Class-9 Science Chapter-2 Cell: The Building Block of Life
Exercise Questions and Answers
Revise, Reflect, Refine
Q.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).
Difference between Cell membrane and cell wall based on permeability
Cell membrane | Cell wall |
The cell membrane is selectively permeable (it allows certain substances to pass through while blocking others). | The cell wall is permeable (it allows water and some dissolved minerals to pass through it) |
Difference between RER and SER based on structure
RER | SER |
The Rough Endoplasmic Reticulum (RER) has ribosomes attached to its surface, which gives it a rough appearance under an electron microscope | The Smooth Endoplasmic Reticulum (SER) does not have ribosomes on its surface, which gives it a smooth appearance |
Difference between chloroplast and chromoplast based on pigments
Chloroplast | Chromoplast |
Chloroplasts contain a green pigment called chlorophyll, which absorbs sunlight for photosynthesis | Chromoplasts contain pigments other than chlorophyll, such as yellow, orange, or red pigments, which give bright colors to fruits and flower petals. |
Q.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.
(iii) Water moved into Cell X and moved out of Cell Y through the cell membrane.
Q.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.
(i) Controlling all the activities of a cell. – Nucleus (b)
(ii) Site of cellular respiration. – Mitochondria (a)
(iii) Storage organelle that also provides rigidity to the cell. – Vacuole (g)
(iv) Separates the cell contents from surroundings. – Cell Membrane (f)
(v) Provides structural rigidity to the cell. - Cell wall (e)
(vi) Packs and stores materials received from ER. – Golgi apparatus (c)
(vii) Helps in manufacturing food. – Chloroplast (d)
Q.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 |
Option | Present in the plant cells | Absent in the animal cells |
(i) | Leucoplast | Cell wall |
Q.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.
Renu is correct.
Plastids have both food synthesis as well as food storage function. Plants parts that perform photosynthesis contains the green plastids called chloroplast. While the roots do not perform photosynthesis, it still contains colourless plastids called leucoplasts that store food materials such as starch, oils, or proteins.
Therefore, even though roots do not photosynthesize, they still contain plastids to store the food that the plant produces.
Q.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 between Mitochondria and Chloroplasts
- Both organelles contain a double membrane structure, an outer layer and an inner layer.
- Both mitochondria and chloroplasts contain their own DNA.
- Both organelles have their own ribosomes.
- As both of these organelles contains their own DNA and ribosomes, they can synthesize their own protein.
- Both mitochondria and chloroplast share an evolutionary history with certain single-celled bacteria.
Differences between Mitochondria and Chloroplasts:
- The inner membrane of the mitochondria is folded into cristae, while any such infoldings are absent in chloroplast.
- Chloroplast contains disc like structure called grana, which is absent in mitochondria.
- Chloroplast contains light absorbing pigment called chlorophylls. No such light absorbing pigments are present in mitochondria.
- These two organelles also differ based on their primary function. Mitochondria is primary involved in energy production while chloroplasts are involved in food synthesis.
Q.7) Which of the following pairs of cell organelles contains DNA?
(i) Chloroplasts, Ribosomes
(ii) Mitochondria, Nucleus
(iii) Golgi bodies, Ribosomes
(iv) Nucleus, Lysosomes
(ii) Mitochondria, Nucleus.
Q.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?
(i) The researcher is testing whether plant cells will absorb water when placed in a dilute solution (plain water) and lose water when placed in a highly concentrated solution (salt water).
(ii) The researcher should measure and record the initial weight of both carrots before placing them in the liquids, and then measure their final weight after 24 hours. Calculating the exact difference in weight would provide concrete evidence of water loss or gain, similar to how the potato piece experiment is conducted in the sources.
(iii) The carrot in plain water is stiff and crunchy because water moves into the carrot cells by the process of osmosis as the plain water is a hypotonic (dilute) solution. The large central vacuoles inside the plant cells store this incoming water, which creates pressure inside the cell and keeps it firm. The plant's rigid cell wall withstands this pressure, preventing the cell from bursting and keeping the carrot structurally stiff and crunchy.
The carrot in concentrated salt solution becomes rubbery and limp as the water inside the carrot cells moves out by osmosis process due to the presence of a hypertonic solution (concentrate salt solution). As the vacuoles lose water, the inner contents of the plant cells shrink and pull away from the cell walls, causing the cells to lose their firmness and becomes rubbery and limp.
Q.9) Indicate the presence or absence of following structures in bacterial and animal cells:
Structures in cell | Bacterial cell | Animal Cell |
Chromosome | ||
Nucleus | ||
Mitochondria | ||
Golgi Complex | ||
Chromoplasts |
Structures in cell | Bacterial cell | Animal Cell |
Chromosome | Absent | Present |
Nucleus | Absent | Present |
Mitochondria | Absent | Present |
Golgi Complex | Absent | Present |
Chromoplasts | Absent | Absent |
Q.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.
(i) Water gathers inside Cup B (sugar) and Cup C (salt) due to the process of osmosis. The addition of sugar or salt to the inside of these cups, it creates a highly concentrated solution inside the hollowed portion compared to the dilute plain water in the beaker. Osmosis causes water to move from the area with more water (the beaker) through the potato cells (semipermeable membrane) and into the area with less water and more solute (the hollowed cup).
(ii) Cup A acts as the control for the experiment. By leaving it completely empty, it proves that water does not naturally leak into or gather inside the hollowed potato on its own.
(iii) Cup A is empty because there is no sugar or salt inside the cup, there is no difference in concentration between the inside of the potato and the outside water. Without a concentration gradient, osmosis does not occur, and water will not move into the cup.
Cup D is empty as boiling the potato kills its cells, which effectively destroys the structure of the selectively permeable cell membrane. Because the process of osmosis strictly requires a functional, selectively permeable membrane to control water movement, water cannot move into the cup even though there is sugar inside.
Q.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
(ii) SER — Lipid and cellulose synthesis.
Q.12) What outcome do you expect, if all the mitochondria are removed from a eukaryotic cell?
Without mitochondria, the cell would not be able to produce enough ATP to perform essential life processes. This will ultimately lead to cell death.
Q.13) Which phenomenon inhibits the formation of tumors in the human body? Can plants also develop tumors? Explain.
The phenomenon that inhibits the formation of tumors in the human body is called contact inhibition.
Yes, Plants can also develop tumors (can be caused by infection with the bacterium Agrobacterium tumefaciens).
Plant cells do not exhibit contact inhibition. Instead, because they possess rigid cell walls, plant cells are restricted from changing shape easily and follow a completely different pattern of growth compared to animal cells. Although, errors in the process of mitosis are what lead to uncontrolled cell growth and the formation of tumors and as plant cells also rely on mitosis to grow and divide, it can be inferred that they could potentially experience uncontrolled cell division,
Q.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.
The cell organelles that help in the synthesis of the cell membrane are the Rough Endoplasmic Reticulum (RER), which is involved in protein synthesis, and the Smooth Endoplasmic Reticulum (SER), which is involved in the synthesis of lipids (fats).
The pathway of proteins and lipids from their site of synthesis to the cell membrane is as follows:
a. Proteins are produced by the ribosomes attached to the RER, and lipids are produced by the SER.
b. These synthesized proteins and lipids are then sent to the Golgi apparatus where they are modified, sorted, and packaged into tiny transport sacs called vesicles.
c. The vesicles transport the packaged proteins and lipids to the plasma membrane to be integrated into its structure or secreted outside the cell.
Q.15) What would happen if gametes are formed by mitotic divisions?
If gametes were formed by mitotic divisions instead of meiosis, there would be two main consequences:
- Mitosis produces cells with the exact same number of chromosomes as the parent cell. If these gametes combined during fertilisation, the resulting offspring would have double the normal chromosome number, and this doubling would continue in every subsequent generation.
- Mitosis creates genetically identical daughter cells. This means the gametes would lack the genetic variations that normally arise from meiosis, eliminating the genetic diversity essential for sexual reproduction.
Q.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?
(i) The farmer applied the scientific concept of osmosis in the preservation of farm produce.
(ii) The addition of high concentrations of salt and sugar to the excess produce, essentially creates a hypertonic environment. The growth of bacteria and fungi is prevented in such environment as there is a higher concentration of water inside the cells of the bacteria and fungi than in the surrounding salty or sugary environment and thus, the water moves out of their cells through their selectively permeable cell membranes via osmosis. Due to this the fungi and bacteria cannot survive and the spoilage of food is prevented.
(iii) One healthy recipe for food preservation is Amla Murabba:
- Wash and cut fresh amla fruits.
- Boil them lightly and remove the seeds.
- Prepare a syrup using jaggery instead of refined sugar.
- Add the amla pieces into the syrup and cook gently.
- Store the prepared murabba in clean glass jars.
This preserved food is rich in Vitamin C and can be stored for a long time.
(iv) The scientific values addressed in this case is as follows:
Application of scientific knowledge: Using the principles of osmosis practically to solve the real-world problem of food spoilage
Resourcefulness and Sustainability : Recognising a problem and turning perishable, excess produce into profitable, long-lasting products, thereby cutting down on post-harvest waste
Socio-economic responsibility: Creating a sustainable model that not only increases her personal income but also strengthens overall food security and boosts the local economy
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