This page has detailed explanations for the NCERT questions of class 9 science chapter 2 "Cell : The Building Block of Life".
In-text Questions (Pause and Ponder)
Q1. 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 are stationary organisms and cannot move to find shelter. They are constantly exposed to environmental stresses like wind, rain, and temperature changes. A rigid cell wall is essential for providing structural strength to plants. This cell wall offers mechanical support and protection against external forces.Animal cells do not need a cell wall because animals can physically move from one place to another to escape unfavorable weather, seek shelter, or protect themselves, meaning their cells can remain flexible to facilitate smooth movement and locomotion.
2. What consequences would you predict for a plant cell if its cell wall were to become as flexible as a cell membrane?
Answer: If the cell wall of a plant cell became as flexible as a cell membrane, the entire plant would lose its mechanical strength and structural rigidity, causing it to fall down under its own weight or wilt completely.
Furthermore, the cell wall protects plant cells from bursting when they take in too much water through osmosis. Rigid cell wall exerts a counter pressure to save the plant from this crushing force. Without a rigid wall, plant cells would easily burst when placed in a hypotonic environment like plain rainwater, leading to tissue damage and death for the plant.
3. 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:
The potato pieces should be the same size and weight at the start. This makes the test fair. If one piece is bigger than another, it may gain or lose more mass. Then we would not know if the change was caused by the liquid or by the size of the potato piece. Keeping the pieces equal helps us see the true effect of the different liquids on the potato.
4. Do white flowers contain any pigment? Give reasons.
Answer: No. White flowers look white because they do not have strong colored pigments.
Inside the petals, there are tiny air spaces between the cells. When sunlight hits these air spaces, all colors of light are reflected and scattered equally. Since all the colors are reflected together, our eyes see the petals as white, just like snow looks white.
5. Draw a well-labelled schematic diagram of a plant or an animal cell using these clues—
(i) Nucleus appears as a dark and round body inside the cell.
(ii) ER spreads like a network of extended nuclear envelope.
(iii) Mitochondria and chloroplasts are rod shaped. You may refer to Fig. 2.10.
Answer: To draw a clear schematic diagram based on these clues, we should choose a plant cell layout since it includes both mitochondria and chloroplasts.
- Draw a large rectangle with a double boundary for the cell wall and cell membrane.
- Draw a large oval inside for the vacuole.
- Draw a dark round shape for the nucleus.
- Draw lines coming from the nucleus and spreading through the cell for the endoplasmic reticulum (ER).
- Draw a few rod-shaped mitochondria with wavy lines inside.
- Draw a few rod-shaped chloroplasts with straight lines inside.
- Label all parts clearly: cell wall, cell membrane, nucleus, ER, vacuole, mitochondria, and chloroplasts.
6. Instead of many small ones, why does a cell not have a single giant mitochondrion? How does this relate to the concept of surface area?
Answer:
- Cells have many small mitochondria instead of one large mitochondrion because small mitochondria work better.
- Many small mitochondria provide more surface area for energy production. This helps the cell make energy quickly and efficiently.
- They also allow oxygen and nutrients to move in and waste materials to move out more easily. As a result, the cell can meet its energy needs more effectively.
7. If the skin cells start dividing by meiosis instead of mitosis, what do you think will happen to a cut on the skin?
Answer:
If skin cells divided by meiosis, cuts and wounds would not heal properly.
- Mitosis makes new skin cells that are exactly the same as the old cells. This helps the body repair damaged skin.
- Meiosis makes cells with only half the normal number of chromosomes and is used to make sex cells. If skin cells used meiosis, the new cells would not work properly and could not repair the skin.
Exercise - Revise, Reflect and 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: Differences between the given components are explained below:
(i) Cell membrane and Cell wall
- Cell membrane: Selectively permeable. It allows some substances to pass through and blocks others.
- Cell wall: Fully permeable. It allows substances to pass through easily.
- Rough ER: Has ribosomes on its surface, so it looks rough.
- Smooth ER: Does not have ribosomes, so it looks smooth.
- Chloroplasts: Contain chlorophyll, a green pigment used in photosynthesis.
- Chromoplasts: Contain yellow, orange, or red pigments that give color to flowers and fruits.
2. Two similar animal cells are placed in two different solutions:
• Cell X is placed in pure water.
• Cell Y is placed in a concentrated salt solution.
Cells are observed after some time. Cell X swells, and Cell Y shrinks. Which statement provides the correct explanation for the above observations?
(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 salt solution entered in it.
(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: The correct option is (iii) Water moved into Cell X and moved out of Cell Y through the cell membrane.
Pure water has more water than the cell, so water moves into Cell X by osmosis. This makes the cell swell. A concentrated salt solution has less water than the cell, so water moves out of Cell Y by osmosis. This makes the cell shrink. Osmosis is the movement of water through a semi-permeable membrane. Water moves, not the salt.
Therefore, statement 3 is correct.
3. Look at the diagram of a cell in Fig. 2.20. Identify the parts labelled from (a) to (g) and correctly match them with their functions given below:
(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:
(a) – Mitochondrion (Site of cellular respiration.)
(b) – Nucleus (Controlling all the activities of a cell.)
(c) – Golgi Apparatus (Packs and stores materials received from ER.)
(d) – Chloroplast (Helps in manufacturing food.)
(e) – Cell Wall (Provides structural rigidity to the cell.)
(f) – Cell Membrane (Separates the cell contents from surroundings.)
(g) – Vacuole (Storage organelle that also provides rigidity to the cell.)
4. Which of the following option(s) of the pairs of cell organelles are correctly placed under the given categories?
Answer:
The correct pairs are:
(ii) Mitochondria – Ribosome ✔️
Both are cell organelles.
(iv) Lysosome – Endoplasmic Reticulum ✔️
Both are cell organelles.
The incorrect pairs are:
(i) Leucoplast – Cell wall ✖️
Leucoplast is a cell organelle, but the cell wall is a cell covering.
(iii) Cell wall – Golgi apparatus ✖️
Cell wall is a cell covering, while Golgi apparatus is a cell organelle.
Answer: (ii) and (iv) ✅
5. Two students, Renu and Rohit, were having a discussion on the plastids. Renu emphasised that all parts of the plants, even roots, contain plastids. However, Rohit did not agree with the statement and told her that plastids are absent in plant roots since the roots are underground and do not need to perform photosynthesis. Who is correct? Justify your answer.
Answer:
Renu is correct. Roots also contain plastids. They do not have green chloroplasts, but they have leucoplasts, which store food such as starch, oils, and proteins. So, plastids are present in roots, even though roots do not perform photosynthesis.
6. Mitochondria and chloroplasts are two important organelles in a plant cell. Discuss how these two organelles are structurally and functionally similar to each other, and different from each other.
Answer:
Similarities:
- Both have a double membrane.
- Both contain their own DNA and ribosomes.
- Both help in energy production.
Differences:
- Mitochondria carry out cellular respiration and release energy from food.
- Chloroplasts carry out photosynthesis and make food using sunlight.
- Chloroplasts contain chlorophyll, while mitochondria do not.
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:
Answer: (ii) Mitochondria, Nucleus
The nucleus contains most of the cell's DNA. Mitochondria also contain a small amount of their own DNA. Ribosomes, Golgi bodies, and lysosomes do not contain DNA.
8. A researcher carried out an experiment in which she took two carrots of similar size. She placed one carrot in plain water and the other carrot in concentrated salt solution (Fig. 2.21). After 24 hours she recorded her observations.
(i) What hypothesis does she want to test through this experiment?
(ii) What would you suggest for the improvement of this experiment?
(iii) Why does the carrot in plain water stay stiff and crunchy, but the carrot in concentrated salt solution become rubbery and limp?
Answer:
(i) The experiment tests whether water moves into or out of plant cells by osmosis depending on the concentration of the surrounding solution.
(ii) The experiment can be improved by:
- Keeping a control carrot in an empty beaker.
- Measuring the mass and size of each carrot before and after the experiment.
(iii) The carrot in plain water becomes hard and crisp because water enters its cells by osmosis.
The carrot in salt solution becomes soft and limp because water leaves its cells by osmosis.
9. Indicate the presence or absence of following structures in bacterial and animal cells:
Answer:
| Feature | Bacterial Cell | Animal Cell |
|---|---|---|
| Chromosomes | Single circular chromosome | Multiple linear chromosomes |
| Nucleus | Absent | Present |
| Mitochondria | Absent | Present |
| Golgi Complex | Absent | Present |
| Chromoplasts | Absent | Absent |
Reason: Bacterial cells are simple cells without membrane-bound organelles. Animal cells are more complex and contain organelles such as the nucleus, mitochondria, and Golgi complex. Chromoplasts are found only in plant cells.
10. Carry out the following experiment: Take four peeled potato halves and scoop each one out to make potato cups. One of these potato cups should be made from a boiled potato. Place each of the potato cups in a beaker containing water (Fig. 2.22). Now, set up the experiment as follows:
(a) Keep Cup A empty.
(b) Add one teaspoon sugar in Cup B.
(c) Add one teaspoon salt in Cup C.
(d) Add one teaspoon sugar in the boiled potato in Cup D. Observe the four potato cups at least two hours and answer the following questions:
(i) Explain why water gathers in the hollowed portion of Cup B and Cup C.
(ii) Why is Cup A necessary for this experiment?
(iii) Explain why water does not gather in the hollowed portions of Cups A and D.
Answer:
(i) Sugar in Cup B and salt in Cup C create a concentrated solution inside the cups. Water moves through the potato tissue into the cups by osmosis.
(ii) Cup A is the control. It shows that water does not collect in the cup unless a solute such as salt or sugar is present.
(iii) No water collects in Cup A because there is no salt or sugar to draw water in. In Cup D, the potato has been boiled, so its cell membranes are damaged. Since osmosis needs a semi-permeable membrane, water does not move into the cup.
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) SER — Lipid and cellulose synthesis
This pair is incorrect because SER (Smooth Endoplasmic Reticulum) helps in the synthesis of lipids, but it does not make cellulose. Cellulose is made by enzymes present in the cell membrane of plant cells.
The other pairs are correct:
- Ribosomes → Protein synthesis
- Lysosomes → Digestion of foreign particles and waste materials.
12. What outcome do you expect, if all the mitochondria are removed from a eukaryotic cell?
Answer: If you remove every single mitochondrion from a eukaryotic cell, the cell will lose its primary energy generation units and will no longer be able to produce ATP through aerobic cellular respiration. Because ATP acts as the fundamental energy currency needed to drive metabolic processes, all internal operations will stop immediately due to the lack of power, leading directly to cell death.
13. Which phenomenon inhibits the formation of tumors in the human body? Can plants also develop tumors? Explain.
Answer:
Apoptosis (programmed cell death) helps prevent tumor formation in humans. It destroys damaged or abnormal cells before they can divide uncontrollably.
Plants can also develop tumors, called galls. These are often caused by parasites or bacteria such as Agrobacterium tumefaciens, which can make plant cells divide rapidly and form abnormal growths.
14. The cell membrane of a cell is made up of proteins and lipids. Which cell organelles help in the synthesis of cell membrane? Write the path of these compounds from their site of synthesis to the cell membrane and show this through a labelled diagram.
Answer:
Membrane biogenesis is the process of making new cell membrane.
- Rough Endoplasmic Reticulum (RER) makes proteins.
- Smooth Endoplasmic Reticulum (SER) makes lipids (fats).
- These materials move to the Golgi apparatus for modification and packaging.
- The Golgi sends them in vesicles to the cell membrane.
- The vesicles fuse with the cell membrane, helping it grow and repair itself.
Pathway:
RER + SER → Golgi Apparatus → Vesicles → Cell Membrane.
15. What would happen if gametes are formed by mitotic divisions?
Answer:
If gametes were formed by mitosis, they would have the full number of chromosomes instead of half.
When two such gametes fused during fertilization, the offspring would receive double the normal number of chromosomes. The chromosome number would keep increasing in each generation, causing serious genetic problems.
Therefore, gametes are formed by meiosis, which reduces the chromosome number by half.
16. A farmer, Deepa, was very happy with the harvest of amla (Indian Gooseberry) and lemons on her farm. However, she could sell only one-fourth of the produce in the local market. Recognising that a significant amount of produce may be lost post-harvest, she employed a traditional yet scientifically sound method to extend the shelf life of amla and lemons. She turned perishable produce into profitable products, such as pickles and sharbat. She used the excess produce to prepare pickles, murabbas, and sharbat by adding appropriate amounts of salt, sugar, or jaggery to small pieces of fruit and their juices. These were then stored in small glass bottles for sale, helping her prevent the wastage of post-harvest produce. This shift from farming to agro-processing would strengthen food security and boost the local economy, creating a sustainable model that cuts waste while increasing her income. Based on the above passage answer the following questions:
(i) Which scientific concept has the farmer applied in the preservation of the farm produce?
(ii) How does the addition of high concentrations of salt and sugar create an environment that prevents the growth of spoilage-causing bacteria and fungi?
(iii) Suggest a healthy recipe of this kind for food preservation.
(iv) What are the scientific values addressed in this case?
Answer:
(i) The farmer is using osmosis to preserve food.
(ii) A large amount of sugar or salt creates a hypertonic solution around the food. Water moves out of bacteria and fungi by osmosis, causing them to shrink and die. This prevents food spoilage.
(iii) One example is Amla Murabba, where amla fruits are preserved in a thick sugar syrup.
(iv) This shows:
- Use of scientific knowledge in daily life.
- Reduction of food wastage.
- Better preservation of food resources.