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Class 9 - Chapter 2: Cell: The Building Block of Life

NCERTChapter 2Solution- Revise, Reflect, Refine

Exercise Solutions

1. Differentiate between the following pairs of terms.

(i) Cell Membrane and Cell Wall (Permeability)

Cell Membrane Cell Wall
It is selectively permeable and allows only certain substances to pass through. It is freely permeable and allows most substances to pass through.

(ii) RER and SER (Structure)

RER (Rough Endoplasmic Reticulum) SER (Smooth Endoplasmic Reticulum)
Ribosomes are present on its surface, giving it a rough appearance. Ribosomes are absent on its surface, giving it a smooth appearance.

(iii) Chloroplasts and Chromoplasts (Pigments)

Chloroplasts Chromoplasts
Contain green pigment chlorophyll. Contain coloured pigments such as yellow, orange and red carotenoids.

2. Osmosis in Animal Cells

Two similar animal cells are placed in different solutions:

  • Cell X is placed in pure water.
  • Cell Y is placed in a concentrated salt solution.

After some time, Cell X swells and Cell Y shrinks.

Correct Answer: (iii)

Water moved into Cell X and moved out of Cell Y through the cell membrane. This occurs due to osmosis, the movement of water from a region of higher water concentration to a region of lower water concentration through a selectively permeable membrane.

3. Identify the labelled parts (a) to (g) and match them with their functions.

Label Cell Part Function
(a) Mitochondrion Site of cellular respiration.
(b) Nucleus Controlling all the activities of a cell.
(c) Golgi Apparatus Packs and stores materials received from ER.
(d) Chloroplast Helps in manufacturing food.
(e) Cell Wall Provides structural rigidity to the cell.
(f) Cell Membrane Separates the cell contents from surroundings.
(g) Vacuole Storage organelle that also provides rigidity to the cell.

Matching Functions:

  1. Controlling all the activities of a cell → Nucleus (b)
  2. Site of cellular respiration → Mitochondrion (a)
  3. Storage organelle that also provides rigidity to the cell → Vacuole (g)
  4. Separates the cell contents from surroundings → Cell Membrane (f)
  5. Provides structural rigidity to the cell → Cell Wall (e)
  6. Packs and stores materials received from ER → Golgi Apparatus (c)
  7. Helps in manufacturing food → Chloroplast (d)

4. Correct Option

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

Answer: Option (iii)

Present in Plant Cells Absent in Animal Cells
Cell Wall Golgi Apparatus ❌

Actually, Golgi apparatus is present in both plant and animal cells. However, among the given options, only Cell Wall is correctly placed under "Present in Plant Cells". The intended NCERT answer is: Option (iii).

5. Two students, Renu and Rohit, were having a discussion on plastids.

Answer:

Renu is correct. Plastids are present in all plant cells, including root cells. However, different types of plastids perform different functions.

Roots do not perform photosynthesis, so they generally do not contain chloroplasts (green plastids containing chlorophyll). Instead, root cells usually contain leucoplasts, which store food materials such as starch, oils and proteins.

Therefore, plastids are present in roots, but they are usually 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.

Answer:

Similarities
  • Both are double-membrane-bound organelles.
  • Both contain their own DNA and ribosomes.
  • Both can synthesize some of their own proteins.
  • Both are involved in energy conversion processes.

Mitochondria Chloroplasts
Present in both plant and animal cells. Present only in plant cells and some algae.
Site of cellular respiration. Site of photosynthesis.
Convert food energy into ATP. Convert solar energy into chemical energy.
Do not contain chlorophyll. Contain chlorophyll pigment.
Known as the "Powerhouse of the Cell". Known as the "Kitchen of the Cell".

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

Both mitochondria and the nucleus contain DNA. 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.

(i) What hypothesis does she want to test through this experiment?

Answer:

The researcher wants to test the hypothesis that water moves across the cell membrane by osmosis. Water enters cells in plain water and leaves cells in concentrated salt solution.

(ii) What would you suggest for the improvement of this experiment?

Answer:

The experiment should be repeated with several carrots of similar size and under identical conditions. The mass, length or firmness of the carrots should also be measured before and after the experiment for more accurate results.

(iii) Why does the carrot in plain water stay stiff and crunchy, but the carrot in concentrated salt solution become rubbery and limp?

Answer:

In plain water, water enters the carrot cells through osmosis. The cells become turgid, 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, causing the carrot to become rubbery and limp.

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

Structures in a Cell Bacterial Cell Animal Cell
Chromosome Present Present
Nucleus Absent Present
Mitochondria Absent Present
Golgi Complex Absent Present
Chromoplasts Absent Absent

10. Potato Cup Experiment

(i) Explain why water gathers in the hollowed portion of Cup B and Cup C.

In Cup B (containing sugar) and Cup C (containing salt), the concentration of solute inside the potato cup is higher than that of the surrounding water. Therefore, water moves from the beaker into the potato cup through the living cells of the potato by the process of osmosis. As a result, water collects in the hollowed portions of Cups B and C.

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

Cup A serves as the control set-up. Since it contains neither sugar nor salt, there is no concentration difference between the inside of the potato cup and the surrounding water. It helps us compare the results and confirm that water accumulation in Cups B and C is due to osmosis.

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

In Cup A, no solute is present inside the potato cup, so there is no osmotic movement of water.

In Cup D, the potato cup is made from a boiled potato. Boiling destroys the selectively permeable cell membranes of the potato cells. Therefore, osmosis cannot occur and water 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. It does not synthesize cellulose. Therefore, this pair is incorrectly matched.

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

Answer:

Mitochondria are known as the powerhouses of the cell because they produce ATP through cellular respiration. If all mitochondria are removed, the cell will not be able to produce sufficient energy. As a result, essential cellular activities will stop and the cell will eventually die.

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

Answer:

The phenomenon that inhibits tumor formation is called cell division control or contact inhibition. Normal cells stop dividing when they come into contact with neighboring cells, preventing uncontrolled growth.

Plants can also develop tumors. One example is Crown Gall Disease, caused by the bacterium Agrobacterium tumefaciens. However, plant tumors generally remain localized and do not spread throughout the body like cancer in animals.

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:

Proteins are synthesized by ribosomes present on the Rough Endoplasmic Reticulum (RER). Lipids are synthesized by the Smooth Endoplasmic Reticulum (SER). Both are transported to the Golgi apparatus for modification and packaging, and then carried by vesicles to the cell membrane.

Pathway:

Proteins: Ribosomes → RER → Golgi Apparatus → Vesicles → Cell Membrane

Lipids: SER → Golgi Apparatus → Vesicles → Cell Membrane 

Proteins:
Ribosomes
    ↓
RER
    ↓
Golgi Apparatus
    ↓
Transport Vesicles
    ↓
Cell Membrane

Lipids:
SER
 ↓
Golgi Apparatus
 ↓
Transport Vesicles
 ↓
Cell Membrane

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

Answer:

If gametes were formed by mitosis, they would contain the same number of chromosomes as the parent cell. After fertilization, the chromosome number would double in every generation. This would disturb the chromosome balance of the species. Therefore, gametes are formed by meiosis, which reduces the chromosome number to half.

16. Case Study Based Questions

(i) Which scientific concept has the farmer applied in the preservation of the farm produce?

Answer:

The farmer has applied the concept of food preservation using high concentrations of salt, sugar and jaggery to prevent spoilage and increase shelf life.

(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?

Answer:

High concentrations of salt and sugar create a hypertonic environment. Water moves out of microbial cells by osmosis, causing plasmolysis. As a result, bacteria and fungi cannot grow and multiply.

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

Answer:

A healthy preserved food recipe is Amla Murabba. Fresh amla fruits are cleaned, boiled and preserved in sugar syrup. It remains edible for a long time and is rich in Vitamin C.

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

Answer:

  • Food preservation and prevention of wastage.
  • Efficient utilization of agricultural produce.
  • Promotion of sustainable development.
  • Improvement of food security.
  • Scientific processing and storage of food.
  • Economic empowerment of farmers.