NCERT Class 9 Science Chapter 2 Cell: The Building Block of Life

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NCERT Class 9 Science Chapter 2 Cell: The Building Block of Life

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Cell: The Building Block of Life

Chapter – 2

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 fixed in one place and cannot move away from physical stress. Therefore, they need a rigid cell wall to provide support, protection, and maintain their shape. Animals move from one place to another, so their cells need to be flexible. Hence, animal cells have only a cell membrane and do not have a cell wall.

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

Ans: (i) The plant cell would lose its fixed shape and proper support.

(ii) The cell could easily burst due to excess water entering by osmosis, making the plant weak and wilted.

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 cut the two potato pieces to roughly equal size and measure their initial weight so that the experiment is fair. This ensures that any change in weight is due only to osmosis and helps in obtaining accurate and reliable results.

Pause And Ponder

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

Ans: Yes, white flowers also contain pigments. They appear white because they do not contain coloured pigments like anthocyanins or because the pigments present do not absorb visible light. The white colour is due to the reflection of almost all visible light.

2. 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. You may refer to Fig. 2.10.

Ans: 

Pause And Ponder

1. 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 instead of one giant mitochondrion because many small mitochondria provide a larger surface area for energy production. They also distribute energy efficiently throughout the cell and function more effectively.

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

Ans: If skin cells divide by meiosis instead of mitosis, the cut on the skin will not heal properly because meiosis produces haploid cells meant for reproduction, not for growth and repair. Skin repair requires mitosis, which produces identical body cells.

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).

Ans: 

Cell membrane	Cell wall
(i) The cell membrane is selectively permeable.	(i) The cell wall is permeable.
(ii)  It allows only some substances to pass through.	(ii) It allows water and dissolved minerals to pass through.
(iii)  It controls the movement of substances in and out of the cell.	(iii)  It provides support and protection to the cell.

(ii) RER and SER (structure).

Ans: 

RER (Rough Endoplasmic Reticulum)	SER (Smooth Endoplasmic Reticulum)
(i)  RER has ribosomes attached to its surface.	(i) SER does not have ribosomes on its surface.
(ii)  It appears rough under an electron microscope.	(ii) It appears smooth under an electron microscope.
(iii) It mainly helps in protein synthesis.	(iii) It mainly helps in lipid synthesis.

(iii) Chloroplasts and chromoplasts (pigments).

Ans: 

Chloroplasts	Chromoplasts
(i) Chloroplasts contain a green pigment called chlorophyll.	(i) Chromoplasts contain pigments other than chlorophyll.
(ii) They are green in colour.	(ii) They are yellow, orange, or red in colour.
(iii) They help in photosynthesis.	(iii) They give colour to fruits and flowers.

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.

Ans: (iii) Water moved into Cell X and moved out of Cell Y through the cell membrane. 

Explanation: Cell X was placed in pure water (a hypotonic solution), causing water to move into the cell via osmosis, which made it swell. Cell Y was placed in a concentrated salt solution (a hypertonic solution), causing water to move out of the cell, making it shrink

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. 

Ans: Nucleus.

(ii) Site of cellular respiration. 

Ans: Mitochondria.

(iii) Storage organelle that also provides rigidity to the cell. 

Ans: Vacuole.

(iv) Separates the cell contents from surroundings. 

Ans: Cell membrane.

(v) Provides structural rigidity to the cell. 

Ans: Cell wall.

(vi) Packs and stores materials received from ER. 

Ans: Golgi apparatus.

(vii) Helps in manufacturing food.

Ans: Chloeoplast.

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

Ans: (i) Leucoplast Cell wall.

Explanation: Animal cells do not have a cell wall. Leucoplasts are a type of plastid found in plant 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.

Ans: Renu is correct. Plant cells, including those in underground roots, contain plastids. While underground parts do not need green chloroplasts for photosynthesis, they contain colorless plastids called leucoplasts which are used to store food materials like starch (for example, in potato and taro cells).

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. They both have their own DNA and ribosomes, meaning they can make some of their own proteins and share an evolutionary history with single-celled bacteria.

Differences: Mitochondria are the “powerhouses of the cell” where cellular respiration takes place to release energy. Chloroplasts contain chlorophyll and are the sites for food synthesis (photosynthesis) using sunlight.

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: (ii) Mitochondria, Nucleus.

Explanation: The nucleus contains DNA in the form of chromosomes, and mitochondria have their own DNA.

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. 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) Hypothesis: She is testing the process of osmosis by observing how plant cells behave in plain water versus a concentrated salt solution.

(ii) Improvement: She should measure and record the initial and final weight of the carrots to accurately determine the amount of water lost or gained, similar to the potato experiment.

(iii) Explanation: In plain water, the carrot stays stiff because water enters the cells via osmosis, and the rigid cell wall maintains the cell’s shape. In the salt solution, the cells lose water due to osmosis; the cell membrane pulls away from the cell wall, causing the carrot to become rubbery and limp.

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

Structures in a cell	Bacterial cell	Animal cell
Chromosome
Nucleus
Mitochondria
Golgi complex
Chromoplasts

Ans: 

Structures in a cell	Bacterial cell	Animal cell
Chromosome	Present (as genetic material in the nucleoid)	Present
Nucleus	Absent (has a primitive nucleoid instead)	Present
Mitochondria	Absent (lacks membrane-bound organelles)	Present
Golgi complex	Absent	Present
Chromoplasts	Absent	Absent (found only in plant cells)

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. 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. 

Ans: Water in Cup B and C: Water from the beaker moves into the hollowed portion containing sugar or salt through the selectively permeable cell membranes of the potato via osmosis.

(ii) Why is Cup A necessary for this experiment? 

Ans: Necessity of Cup A: Cup A is kept empty to serve as a control, proving that water does not gather in the cup when there is no concentration difference.

(iii) Explain why water does not gather in the hollowed portions of Cups A and D.

Ans: No water in Cup A and D: Water does not gather in Cup A because there is no solute. Water does not gather in Cup D because boiling kills the potato cells and destroys the selective permeability of their membranes.

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: (ii) SER Lipid and cellulose synthesis.

Explanation: The Smooth Endoplasmic Reticulum (SER) synthesizes lipids and some hormones, but it does not synthesize cellulose. Cellulose is the carbohydrate that makes up the plant cell wall.

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

Ans: Mitochondria act as the ‘powerhouses of the cell’, breaking down glucose to release energy (ATP) through cellular respiration. If they are removed, the cell would not be able to produce the energy required for most cellular activities.

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

Ans: (i) In animal cells, cell division is inhibited when cells come in contact with neighboring cells, a process called contact inhibition. When cells lose this control, they divide uncontrollably, leading to tumors.

(ii) Plants can also develop tumours (for example, crown gall disease), but unlike animal tumours, they usually remain localized because rigid cell walls prevent the cells from spreading to other parts of the plant.

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: The cell membrane is made up of proteins and lipids.

Organelles involved in membrane synthesis:

(i) Ribosomes (present on the Rough Endoplasmic Reticulum – RER) synthesize proteins.

(ii) Smooth Endoplasmic Reticulum (SER) synthesizes lipids.

(iii) Golgi Apparatus modifies, packages, and transports these materials to the cell membrane.

Path of proteins and lipids to the cell membrane:

(i) Proteins are synthesized by ribosomes on the RER.

(ii) Lipids are synthesized in the SER.

(iii) Both are transported in vesicles to the Golgi apparatus.

(iv) The Golgi modifies and packages them into secretory vesicles.

(v) These vesicles move to and fuse with the plasma membrane (cell membrane), thereby adding proteins and lipids to it.

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

Ans: Mitosis produces daughter cells with the exact same number of chromosomes as the parent cell. Meiosis is normally required to reduce the chromosome number by half to produce gametes. If gametes were formed by mitosis, fertilization would result in an offspring with double the normal number of chromosomes.

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? 

Ans: Scientific concept: The farmer applied the concept of osmosis to preserve the 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? 

Ans: The high concentration of salt and sugar creates a hypertonic environment. This causes water to move out of the cells of any spoilage-causing bacteria and fungi through osmosis, inhibiting their growth.

(iii) Suggest a healthy recipe of this kind for food preservation. 

Ans: A healthy recipe is lemon pickle prepared with salt and natural spices or amla murabba made with jaggery instead of refined sugar.

(iv) What are the scientific values addressed in this case?

Ans: She applied a traditional yet scientifically sound method to prevent the wastage of post-harvest produce, strengthen food security, create a sustainable model, and boost the local economy.