Class 9 Science Chapter 2: Cell — The Building Block of Life Solutions

These Class 9 Science Exploration Chapter 2 solutions cover Cell: The Building Block of Life from NCERT’s Exploration, first edition April 2026 (printed pages 8–27).

Class: 9 Subject: Science Book: Exploration Chapter: 2 Exercise: Revise, Reflect, Refine (16 Qs) Session: 2026–27
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Class 9 Science Exploration Chapter 2 Solutions – Overview

Chapter 2 of Exploration, Cell: The Building Block of Life, establishes the cell as the basic structural and functional unit of all living things. It covers how cells are studied with light and electron microscopes, the cell membrane (selectively permeable) and cell wall, the nucleus and the main organelles (mitochondria, endoplasmic reticulum, Golgi apparatus, ribosomes, plastids, vacuoles, lysosomes), the processes of diffusion and osmosis, and cell division (mitosis and meiosis). The 16 end-of-chapter questions, introductory prompts, activities and Pause and Ponder questions are explained below.

Key Concepts & Definitions

Cell: the basic structural and functional unit of life; every cell arises from a pre-existing cell.

Cell membrane: the selectively permeable boundary controlling what enters and leaves the cell. Cell wall: a rigid outer layer in plants, fungi and most bacteria, permeable to water and many small dissolved substances.

Nucleus: contains most of a eukaryotic cell’s DNA and regulates many activities through gene expression. Prokaryotic cells (bacteria) have no true nucleus; eukaryotic cells do.

Key organelles: mitochondria (respiration/energy), ribosomes (protein synthesis), ER (transport; RER has ribosomes, SER does not), Golgi apparatus (packaging), plastids (chloroplasts, leucoplasts, chromoplasts), vacuole (storage/rigidity), lysosome (digestion).

Diffusion: net movement of particles from higher to lower concentration (no membrane needed). Osmosis: movement of water across a selectively permeable membrane.

Cell division: mitosis (growth and repair) and meiosis (halves chromosome number; contributes to gamete formation in animals).

“Think It Over” — Answers

Where does a cell come from?

ANSWEREvery cell arises from a pre-existing cell by cell division — a central idea of the cell theory.

How have technological interventions facilitated the creation of new knowledge in understanding the world beyond the naked eye?

ANSWERInstruments like the light microscope and the more powerful electron microscope let us see cells and tiny organelles that are invisible to the naked eye, revealing their detailed structure and how they work.

How is the cell the structural and functional unit of life?

ANSWERIt is structural because all living organisms are built from cells, and functional because every life process (nutrition, respiration, growth, reproduction) is carried out within cells.

How does a cell multiply?

ANSWER

Before division, DNA is copied. In mitosis, duplicated chromosomes are separated into two nuclei and the cytoplasm divides, producing two daughter cells with the parent cell’s chromosome number. Bacteria multiply by binary fission; meiosis is a distinct two-division process that halves chromosome number.

Activities 2.1–2.5 and Pause and Ponder

Activity 2.1: Estimating cell size

Use the field diameter measured at the same microscope setting as the cell count. Here D is the field diameter, N is the number of cells across it, and ℓ is the estimated cell size. The estimate assumes cells lie approximately end to end along the diameter.

ℓ=DN\ell=\frac{D}{N}5 mm=5000 μm,5000 μm25=200 μm5\,\mathrm{mm}=5000\,\mu\mathrm{m},\qquad \frac{5000\,\mu\mathrm{m}}{25}=200\,\mu\mathrm{m}Total magnification=10×10=100\text{Total magnification}=10\times10=100

The numerical values above are the textbook’s worked example, not measurements made for this page.

Activities 2.2 and 2.3: Water movement and cell walls

Record actual initial and final masses; do not invent observations. A potato piece in plain water is expected to gain mass, whereas one in a sufficiently concentrated salt or sugar solution is expected to lose mass through osmosis. Equal starting dimensions and recorded initial masses support a fair comparison.

In concentrated sugar solution, the plant protoplast can pull away from its wall (plasmolysis), while the wall retains its outline. Cheek cells lack a wall and shrink. Carry out slide preparation and staining with teacher supervision.

Activity 2.4: Comparing cells

StructureTypical bacteriumTypical plant cellTypical animal cell
Cell membrane and cytoplasmPresentPresentPresent
Cell wallUsually presentPresentAbsent
Membrane-bound nucleusAbsentPresentPresent
NucleoidPresentAbsentAbsent
Membrane-bound organellesAbsentPresentPresent

Bacteria are prokaryotic; plant and animal cells are eukaryotic. A nucleoid is a DNA-containing region, not a membrane-bound “primitive nucleus”.

Activity 2.5: Growing onion roots

New cells form by mitosis in the root-tip meristem, helping roots grow. A prepared or teacher-guided root-tip slide can show cells at different stages; record what is actually observed. Growth also includes cell enlargement. In mitosis, prophase comes first: chromosomes condense before alignment and separation. Root-tip preparation uses chemicals and heat and must be teacher-led.

Pause and Ponder: Answers 1–5

  1. Why a cell wall? It gives plant cells mechanical support and helps resist excessive expansion during water uptake. Animal cells require flexible boundaries for movement and changes of shape.
  2. If the wall became as flexible as the membrane: the cell would lose much of its rigid support and protection against osmotic expansion; plant tissues would be less firm.
  3. Why equal potato pieces and initial masses? To reduce differences in surface area and starting size, and calculate comparable changes in mass.
  4. Do white flowers contain pigment? White appearance usually reflects the absence or low amount of visible coloured pigments in the petals and scattering of light by their tissues. It does not prove every cell or every part of the flower lacks all pigments; other parts may contain chlorophyll and some petals have pigments not obvious to human vision.
  5. Labelled cell: the schematic below shows a plant cell. The nucleus is rounded; ER lies near it; mitochondria and chloroplasts have their characteristic internal membranes. Shapes and sizes vary in real cells.
Labelled plant-cell schematic showing outer wall, inner membrane, nucleus, ER, mitochondrion, chloroplast, Golgi, cytoplasm and large central vacuole.
Original plant-cell schematic, not to scale. Colours distinguish structures and do not represent a stained micrograph. Text descriptions and the label table provide the same information. Open the full-size diagram.

Open-ended extension: Sample response

A cell-division model can use reusable tokens for chromosomes and show DNA replication before division. A synthetic-cell discussion should distinguish a synthetic genome placed in an existing cell from making an entire cell from non-living materials, and consider containment, environmental effects and responsible oversight. Students’ models and ethical arguments may differ.

Class 9 Science Exploration Chapter 2 Solutions — Revise, Reflect, Refine

1. Differentiate between the following pairs of terms based on the clues given in parentheses: (i) Cell membrane and cell wall (permeability)  (ii) RER and SER (structure)  (iii) Chloroplasts and chromoplasts (pigments)

ANSWER

(i) The cell membrane is selectively permeable: passage depends on the substance and membrane transport mechanisms. The cell wall is relatively freely permeable to water and many small solutes, while providing support.

(ii) RER has ribosomes attached to its cytoplasmic surface and commonly forms flattened sacs; SER lacks attached ribosomes and is usually tubular.

(iii) Chloroplasts contain green chlorophyll (as well as accessory pigments). Chromoplasts accumulate coloured carotenoids, giving yellow, orange or red colours to many fruits and flowers.

2. Two similar animal cells are placed in two different solutions — Cell X in pure water, Cell Y in concentrated salt solution. Cell X swells and Cell Y shrinks. Which statement correctly explains this? (i) Salt molecules moved into Cell Y, causing it to shrink.  (ii) Water moved into Cell X and more water moved out of Cell Y than the salt solution entered.  (iii) Water moved into Cell X and moved out of Cell Y through the cell membrane.  (iv) Solute movement caused osmosis in both cells.

ANSWER (iii) — by osmosis, water moves into Cell X (surrounded by pure water, so it swells) and out of Cell Y (surrounded by concentrated salt solution, so it shrinks), across the selectively permeable cell membrane.

3. Look at the diagram of a cell in Fig. 2.20. Identify the parts labelled (a) to (g) and match them with their functions: (i) Controlling all the activities of a cell.(ii) Site of cellular respiration.(iii) Storage organelle that also provides rigidity to the cell.(iv) Separates the cell contents from surroundings.(v) Provides structural rigidity to the cell.(vi) Packs and stores materials received from ER.(vii) Helps in manufacturing food.

ANSWER

Reading the arrow tips in textbook Fig. 2.20:

LabelStructureFunction numberRole
aMitochondrioniiMost aerobic ATP production
bNucleusiRegulates cell activities
cGolgi apparatusviModifies, sorts and packages ER materials
dChloroplastviiPhotosynthesis
eCell wallvStructural support and rigidity
fCell membraneivSelective boundary of the cell
gCentral vacuoleiiiStores cell sap and maintains turgor

The outer wall is (e); the membrane just inside it is (f).

4. Which of the following option(s) of pairs of cell organelles are correctly placed under the categories “Present in plant cells” and “Absent in animal cells”? (i) Leucoplast / Cell wall (ii) Mitochondria / Ribosome (iii) Cell wall / Golgi apparatus (iv) Lysosome / Endoplasmic reticulum

ANSWER (i) — a leucoplast is present in plant cells, and a cell wall is absent in animal cells, so both entries are correct. The others fail (mitochondria, ribosomes, Golgi and ER are present in animal cells too).

5. Renu says all parts of plants, even roots, contain plastids; Rohit says plastids are absent in roots because roots do not perform photosynthesis. Who is correct? Justify.

ANSWER Renu is correct. Roots do contain plastids — in the form of leucoplasts (colourless plastids that store starch). Rohit confuses “plastids” with “chloroplasts”: typical underground roots have non-green plastids, often starch-storing amyloplasts, rather than photosynthetic chloroplasts. This does not mean every specialised plant cell contains every kind of plastid.

6. Discuss how mitochondria and chloroplasts are structurally and functionally similar to, and different from, each other.

ANSWER

Similarities: both have an outer and inner membrane, their own DNA and ribosomes, and roles in energy conversion.

Mitochondria: the inner membrane forms cristae around the matrix. They release energy from organic molecules during aerobic respiration, supplying ATP. They occur in most eukaryotic cells; mature mammalian red blood cells are an exception.

Chloroplasts: contain chlorophyll-bearing thylakoid membranes in the stroma. They use light energy to make organic food during photosynthesis, and occur in photosynthetic plant and algal cells. Plant cells also respire and generally contain mitochondria.

7. Which of the following pairs of cell organelles contains DNA? (i) Chloroplasts, Ribosomes (ii) Mitochondria, Nucleus (iii) Golgi bodies, Ribosomes (iv) Nucleus, Lysosomes

ANSWER (ii) Mitochondria, Nucleus — both contain DNA. (Chloroplasts also have DNA, but ribosomes, Golgi bodies and lysosomes do not.)

8. A researcher placed one carrot in plain water and another in concentrated salt solution (Fig. 2.21) and recorded observations after 24 hours. (i) What hypothesis does she want to test?(ii) What would you suggest to improve the experiment?(iii) Why does the carrot in plain water stay stiff and crunchy, but the one in salt solution become rubbery and limp?

ANSWER (i) Hypothesis: water moves into or out of the carrot’s cells by osmosis depending on the surrounding solution’s concentration. (ii) Improvements: use carrots of equal size/mass; measure and record mass and length before and after; keep the same volume of solution and the same temperature; repeat the trial for reliability. (iii) In plain water, water enters the carrot cells by osmosis, making them turgid → stiff and crunchy. In concentrated salt solution, water leaves the cells by osmosis, making them flaccid → rubbery and limp.

9. Indicate the presence or absence of the following structures in bacterial and animal cells: Chromosome, Nucleus, Mitochondria, Golgi complex, Chromoplasts.

ANSWER
StructureBacterial cellAnimal cell
ChromosomePresent (typically a circular chromosome)Present
Nucleus (true)AbsentPresent
MitochondriaAbsentPresent
Golgi complexAbsentPresent
ChromoplastsAbsentAbsent

10. Four peeled potato cups are placed in water: Cup A empty; Cup B + 1 tsp sugar; Cup C + 1 tsp salt; Cup D (boiled potato) + 1 tsp sugar. (i) Why does water gather in Cup B and Cup C?(ii) Why is Cup A necessary?(iii) Why does water not gather in Cups A and D?

ANSWER (i) The sugar/salt makes the contents of Cups B and C more concentrated than the surrounding water, so water moves into the cups by osmosis through the living potato cells and gathers there. (ii) Cup A (empty) is the control — it shows that without sugar or salt no water gathers, supporting the inference that added solute creates the conditions for osmotic water movement. (iii) Cup A lacks the concentrated sugar or salt solution needed to produce the same accumulation of water in the hollow. Cup D’s potato is boiled, which kills the cells and destroys their selectively permeable membranes, so osmosis cannot occur even though sugar is added.

11. Identify the pair that incorrectly matches the cell organelle with its function. (i) Ribosome — Protein synthesis (ii) SER — Lipid and cellulose synthesis (iii) Lysosome — Digestion of foreign agents

ANSWER (ii) is incorrect. The SER does synthesise lipids, but it does not synthesise cellulose, so this match is wrong. (Ribosome–protein synthesis and lysosome–digestion are correct.)

12. What outcome do you expect if all the mitochondria are removed from a eukaryotic cell?

ANSWER

Removing mitochondria from a typical mitochondria-containing eukaryotic cell would stop mitochondrial aerobic respiration and greatly reduce its ATP supply. Many energy-dependent processes would fail, and the cell would usually die.

Important correction: ATP production would not necessarily become zero. Glycolysis takes place in the cytoplasm and can still make a small amount of ATP. Some specialised cells, such as mature mammalian red blood cells, naturally function without mitochondria.

13. Which phenomenon inhibits the formation of tumours in the human body? Can plants also develop tumours? Explain.

ANSWER

Contact inhibition helps regulate growth: normal animal cells generally stop proliferating when crowded by neighbouring cells. Loss of this regulation can contribute to tumour formation, alongside other changes in cell-cycle control.

Plants can also develop tumours or tumour-like galls. For example, crown gall results from bacterial infection that alters plant growth regulation. Rigid walls do not make plants immune to abnormal growth.

14. The cell membrane is made of proteins and lipids. Which organelles help synthesise it? Write the path of these compounds from their site of synthesis to the cell membrane (with a labelled diagram).

ANSWER

Ribosomes attached to the rough ER synthesise membrane proteins; the smooth ER is important in lipid synthesis. Materials move from ER in transport vesicles to the Golgi apparatus for modification and sorting, then in vesicles that fuse with the plasma membrane.

Membrane proteins from rough ER and lipids from smooth ER pass through transport vesicles, Golgi and secretory vesicles to the plasma membrane.
Membrane biosynthesis: a simplified secretory pathway. Arrows show the direction of transport; not to scale.

15. What would happen if gametes are formed by mitotic divisions?

ANSWER

Assume the question concerns a diploid animal whose gametes normally arise through meiosis. Mitosis would retain the diploid chromosome number instead of producing haploid gametes. Fusion of two such gametes would give a tetraploid zygote:

2n+2n=4n2n+2n=4n

Normal meiosis and fertilisation instead maintain the species’ chromosome number:

n+n=2nn+n=2n

Without a reduction step, chromosome number could double in successive generations. Scope: gametes are not universally formed directly by meiosis; in flowering plants, meiosis produces spores and subsequent mitoses contribute to gamete formation.

16. Farmer Deepa preserved amla and lemons by making pickles, murabbas and sharbat with salt, sugar or jaggery. Based on the passage: (i) Which scientific concept did she apply?(ii) How does adding high concentrations of salt and sugar prevent the growth of spoilage bacteria and fungi?(iii) Suggest a healthy recipe of this kind.(iv) What scientific values are addressed?

ANSWER

(i) Deepa uses osmosis and reduced water availability to help preserve produce.

(ii) Concentrated salt or sugar creates a hypertonic environment, drawing water out of many microbial cells and inhibiting their growth. It also lowers water activity. This does not sterilise food: some yeasts, moulds and salt-tolerant microbes may still grow.

(iii) Sample proposal: prepare small portions of amla murabba using a tested preservation recipe, with fruit as the main ingredient, and serve it in moderation because it contains added sugar. A preserved food is not automatically healthy merely because it contains fruit. Follow a tested recipe’s processing and storage directions rather than guessing salt or sugar quantities.

(iv) Scientific reasoning, reducing post-harvest waste, resourcefulness, sustainable use of produce, and improving local livelihoods.

Further reading: National Center for Home Food Preservation: pickling principles.

Common Misconceptions to Avoid

Watch out for these

  • Treating the cell wall and cell membrane as the same — the wall is rigid and freely permeable; the membrane is selectively permeable.
  • Calling osmosis the same as diffusion — osmosis is specifically the movement of water across a selectively permeable membrane.
  • Thinking plastids occur only in leaves — roots have colourless plastids (leucoplasts).
  • Believing only the nucleus has DNA — mitochondria and chloroplasts have their own DNA too.
  • Assuming chloroplasts are in animal cells — they are characteristic of photosynthetic plant and algal cells.
  • Confusing mitosis with meiosis — mitosis is for growth/repair; meiosis halves chromosome number and contributes to sexual reproduction.

Extra Practice Questions

Very Short Answer Type Questions

Q1. Which organelle is called the “powerhouse of the cell”?

ANSWERThe mitochondrion (it releases energy as ATP through respiration).

Q2. Name the organelle that controls all the activities of the cell.

ANSWERThe nucleus.

Q3. What is the green pigment present in chloroplasts called?

ANSWERChlorophyll.

Short Answer Type Questions

Q1. Differentiate between diffusion and osmosis.

ANSWERDiffusion is the net movement of any particles from a higher to a lower concentration and needs no membrane. Osmosis is the movement of water across a selectively permeable membrane from a more dilute to a more concentrated solution.

Q2. Why is the cell membrane called selectively permeable?

ANSWERBecause it allows only certain substances to pass through while restricting others, thus controlling what enters and leaves the cell.

Long Answer Type Question

Q1. Compare mitosis and meiosis on the basis of where they occur, the number of divisions, and the daughter cells produced.

ANSWER Mitosis occurs in body (somatic) cells, involves one division, and produces two daughter cells that are genetically identical to the parent with the same chromosome number — used for growth and repair. Meiosis occurs in reproductive cells, involves two successive divisions, and produces four daughter cells with half the chromosome number and genetic variation — contributes to gamete production in animals; in plants it produces spores.

MCQs & Assertion–Reason

1. The basic structural and functional unit of life is the:

(a) tissue    (b) cell    (c) organ    (d) molecule

2. Which organelle is known as the “powerhouse of the cell”?

(a) nucleus    (b) ribosome    (c) mitochondrion    (d) Golgi apparatus

3. A cell wall is present in:

(a) animal cells only    (b) plant (and bacterial) cells    (c) no cells    (d) only nerve cells

4. The movement of water across a selectively permeable membrane is called:

(a) diffusion    (b) osmosis    (c) respiration    (d) transpiration

5. Which organelle controls all the activities of the cell?

(a) vacuole    (b) nucleus    (c) lysosome    (d) ribosome

6. Chlorophyll is found in:

(a) mitochondria    (b) chloroplasts    (c) ribosomes    (d) lysosomes

7. Which cell does NOT have a true nucleus?

(a) plant cell    (b) animal cell    (c) bacterial cell    (d) muscle cell

8. Cell division for growth and repair is:

(a) meiosis    (b) mitosis    (c) osmosis    (d) diffusion

9. Which organelle packs and dispatches materials received from the ER?

(a) ribosome    (b) Golgi apparatus    (c) mitochondrion    (d) vacuole

10. Ribosomes are the site of:

(a) respiration    (b) photosynthesis    (c) protein synthesis    (d) digestion

Answer key: 1-(b), 2-(c), 3-(b), 4-(b), 5-(b), 6-(b), 7-(c), 8-(b), 9-(b), 10-(c).

For each Assertion–Reason question, choose: (A) Both true and the Reason correctly explains the Assertion; (B) Both true but the Reason is not the correct explanation; (C) Assertion true, Reason false; (D) Assertion false, Reason true.

A-R 1. Assertion: Mitochondria are called the powerhouse of the cell.

Reason: They release energy in the form of ATP through cellular respiration.

A-R 2. Assertion: The cell membrane is selectively permeable.

Reason: It controls the movement of substances into and out of the cell.

A-R 3. Assertion: A bacterial cell has no true nucleus.

Reason: Bacteria are prokaryotes whose genetic material is not enclosed in a nuclear membrane.

A-R 4. Assertion: Osmosis is the movement of any particles from a higher to a lower concentration.

Reason: Osmosis is the movement of water across a selectively permeable membrane.

A-R 5. Assertion: Chloroplasts are found in animal cells.

Reason: Chloroplasts contain chlorophyll for photosynthesis.

Answer key: 1-(A), 2-(A), 3-(A), 4-(D), 5-(D).

Quick Revision Summary

  • The cell is the basic structural and functional unit of life; new cells come from existing cells.
  • Cell membrane = selectively permeable; cell wall = rigid and freely permeable (plants/bacteria).
  • Nucleus controls activities; prokaryotes (bacteria) lack a true nucleus.
  • Organelles: mitochondria (respiration), ribosomes (protein), ER & Golgi (synthesis/packing), plastids, vacuole, lysosome.
  • Diffusion = particles high→low; osmosis = water across a selectively permeable membrane.
  • Mitosis retains chromosome number for growth and repair; meiosis halves chromosome number.

Real-life Applications

Osmosis explains everyday preservation: salting fish or making fruit pickles, murabbas and jams draws water out of spoilage microbes so food lasts longer. It also explains why vegetables become crisp in plain water and limp in salty water, why salty soil can make water uptake difficult. Waterlogging mainly harms roots by reducing oxygen availability; it should not be explained simply as osmosis.

How to revise this chapter

Learn the organelles with one function each and be able to draw a labelled plant and animal cell. State the exact difference between diffusion and osmosis, and between mitosis and meiosis. For experiment questions, always identify the hypothesis, the control and the role of the selectively permeable membrane.

Frequently Asked Questions

What is Class 9 Science Exploration Chapter 2 about?

The cell as the unit of life — its structure, the membrane and wall, the nucleus and organelles, diffusion and osmosis, and cell division (mitosis and meiosis).

What is the difference between diffusion and osmosis?

Diffusion is the movement of particles from higher to lower concentration (no membrane needed); osmosis is the movement of water across a selectively permeable membrane.

How many questions are in the Revise, Reflect, Refine exercise?

16 questions, all solved on this page along with the “Think It Over” prompts.

Are these Class 9 Science Exploration Chapter 2 solutions free?

Yes. All solutions are free and follow the official NCERT Exploration textbook for 2026–27.

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