Cell: The Building Block of Life Class 9 Questions and Answers | NCERT Exploration Chapter 2

Cell: The Building Block of Life Class 9 Questions and Answers: Looking for complete solutions to Class 9 Science Chapter 2 Cell: The Building Block of Life from the NCERT Exploration textbook? You are at the right place. In this post, we have provided complete and easy-to-understand NCERT solutions to all the important questions given in the chapter, including Think It Over, What If…, Pause and Ponder, and Revise, Reflect, Refine. The answers are explained in simple language to help Class 9 students understand the concepts clearly, revise effectively, and prepare better for their school examinations. Whether you are looking for Class 9 Cell questions and answers, NCERT Exploration Chapter 2 solutions, or Cell: The Building Block of Life answers, this guide covers the complete exercise in one place.

Think It Over

1. Where does a cell come from?

Ans. A cell comes from a pre-existing cell. New cells are formed through the process of cell division. This principle was later included in the Cell Theory by Rudolf Virchow.

2. How have technological interventions facilitated the creation of new knowledge in understanding the world beyond the naked eye?

Ans. Technological developments, especially microscopes, have enabled scientists to observe objects that cannot be seen with the naked eye. Light microscopes help us study cells, while powerful electron microscopes reveal very fine details of cell structures at the nanometre scale. Thus, technology has greatly expanded our understanding of cells and the microscopic world.

3. How is the cell structural and functional unit of life?

Ans. The cell is called the structural and functional unit of life because:

• All living organisms are made up of one or more cells.
• A cell forms the basic structure of an organism.
• Cells perform essential life processes such as obtaining energy, synthesising substances, removing waste and reproducing.
• In multicellular organisms, cells combine to form tissues, organs and organ systems.

Therefore, the cell is the basic unit responsible for both the structure and functioning of living organisms.

4. How does a cell multiply?

Ans. A cell multiplies through cell division. During cell division, a pre-existing cell divides to form new cells. There are two major types of cell division:

• Mitosis — produces two genetically identical daughter cells and is important for growth, repair and maintenance.
• Meiosis — produces four daughter cells with half the number of chromosomes and is important for sexual reproduction.

What if

5. What will happen if mung bean seeds are kept in a concentrated solution after soaking in water for 12 hours?

Ans. The mung bean seeds will lose water and shrink.

This happens because the concentrated solution is hypertonic compared with the cells of the soaked seeds. Water moves out of the cells through the selectively permeable cell membrane by osmosis. As a result, the seeds may become less swollen and wrinkled.

6. What happens if a cell is kept in salt or sugar solutions of different concentrations?

Ans. The effect depends on the concentration of the surrounding solution:

This occurs because of osmosis, the movement of water through a selectively permeable membrane.

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?

Ans. Plants are generally fixed in one place and cannot move away from environmental stresses such as wind and rain. Therefore, they need a rigid cell wall to provide strength, support and shape.

Animals usually move from one place to another. Their cells therefore need greater flexibility rather than a rigid cell wall. Animal cells can change their shape easily, which supports the movement and functioning of animal tissues.

2. What consequences would you predict for a plant cell if its cell wall were to become as flexible as a cell membrane?

Ans. If the plant cell wall became as flexible as the cell membrane:

• The cell would lose much of its rigidity and structural support.
• It would not be able to maintain its normal shape effectively.
• Plant tissues would become less firm.
• The plant might have difficulty remaining upright and resisting environmental stresses.

Thus, the rigid cell wall is essential for providing strength, shape and support to plant cells.

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?

Ans. It is important to use roughly equal-sized potato pieces and measure their initial weights so that we can make a fair comparison.

• Both pieces start with nearly the same size and weight.
• This helps us know that any change in weight is due to the different liquids, not because the pieces were different at the beginning.

4. Do white flowers contain any pigment? Give reasons.

Ans. Yes, white flowers can contain pigments. Their colour may be due to very small amounts or different types of pigments that do not produce strong visible colours. White flowers may also appear white because their petals reflect or scatter most of the visible light rather than absorbing a particular colour strongly.

5. Draw a well-labelled schematic diagram of a plant or an animal cell using these clues:

• Nucleus appears as a dark and round body inside the cell.
• ER spreads like a network of extended nuclear envelope.
• Mitochondria and chloroplasts are rod shaped.

Ans.

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?

Ans. A cell has many small mitochondria rather than one giant mitochondrion because smaller mitochondria provide a greater surface area in relation to their volume.

The inner membrane of mitochondria contains structures called cristae, which increase the surface area available for chemical reactions and energy production. Therefore, having many smaller mitochondria helps provide a large surface area for efficient cellular respiration and ATP production.

7. If the skin cells start dividing by meiosis instead of mitosis, what do you think will happen to a cut on the skin?

Ans. The cut would not heal normally or efficiently.

Skin cells need to divide by mitosis to produce new cells that are genetically similar to the existing cells. Mitosis maintains the chromosome number and helps in growth and repair.

If skin cells divided by meiosis instead, the resulting cells would have half the number of chromosomes and would not be suitable for normal growth and repair of skin tissue. Therefore, healing of the cut would be severely affected.

Revise, Reflect, Refine

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)

Ans. (i)

(ii)

(iii)

2. Two similar animal cells are placed in two different solutions.

• Cell X is placed in pure water.
• Cell Y is placed in 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.

Ans. Option (iii)

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.

Ans.

4. Which of the following option(s) of the pairs of cell organelles are correctly placed under the given categories?

Ans. Option (i) only.

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.

Ans. Renu is correct.

Plastids are not restricted to the green parts of plants. Different types of plastids perform different functions. Chloroplasts contain chlorophyll and perform photosynthesis, while leucoplasts are colourless plastids that store substances such as starch, oils and proteins.

Therefore, roots can contain plastids, particularly leucoplasts, even though they are underground and do not normally perform photosynthesis. For example, leucoplasts in potato and taro cells store starch.

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.

Ans. Similarities

• Both are double-membrane-bound organelles.
• Both contain their own DNA and ribosomes.
• Both can make some of their own proteins.
• Both are involved in important processes related to energy conversion in the cell.

Differences

7. Which of the following pairs of cell organelles contains DNA?

(i) Chloroplasts, Ribosomes

(ii) Mitochondria, Nucleus

(iii) Golgi bodies, Ribosomes

(iv) Nucleus, Lysosomes

Ans. Option (ii)

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?

Ans. (i) She wants to test whether the concentration of the surrounding solution affects the movement of water into or out of plant cells through osmosis.

(ii) The experiment can be improved by:

• Taking more than one carrot for each condition.
• Keeping the carrots of similar size and mass.
• Measuring their initial and final masses.
• Keeping the volume and concentration of the solutions controlled.
• Keeping the duration and environmental conditions the same.

This would make the results more reliable and allow a better comparison.

(iii) In plain water, water enters the carrot cells by osmosis, making the cells firm and maintaining their turgidity. Therefore, the carrot remains stiff and crunchy.

In concentrated salt solution, water moves out of the carrot cells by osmosis. The cells lose water and become less firm, so the carrot becomes rubbery and limp.

9. Indicate the presence or absence of following structures in bacterial and animal cells:

Ans.

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.

Ans. (i) Cups B and C contain sugar and salt, respectively. These substances make the solution inside the potato cup more concentrated than the surrounding water.

Water therefore moves from the surrounding water into the potato cup through the selectively permeable membranes of the potato cells by osmosis. As a result, water gathers in the hollow portions of Cups B and C.

(ii) Cup A acts as the control of the experiment.

It helps us compare the cups containing sugar or salt with a cup containing no added solute. This allows us to determine whether the accumulation of water is actually due to the presence of sugar or salt.

(iii) Cup A:
Cup A is empty, so there is no concentrated solution inside it to create a significant osmotic gradient. Therefore, water does not accumulate in its hollow portion.

Cup D:
Cup D is made from a boiled potato. Boiling damages the cell membranes, which are essential for osmosis. Therefore, the selectively permeable membrane cannot function normally, so water does not gather in the hollow portion.

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

Ans. Option (ii).

12. What outcome do you expect, if all the mitochondria are removed from a eukaryotic cell?

Ans. If all mitochondria were removed, the cell would be unable to carry out normal cellular respiration efficiently and would have a severe shortage of usable energy (ATP).

As mitochondria are the major sites of cellular respiration, many energy-requiring cellular activities would stop or become severely impaired. Eventually, the cell would lose its ability to function normally and die.

13. Which phenomenon inhibits the formation of tumors in the human body? Can plants also develop tumors? Explain.

Ans. The phenomenon that helps inhibit uncontrolled cell division in many animal cells is called contact inhibition.

In contact inhibition, cell division generally stops when cells come into contact with neighbouring cells. Cancer cells lose this control and continue dividing uncontrollably, which can lead to tumour formation.

Yes, plants can also develop abnormal tumour-like growths. However, plant cells do not show contact inhibition in the same way as many animal cells because their rigid cell walls restrict their growth differently.

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.

Ans.

• Proteins of the cell membrane are mainly synthesised by ribosomes attached to the Rough Endoplasmic Reticulum (RER).
• Lipids are mainly synthesised by the Smooth Endoplasmic Reticulum (SER).

• Path of membrane proteins:
  • Ribosomes on RER → Endoplasmic Reticulum → Transport vesicles → Golgi apparatus → Secretory/transport vesicles → Cell membrane
• Path of membrane lipids:
  • SER → Transport vesicles → Golgi apparatus → Vesicles → Cell membrane

The Golgi apparatus modifies, sorts and packages materials received from the ER before they are transported to their destination.

15. What would happen if gametes are formed by mitotic divisions?

Ans. If gametes were formed by mitosis instead of meiosis, they would retain the same number of chromosomes as the parent cell.

When two such gametes fused during fertilisation, the chromosome number would double in every generation.

Meiosis normally produces gametes with half the number of chromosomes, so that fertilisation restores the original chromosome number. Therefore, mitosis cannot normally replace meiosis in gamete formation.

16. A farmer, Deepa, was very happy with the harvest of amla (Indian Gooseberry) and lemons on her farm. However, she could sell only onefourth 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?

Ans. (i) The farmer has applied the principle of osmosis and the use of high solute concentration for food preservation.

(ii) High concentrations of salt and sugar create a hypertonic environment around microorganisms.

Water moves out of the microbial cells by osmosis. The cells lose water and their growth and metabolic activities are inhibited. As a result, the growth of many spoilage-causing bacteria and fungi is reduced, helping to preserve the food.

(iii) One example is lemon pickle.

Method:
Cut fresh lemons into pieces and add an appropriate amount of salt and spices. Store them in a clean, dry glass jar. The salt creates a concentrated environment that helps reduce microbial growth and extends the shelf life of the lemons.

(iv) The case promotes several important scientific values:

• Application of scientific knowledge in daily life.
• Scientific thinking and understanding of osmosis.
• Food preservation and reduction of food wastage.
• Sustainable use of resources.
• Innovation and problem-solving.
• Food security.
• Value addition and agro-processing.
• Improvement of income and local economy.

Related posts:

Matter in Our Surroundings Class 9 Important Questions Is Matter Around Us Pure? Class 9 Important Questions The Fundamental Unit of Life (Class 9) Important Questions Science Class 9 Important Questions Motion Class 9 Science Important Questions Force and Laws of Motion Class 9 Important Questions Gravitation Class 9 Important Questions Atoms and Molecules Class 9 – Most Important Questions and Answers