Page 8 – Think It Over
Q. Where does a cell come from?
Answer: A cell comes from a pre-existing cell — new cells are formed only by division of existing cells, not from non-living material.
Q. How have technological interventions facilitated the creation of new knowledge in understanding the world beyond the naked eye?
Answer: Microscopes (light and electron) let scientists see objects smaller than the human eye’s limit of resolution (0.1 mm), revealing cell structures down to the nanometre scale.
Q. How is the cell structural and functional unit of life?
Answer: All life processes — growth, respiration, reproduction — happen through cells. Even in tissues and organs, the cell stays the basic unit of structure and function.
Q. How does a cell multiply?
Answer: By cell division — mitosis (growth/repair, 2 identical daughter cells) and meiosis (forms gametes, 4 daughter cells with half chromosomes).
Page 9 – Limit of Resolution
Q. What do we call the ability of the human eye to see two very close objects as separate and distinct?
Answer: It is called the limit of resolution of the human eye, which is 0.1 mm.
Page 10 – Magnification
Q. Under the microscope, you will see a magnified image of an object. Can you estimate its actual size?
Answer: Yes — Estimated size = Diameter of visible field (µm) ÷ Number of cells along the diameter. For example, 5000 µm ÷ 25 cells = 200 µm per cell.
Page 11 – Cell Membrane
Q. How does the structure of the cell membrane in the cells of alveoli control the movement of substances across it?
Answer: The membrane is selectively permeable — it lets gases like oxygen and carbon dioxide pass while controlling what else enters or leaves.
Page 12 – What If…
Q. What if mung bean seeds are kept in a concentrated solution after soaking in water for 12 hours? What will happen to them?
Answer: The swollen (water-filled) seeds will lose water by osmosis into the concentrated solution and shrink/become less swollen.
Q. What if a cell is kept in salt or sugar solutions of different concentrations?
Answer:
- Isotonic solution – equal concentration inside/outside, no change.
- Hypotonic solution – outside concentration lower, water enters, cell swells.
- Hypertonic solution – outside concentration higher, water leaves, cell shrinks.
Page 13 – Cell Wall
Q. Cells of plant, fungi, and bacteria have an additional layer around the cell membrane, called the cell wall. What do you think is the necessity of the cell wall in these cells?
Answer: Since these organisms can’t move, the rigid cell wall protects them and gives structural support against wind, rain, and other stress.
Q. Onion peel cells are box-shaped and regularly arranged, whereas cheek cells are irregularly arranged. Why do you think this difference exists?
Answer: Onion cells have a rigid cell wall giving a fixed shape; cheek cells only have a flexible membrane, so they can change shape easily.
Page 14 – Pause and Ponder
1. 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 can’t move, so they need a rigid wall for support against environmental stress. Animals need to move, so flexible membranes (no wall) allow shape change.
2. 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 its fixed shape and rigidity, droop/wilt more easily, and cells would shrink during osmosis like animal cells do.
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: Equal size ensures a fair comparison, and recording initial weight gives a baseline to measure the actual change caused by osmosis.
Page 15 – Prokaryotic vs Eukaryotic
Q. Which of the cells given in Fig. 2.10 fall under the categories of prokaryotic and eukaryotic cells?
Answer: Bacterial cell = prokaryotic (no defined nucleus). Plant cell and animal cell = eukaryotic (have a defined, membrane-bound nucleus).
Page 16 – Threads of Curiosity
Q. Do you know any other cells without nucleus?
Answer: The chapter gives mature RBCs as the example of nucleus-less (enucleate) cells, used so they can carry more haemoglobin/oxygen.
Page 19 – Plastids & Pause and Ponder
Q. Are there any other plastids in plant cells that contain any pigments other than the green pigments?
Answer: Yes — chromoplasts (yellow/orange/red pigments in flowers and fruits) and leucoplasts (colourless, store starch/oils/proteins).
4. Do white flowers contain any pigment? Give reasons.
Answer: No coloured pigment — their plastids are likely leucoplasts (colourless), so no wavelengths of light are specifically absorbed, making petals appear white.
5. Draw a well-labelled schematic diagram of a plant or an animal cell using the given clues.
Answer: Draw the cell with a dark round nucleus near the centre, ER as a branching network from the nuclear membrane, and rod-shaped mitochondria (and chloroplasts, for plant cells). Label all parts using Fig. 2.10 as reference.
Page 21 – Activity 2.5 & Cell Division
Q. Do you observe the cells of the onion root tip? Are they similar in structure? Do you find any structural differences in these cells? If yes, why is it so?
Answer: No, they are not all similar — different cells are caught at different stages of continuous cell division, so each looks structurally different.
Q. Can you identify which stage comes first during cell division?
Answer: The non-dividing stage, where chromatin appears as an entangled thread-like mass, comes first — before it organises into visible rod-shaped chromosomes.
Page 22 – Pause and Ponder
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: Many small mitochondria give more total surface area (cristae) for energy-producing reactions than one giant mitochondrion would, making energy production more efficient.
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: New cells would have only half the chromosomes and incomplete genetic information, so the cut would not heal properly with normal, functional skin cells.
Page 24 – Exercise Questions
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) Cell membrane: selectively permeable (allows only some substances). Cell wall: permeable (lets water/minerals pass freely).
(ii) RER: has ribosomes attached, looks rough. SER: no ribosomes, looks smooth.
(iii) Chloroplasts: contain green chlorophyll, for photosynthesis. Chromoplasts: contain yellow/orange/red pigments, give colour to flowers/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 the 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: Option (iii) — Water moved into Cell X and moved out of Cell Y through the cell membrane (by osmosis, due to concentration difference, not solute movement).
Page 25 – Exercise Questions
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: (i) Nucleus – controls activities; (ii) Mitochondria – respiration; (iii) Vacuole – storage + rigidity; (iv) Cell membrane – separates cell from surroundings; (v) Cell wall – rigidity; (vi) Golgi body – packs ER materials; (vii) Chloroplast – food synthesis.
4. Which of the following option(s) of the pairs of cell organelles are correctly placed under the given categories? Option | Present in the plant cells | Absent in the animal cells — (i) Leucoplast | Cell wall (ii) Mitochondria | Ribosome (iii) Cell wall | Golgi apparatus (iv) Lysosome | Endoplasmic reticulum
Answer: Option (i) — Leucoplast / Cell wall. Both are found in plant cells and genuinely absent in animal cells.
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 don’t have chloroplasts (no sunlight), but they do have leucoplasts, which store food like starch — so plastids are still present.
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: Similar: both double-membrane-bound, contain their own DNA & ribosomes, linked to energy processes. Different: Mitochondria have cristae and do cellular respiration (release energy as ATP); chloroplasts have stroma with chlorophyll and do photosynthesis (make food using light).
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: Option (ii) — Mitochondria, Nucleus. (Plastids also have DNA, but among these options, mitochondria and nucleus is correct.)
Page 26 – Exercise Questions
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) Tests if osmosis (water movement) depends on solution concentration and affects firmness.
(ii) Use repeated trials, record exact weights, keep size/time/temperature same.
(iii) Plain water carrot: water enters cells by osmosis, stays firm. Salt water carrot: water leaves cells by osmosis, becomes limp.
9. Indicate the presence or absence of following structures in bacterial and animal cells: Structures in a cell — Chromosome, Nucleus, Mitochondria, Golgi complex, Chromoplasts.
Answer: Bacterial cell: all five — Absent (bacteria have a nucleoid, not true chromosomes/nucleus/organelles). Animal cell: Chromosome, Nucleus, Mitochondria, Golgi complex – Present; Chromoplasts – Absent (plant-only organelle).
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) Added sugar/salt raises solute concentration in the hollow, so water moves in by osmosis.
(ii) Cup A is the control — shows no water collects without an added solute.
(iii) Cup A has no concentration difference; Cup D’s potato is boiled (dead cells, damaged membrane), so osmosis can’t happen.
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: Option (ii) is incorrect. SER makes/stores lipids and hormones, not cellulose.
12. What outcome do you expect, if all the mitochondria are removed from a eukaryotic cell?
Answer: The cell can’t carry out cellular respiration, so it can’t produce ATP (energy). It would lack energy for normal functions and eventually couldn’t survive.
13. Which phenomenon inhibits the formation of tumors in the human body? Can plants also develop tumors? Explain.
Answer: Contact inhibition stops normal cell division when cells touch neighbours. Plant cells don’t show contact inhibition (due to rigid cell walls) and grow differently.
Page 27 – Exercise Questions
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: Ribosomes (on RER) make proteins; SER makes lipids → both go to the Golgi apparatus → packaged into vesicles → vesicles fuse with the cell membrane. (See Fig. 2.13: Ribosome → RER → SER → Golgi → Vesicles → Plasma membrane.)
15. What would happen if gametes are formed by mitotic divisions?
Answer: Gametes would have the full chromosome number instead of half. Fertilisation would then double the normal chromosome number, causing genetic imbalance and developmental problems.
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) Osmosis.
(ii) High salt/sugar makes the solution hypertonic, so water moves out of microbial cells by osmosis, shrinking and killing them, stopping spoilage.
(iii) E.g., amla murabba in sugar syrup, or lemon pickle with salt.
(iv) Practical use of science, reducing food waste, supporting sustainability, and using traditional knowledge effectively.