NCERT Solutions - Class 9 Science Chapter 2: Cell: The Building Block of Life
Think It Over (Page No. 08)
Q1. Where does a cell come from?
Answer - According to cell theory, new cells arise from cells that already exist. A parent cell divides and passes its genetic material to the new cells.
Q2. How have technological interventions facilitated the creation of new knowledge in understanding the world beyond the naked eye?
Answer - Tools such as light microscopes and electron microscopes let scientists observe cells and structures too small to see with the naked eye. Better magnification, resolution, contrast and imaging methods have revealed cell parts and their organisation, helping scientists develop and improve explanations about how cells work.
Q3. How is the cell structural and functional unit of life?
Answer - Cells are the structural units of life because they make up every living organism. They are the functional units because life processes, such as taking in materials, releasing energy and making new cells, take place in cells.
Q4. How does a cell multiply?
Answer - A cell grows, copies its DNA and divides to form new cells. Mitosis makes two cells used for growth and repair. Meiosis makes reproductive cells with half the usual chromosome number.
What if... (Page No. 12)
Q1. What if mung bean seeds are kept in a concentrated solution after soaking in water for 12 hours?
Answer - The soaked seeds would lose water to the concentrated solution by osmosis. They would become less swollen and may shrink or wrinkle. The solution outside has less available water than the seed cells, so water moves out through their selectively permeable membranes.
Q2. What if a cell is kept in salt or sugar solutions of different concentrations?
Answer - The direction of water movement depends on the solution outside the cell:
(i) In an isotonic solution, the solute concentration is about equal inside and outside the cell, so there is no net movement of water and the cell stays about the same size.
(ii) In a hypotonic solution, the outside solution is more dilute, so water enters the cell. An animal cell may swell; a plant cell becomes firm because its wall resists expansion.
(iii) In a hypertonic solution, the outside solution is more concentrated, so water leaves the cell. The cell shrinks; in a plant cell, the contents pull away from the cell wall.
(i) In an isotonic solution, the solute concentration is about equal inside and outside the cell, so there is no net movement of water and the cell stays about the same size.
(ii) In a hypotonic solution, the outside solution is more dilute, so water enters the cell. An animal cell may swell; a plant cell becomes firm because its wall resists expansion.
(iii) In a hypertonic solution, the outside solution is more concentrated, so water leaves the cell. The cell shrinks; in a plant cell, the contents pull away from the cell wall.
In-Text Activity (Page No. 15)
Q1. Study the bacterial, plant and animal cells in Fig. 2.10. Observe the structures in each and record whether they are present or absent in Table 2.1.
| S. No. | Cell structures | Bacterial cell | Plant cell | Animal cell |
|---|---|---|---|---|
| 1 | Cell membrane | |||
| 2 | Cell wall | |||
| 3 | Cytoplasm | |||
| 4 | Well-defined nucleus (genetic material enclosed by a membrane) | |||
| 5 | Primitive nucleus or nucleoid (genetic material without a membrane around it) | |||
| 6 | Membrane-bound organelles |
Answer - Table 2.1 is completed below. A tick means the structure is present; a cross means it is absent.
| S. No. | Cell structures | Bacterial cell | Plant cell | Animal cell |
|---|---|---|---|---|
| 1 | Cell membrane | ✓ | ✓ | ✓ |
| 2 | Cell wall | ✓ | ✓ | ✗ |
| 3 | Cytoplasm | ✓ | ✓ | ✓ |
| 4 | Well-defined nucleus | ✗ | ✓ | ✓ |
| 5 | Nucleoid | ✓ | ✗ | ✗ |
| 6 | Membrane-bound organelles | ✗ | ✓ | ✓ |
Q2. Which of the cells shown in Fig. 2.10 are prokaryotic and which are eukaryotic?
Answer - The bacterial cell is prokaryotic because it has no membrane-enclosed nucleus. The plant and animal cells are eukaryotic because each has a well-defined nucleus and membrane-bound organelles.
Pause and Ponder (Page No. 14)
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 need firm support because they remain rooted in one place and must hold their stems and leaves upright. A strong cell wall helps each plant cell keep its shape and resist the pressure created when water enters. Animal cells lack a rigid wall, which lets them change shape and supports movement of animal tissues.
Q2. What consequences would you predict for a plant cell if its cell wall were to become as flexible as a cell membrane?
Answer - The cell would lose much of its rigid support and would not keep its usual shape as well. When water entered by osmosis, the flexible boundary might stretch too far, so the cell could swell or even burst. At the level of the plant, weakly supported cells could make stems and leaves less firm.
Q3. 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 two pieces should begin as alike as possible so their size or starting water content does not unfairly affect the comparison. Measuring their initial weights lets us calculate how much each piece gains or loses. Then a difference in weight can be linked more confidently to the liquid and osmosis.
Pause and Ponder (Page No. 19)
Q4. Do white flowers contain any pigment? Give reasons.
Answer - White petals usually have little or no pigment that absorbs visible light. They appear white because their surfaces scatter or reflect most visible colours back to our eyes. Some white flowers may still contain pigments that absorb ultraviolet light, which people cannot see.
Q5. 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.
(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 - One possible answer is a labelled plant cell. It shows the nucleus, ER, Golgi apparatus, mitochondrion, chloroplast, vacuole, cell membrane and cell wall.
Pause and Ponder (Page No. 22)
Q6. 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 - Many small mitochondria provide more total membrane surface area for their volume than one giant mitochondrion. The folded inner membranes are where many respiration reactions take place, so greater surface area allows more reactions at the same time. Small mitochondria can also be distributed near the parts of the cell that need energy.
Q7. If the skin cells start dividing by meiosis instead of mitosis, what do you think will happen to a cut on the skin?
Answer - A cut would not repair normally. Skin cells are body cells and use mitosis to make replacement cells with the same chromosome number. Meiosis makes reproductive cells with half the chromosome number, so using it in skin would produce cells with the wrong chromosome content and disrupt tissue repair.
Revise, Reflect, Refine
Q1. 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)
(i) Cell membrane and cell wall (permeability)
(ii) RER and SER (structure)
(iii) Chloroplasts and chromoplasts (pigments)
Answer - The differences are as follows:
| Cell membrane | Cell wall |
|---|---|
| Selectively permeable: it allows some substances to cross more easily than others. | Generally freely permeable to water and many dissolved substances through its pores. |
| Rough endoplasmic reticulum (RER) | Smooth endoplasmic reticulum (SER) |
|---|---|
| Ribosomes are attached to its surface, giving it a rough appearance. | Ribosomes are not attached, so its surface looks smooth. |
| Chloroplasts | Chromoplasts |
|---|---|
| Contain green chlorophyll, which absorbs light for photosynthesis. | Contain pigments, often yellow, orange or red, that colour many flowers and fruits. |
Q2. Two similar animal cells are placed in two different solutions: Cell X is placed in pure water, and Cell Y is placed in a concentrated salt solution. After some time, Cell X swells and Cell Y shrinks. Which statement correctly explains these 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 the salt solution entered 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.
(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 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 - (iii) is correct. Water enters Cell X from the pure water and leaves Cell Y for the more concentrated salt solution. This movement of water through the cell membrane is osmosis.
Q3. Look at the diagram of a cell in Fig. 2.20. Identify parts (a) to (g) and match them with these functions:
(i) Controls 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 the surroundings.
(v) Provides structural rigidity to the cell.
(vi) Packs and stores materials received from ER.
(vii) Helps in manufacturing food.
(i) Controls 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 the surroundings.
(v) Provides structural rigidity to the cell.
(vi) Packs and stores materials received from ER.
(vii) Helps in manufacturing food.
Answer - The labelled parts are: (a) mitochondrion, (b) nucleus, (c) Golgi body, (d) chloroplast, (e) cell wall, (f) cell membrane and (g) vacuole.
(i) (b) nucleus
(ii) (a) mitochondrion
(iii) (g) vacuole
(iv) (f) cell membrane
(v) (e) cell wall
(vi) (c) Golgi body
(vii) (d) chloroplast
(i) (b) nucleus
(ii) (a) mitochondrion
(iii) (g) vacuole
(iv) (f) cell membrane
(v) (e) cell wall
(vi) (c) Golgi body
(vii) (d) chloroplast
Q4. Which option(s) correctly place the cell organelles under the given categories?
Option (i): Present in plant cells — Leucoplast; absent in animal cells — Cell wall
Option (ii): Present in plant cells — Mitochondria; absent in animal cells — Ribosome
Option (iii): Present in plant cells — Cell wall; absent in animal cells — Golgi apparatus
Option (iv): Present in plant cells — Lysosome; absent in animal cells — Endoplasmic reticulum
Option (i): Present in plant cells — Leucoplast; absent in animal cells — Cell wall
Option (ii): Present in plant cells — Mitochondria; absent in animal cells — Ribosome
Option (iii): Present in plant cells — Cell wall; absent in animal cells — Golgi apparatus
Option (iv): Present in plant cells — Lysosome; absent in animal cells — Endoplasmic reticulum
Answer - Option (i) is correct. Leucoplasts occur in plant cells, and animal cells do not have a cell wall. The other options each include a structure that is present in animal cells, such as ribosomes, Golgi apparatus or endoplasmic reticulum.
Q5. Two students, Renu and Rohit, discuss plastids. Renu says all parts of plants, even roots, contain plastids. Rohit says plastids are absent from plant roots because roots are underground and do not need photosynthesis. Who is correct? Justify your answer.
Answer - Renu is correct that plant roots can contain plastids, but Rohit's statement is incorrect. Roots usually do not photosynthesise, so they generally lack green chloroplasts. They can contain colourless leucoplasts, which store food such as starch.
Q6. Mitochondria and chloroplasts are important organelles in a plant cell. Discuss how they are structurally and functionally similar to each other, and how they differ.
Answer - Similarities: Both have a double membrane and contain some of their own DNA and ribosomes, so they can make some of their own proteins. Both are involved in energy transformations in the cell.
Differences: Mitochondria carry out cellular respiration, releasing usable energy from food. Chloroplasts contain chlorophyll and use light energy to make sugars by photosynthesis. Mitochondria have a folded inner membrane; chloroplasts contain internal thylakoid membranes arranged in stacks within the stroma.
Differences: Mitochondria carry out cellular respiration, releasing usable energy from food. Chloroplasts contain chlorophyll and use light energy to make sugars by photosynthesis. Mitochondria have a folded inner membrane; chloroplasts contain internal thylakoid membranes arranged in stacks within the stroma.
Q7. Which of the following pairs of cell organelles contains DNA?
(i) Chloroplasts, ribosomes
(ii) Mitochondria, nucleus
(iii) Golgi bodies, ribosomes
(iv) Nucleus, lysosomes
(i) Chloroplasts, ribosomes
(ii) Mitochondria, nucleus
(iii) Golgi bodies, ribosomes
(iv) Nucleus, lysosomes
Answer - (ii) Mitochondria, nucleus. Both contain DNA. Chloroplasts also contain their own DNA, but ribosomes do not, so option (i) is not a pair in which both structures contain DNA.
Q8. A researcher takes two carrots of similar size and places one in plain water and the other in concentrated salt solution. After 24 hours, she records her observations.
(i) What hypothesis does she want to test through this experiment?
(ii) What would you suggest to improve 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?
(i) What hypothesis does she want to test through this experiment?
(ii) What would you suggest to improve 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) She is testing whether the concentration of the surrounding solution affects water movement into or out of carrot cells, and therefore affects how firm the carrot is.
(ii) Use equal-sized carrot pieces, the same volume of each solution, the same temperature and the same soaking time. Measure each piece's mass before and after, and repeat the experiment to make the result more reliable.
(iii) In plain water, water enters the carrot cells by osmosis and makes them firm. In concentrated salt solution, water leaves the cells by osmosis, so the cells lose firmness and the carrot becomes limp.
(ii) Use equal-sized carrot pieces, the same volume of each solution, the same temperature and the same soaking time. Measure each piece's mass before and after, and repeat the experiment to make the result more reliable.
(iii) In plain water, water enters the carrot cells by osmosis and makes them firm. In concentrated salt solution, water leaves the cells by osmosis, so the cells lose firmness and the carrot becomes limp.
Q9. Indicate whether each structure is present or absent in bacterial and animal cells: chromosome, nucleus, mitochondria, Golgi complex and chromoplasts.
Answer -
| Structure | Bacterial cell | Animal cell |
|---|---|---|
| Chromosome | Present (usually a circular DNA chromosome) | Present (linear chromosomes in the nucleus) |
| Nucleus | Absent | Present |
| Mitochondria | Absent | Present |
| Golgi complex | Absent | Present |
| Chromoplasts | Absent | Absent |
Q10. Carry out this experiment with four peeled potato halves scooped into cups. Place each cup in a beaker of water; use a boiled potato for Cup D. Keep Cup A empty, add one teaspoon of sugar to Cup B, one teaspoon of salt to Cup C and one teaspoon of sugar to Cup D. Observe the cups for at least two hours and answer:
(i) Explain why water gathers in the hollowed portions of Cups B and 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.
(i) Explain why water gathers in the hollowed portions of Cups B and 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 and salt make the solution inside Cups B and C more concentrated than the water outside. Water moves through the selectively permeable membranes of the potato cells towards the more concentrated solution, so it collects in the hollows. This is osmosis.
(ii) Cup A is the control. It shows what happens when no sugar or salt is added, so the results in Cups B and C can be compared with it.
(iii) Cup A has no concentrated solute in its hollow to draw water into it. In Cup D, boiling has damaged the cell membranes, so they can no longer act as selectively permeable barriers for osmosis.
(ii) Cup A is the control. It shows what happens when no sugar or salt is added, so the results in Cups B and C can be compared with it.
(iii) Cup A has no concentrated solute in its hollow to draw water into it. In Cup D, boiling has damaged the cell membranes, so they can no longer act as selectively permeable barriers for osmosis.
Q11. 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
(i) Ribosome — Protein synthesis
(ii) SER — Lipid and cellulose synthesis
(iii) Lysosome — Digestion of foreign agents
Answer - (ii) is incorrectly matched. Smooth ER helps make lipids, but cellulose is made at the cell membrane by enzymes located there; it is not synthesised by the SER. Ribosomes make proteins, and lysosomes digest waste and some foreign materials.
Q12. What outcome do you expect if all the mitochondria are removed from a eukaryotic cell?
Answer - The cell would lose its main site for aerobic cellular respiration and would make far less usable energy. It might continue some processes briefly using energy from glycolysis in the cytoplasm, but it could not meet its normal energy needs and would eventually stop functioning or die.
Q13. Which phenomenon inhibits the formation of tumours in the human body? Can plants also develop tumours? Explain.
Answer - Contact inhibition helps prevent uncontrolled growth in many normal animal cells: they usually stop dividing when they contact neighbouring cells. Some cancer cells lose this response and continue dividing, forming tumours. Plants can also develop tumour-like growths, for example after infection by the bacterium that causes crown gall. Plant cells do not rely on contact inhibition in the same way; their rigid walls and growth controls shape their tissues.
Q14. The cell membrane is made of proteins and lipids. Which cell organelles help synthesise these components? Show the path from their sites of synthesis to the cell membrane in a labelled diagram.
Answer - Ribosomes attached to the rough endoplasmic reticulum make many membrane proteins. The smooth endoplasmic reticulum makes lipids. These components move in transport vesicles to the Golgi apparatus, where they can be modified and sorted. Vesicles then carry them to the cell membrane and fuse with it, adding their components to the membrane.
Q15. What would happen if gametes were formed by mitotic divisions?
Answer - Mitosis keeps the chromosome number unchanged, so the gametes would have the full chromosome number instead of half. When two such gametes joined during fertilisation, the offspring would receive twice the usual chromosome number. The chromosome number would keep increasing in later generations, disrupting normal development.
Q16. A farmer, Deepa, had a good harvest of amla (Indian gooseberry) and lemons but could sell only one-fourth of the produce locally. To reduce post-harvest loss, she used the excess fruit to prepare pickles, murabbas and sharbat by adding suitable amounts of salt, sugar or jaggery, then storing them in glass bottles. Based on the passage, answer:
(i) Which scientific concept has the farmer applied to preserve the produce?
(ii) How do high concentrations of salt and sugar create an environment that prevents spoilage-causing bacteria and fungi from growing?
(iii) Suggest a healthy recipe of this kind for food preservation.
(iv) What scientific values are addressed in this case?
(i) Which scientific concept has the farmer applied to preserve the produce?
(ii) How do high concentrations of salt and sugar create an environment that prevents spoilage-causing bacteria and fungi from growing?
(iii) Suggest a healthy recipe of this kind for food preservation.
(iv) What scientific values are addressed in this case?
Answer - (i) She has used the effect of a concentrated solution on cells, especially osmosis, as a food-preservation method.
(ii) A concentrated salt or sugar solution has less available water than the inside of microbial cells. Water moves out of those cells, making it difficult for the microbes to grow and multiply. The high solute concentration also lowers the water available for spoilage.
(iii) A healthier option is a small batch of amla pieces with a modest amount of salt, spices and lemon juice, stored in a clean, sealed jar in the refrigerator. Use safe food-handling practices and do not treat a low-salt recipe as shelf-stable.
(iv) The example shows resourcefulness, reducing food waste, responsible use of a harvest, scientific thinking and sustainable livelihood.
(ii) A concentrated salt or sugar solution has less available water than the inside of microbial cells. Water moves out of those cells, making it difficult for the microbes to grow and multiply. The high solute concentration also lowers the water available for spoilage.
(iii) A healthier option is a small batch of amla pieces with a modest amount of salt, spices and lemon juice, stored in a clean, sealed jar in the refrigerator. Use safe food-handling practices and do not treat a low-salt recipe as shelf-stable.
(iv) The example shows resourcefulness, reducing food waste, responsible use of a harvest, scientific thinking and sustainable livelihood.