Q: Where does a cell come from? A: A new cell comes from a pre-existing (already existing) cell through a process called cell division. When a living cell grows, it divides into new cells to help an organism grow and heal.
Q: How have technological interventions facilitated the creation of new knowledge in understanding the world beyond the naked eye? A: Our eyes cannot see very tiny things clearly if they are smaller than 0.1 mm. Technological inventions like light microscopes and powerful electron microscopes use lenses and beams of electrons to magnify tiny objects by hundreds or thousands of times. This allowed scientists to see cells and their inner parts clearly, creating new knowledge about how living things work.
Q: How is the cell structural and functional unit of life? A: All living things — from tiny bacteria to humans and large trees — are made up of cells, making the cell the basic building block (structural unit) of life. Every cell carries out important living activities like producing energy, clearing waste, and making nutrients, which keeps the body alive and healthy (functional unit).
Q: How does a cell multiply? A: A cell multiplies by dividing its parts and splitting into new cells through a process called cell division. The most common type of cell division is mitosis, where one parent cell makes a copy of its DNA and splits to form two identical daughter cells.
Q: Can you estimate its actual size? (When observing a cell under a microscope) A: Yes, you can estimate the actual size of a cell by measuring the diameter of the visible circle (field of view) under the microscope in micrometres (µm) using a ruler. Then, you count how many cells fit in a straight line across that circle and divide the total diameter by the number of cells.
Q: If the estimated size of an onion peel cell is 200 µm, how much does a light microscope magnify this cell? (If both the eyepiece and objective lens have a magnifying power of 10X) A: The total magnification of a microscope is calculated by multiplying the power of the eyepiece by the power of the objective lens. Since both lenses are 10X, the microscope magnifies the cell 10 × 10 = 100 times. This makes the 200 µm cell appear 100 times larger to our eyes.
Q: How does the structure of the cell membrane in the cells of alveoli control the movement of substances across it? A: The cell membrane is very thin and selectively permeable, which means it acts like a gatekeeper that allows only specific substances to pass through while blocking others. In the alveoli (air sacs in our lungs), the cell membrane allows gases like oxygen and carbon dioxide to easily move in and out based on their concentration differences (diffusion), helping us breathe.
Q: What do you observe? You may observe that — Beaker A: the potato piece swells. Beaker B: the potato piece shrinks. What do you infer? What do you expect in terms of changes in their weight? A: Observation: The potato piece in Beaker A (plain water) increases in size and swells, while the potato piece in Beaker B (20 per cent salt solution) decreases in size and shrinks. Inference: Water moves through the selectively permeable cell membrane from a place where there is more water (dilute solution) to a place where there is less water (concentrated solution). This special movement of water is called osmosis. Weight changes: The weight of the potato piece in Beaker A increases because water entered the cells. The weight of the potato piece in Beaker B decreases because water moved out of the cells into the salt solution.
Q: What if a cell is kept in salt or sugar solutions of different concentrations? A: In an isotonic solution (equal solute concentration outside and inside): Water moves in and out at the same rate, so the cell stays the exact same size. In a hypotonic solution (less solute and more water outside): Water moves into the cell, causing it to swell up. In a hypertonic solution (more solute and less water outside): Water moves out of the cell, causing it to shrink.
Q: What if mung bean seeds are kept in a concentrated solution after soaking in water for 12 hours? What will happen to them? A: The mung bean seeds that soaked in water for 12 hours are filled with water. If you put them into a concentrated salt or sugar solution, water will move out of the swollen bean cells into the solution due to osmosis. As a result, the seeds will lose water and shrink.
Q: What do you think is the necessity of the cell wall in these cells? (Plant, fungi, and bacteria cells) A: Plants, fungi, and bacteria cannot move or run away to hide from rough weather. The rigid cell wall forms a strong outer cover that protects them from strong winds, heavy rains, and temperature changes. It also gives the cells a fixed shape and keeps plant stems upright.
Q: What do you observe? Onion peel cells or Rhoeo leaf peel cells are box-shaped and regularly arranged, whereas cheek cells are irregularly arranged. Why do you think this difference exists? A: Plant cells (like onion and Rhoeo leaves) have a hard and rigid outer layer called a cell wall made of cellulose, which gives them a fixed, box-like shape and allows them to stack neatly. Animal cells (like human cheek cells) do not have a cell wall; they only have a thin, flexible cell membrane, so they can easily change shape and look irregular.
Q: Prepare two slides of a Rhoeo leaf peel and human cheek cells again, and put 20 per cent sugar solution on them. Observe them under a microscope after half an hour. What do you observe? A: In the Rhoeo plant cells, water moves out through osmosis, causing the inner jelly-like contents (cytoplasm and membrane) to shrink away from the hard outer cell wall, while the outer cell wall stays the same size. In the human cheek cells, because there is no cell wall to hold their shape, the entire cell loses water and shrinks considerably.
Q: 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? A: Since plants are fixed in one place, they need a strong cell wall to stay standing tall against gravity and to survive heavy rain, strong winds, and scorching heat without moving. Animals move from place to place to find food and shelter, so their cells need to be flexible without a rigid cell wall to allow muscles and tissues to bend and move easily.
Q: What consequences would you predict for a plant cell if its cell wall were to become as flexible as a cell membrane? A: If a plant cell wall became soft and flexible, the plant would lose its strength and would not be able to stand upright. Trees and plants would droop and fall over. Additionally, if plant cells absorb too much water during heavy rain, they might swell too much and burst, because there would be no rigid wall to control the water pressure.
Q: 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? A: Cutting the pieces to roughly equal size ensures a fair test so that both pieces have a similar amount of surface area exposed to the liquid. Measuring their initial weight is important so we can compare it with the final weight and calculate exactly how much water was gained or lost by osmosis.
Q: Study the given diagrams of a bacterial cell, a plant cell, and an animal cell (Fig. 2.10a, b and c). Observe the different structures present in each of them. Record your observations in Table 2.1. A:
- Cell membrane: Present in bacterial cell, plant cell, and animal cell.
- Cell wall: Present in bacterial cell and plant cell; absent in animal cell.
- Cytoplasm: Present in bacterial cell, plant cell, and animal cell.
- Well-defined nucleus (genetic material enclosed by a membrane): Absent in bacterial cell; present in plant cell and animal cell.
- Primitive nucleus or nucleoid (genetic material without membrane around it): Present in bacterial cell; absent in plant cell and animal cell.
- Membrane-bound organelles: Absent in bacterial cell; present in plant cell and animal cell.
Q: Which of the cells given in Fig. 2.10 fall under the categories of prokaryotic and eukaryotic cells? A: The bacterial cell is a prokaryotic cell because it has a primitive nucleus without a nuclear membrane. The plant cell and animal cell are eukaryotic cells because they have a true, well-defined nucleus and membrane-bound organelles.
Q: Do you know any other cells without nucleus? A: Yes, mature sieve tube cells found in the phloem tissue of plants (which transport food) do not have a nucleus. Another example is the platelets found in our blood.
Q: How does the cell prevent these wastes from accumulating inside it? (Reference to lysosomes) A: The cell uses special round organelles called lysosomes, which act like a clean-up system. Lysosomes are filled with digestive enzymes that chop up and break down unwanted waste materials, damaged proteins, and worn-out cell parts to keep the inside of the cell clean.
Q: But where do plants synthesise their food and obtain energy for cellular activities? A: Plants synthesise (make) their food inside special green organelles called chloroplasts (a type of plastid) using sunlight, water, and carbon dioxide through a process called photosynthesis. To get energy from this food for cellular activities, both plant and animal cells use mitochondria, which break down food molecules to release stored energy.
Q: Are there any other plastids in plant cells that contain any pigments other than the green pigments? How do flowers, fruits, and vegetables acquire varied colours? A: Yes, plant cells contain plastids called chromoplasts that store non-green pigments like red, orange, and yellow. These colourful chromoplasts give bright and beautiful colours to flower petals, fruits, and vegetables like tomatoes and carrots. This helps attract insects for pollination and animals for seed dispersal.
Q: But where are water, minerals, and waste materials stored in the cell? Why do plants look wilted when they do not get enough water? A: In a plant cell, water, minerals, sugars, and waste materials are stored in a large sac called the central vacuole. When the central vacuole is full of water, it pushes against the cell wall, keeping the cell firm and stiff. When a plant does not get enough water, the vacuole loses water and shrinks, causing the cells to become soft, which makes the whole plant look wilted and droopy.
Q: Do white flowers contain any pigment? Give reasons. A: White flowers do not contain coloured pigments in their petals. They look white because their cells have tiny air pockets between them that scatter and reflect all visible light, making them appear white to our eyes. They may contain colourless plastids called leucoplasts.
Q: 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. A: To draw a well-labelled plant cell:
- Draw a rigid outer box and label it Cell wall.
- Draw a second inner line just inside the wall and label it Cell membrane.
- In the centre, draw a large empty circle and label it Vacuole.
- To one side, draw a dark round circle with a double boundary and label it Nucleus.
- Attached to the nucleus, draw a folded network of tubes and label it Endoplasmic Reticulum (ER).
- In the jelly-like space (Cytoplasm), draw a few rod-shaped ovals with inner folds labelled Mitochondria and rod-shaped ovals containing green coin-like stacks labelled Chloroplasts.
Q: What do you observe? 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? Can you identify which stage comes first during cell division? A: Observation: Yes, under the microscope, we can see many rectangular cells of the growing onion root tip. Differences: They are not all similar in structure; we can see differences in how their dark nuclear material (chromosomes) looks. Some cells have a round nucleus, while others have thread-like chromosomes lining up or pulling apart. Reason: This happens because the cells at the root tip are continuously growing and dividing to make the root longer. Each cell is caught at a different stage of the division process. First stage: The stage where the cell has a clear, round, normal-looking nucleus with tangled chromatin threads (before chromosomes form thick rods) comes first during cell division.
Q: 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? A: Having many small mitochondria instead of one giant mitochondrion creates a much larger total surface area. More surface area allows the cell to take in nutrients and oxygen much faster and carry out chemical reactions quickly. This produces energy much more efficiently for the whole cell.
Q: If the skin cells start dividing by meiosis instead of mitosis, what do you think will happen to a cut on the skin? A: Mitosis makes exact copies of body cells with the full amount of DNA to heal wounds. If skin cells divided by meiosis, the new skin cells would only have half the normal number of chromosomes. These half-DNA cells would not function properly as skin cells, so the cut would not heal correctly, and the skin tissue would be damaged or abnormal.
Q: What happens if meiosis and mitosis do not happen properly? A: If cell division does not happen properly, it causes serious health problems. Errors in mitosis can make cells divide out of control, leading to lumps called tumours and cancer. Errors in meiosis can lead to genetic disorders in babies (where a child is born with physical or developmental problems) or cause reduced fertility.
Q: Do cells grow and reproduce forever? A: No, normal cells do not grow and divide forever. They grow in a controlled way, do their job, stop dividing when they touch other cells (a rule called contact inhibition), and eventually die naturally when they get old or damaged.
Q: Ready to Go Beyond: How do cells monitor their growth to maintain a balance? A: Cells monitor their growth using built-in chemical signals and checkpoints during the cell cycle. They also use a natural process called Programmed Cell Death (PCD). If a cell is old, damaged, or no longer needed, it selectively destroys itself in an organized way so new, healthy cells can take its place, maintaining a healthy balance in the body.
Q: Threads of Curiosity: How do cancer cells grow and spread? A: Normal cells stop dividing when they touch other cells. Cancer cells lose this natural control and ignore signals to stop growing. They keep dividing rapidly and uncontrollably to form lumps called tumours. Cancerous cells can break away from the tumour, invade nearby healthy tissues, and travel through the blood to spread and form new tumours in other parts of the body.
Revise, Reflect, Refine (Exercises)
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)
A: (i) Cell membrane and cell wall: The cell membrane is selectively permeable, meaning it allows only certain chosen substances to pass through. The cell wall is generally permeable, allowing water and most dissolved substances to pass through easily. (ii) RER and SER: Rough Endoplasmic Reticulum (RER) looks rough under a microscope because it has tiny round dots called ribosomes attached to its surface. Smooth Endoplasmic Reticulum (SER) looks smooth because it does not have ribosomes on its surface. (iii) Chloroplasts and chromoplasts: Chloroplasts contain the green pigment chlorophyll, which helps plants trap sunlight for photosynthesis. Chromoplasts contain non-green pigments (like yellow, orange, or red) that give bright colours to flowers and fruits.
Q2. 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.
A: (iii) Water moved into Cell X and moved out of Cell Y through the cell membrane. (This movement of water from higher water concentration to lower water concentration across the cell membrane is called osmosis.)
Q3. 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.
A: (a) Mitochondria — matches with (ii) Site of cellular respiration. (b) Nucleus — matches with (i) Controlling all the activities of a cell. (c) Golgi apparatus — matches with (vi) Packs and stores materials received from ER. (d) Chloroplast — matches with (vii) Helps in manufacturing food. (e) Cell membrane (Plasma membrane) — matches with (iv) Separates the cell contents from surroundings. (f) Cell wall — matches with (v) Provides structural rigidity to the cell. (g) Vacuole — matches with (iii) Storage organelle that also provides rigidity to the cell.
Q4. Which of the following option(s) of the pairs of cell organelles are correctly placed under the given categories? (i) Present in plant cells: Leucoplast | Absent in animal cells: Cell wall (ii) Present in plant cells: Mitochondria | Absent in animal cells: Ribosome (iii) Present in plant cells: Cell wall | Absent in animal cells: Golgi apparatus (iv) Present in plant cells: Lysosome | Absent in animal cells: Endoplasmic reticulum
A: Option (i) is correctly placed. Leucoplasts are present in plant cells (for storing food), and cell walls are absent in animal cells. (Note: Animal cells do have ribosomes, Golgi apparatus, and endoplasmic reticulum, so options ii, iii, and iv are incorrect.)
Q5. 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. A: Renu is correct. While underground roots do not get sunlight and do not need green chloroplasts for photosynthesis, they still need to store food. Roots contain colourless plastids called leucoplasts that store food materials like starch, oils, or proteins (for example, starch stored in potato or taro roots). Therefore, plastids are present in plant roots.
Q6. 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. A: Structural similarities: Both mitochondria and chloroplasts are surrounded by a double membrane (an outer and an inner membrane). Both have their own DNA and ribosomes, allowing them to make some of their own proteins. Functional similarities: Both are involved in energy management inside plant cells. Differences: Function: Chloroplasts capture sunlight to make food (sugar) through photosynthesis. Mitochondria break down food (glucose) to release stored energy (ATP) through cellular respiration. Pigment and structure: Chloroplasts contain the green pigment chlorophyll and a fluid called stroma with coin-like discs. Mitochondria do not have green pigments; instead, their inner membrane is folded into finger-like projections called cristae.
Q7. Which of the following pairs of cell organelles contains DNA? (i) Chloroplasts, Ribosomes (ii) Mitochondria, Nucleus (iii) Golgi bodies, Ribosomes (iv) Nucleus, Lysosomes
A: (ii) Mitochondria, Nucleus. (Note: Chloroplasts also contain DNA, but ribosomes do not, which makes option (ii) the correct choice among the pairs.)
Q8. 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?
A: (i) Hypothesis: She wants to test how solutions of different solute concentrations (plain water vs. concentrated salt solution) cause water to move in or out of plant cells through osmosis. (ii) Improvement: She should accurately measure and record the initial weight and exact length/width of both carrots before placing them in the liquids, and then measure their final weight and size after 24 hours to get exact number measurements. (iii) Reason: In plain water, water enters the carrot cells by osmosis, filling up their central vacuoles and making the cells swollen and firm (stiff and crunchy). In the concentrated salt solution, water leaves the carrot cells by osmosis, causing the cells to lose water and shrink, which makes the carrot limp and rubbery.
Q9. Indicate the presence or absence of following structures in bacterial and animal cells: (Structures: Chromosome, Nucleus, Mitochondria, Golgi complex, Chromoplasts)
A: Chromosome: Present in bacterial cell (as a single circular DNA/nucleoid); present in animal cell. Nucleus (well-defined with membrane): Absent in bacterial cell; present in animal cell. Mitochondria: Absent in bacterial cell; present in animal cell. Golgi complex: Absent in bacterial cell; present in animal cell. Chromoplasts: Absent in bacterial cell; absent in animal cell (only found in plants).
Q10. 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.
A: (i) Water gathers in Cup B (sugar) and Cup C (salt) because the liquid inside the cup is very concentrated compared to the water in the beaker outside. Through osmosis, water moves from the beaker through the living, selectively permeable potato cells into the hollow cup to balance the concentration. (ii) Cup A is kept empty as a control. It proves that water does not just leak inside the potato cup on its own; water only moves in when a solute like salt or sugar is present to attract it. (iii) In Cup A, there is no sugar or salt inside to pull water in, so no osmosis happens. In Cup D, the potato was boiled, which killed the potato cells and destroyed their selectively permeable cell membranes. Dead cells cannot perform osmosis, so water does not move into Cup D.
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
A: (ii) SER — Lipid and cellulose synthesis is incorrectly matched. (Smooth Endoplasmic Reticulum synthesises fats/lipids and hormones, but cellulose is made at the plasma membrane for the cell wall, not by SER.)
Q12. What outcome do you expect, if all the mitochondria are removed from a eukaryotic cell? A: Mitochondria are the “powerhouses of the cell” that produce energy (ATP) through cellular respiration. If all mitochondria are removed, the cell will not be able to produce the energy it needs to carry out basic life activities like making proteins, transporting nutrients, or repairing itself. As a result, the cell will quickly stop working and die.
Q13. Which phenomenon inhibits the formation of tumors in the human body? Can plants also develop tumors? Explain. A: The phenomenon of contact inhibition inhibits (stops) the formation of tumours in animals and humans. When normal animal cells touch neighbouring cells, they get a signal to stop dividing. Yes, plants can also develop tumour-like growths. Because plant cells have rigid cell walls, they do not show contact inhibition. If plant cells get infected by certain bacteria or get wounded, their cells can divide rapidly and form large lumps or knots on stems and roots.
Q14. 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. A: The Endoplasmic Reticulum (ER) and the Golgi apparatus help make the cell membrane.
Path of compounds:
- Rough Endoplasmic Reticulum (RER) makes proteins.
- Smooth Endoplasmic Reticulum (SER) makes lipids (fats).
- These proteins and lipids travel in small sacs (vesicles) to the Golgi apparatus.
- The Golgi apparatus modifies, sorts, and packages them into new transport vesicles.
- Finally, these vesicles travel to and fuse with the plasma membrane (cell membrane) to repair or build it.
Diagram flow: RER (Proteins) / SER (Lipids) → Transport Vesicles → Golgi Apparatus → Secretory Vesicles → Plasma Membrane
Q15. What would happen if gametes are formed by mitotic divisions? A: Gametes (sperm and egg cells) normally form by meiosis so they contain only half the number of chromosomes. If gametes were formed by mitosis, they would contain the full normal number of chromosomes. When a sperm and an egg combine during fertilisation, the baby would end up with double the number of chromosomes compared to the parents. This doubling would repeat every generation, causing severe abnormalities or making survival impossible.
Q16. A farmer, Deepa, was very happy with the harvest of amla (Indian Gooseberry) and lemons on her farm… 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?
A: (i) Scientific concept: She applied the concept of osmosis (specifically, creating a hypertonic environment) to preserve food. (ii) How it prevents spoilage: High concentrations of salt or sugar create a hypertonic solution surrounding any spoilage bacteria or fungi. Through osmosis, water is drawn out of the bacterial and fungal cells into the salty or sugary syrup. This severe loss of water dehydrates and kills the germs, preventing them from spoiling the food. (iii) Healthy recipe suggestion: You can make Amla Murabba (Sweet Gooseberry Preserve) by washing and steaming fresh amla, poking small holes in them with a fork, and soaking them in a concentrated sugar or jaggery syrup flavoured with cardamom. The sweet hypertonic syrup preserves the vitamin-rich amla for months without spoiling. (iv) Scientific values addressed: The case addresses values like applying scientific knowledge to solve everyday real-life problems, reducing food wastage, promoting sustainability, showing self-reliance, and improving economic well-being through smart resource management.
Q: The Quest Continues… What is the future of the development of synthetic cells using non-living chemicals? If a synthetic cell is developed, what may be the related ethical issues? A: Future of synthetic cells: In the future, scientists might be able to create fully artificial living cells from scratch using basic non-living chemicals. These synthetic cells could be custom-designed to produce clean bio-fuels, manufacture life-saving medicines, clean up environmental pollution (like eating plastic or oil spills), and help us understand how life first originated on Earth. Ethical issues: Creating artificial life raises important questions: Safety: What if a synthetic cell escapes from the laboratory, mutates, and harms natural plants, animals, or humans? Environment: Could man-made cells replace or destroy natural ecosystems? Moral boundaries: Is it right for humans to “play God” by creating artificial life, and who should control or own the rights to man-made living organisms?