pp. 44–47

Q1
Differentiate between cell membrane and cell wall (permeability), RER and SER (structure), chloroplasts and chromoplasts (pigments).
✅ Answer
Cell membrane: selectively permeable; cell wall: fully permeable. RER has ribosomes (protein synthesis), SER lacks ribosomes (lipid synthesis). Chloroplasts contain chlorophyll (green), chromoplasts contain other pigments (red, yellow, orange).
Q2
Two similar animal cells placed in pure water (Cell X) and concentrated salt solution (Cell Y). Cell X swells, Cell Y shrinks. Correct explanation: (i) Salt moved into Y (ii) Water moved into X and more water moved out of Y (iii) Water moved into X and moved out of Y (iv) Solute movement caused osmosis in both.
✅ Answer
(iii) Water moved into X and moved out of Y through the cell membrane.
Q3
Identify parts (a) to (g) in Fig. 2.20 and match functions: controlling activities, respiration, storage, separates contents, rigidity, packing, food manufacturing.
✅ Answer
(a) Nucleus – controlling activities; (b) Mitochondria – respiration; (c) Vacuole – storage/rigidity; (d) Cell membrane – separates contents; (e) Cell wall – rigidity; (f) Golgi apparatus – packing; (g) Chloroplast – food manufacturing.
Q4
Which pair of organelles correctly placed under "present in plant cells, absent in animal cells"? (i) Leucoplast – Cell wall (ii) Mitochondria – Ribosome (iii) Cell wall – Golgi (iv) Lysosome – ER
✅ Answer
(i) Leucoplast and cell wall are present in plants, absent in animals.
Q5
Two students discuss plastids. Renu says all plant parts even roots contain plastids. Rohit disagrees. Who is correct? Justify.
✅ Answer
Renu is correct. Roots contain leucoplasts (colourless plastids) for starch storage, even though they lack chloroplasts.
Q6
Mitochondria and chloroplasts – structurally and functionally similar and different.
✅ Answer
Similar: double membrane, own DNA, produce energy (ATP). Different: mitochondria (cellular respiration, all eukaryotes), chloroplasts (photosynthesis, only plants).
Q7
Which pair contains DNA? (i) Chloroplasts, Ribosomes (ii) Mitochondria, Nucleus (iii) Golgi, Ribosomes (iv) Nucleus, Lysosomes
✅ Answer
(ii) Mitochondria and Nucleus.
Q8
Carrot experiment: one in plain water, one in salt solution. (i) Hypothesis? (ii) Improvement? (iii) Why carrot in water stays stiff, in salt becomes limp?
✅ Answer
(i) Osmosis causes water movement. (ii) Use multiple carrots, measure weight change. (iii) Water enters by osmosis in plain water (turgid); water leaves in salt solution (plasmolysis, limp).
Q9
Indicate presence/absence of structures in bacterial and animal cells: Chromosome, Nucleus, Mitochondria, Golgi complex, Chromoplasts.
✅ Answer
Bacterial: Chromosome (present, nucleoid), Nucleus (absent), Mitochondria (absent), Golgi (absent), Chromoplasts (absent). Animal: Chromosome (present), Nucleus (present), Mitochondria (present), Golgi (present), Chromoplasts (absent).
Q10
Potato cup experiment with sugar/salt/boiled potato. (i) Why water gathers in B and C? (ii) Why is Cup A necessary? (iii) Why no water in A and D?
✅ Answer
(i) Osmosis – water moves into hypertonic solution. (ii) Control – shows no water movement without solute. (iii) Cup A: no solute gradient; Cup D: boiled potato cells are dead, membranes non-functional.
Q11
Identify incorrect match of organelle with function: (i) Ribosome – Protein synthesis (ii) SER – Lipid and cellulose synthesis (iii) Lysosome – Digestion of foreign agents
✅ Answer
(ii) SER is involved in lipid synthesis, but cellulose synthesis occurs in plasma membrane (not SER).
Q12
What outcome if all mitochondria are removed from a eukaryotic cell?
✅ Answer
The cell would lack ATP production, leading to energy failure and cell death.
Q13
Which phenomenon inhibits tumor formation in human body? Can plants also develop tumors? Explain.
✅ Answer
Contact inhibition stops cell division when cells touch. Plants can develop tumors (e.g., crown gall disease) but they do not have contact inhibition due to cell walls.
Q14
Cell membrane synthesis – which organelles help? Write path from synthesis to membrane.
✅ Answer
RER (protein synthesis) → vesicles → Golgi apparatus (modification) → vesicles → cell membrane. Lipids from SER also contribute.
Q15
What would happen if gametes are formed by mitotic divisions?
✅ Answer
Gametes would be diploid; after fertilisation, zygote would have tetraploid number, doubling each generation, leading to genetic instability.
Q16
Farmer Deepa uses salt/sugar to preserve amla and lemons. (i) Which scientific concept? (ii) How does high salt/sugar prevent spoilage? (iii) Suggest a healthy recipe. (iv) Scientific values addressed.
✅ Answer
(i) Osmosis. (ii) High concentration causes plasmolysis in microbes, killing them. (iii) Pickle with turmeric, salt, and oil. (iv) Waste reduction, sustainable agriculture, economic upliftment.

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Chapter 2: Cell – The Building Block of Life (Complete Q&A)

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