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NCERT Class 9 Science Cell: The Building Block of Life: Exercise Solutions

Step-by-step working for every question of End-of-chapter exercises in NCERT's Class 9 Science book, chapter “Cell: The Building Block of Life”, written by Shishya. Each question is in our own short words; open the NCERT chapter for the exact wording and figures.

1. Exercises Q1

   Tell apart, using the clue in brackets: (i) cell membrane and cell wall (permeability) (ii) RER and SER (structure) (iii) chloroplasts and chromoplasts (pigments).

   1. (i) The cell membrane is selectively permeable, allowing only specific substances to pass, while the cell wall is permeable, letting water and dissolved minerals through.
   2. (ii) RER has ribosomes attached to its surface, giving it a rough appearance, whereas SER lacks ribosomes and appears smooth.
   3. (iii) Chloroplasts contain green pigments like chlorophyll, while chromoplasts contain other colored pigments such as red, orange, or yellow.

   Answer: (i) Cell membrane is selectively permeable; cell wall is permeable; (ii) RER has ribosomes; SER does not; (iii) Chloroplasts have green pigments; chromoplasts have other colors.

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2. Exercises Q2

   Animal cell X is put in pure water and swells; similar cell Y is put in concentrated salt solution and shrinks. Best explanation? (i) salt moved into Y and made it shrink (ii) water entered X, and more water left Y than salt entered it (iii) water entered X and left Y through the cell membrane (iv) solute movement caused osmosis in both cells

   1. Osmosis is the movement of water through a selectively permeable membrane from a dilute to a concentrated solution.
   2. Cell X in pure water swells because water enters the cell as the external solution is less concentrated than the cell contents.
   3. Cell Y in concentrated salt solution shrinks because water leaves the cell to balance the higher solute concentration outside.
   4. The cell membrane allows water to pass but restricts the movement of solute molecules like salt.
   5. Option (iii) correctly identifies that water movement through the membrane in both directions explains the swelling and shrinking.

   Answer: (iii)

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3. Exercises Q3

   In the plant-cell diagram (Fig. 2.20) the labels read as (a) mitochondrion (b) nucleus (c) Golgi apparatus (d) chloroplast (e) cell membrane (f) cell wall (g) vacuole. Match them with: (i) controls all cell activities (ii) site of cellular respiration (iii) storage organelle that also gives rigidity (iv) separates cell contents from the surroundings (v) gives structural rigidity (vi) packs and stores material from the ER (vii) helps make food.

   Uses a figure printed in the NCERT book — open the chapter above to see it.

   1. (i) The nucleus is the control centre that directs all cell activities.
   2. (ii) Mitochondria are the sites where cellular respiration occurs to release energy.
   3. (iii) The vacuole stores materials and helps maintain cell rigidity in plant cells.
   4. (iv) The cell membrane acts as a barrier separating the cell contents from the outside.
   5. (v) The cell wall provides structural rigidity and protection to the plant cell.
   6. (vi) The Golgi apparatus modifies, sorts, and packages materials from the ER.
   7. (vii) Chloroplasts are the organelles responsible for making food through photosynthesis.

   Answer: (i)-(b), (ii)-(a), (iii)-(g), (iv)-(e), (v)-(f), (vi)-(c), (vii)-(d)

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4. Exercises Q4

   Which pair is correctly placed as "present in plant cells" / "absent in animal cells"? (i) leucoplast / cell wall (ii) mitochondria / ribosome (iii) cell wall / Golgi apparatus (iv) lysosome / endoplasmic reticulum

   1. (i) Plant cells have a cell wall, which is absent in animal cells, so the pair is correctly placed.
   2. (ii) Mitochondria and ribosomes are found in both plant and animal cells, so this pair is incorrect.
   3. (iii) Golgi apparatus is present in both plant and animal cells, so this pair is incorrect.
   4. (iv) Lysosomes are present in both plant and animal cells, so this pair is incorrect.

   Answer: (i) leucoplast / cell wall

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5. Exercises Q5

   Renu says every part of a plant, even the root, has plastids; Rohit says roots have none because they are underground and do not photosynthesise. Who is right? Justify.

   1. Renu is correct because plastids are found in all parts of the plant, including the roots.
   2. Rohit is incorrect because the absence of photosynthesis does not mean the absence of plastids.
   3. Roots contain leucoplasts, which are a type of plastid responsible for storing food.
   4. Only chloroplasts are specific to photosynthesis, while other plastids serve different functions in non-photosynthetic tissues.

   Answer: Renu is correct; roots contain plastids like leucoplasts for storage, even though they do not have chloroplasts for photosynthesis.

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6. Exercises Q6

   How are mitochondria and chloroplasts alike, and how are they different, in structure and in function?

   1. Alike in structure: both are organelles bounded by membranes, and both have their own DNA and ribosomes, so they can make some of their own proteins.
   2. Alike in function: both deal with energy in the cell.
   3. Different in function: chloroplasts contain green pigment and use light to make food (sugars) by photosynthesis, while mitochondria release energy from food by cellular respiration.
   4. Different in where they occur: chloroplasts are found only in plant cells (and some protists), while mitochondria are found in both plant and animal cells.

   Answer: Both are membrane-bound energy organelles with their own DNA and ribosomes; chloroplasts make food by photosynthesis (plants only), mitochondria release energy by respiration (plants and animals).

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7. Exercises Q7

   Which pair of organelles contains DNA? (i) chloroplasts, ribosomes (ii) mitochondria, nucleus (iii) Golgi bodies, ribosomes (iv) nucleus, lysosomes

   1. The nucleus contains chromosomes which are made up of DNA molecules that hold genetic information.
   2. Mitochondria are double-membrane-bound organelles that also possess their own DNA.
   3. Ribosomes, Golgi bodies, and lysosomes do not contain DNA as they are involved in protein synthesis, packaging, or digestion.
   4. Therefore, the pair of organelles that contains DNA is mitochondria and the nucleus.

   Answer: (ii) mitochondria, nucleus

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8. Exercises Q8

   One carrot is kept in plain water and a similar carrot in concentrated salt solution for 24 hours. (i) What hypothesis is being tested? (ii) How could the experiment be improved? (iii) Why does the carrot in water stay stiff and crunchy while the one in salt solution turns rubbery and limp?

   Uses a figure printed in the NCERT book — open the chapter above to see it.

   1. (i) The hypothesis being tested is that the concentration of the external solution affects the water movement in and out of plant cells, thereby changing the cell's turgidity.
   2. (ii) To improve the experiment, one should use multiple carrots in each setup to ensure the results are consistent and not due to chance, and measure the mass or length of the carrots before and after the 24 hours to quantify the change.
   3. (iii) In plain water, the external solution is hypotonic compared to the cell sap, so water enters the carrot cells by endosmosis, making them turgid and stiff.
   4. (iii) In the concentrated salt solution, the external solution is hypertonic, so water leaves the carrot cells by exosmosis, causing them to lose turgor pressure and become flaccid, rubbery, and limp.

   Answer: (i) Hypothesis: External solution concentration affects water movement and cell turgidity; (ii) Improvement: Use multiple samples and measure mass/length changes; (iii) Water carrot is stiff due to endosmosis causing turgidity, while salt carrot is limp due to exosmosis causing flaccidity.

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9. Exercises Q9

   Say whether each is present or absent in a bacterial cell and in an animal cell: chromosome, nucleus, mitochondria, Golgi complex, chromoplasts.

   1. (i) Chromosome: present in both; a bacterial cell has its genetic material in a nucleoid region without a nuclear membrane, and an animal cell has chromosomes inside its nucleus.
   2. (ii) Animal cells are eukaryotic and possess a well-defined nucleus, so the nucleus is present, while bacterial cells do not have one.
   3. (iii) Mitochondria are membrane-bound organelles found in eukaryotic cells like animal cells, so they are present in animal cells but absent in bacterial cells.
   4. (iv) The Golgi complex is a membrane-bound organelle involved in packaging, which is present in animal cells but absent in bacterial cells.
   5. (v) Chromoplasts are a type of plastid found in plant cells, so they are absent in both bacterial and animal cells.

   Answer: Chromosome: present in both (bacteria have it in a nucleoid, not a nucleus); nucleus: absent in bacteria, present in animal cells; mitochondria: absent in bacteria, present in animal cells; Golgi complex: absent in bacteria, present in animal cells; chromoplasts: absent in both.

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10. Exercises Q10

    Four potato cups stand in water: A empty, B with a teaspoon of sugar, C with a teaspoon of salt, D (a boiled potato) with sugar. After two hours: (i) why does water collect in cups B and C? (ii) why is cup A needed? (iii) why does no water collect in cups A and D?

    Uses a figure printed in the NCERT book — open the chapter above to see it.

    1. (i) Water moves out of the potato cells in B and C into the sugar or salt solution because the cell membrane is selectively permeable and allows water to pass but blocks solute molecules.
    2. (i) This movement occurs from the area of higher water concentration inside the potato to the area of lower water concentration in the concentrated solution, a process known as osmosis.
    3. (ii) Cup A acts as a control because it contains only plain water without any solute, allowing us to compare the effects of adding sugar or salt on water movement.
    4. (iii) No water gathers in Cup A because there is no concentration difference between the plain water inside and outside the potato, so water movement is balanced.
    5. (iii) No water gathers in Cup D because the potato was boiled, which kills the cells and destroys the selectively permeable cell membrane, preventing osmosis.

    Answer: (i) Water gathers in B and C due to osmosis where water moves out of the potato into the concentrated solution; (ii) Cup A is necessary as a control to show the effect of solutes; (iii) No water gathers in A due to no concentration difference and in D because boiling destroys the cell membrane.

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11. Exercises Q11

    Which pair matches an organelle with its function INCORRECTLY? (i) ribosome — protein synthesis (ii) SER — lipid and cellulose synthesis (iii) lysosome — digestion of foreign agents

    1. (i) Ribosomes are the sites of protein synthesis, so this pair is correct.
    2. (ii) The smooth ER makes fats (lipids) and hormones; cellulose, the material of the plant cell wall, is not made by the SER, so this pair is incorrect.
    3. (iii) Lysosomes contain enzymes that digest foreign material and worn-out parts, so this pair is correct.

    Answer: (ii) SER — lipid and cellulose synthesis

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12. Exercises Q12

    What would happen if all the mitochondria were removed from a eukaryotic cell?

    1. Mitochondria are membrane-bound organelles that provide energy to the cell.
    2. Eukaryotic cells rely on these organelles to carry out life processes independently.
    3. Without mitochondria, the cell cannot produce the energy required for its functions.
    4. The cell would be unable to perform essential activities like building materials or removing waste.
    5. Consequently, the eukaryotic cell would die due to the lack of energy.

    Answer: The cell would die because it would not be able to produce energy.

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13. Exercises Q13

    Which phenomenon stops tumours from forming in the human body? Can plants also develop tumours? Explain.

    1. In many animal cells, cell division stops when a cell touches its neighbouring cells; this is called contact inhibition, and it keeps cells from piling up into a tumour.
    2. Cancer cells lose this control and keep dividing, which forms tumours.
    3. Plant cells have rigid cell walls and do not show contact inhibition; they follow a different pattern of growth.
    4. So plants do not form tumours by losing contact inhibition the way animal cells do, although abnormal, uncontrolled growths can still occur in plants.

    Answer: Contact inhibition prevents tumours in the human body; plant cells, with rigid walls, do not show contact inhibition and grow differently, so they do not form tumours the way animal cells do, though abnormal growths can occur.

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14. Exercises Q14

    The cell membrane is made of proteins and lipids. Which organelles help make it? Describe the path of these compounds from where they are made to the cell membrane (with a labelled diagram).

    1. Lipids of the membrane are made in the smooth endoplasmic reticulum (SER), and membrane proteins are made by ribosomes on the rough endoplasmic reticulum (RER).
    2. The ER sends these lipids and proteins to the Golgi apparatus, which modifies, sorts and packs them into small vesicles.
    3. The vesicles move to the cell membrane and fuse with it, adding the new lipids and proteins.
    4. Labelled diagram (draw it as a flow): RER (proteins) and SER (lipids) → Golgi apparatus → vesicles → cell membrane.

    Answer: The ER (RER for proteins, SER for lipids) and the Golgi apparatus; path: ER → Golgi apparatus → vesicles → cell membrane.

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15. Exercises Q15

    What would happen if gametes were formed by mitosis?

    1. Mitosis produces two daughter cells that are genetically identical to the parent cell.
    2. Each cell formed by mitosis retains the same number of chromosomes as the parent cell.
    3. If gametes were formed by mitosis, they would have the full set of chromosomes instead of half.
    4. During fertilization, the fusion of two such gametes would result in a zygote with double the normal number of chromosomes.
    5. This abnormal chromosome number would likely cause developmental problems or genetic disorders in the offspring.

    Answer: The zygote would have double the normal number of chromosomes, leading to genetic abnormalities.

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16. Exercises Q16

    A farmer, Deepa, could sell only a quarter of her amla and lemon harvest, so she made pickles, murabbas and sharbat with salt, sugar or jaggery and bottled them. (i) Which scientific concept did she use? (ii) How do high salt and sugar levels stop spoilage bacteria and fungi? (iii) Suggest a healthy preserved recipe of this kind. (iv) What scientific values does this case show?

    1. (i) Deepa used the concept of food preservation.
    2. (ii) High salt and sugar levels create a hypertonic environment.
    3. (ii) Water moves out of bacterial and fungal cells by osmosis.
    4. (ii) This causes plasmolysis, killing the microbes and stopping spoilage.
    5. (iii) A healthy recipe is lemon pickle made with minimal oil and spices.
    6. (iii) Using less salt and no preservatives keeps it nutritious.
    7. (iv) The case shows the value of scientific application in daily life.
    8. (iv) It also highlights the importance of resourcefulness and health awareness.

    Answer: (i) Food preservation; (ii) Osmosis causes plasmolysis in microbes; (iii) a lemon pickle made with less salt; (iv) Scientific application and health awareness.

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