Class 9 Science Chapter 2 Question Answer Cell The Building Block of Life

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Revise, Reflect, Refine (NCERT Textbook Page No. 24)

Question 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) Chioroplasts and chromoplasts (pigments)
Answer:
(i) The cell membrane is selectively permeable, allowing only specific substances to pass through based on the cell’s needs. The cell wall is freely permeable, allowing almost all substances to pass through it without any restriction.

(ii) The RER (Rough Endoplasmic Reticulum) has ribosomes attached to its outer surface, giving it a rough appearance, and it is made up of flattened, sac-like structures called cisternae. The SER (Smooth Endoplasmic Reticulum) lacks ribosomes on its surface, giving it a smooth appearance, and it is made up of a network of interconnected tubular structures.

(iii) Chloroplasts contain the green pigment chlorophyll, which enables the process of photosynthesis in the cell. Chromoplasts contain pigments other than chlorophyll, such as carotenoids, which give fruits and flowers their red, orange, and yellow colours.

Question 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.

Question 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:
(i) Controlling all the activities of a cell – (b) Nucleus
(ii) Site of cellular respiration – (a) Mitochondria
(iii) Storage organelle that also provides rigidity to the cell – (g) Vacuole
(iv) Separates the cell contents from surroundings – (f) Cell membrane
(v) Provides structural rigidity to the cell – (e) Cell wall
(vi) Packs and stores materials received from ER – (c) Golgi apparatus
(vii) Helps in manufacturing food – (d) Chloroplast

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

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

Answer:
(i) Leucoplasts, Cell wall

Question 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 plasrids 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. All parts of plants, including roots, contain plastids. However, roots do not contain chloroplasts (which carry out photosynthesis) because they are underground and lack light. Instead, root cells have leucoplasts. which store food materials like starch, oils, and proteins.

Question 6.
Mitochondria and chloroplasts are two important organelles in a plant cell. Discuss how these two Organalles are structurally and functionally similar to each other, and different from each other.
Answer:
Similarities: Both mitochondria and chloroplasts are double-membrane organelles and have their own DNA and ribosomes. allowing them to synthesise some of their own proteins. They are involved in energy conversion processes and are considered semi-autonomous organelles.

Differences: Mitochondria are found in both plant and animal cells and carry out cellular respiration, releasing energy in the form of ATP. Chloroplasts are found only in plant cells and perform photosynthesis, using sunlight to produce glucose. Structurally, inside the chloroplast, there is a semi-fluid substance called the stroma. Within the stroma are disc- shaped membrane structures, which contain chlorophyll. In contrast. Mitochondria have folded inner membranes (cristae) to increase surface area for respiration.

Question 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

Question 8.
A researcher conducted an art 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 hr obserrations

(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 how osmosis affects plant tissue in hypotonic (plain water) and hypertonic (salt solution) conditions. She expects water to enter the carrot cells in plain water and leave them in salt solution.

(ii) She can do some improvements in this experiment as given below:

• Use a digital balance to measure initial and final weights precisely.
• Cut carrots into equal-sized pieces for fair comparison.
• Use the same concentration of salt solution each time.
• Repeat the experiment for accuracy.

(iii) The carrot in plain water stays stiff and crunchy because water enters the cells by osmosis, increasing turgidity. The carrot in salt solution becomes rubbery and limp because water leaves the cells due to the higher external salt
concentration. causing loss of turgidity (plasmolysis).

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

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

Answer:

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

Question 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) Water gathers in Cups B and C due to osmosis. The sugar and salt solutions inside the cups are more concentrated than the surrounding water, so water moves from the beaker (higher water concentration) into the potato cup (lower water concentration) through the semipermeable membranes of potato cells.

(ii) Cup A acts as a control helps in comparison and shows that without any solute (sugar or salt), water does not accumulate. This confirms that the movement of water in Cups B and C is due to osmosis.

(iii) Water does not gather in the hollowed portions of Cups A and D, respectively, because:

• In Cup A, there is no concentration difference between inside and outside, so osmosis does not occur.
• In Cup D, boiling destroys the semipermeable membranes of the potato cells, so osmosis cannot take place.

Question 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:
The incorrect pair is (ii) Lipid synthesis occurs in SER, but cellulose synthesis occurs in the cell membrane in plant cells, not by SER.

Question 12.
What outcome do you expect if all the mitochondria are removed from a eukaryotic cell?
Answer:
If all mitochondria are removed from a eukaryotic cell, the cell will not be able to produce ATP energy through cellular respiration. As a result, all cellular activities, such as metabolism, active transport, and cell division, will stop, and the cell will eventually die.

Question 13.
Which phenomenon inhibits the formation of tumours in the human body? Can plants also develop tumours? Explain.
Answer:
The formation of tumours in the human body is inhibited by contact inhibition, where cells stop dividing when they come in contact with each other, preventing uncontrolled growth. Yes, plants can also develop tumours, but they are different from animal cancers. These are usually caused by infections (such as bacteria) or uncontrolled cell division. However, they are generally less harmful because the rigid cell wall prevents the tumour from spreading to other parts.

Question 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:
Endoplasmic Reticulum plays key role in the synthesis of lipids and proteins. Smooth endoplasmic reticulum and rough endoplasmic reticulum synthesise the lipids and proteins of the cell membrane, respectively. The Golgi apparatus modifies sorts, and packages them into vesicles for delivery.

Pathway: Nucleus → SER/RER (Synthesis of lipids/proteins) → Transport Vesicle – Golgi Apparatus (Modification and packaging)

Secretory Vesicles → Cell/Plasma Membrane Endoplasmic Reticulum and Golgi apparatus – pathway for protein processing and secretion.

Question 15.
Which would happen if gametes are formed by mitotic divisions?
Answer:
If gametes were formed by mitotic divisions instead of meiosis, they would be diploid (2n) rather than haploid (n). When two such gametes fuse during fertilisation, the zygote would become tetraploid (4n).

In the next generation, this doubling would continue, leading to 8n, 16n, 32n, etc. The cells would become genetically unstable, too large to function properly, and species survival would be difficult due to lethal chromosome imbalances. Genetic diversity would also be lost since mitosis produces identical copies without recombination.

Question 16.
A farmer, Deepa, was very happy with the harvest of amia (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, mura bbas, and sharhat 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 applied the principle of osmosis for food preservation.
(ii) High concentrations of salt and sugar create a hypertonic environment around microorganisms. Water moves out of the bacterial and fungal cells by osmosis. causing dehydration, which inhibits their growth and prevents spoilage.
(iii) Healthy amia pickle recipe:

• Take 500 g amla chopped)
• Add 2 tbsp mustard oil, I tbsp turmeric, 1 tbsp red chilli powder, 2 tbsp salt,1 tbsp jaggery (instead of excess sugar).
• Heat the oil and add spices, mix it with amla. Store in clean, dry glass jar and keep in sunlight for a few days. This retains vitamin C while preventing microbial growth. also enhancing its flavour.

(iv) Scientific values addressed are resource conservation (zero waste), application of scientific principles (osmosis for preservation). sustainability (agro- processing for food security), and innovation (traditional knowledge + science), economic awareness and entrepreneurship.

Class 9 Science Chapter 2 Cell The Building Block of Life Question Answer InText Questions

Think it Over (NCERT Textbook Page No. 8)

Question 1.
Where does a cell come from?
Answer:
A cell comes from pre-existing cells by a process called cell division. New cells do not appear on their own, they arise from cells that already exist.

Question 2.
How have technological interventions facilitated the creation of rich knowledge in understanding the world beyond the naked eye?
Answer:
Technological advancements, like the microscope, electron microscope, and advanced imaging techniques, have helped scientists observe cells/organisms that cannot be seen with the naked eye.

• Early microscopes revealed the existence of cells.
• Advanced microscopes (like electron microscopes) showed tiny cell structures (organelles).
• This helped us understand cell structure, functions and microorganisms expanding biological knowledge greatly.

Question 3.
‘How is the cell structural and functional unit of life’
Answer:
The cell is the smallest unit that makes up living organisms and carries out life processes such as taking in nutrients, releasing wastes. responding to the environment, growing, and reproducing. In multicellular organisms. Cells also form tissues, organs, and organ systems. So, it is said to be the structural and functional unit of life.

Question 4.
How does a cell multiply?
Answer:
A cell multiplies by cell division, where one parent cell divides to form two or more new cells. This helps in growth and repair, and reproduction in living organisms.

Think it Over (NCERT Textbook Page No. 11)

Question 5.
How does the structure of the cell membrane in the cells of alveoli control the movement of substances across it?
Answer:
The cell membrane in alveolar cells is very thin and selectively permeable, so it allows gases like oxygen and carbon dioxide to diffuse quickly while restricting most water-soluble substances. The thin structure and large surface area of the cell membrane make gas exchange efficient, enabling oxygen to enter the blood easily and carbon dioxide to leave it.

What If. (NCERT Textbook Page No. 12)

Question 1.
Mung bean seeds are kept in a concentrated solution after soaking in water for 12 hours? What will happen to them?
Answer:
If mung bean seeds are kept in a concentrated solution after soaking for 12 hours, they lose water from their cells by osmosis. As a result, the seeds may shrink, stop swelling properly, and their germination may slow down or fail. This happens because a concentrated solution has less water than the seed cells, so water moves out of the seeds into the surrounding solution.

Pause and Ponder (NCERT Textbook Page No. 14)

Question 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?
Answer:
Plants are usually fixed in one place, so a cell wall is needed to give their cells rigidity, support, and protection. Animals, on the other hand, move from place to place, so they need more flexible cells and tissues instead of a rigid outer wall.

Question 2.
What consequences would you predict for a plant cell if its cell wall were to become as flexible as a cell membrane?
Answer:
If a plant cell wall became as flexible as the cell membrane, the cell would lose its firmness and shape more easily, and the plant could wilt or collapse under pressure changes. It would also be less able to resist bursting when water enters by osmosis, so the plant would become weak and poorly supported.

Question 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?
Answer:
The two potato pieces should be cut to roughly equal size and weighed at the start so that the comparison is fair. If they are different in size or starting mass, any change after being placed in liquids could be due to size differences rather than the effect of the liquids themselves.

Pause and Ponder (NCERT Textbook Page No. 19)

Question 4.
Do white flowers contain any pigment? Give reasons.
Answer:
Yes, white flowers do contain pigments, but they lack coloured pigments like anthocyanins. White colour appears because petals reflect all wavelengths of light. They may contain colourless or very faint pigments that do not produce visible colour.

Question 5.
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.

Pause and Ponder (NCERT Textbook Page No. 21)

Question 6.
Can you identify which stage comes first during cell division?
Answer:
Prophase is the first stage of cell division in which chromosomes become visible and the nuclear membrane starts breaking down.

Pause and Ponder (NCERT Textbook Page No. 22)

Question 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 does not usually have one giant mitochondrion because many small mitochondria provide a larger total surface area for respiration and energy release. This increased surface area enhances the efficiency of metabolic reactions, making Energy production more effective than in a single large mitochondrion.

Question 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:
If skin cells divide by meiosis instead of mitosis, the cells formed would have half the chromosome number and would not be suitable for body repair; therefore, the cut would not heal properly.

Ready to Go Beyond (NCERT Textbook Page No. 23)

Question 1.
Explore different ways by which cells maintain themselves.
Answer:
Cells maintain themselves by regulating intake and output, producing energy, repairing damage. removing waste, growing/dividing. and responding to their environment, all these are under the control of genetic instructions.

Class 9 Science Chapter 2 Question Answer (Activities)

Activity 2.1:
Let Us Estimate The Size Of a Cell (NCERT Textbook Page No. 10)

Aim: To estimate the size of an onion peel cell using Estimated size of the onion peel cell a microscope by measuring the field of view and counting the number of cells across it.

Estimation: The formula to calculate the estimated size of the cell is as follows:

Estimated size of the onion peel cell =  Diameter of the visible field in micrometre  Number of cells along the diameter 

Observation: The diameter of the field of view is measured (e.g., 5 mm = 5000 μm), The number of onion peel cells along the diameter is counted (e.g., suppose 10 cells). Cells appear rectangular and are arranged closely without gaps.

Conclusion: The size of a single onion peel cell can be estimated by dividing the diameter of the field of view
by the number of cells counted (e.g., 5000 pm ÷ 10 = 500 μm), showing that cells are microscopic and require a microscope for observation.

Activity 2.2:
Let Us Experiment (NCERT Textbook Page No. 11)

Aim: To observe the effect of osmosis by studying the change in weight of potato pieces placed in plain water and concentrated salt/sugar solution.

Observation: The potato piece kept in Beaker A containing plain water will absorb water, swell, and gain weight, so it becomes firm or turgid. The piece kept in Beaker B, containing a 20% salt or sugar solution, will lose water, shrink and decrease in weight, so it becomes soft or flaccid.

Conclusion: The potato experiment demonstrates osmosis. Water moves from a dilute solution (plain water) into the potato cells, making them turgid, while it moves out of the potato cells in a concentrated salt/sugar solution, making them flaccid. This shows that water always moves from a higher water concentration to a lower water concentration through a semipermeable membrane.

Activity 2.3:
Let Us Investigate (NCERT Textbook Page No. 13)

Aim: To prepare and observe temporary slides of plant cells (onion/ Rhoeo leaf) and human cheek cells, compare their structures, and study the effect of a concentrated sugar solution on them.

Observation:
Onion/Rhoeo leaf (plant cells): The cells are rectangular/brick-shaped with a distinct, rigid cell wall, clear cell membrane, cytoplasm, and a visible nucleus. Cells are regularly arranged.

Human cheek cells: The cells are irregular or rounded in shape and do not have a cell wall. A cell membrane, cytoplasm, and a distinct nucleus are visible. Cells are loosely arranged.

Effect of concentrated sugar solution:

• In plant cells, the cytoplasm shrinks away from the cell wall (plasmolysis), and the cell becomes flaccid.
• In human cheek cells, the cell shrinks due to water loss (exosmosis) and becomes distorted.

Conclusion: Plant cells and animal cells differ in structure mainly due to the presence of a cell wall in plant cells. Both types of cells lose water when placed in a concentrated sugar solution due to osmosis, but plant cells show plasmolysis because of the rigid cell wall, while animal cells simply shrink and get distorted.

Activity 2.4:
Let Us Study (NCERT Textbook Page No. 14-15)

Aim: To study and compare the structures of bacterial, plant, and animal cells using diagrams and record the observations.

Observation:
Table 2.1: Comparison of different kinds of cells based on their structure

Cell structures	Bacterial Cell	Plant Cell	Animal Cell
1. Cell membrane	Present	Present	Present
2. Cell wall	Present	Present	Absent
3. Cytoplasm	Present	Present	Present
4. Well-defined nucleus (genetic material enclosed by a membrane)	Absent	Present	Present
5. Primitive nucleus (nucleoid) (genetic material without membrane around it)	Present	Absent	Absent
6. Membrane-bound organelles	Absent	Present	Present

Conclusion: Bacterial cells are prokaryotic and structurally simpler as they lack a true nucleus and membrane-bound organelles. Plant and animal cells are eukaryotic and more complex, but plant cells differ from animal cells due to the presence of a cell wall, chloroplasts, and a large central vacuole.

Activity 2.5:
Let Us Enhance Our Skills (NCERT Textbook Page No. 20)

Aim: To study the structure of actively dividing cells in the onion root tip and observe different stages of mitosis under a microscope.

Observation: The onion root tip shows a large number of actively dividing cells. Cells are small and closely packed. and actively growing in the meristematic region.

Different stages of mitosis can be clearly observed:

• Interphase: Nucleus is distinct: chromatin network is visible.
• Prophase: Chromosomes become short, thick, and visible.
• Metaphase: Chromosomes align at the centre equatorial plate.
• Anaphase: Sister chromatids move toward opposite poles.
• Telophase: Two new nuclei form: cell begins to divide.

The chromosomes appear darkly stained due to acetocarmine stain, making them clearly visible under the microscope.

The highest number of cells is usually seen in interphase, while and phase and telophase stages are less frequently observed.

Conclusion: The onion root tip consists of a rapidly dividing meristematic region where cells undergo mitosis continuously. Mitosis produces two genetically identical daughter cells, ensuring growth and repair in plants while maintaining the same chromosome number. Hence, the root tip is an ideal region to study different stages of mitotic cell division.