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?

Answer

A cell contains many small mitochondria instead of one giant mitochondrion because many small mitochondria provide a larger total surface area. A larger surface area allows more cellular respiration to occur and enables efficient production of energy (ATP).

Small mitochondria can also be distributed throughout the cell wherever energy is needed. If there were only one giant mitochondrion, its surface area relative to its volume would be smaller, making energy production and distribution less efficient.

Thus, having many small mitochondria increases the surface area available for metabolic reactions and helps the cell meet its energy requirements effectively.

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

Answer

Normally, skin cells divide by mitosis to produce new cells with the same chromosome number. This helps in growth and repair of damaged tissues.

If skin cells start dividing by meiosis, the daughter cells will contain only half the chromosome number. These cells will not function properly and will not be able to replace the damaged skin cells effectively.

As a result, the cut on the skin will heal very slowly or may not heal properly. Tissue repair and regeneration will be affected because meiosis is meant for the formation of gametes, not for body cell repair.

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)

Answer

(i) Cell membrane and cell wall (permeability)

(ii) RER and SER (structure)

(iii) Chloroplasts and chromoplasts (pigments)

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.

Answer

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

Pure water is a hypotonic solution compared to the cell contents. Therefore, water enters Cell X through osmosis, causing it to swell.

The concentrated salt solution is a hypertonic solution. Therefore, water moves out of Cell Y through osmosis, causing it to shrink.

Thus, the correct explanation is that water moved into Cell X and moved out of Cell Y through the cell membrane.

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.

Answer

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

Answer

Leucoplasts are plastids found in plant cells and are absent in animal cells. Similarly, the cell wall is present in plant cells but absent in animal cells.

Mitochondria, ribosomes, Golgi apparatus and endoplasmic reticulum are present in both plant and animal cells. Therefore, options (ii), (iii) and (iv) are incorrect.

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.

Answer

Renu is correct.

Plastids are present in all parts of plants, including roots. However, different types of plastids are found in different plant parts depending on their functions.

Roots generally contain leucoplasts, which are colourless plastids that store food materials such as starch, oils, and proteins. Since roots are underground and do not receive sunlight, they usually do not contain chloroplasts and therefore do not perform photosynthesis.

Thus, plastids are present in roots, but they are mainly in the form of leucoplasts rather than chloroplasts.

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.

Similarities

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

Answer

(ii) Mitochondria, Nucleus

The nucleus contains the main genetic material of the cell in the form of DNA. Mitochondria also contain their own DNA, which allows them to replicate independently.

Ribosomes, Golgi bodies, and lysosomes do not contain 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 (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?

Answer

(i) The researcher wants to test the hypothesis that water moves across a selectively permeable membrane by osmosis. Cells gain water when placed in a dilute solution and lose water when placed in a concentrated solution.

(ii) Suggestion for the improvement of this experiment:

• Use several carrots instead of only one carrot in each setup.
• Measure and record the initial and final mass of the carrots.
• Use carrots of exactly the same size and weight.
• Keep all other conditions such as temperature and duration constant.
• Repeat the experiment several times to obtain more reliable results.

(iii) In plain water, water enters the carrot cells through osmosis. The cells become turgid and firm, making the carrot stiff and crunchy.

In concentrated salt solution, water moves out of the carrot cells through osmosis. The cells lose water and become flaccid. As a result, the carrot becomes rubbery and limp.

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

Answer

Bacterial cells are prokaryotic cells. They do not possess a true nucleus or membrane-bound organelles such as mitochondria and Golgi complex. However, they do contain chromosomes.

Animal cells are eukaryotic cells and possess a nucleus, mitochondria, Golgi complex, and chromosomes.

Chromoplasts are plastids found only in plant cells; therefore, they are absent in both bacterial and animal 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 (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.

Answer

(i) In Cups B and C, sugar and salt create a concentrated solution inside the hollowed portions. The water outside the potato cups is less concentrated. Due to osmosis, water moves through the living potato cells from the dilute solution to the concentrated solution. Therefore, water collects in the hollowed portions of Cups B and C.

(ii) Cup A acts as a control setup. It helps us compare the results with the other cups. Since Cup A contains neither sugar nor salt, it shows that water collection in Cups B and C occurs due to osmosis caused by the concentration difference.

(iii) In Cup A, there is no concentration difference between the inside and outside of the potato cup. Therefore, osmosis does not occur and water does not collect.

In Cup D, the potato has been boiled. Boiling destroys the selectively permeable cell membranes required for osmosis. As a result, water cannot move through the cells and does not collect in the hollowed 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

Answer

(ii) SER — Lipid and cellulose synthesis

Smooth Endoplasmic Reticulum (SER) is involved in the synthesis of lipids and certain hormones, but it does not synthesise cellulose. Therefore, option (ii) is incorrectly matched.

Ribosomes perform protein synthesis, and lysosomes help in the digestion of foreign particles and worn-out cell organelles.

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

Answer

Mitochondria are known as the powerhouse of the cell because they produce ATP, which provides energy for cellular activities.

If all the mitochondria are removed from a eukaryotic cell:

• Cellular respiration will stop.
• ATP production will cease or decrease drastically.
• The cell will not receive sufficient energy to perform its activities.
• Processes such as growth, repair, transport of substances, and metabolism will be affected.
• Eventually, the cell will die due to lack of energy.

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

Answer

The phenomenon that inhibits the formation of tumors in the human body is contact inhibition.

In normal cells, cell division stops when cells come into contact with neighbouring cells. This prevents uncontrolled cell division and helps maintain the normal structure of tissues.

Yes, plants can also develop tumors. Plant tumors are usually caused by infections such as crown gall disease caused by bacteria. However, unlike many animal tumors, plant tumors generally remain localized and do not spread from one part of the plant to another.

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.

Answer

Cell organelles involved in the synthesis of the cell membrane:

• Rough Endoplasmic Reticulum (RER) – synthesises proteins.
• Smooth Endoplasmic Reticulum (SER) – synthesises lipids.
• Golgi apparatus – modifies, packages and transports proteins and lipids.

Path of proteins:

Ribosomes → Rough Endoplasmic Reticulum → Transport Vesicles → Golgi Apparatus → Secretory Vesicles → Cell Membrane

Path of lipids:

Smooth Endoplasmic Reticulum → Transport Vesicles → Golgi Apparatus → Secretory Vesicles → Cell Membrane

Labelled Diagram:

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

Gametes are normally formed by meiosis and contain half the chromosome number of the parent cell.

If gametes are formed by mitotic divisions:

• The gametes will contain the full chromosome number.
• After fertilisation, the zygote will have double the normal chromosome number.
• The chromosome number will continue to increase in every generation.
• The genetic stability of the species will be disturbed.
• Normal growth, development and reproduction will be affected.

Therefore, meiosis is necessary to maintain a constant chromosome number in a species.

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?

(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?

Answer

(i) The farmer has applied the principle of food preservation through osmosis. High concentrations of salt, sugar, or jaggery help preserve food by preventing the growth of microorganisms.

(ii) High concentrations of salt and sugar create a hypertonic environment around microbial cells. Water moves out of the cells through osmosis, causing plasmolysis. As a result, bacteria and fungi are unable to grow and reproduce, thereby preventing food spoilage.

(iii) Healthy recipe:

Amla Murabba

• Wash fresh amla thoroughly.
• Boil the amla until soft.
• Prepare a concentrated sugar syrup.
• Add the boiled amla to the syrup.
• Store it in clean, airtight glass jars.
• The high sugar concentration helps preserve the fruit for a long period.

(iv) Scientific values addressed in this case: