Chapter 2, Cell : The Building Block of life, Class 9 : Curiosity

Intext Questions and Answers

Think It Over (Chapter Title Page / Page 8)

• Where does a cell come from?
• Answer:- According to the classical Cell Theory, all cells arise from pre-existing cells.

In-Text Questions

• What are the different components of a cell? (Page 9)
• Most cells have three basic parts: a selectively permeable plasma membrane, a semi-fluid cytoplasm, and a prominent nucleus. Eukaryotic cells also contain membrane-bound organelles like mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, and plastids.
• How do cells in our body communicate with each other? (Page 9)
• All living cells communicate with their surroundings and their neighbouring cells through the cell membrane.
• Do cells live forever, or do they die? (Page 9)
• Cells do not live forever. They grow, divide, carry out their functions, and eventually die naturally or are replaced by new cells when they are dead or damaged.
• Under the microscope, you will see a magnified image of an object. Can you estimate its actual size? (Page 10)
• Yes, you can estimate the real size using the formula: Estimated size = Diameter of the visible field in micrometre / Number of cells along the diameter.
• 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? (Page 19)
• Yes, there are plastids called chromoplasts that contain pigments other than chlorophyll, such as yellow, orange, or red. These pigments are the source of bright colours in flowers and fruits.
• Where are water, minerals, and waste materials stored in the cell? Why do plants look wilted when they do not get enough water? (Page 19)
• These materials are stored in vacuoles. A plant looks wilted when it lacks water because the large central vacuole loses water, causing the cell to lose internal pressure and firmness.
• When you get a small cut on your skin, it heals after a few days. When hair fall out, new hair grow back. How does this happen? (Page 20)
• This happens because cells in our body can grow and divide to replace the old, dead, or damaged cells.

Activities

Activity 2.2: Let us experiment (Page 11)

• What do you observe?
• Beaker A (potato in plain water): The potato piece swells. Beaker B (potato in salt/sugar solution): The potato piece shrinks.
• What do you infer? What do you expect in terms of changes in their weight?
• The weight of the potato in Beaker A increases because water moved into the cell, while the weight of the potato in Beaker B decreases because water moved out of the cell into the concentrated solution.

Activity 2.3: Let us investigate (Page 13)

• What do you observe? Why do you think this difference exists?
• Onion peel cells are box-shaped and regularly arranged, whereas cheek cells are irregularly arranged. This difference exists because plant cells have a rigid outer cell wall that maintains their shape, while animal cells lack a cell wall.

Activity 2.5: Let us enhance our skills (Page 20-21)

• 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?
• The cells exhibit different structures corresponding to different stages of cell division. This is because the cells of a growing root tip divide continuously.

What if… / Pause and Ponder / Threads of Curiosity

• What if mung bean seeds are kept in a concentrated solution after soaking in water for 12 hours? What will happen to them? (Page 12)
• The cells of the soaked seeds would lose water due to osmosis and shrink.
• What do you think is the necessity of the cell wall in these cells? (Page 13)
• Because plants cannot move to escape environmental stresses like wind and rain, they need a rigid structure to withstand these stresses, stay upright, and prevent the cells from bursting.
• What consequences would you predict for a plant cell if its cell wall were to become as flexible as a cell membrane? (Pause and Ponder, Page 14)
• The plant would lose its structural support; leaves and flowers would not remain firm, and the plant would not be able to stay upright.
• 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? (Pause and Ponder, Page 14) This creates a controlled experiment, ensuring that any weight change calculated at the end is purely due to the movement of water and not due to starting with vastly different amounts of potato.
• Do white flowers contain any pigment? Give reasons. (Pause and Ponder, Page 19) They often contain colourless plastids called leucoplasts which reflect light rather than possessing a specific colored pigment.
• 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? (Pause and Ponder, Page 22)
• Having many small mitochondria increases the surface area for chemical reactions, facilitating faster and more efficient energy production than one large structure.
• If the skin cells start dividing by meiosis instead of mitosis, what do you think will happen to a cut on the skin? (Pause and Ponder, Page 22)
• Meiosis produces cells with half the number of chromosomes that are genetically varied. The new cells would not be identical to the parent skin cells, disrupting the tissue’s structural integrity, and the cut would not heal properly.
• How do cancer cells grow and spread? (Threads of Curiosity, Page 23)
• Cancer cells lose contact inhibition control and divide uncontrollably, forming tumours. These tumours can invade nearby tissues and spread to other parts of the body.

Revise, Reflect, Refine (End of Chapter Exercises)

Page 24

• 1. Differentiate between:
  • (i) Cell membrane and cell wall: The cell membrane is selectively permeable, whereas the cell wall is permeable.
  • (ii) RER and SER: Rough Endoplasmic Reticulum (RER) has ribosomes attached making it rough; Smooth Endoplasmic Reticulum (SER) lacks ribosomes making it smooth.
  • (iii) Chloroplasts and chromoplasts: Chloroplasts contain green chlorophyll; chromoplasts contain other colored pigments like yellow, orange, or red.
• 2. Cell X in pure water swells, Cell Y in concentrated salt shrinks. Which statement provides the correct explanation?
• (iii) Water moved into Cell X and moved out of Cell Y through the cell membrane.

Page 25

• 3. Match diagram parts to functions: *
• Controlling all activities: Nucleus.
  • Site of cellular respiration: Mitochondria.
  • Storage organelle that provides rigidity: Vacuole.
  • Separates cell contents from surroundings: Cell membrane.
  • Provides structural rigidity: Cell wall.
  • Packs and stores materials received from ER: Golgi apparatus.
  • Helps in manufacturing food: Chloroplast.
• 4. Which of the following option(s) of the pairs of cell organelles are correctly placed under the given categories?
• Option (i) is correct: Leucoplast is present in plant cells, and Cell wall is absent in animal cells.
• 5. Two students, Renu and Rohit, were discussing plastids… Who is correct?
• Renu is technically more accurate. While roots lack chloroplasts, they do contain a type of colourless plastid called a leucoplast, which stores food materials.
• 6. Discuss how mitochondria and chloroplasts are structurally and functionally similar and different.
  • Similarities: Both have double membranes, contain their own DNA and ribosomes, and can make their own proteins.
  • Differences: Chloroplasts synthesize food using sunlight, whereas mitochondria break down glucose to release energy.
• 7. Which of the following pairs of cell organelles contains DNA?
• (ii) Mitochondria, Nucleus.

Page 26

• 8. A researcher placed one carrot in plain water and another in concentrated salt solution.
  • (i) Hypothesis: Testing the effect of osmosis across a selectively permeable membrane in hypotonic vs. hypertonic solutions.
  • (ii) Improvement: Measure the initial and final weights of the carrots to acquire quantitative data.
  • (iii) Observation reason: The carrot in plain water gains water via osmosis, keeping it turgid. The carrot in the salt solution loses water to the hypertonic environment, causing cells to shrink.
• 9. Indicate the presence or absence of following structures in bacterial and animal cells:
  • Chromosome: Present in bacteria / Present in animals.
  • Nucleus: Absent in bacteria / Present in animals.
  • Mitochondria: Absent in bacteria / Present in animals.
  • Golgi complex: Absent in bacteria / Present in animals.
  • Chromoplasts: Absent in bacteria / Absent in animals.
• 10. Potato cups experiment.
  • (i) Explain why water gathers in Cup B and Cup C. The sugar/salt creates a hypertonic solution inside the cups. Water from the beaker moves through the potato cells via osmosis to balance the concentration.
  • (ii) Why is Cup A necessary? Acts as a control to prove water doesn’t gather in a hollowed potato without a concentration gradient.
  • (iii) Explain why water does not gather in Cup D (boiled). Boiling kills the potato cells and destroys the selectively permeable cell membrane, halting osmosis.
• 11. Identify the pair that incorrectly matches the cell organelle with its function.
• (ii) SER — Lipid and cellulose synthesis. (SER does not synthesize cellulose).
• 12. What outcome do you expect if all mitochondria are removed from a eukaryotic cell?
• The cell would be unable to break down glucose to release energy in the form of ATP, leading to cell death.
• 13. Which phenomenon inhibits the formation of tumors in the human body? Can plants also develop tumors?
• Contact inhibition prevents cells from dividing uncontrollably, inhibiting tumors. Plant cells do not show contact inhibition and follow a different pattern of growth.

Page 27

• 14. Which cell organelles help in the synthesis of cell membrane?
• The Rough ER synthesises proteins, and the Smooth ER synthesises lipids. These are sent to the Golgi apparatus, where they are packaged and shipped via vesicles to the cell membrane.
• 15. What would happen if gametes are formed by mitotic divisions?
• If gametes were formed by mitosis, they would contain a full set of chromosomes. When two gametes fused, the resulting offspring would have double the normal chromosome number.
• 16. Deepa the farmer preserving produce.
  • (i) Scientific concept: Osmosis / Plasmolysis.
  • (ii) How does salt/sugar prevent bacteria? It creates a hypertonic environment causing water to move out of the spoilage-causing bacteria via osmosis, dehydrating them.
  • (iii) Recipe: Making fruit preserves or natural pickles.
  • (iv) Scientific values: Reducing food waste, increasing economic efficiency, and promoting sustainability.