Cell: The Building Block of Life
NCERT Question Answers
Revise, Reflect and Refine — all 16 questions solved with clear explanations, comparison tables and diagrams.
Try each question before opening its answer. For multiple-choice questions, tap an option first. Use “Open all answers” when revising.
Question 1 — Differentiate between the pairs
Differentiate between:
- Cell membrane and cell wall — permeability.
- Rough ER and smooth ER — structure.
- Chloroplasts and chromoplasts — pigments.
Show answer and explanation
(i) Cell membrane and cell wall
| Cell membrane | Cell wall |
|---|---|
| Selectively permeable: regulates the passage of substances into and out of the cell. | Generally permeable to water and many small dissolved substances. |
| Acts as the main selective barrier. | Provides support and protection; does not regulate exchange in the same selective way. |
(ii) Rough ER and smooth ER
| Rough ER — RER | Smooth ER — SER |
|---|---|
| Has ribosomes attached to its surface. | Lacks attached ribosomes. |
| Appears rough; commonly has flattened membrane sacs. | Appears smooth; commonly forms a network of membrane tubules. |
(iii) Chloroplasts and chromoplasts
| Chloroplasts | Chromoplasts |
|---|---|
| Contain green chlorophyll, which absorbs light for photosynthesis. | Contain pigments such as yellow, orange and red carotenoids. |
| Common in photosynthetic plant tissues. | Contribute colour to many fruits, flowers and other plant parts. |
Question 2 — Animal cells in different solutions
Cell X is placed in pure water and swells. Cell Y is placed in a concentrated salt solution and shrinks. Which statement correctly explains this?
Show complete answer
- Cell X: Pure water is hypotonic relative to its contents. Net water movement is inward by osmosis, so the cell swells and may eventually burst.
- Cell Y: Concentrated salt solution is hypertonic relative to its contents. Net water movement is outward by osmosis, so the cell shrinks.
Osmosis concerns the movement of water, rather than solute.
Question 3 — Identify cell parts and their functions
Identify the parts labelled (a)–(g) in the given plant-cell diagram and match them with their functions.
The answer table follows the labels in the question’s diagram. The illustration below is a simplified revision diagram.
Show labels, diagram and functions
| Label | Part | Matching function |
|---|---|---|
| (a) | Mitochondrion | (ii) Site of cellular respiration. |
| (b) | Nucleus | (i) Regulates cell activities through genetic instructions. |
| (c) | Golgi apparatus | (vi) Processes and packages materials received from ER. |
| (d) | Chloroplast | (vii) Helps manufacture food through photosynthesis. |
| (e) | Cell wall | (v) Provides structural rigidity. |
| (f) | Cell membrane | (iv) Separates the contents from the surroundings. |
| (g) | Vacuole | (iii) Stores materials and helps maintain firmness through turgor pressure. |
Question 4 — Correctly classified cell structures
Which pair correctly lists a structure present in plant cells and a structure absent in animal cells?
Show complete answer
Leucoplasts occur in suitable plant cells. Animal cells lack a cell wall.
The other options are incorrect because animal cells have ribosomes, Golgi apparatus and endoplasmic reticulum.
Question 5 — Do roots contain plastids?
Renu says plastids occur even in roots. Rohit says underground roots lack plastids because they do not need photosynthesis. Who is correct? Explain.
Show answer and explanation
Renu is correct in the intended comparison. Non-green plant parts, including roots, can contain plastids.
Roots generally do not require chloroplasts for photosynthesis, but their cells may contain colourless plastids, including leucoplasts that store materials such as starch.
Rohit has confused plastids with chloroplasts. Chloroplasts are only one type of plastid.
Question 6 — Mitochondria and chloroplasts
Explain how mitochondria and chloroplasts are structurally and functionally similar and different.
Show comparison
Similarities
- Both are surrounded by two membranes.
- Both contain their own DNA and ribosomes.
- Both can make some of their own proteins.
- Both take part in energy-conversion processes.
Differences
| Feature | Mitochondria | Chloroplasts |
|---|---|---|
| Main process | Key steps of cellular respiration. | Photosynthesis. |
| Energy role | Help convert energy from food into ATP. | Convert light energy into chemical energy used to make sugars. |
| Internal structure | Inner membrane folds called cristae. | Thylakoid membranes, often arranged in grana, within the stroma. |
| Chlorophyll | Absent. | Present in thylakoid membranes. |
| Occurrence | Most plant and animal cells. | Photosynthetic plant cells and many algae. |
Question 7 — Which pair contains DNA?
Select the pair in which both organelles contain DNA.
Show complete answer
The nucleus contains most of the genetic material of a eukaryotic cell. Mitochondria also have their own DNA.
Chloroplasts contain DNA too, but option (i) is incorrect because ribosomes do not contain DNA.
Question 8 — Carrots in water and salt solution
Two similar carrots are placed in plain water and concentrated salt solution for 24 hours.
- What hypothesis is being tested?
- How could the experiment be improved?
- Why does one carrot remain firm while the other becomes limp?
Show all three answers
(i) Hypothesis
Water moves into or out of carrot cells by osmosis depending on the surrounding solution, changing their mass and firmness.
(ii) Improvements
- Use similar pieces from the same carrot.
- Record their initial masses.
- Use equal liquid volumes and a measured salt concentration.
- Keep soaking time and temperature the same.
- Repeat with several pieces.
- Blot away surface liquid before weighing again.
(iii) Explanation
In plain water: water enters the cells. Vacuoles expand and turgor pressure increases, making the carrot stiff and crunchy.
In concentrated salt solution: water leaves the cells. Turgor pressure falls, making the carrot limp and rubbery. Strong water loss may cause plasmolysis.
Question 9 — Bacterial and animal cells
Indicate whether chromosomes, nucleus, mitochondria, Golgi complex and chromoplasts occur in bacterial and animal cells.
Show presence-and-absence table
| Structure | Typical bacterial cell | Typical animal cell |
|---|---|---|
| Chromosome | Present: usually a circular chromosome in the nucleoid. | Present: usually several linear chromosomes in the nucleus. |
| Membrane-bound nucleus | Absent. | Present. |
| Mitochondria | Absent. | Present. |
| Golgi complex | Absent. | Present. |
| Chromoplasts | Absent. | Absent. |
These answers describe typical cells. Specialised animal cells, such as mature human red blood cells, are exceptions to some features.
Question 10 — Potato-cup experiment
Four hollow potato cups are placed in water:
- A: raw potato, hollow left empty.
- B: raw potato, sugar added.
- C: raw potato, salt added.
- D: boiled potato, sugar added.
Explain water collection in B and C, the purpose of A, and the lack of appreciable collection in A and D.
Show all parts of the answer
(i) Why water collects in B and C
The sugar and salt create concentrated solutions in the hollows. Water moves through the living potato tissue toward these solutions by osmosis and accumulates inside.
(ii) Why Cup A is necessary
Cup A is the control. It helps show that accumulation in B and C is associated with the added solute rather than simply with a hollow potato placed in water.
(iii) Why water does not collect appreciably in A and D
- A: no solute has been added to establish the same inward osmotic driving force.
- D: boiling damages selectively permeable cell membranes. The tissue no longer demonstrates osmosis in the same way as living tissue.
Question 11 — Incorrect organelle–function pair
Which pair is incorrectly matched?
Show complete answer
Smooth ER helps synthesise lipids. Cellulose is synthesised by cellulose-synthesising complexes at the plant cell membrane, rather than by the SER.
Ribosomes synthesise proteins. Lysosomes digest materials, including foreign material taken into cells.
Question 12 — Removing mitochondria
What outcome would you expect if all mitochondria were removed from a eukaryotic cell?
Show answer and explanation
The cell would lose its normal capacity for mitochondrial respiration and oxidative ATP production.
Its energy supply would fall sharply. Activities such as active transport, synthesis and maintenance would be disrupted. A cell heavily dependent on mitochondria would eventually die.
Question 13 — Growth control and tumours
Which phenomenon helps inhibit tumour formation in humans? Can plants also develop tumours?
Show answer and explanation
Contact inhibition is the intended answer. Many normal animal cells reduce or stop division when crowded by neighbouring cells. Cancer cells can lose this control.
Cell-cycle checkpoints and programmed cell death also help prevent abnormal growth.
Yes, plants can develop tumours. Crown gall disease is an example of abnormal growth caused by infection with certain bacteria.
Plant growth regulation differs from animal contact inhibition. Rigid cell walls limit cell movement, so such growths generally do not spread like animal cancers.
Question 14 — Making the cell membrane
Which organelles help synthesise the proteins and lipids of the cell membrane? Show their path to the membrane using a labelled diagram.
Show answer and pathway
- Smooth ER: synthesises many membrane lipids.
- Ribosomes associated with rough ER: synthesise many membrane proteins, which enter or are inserted into the ER membrane.
- Golgi apparatus: modifies and sorts materials arriving from the ER.
- Transport vesicles: carry components between compartments and to the surface.
Ribosomes on rough ER: membrane proteins
When a vesicle fuses with the surface membrane, its lipids and membrane proteins become part of the cell membrane.
Question 15 — Gametes formed by mitosis
What would happen if gametes were formed by mitotic divisions?
Show answer and chromosome example
If gametes formed directly by mitosis from diploid cells, they would retain the full chromosome number rather than half.
Fusion of two such gametes would double the chromosome number.
Hypothetical diploid gametes: 46 + 46 = 92
This would disrupt the normal chromosome number. The variation generated by meiotic recombination and chromosome assortment would also be absent from that process.
Question 16 — Deepa’s fruit-preservation case
Deepa uses surplus amla and lemons to make products such as pickles, murabba and sharbat with salt, sugar or jaggery.
- Which scientific concept is applied?
- How do concentrated salt and sugar inhibit spoilage organisms?
- Suggest a healthy preparation.
- Which scientific values are demonstrated?
Show all four answers
(i) Scientific concept
Osmosis and reduced water availability caused by high concentrations of salt or sugar.
(ii) Inhibiting microbial growth
High concentrations create a hypertonic environment. Many microbial cells lose water, interfering with their normal activities.
Salt and sugar also lower water activity — the water available for microbial growth. This inhibits many spoilage-causing bacteria and fungi.
It does not necessarily kill every microorganism. Concentration, cleanliness and appropriate storage remain important.
(iii) A healthy preparation
A small batch of fresh lemon-and-amla relish is a suitable example:
- Wash and chop the fruit using clean utensils.
- Add modest amounts of salt and spices.
- Keep it covered in a clean container in the refrigerator.
(iv) Scientific values
- Applying science to practical problems.
- Reducing food waste.
- Responsible use of resources.
- Hygienic preparation and careful handling.
- Creativity and problem-solving.
- Supporting livelihoods through value-added products.
Close the answers and explain each concept in your own words. For the experiments, identify the conditions, the expected observation and the scientific reason.
