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Updated 6 August 2026

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Cell :The Building Block of Life

CBSE · Class 9 · Science

NCERT Solutions for Cell :The Building Block of Life — CBSE Class 9 Science.

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Think It Over

1Where does a cell come from?Show solution

According to Cell Theory, new cells are formed only from pre-existing cells. So a cell comes from another cell by cell division.

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2How have technological interventions facilitated the creation of new knowledge in understanding the world beyond the naked eye?Show solution

Technological interventions, especially the development of the microscope, helped scientists study cells and other tiny structures that are beyond the limit of resolution of the naked eye. Robert Hooke first observed cells using a microscope, and later electron microscopes revealed fine details of cell structure at the nanometre scale. These tools improved magnification, resolution, and contrast, creating new knowledge about the cell and the world beyond what we can see directly.

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3How is the cell structural and functional unit of life?Show solution

The cell is the structural and functional unit of life because all living organisms are made of cells, and all life activities are carried out by cells. In multicellular organisms, groups of similar cells form tissues, tissues form organs, and organs form organ systems, but the cell still remains the basic unit. Cells perform essential functions such as taking in materials, producing energy, making substances, removing wastes, communicating, growing, and dividing. Therefore, life exists and functions at the level of the cell.

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4How does a cell multiply?Show solution

A cell multiplies by cell division. The chapter states that cell division is the process by which new cells are formed from pre-existing cells. In body cells this usually occurs by mitosis.

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Ready to Go Beyond

1How do flowers, fruits, and vegetables acquire varied colours?Show solution

Flowers, fruits, and vegetables acquire varied colours because their plastids contain pigments other than chlorophyll. These plastids are called chromoplasts. Their pigments may be yellow, orange, or red, and these pigments give bright colours to petals, fruits, and vegetables. Such colours also help attract pollinators and fruit-eating animals.

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2If the estimated size of an onion peel cell is 200 μm, how much does a light microscope magnify this cell?Show solution

If the eyepiece has magnification 10X and the objective lens also has 10X, then total magnification is:

10 \times 10 = 100

So the light microscope magnifies the cell 100 times.

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3In eukaryotic cells, a network of fine fibres forms the cytoskeleton, which provides structural support, maintains cell shape, and enables cell movement and internal transport. It is visible only under an electron microscope as a separate entity. The cytoplasm may also store starch (in plant cells), or crystals of calcium oxalate or silica (in some plant cells). These are known as cell inclusions.Show solution

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4Viruses, viroids, and prions are acellular (no cells) infectious agents that are too small to be seen under a light microscope. Viruses are composed of some genetic material with a protein coat. Viroids lack protein coat around its genetic material, while prions are misfolded proteins which lack genetic material.Show solution

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5How do cells monitor their growth to maintain a balance?Show solution

Cells maintain a balance by controlled growth and death. The chapter explains that cells grow and divide in a controlled manner, stay in the right place, carry out their functions, and eventually die when they are no longer needed. This programmed cell death (PCD) helps with normal development, cellular quality control, and immune function. In this way, the body keeps a balance between making new cells and removing old ones.

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6Can you estimate its actual size?Show solution

The chapter gives the example directly: if 25 cells are seen along the diameter of a visible field of 5000 μm, then the size of one onion cell is:

5000 \div 25 = 200 \ \mu m

So the actual size of the cell is 200 μm.

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7In eukaryotic cells, a network of fine fibres forms the cytoskeleton, which provides structural support, maintains cell shape, and enables cell movement and internal transport.Show solution

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8Viruses, viroids, and prions are acellular (no cells) infectious agents that are too small to be seen under a light microscope.Show solution

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9What will happen to them?Show solution

If mung bean seeds are soaked in water and then kept in a concentrated solution, they will lose water by osmosis and shrink. Their weight will decrease because water moves out from the seeds into the more concentrated surrounding solution.

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10Are there any other plastids in plant cells that contain any pigments other than the green pigments?Show solution

Yes. The chapter says that in flower petals and fruits, plastids contain pigments other than chlorophyll; these are called chromoplasts. Their pigments can be yellow, orange, or red. So there are other plastids in plant cells that contain pigments other than the green pigment chlorophyll.

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2.1Let us estimate the size of a cellShow solution

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2.2Let us experimentShow solution

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2.3Let us investigateShow solution

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2.4Let us studyShow solution

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2.5Let us enhance our skillsShow solution

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16What if...Show solution

If a cell is kept in salt or sugar solutions of different concentrations, water moves in or out of the cell by osmosis:

- In an isotonic solution, the solute concentration inside and outside is equal, so there is no net movement of water.
- In a hypotonic solution, the outside has less solute, so water enters the cell and it swells.
- In a hypertonic solution, the outside has more solute, so water leaves the cell and it shrinks.

This explains why cells change size in different solutions.

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2.2.2Cell wall — The outer covering of cellsShow solution

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18Explore different ways by which cells maintain themselves.Show solution

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Activity 2.1: Let us estimate the size of a cell

1Take a transparent ruler with millimetre (mm) markings.Show solution

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2Place the ruler on the stage of microscope, focus on it using the adjustment knob and observe the diameter of the circular field of view through the eyepiece and measure it in mm.Show solution

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3Convert the diameter from mm to micrometre (μm). Suppose the diameter of the visible field is 5 mm, meaning 5 × 1000 = 5000 μm.Show solution

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4Remove the ruler and place an onion peel slide on the stage of the microscope.Show solution

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5Focus on the slide and count the number of cells present along the diameter of the field of view in one straight line.Show solution

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6Estimate the real size of the cell using the formula:Show solution

Using the formula given in the chapter:

Estimated size of onion peel cell = $
\frac{\text{Diameter of the visible field in micrometre}}{\text{Number of cells along the diameter}}$

If the diameter is 5000 μm and 25 cells are along the diameter:

\frac{5000}{25} = 200 \ \mu m

So the estimated real size of the cell is 200 μm.

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What if...

1mung bean seeds are kept in a concentrated solution after soaking in water for 12 hours? What will happen to them?Show solution

If mung bean seeds are kept in a concentrated solution after soaking in water for 12 hours, they will lose water because the concentrated solution is hypertonic compared with the seed cells. By osmosis, water moves out of the seeds, so they will shrink and become less firm. Their weight will also decrease.

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2a cell is kept in salt or sugar solutions of different concentrations?Show solution

The chapter explains three kinds of solutions based on solute concentration relative to the cell:

- Isotonic: extracellular and intracellular concentrations are equal.
- Hypotonic: extracellular concentration is less than intracellular concentration.
- Hypertonic: extracellular concentration is greater than intracellular concentration.

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3What do you infer?Show solution

The inference is that water movement depends on solute concentration. A cell in a dilute medium takes in water and swells, while a cell in a concentrated medium loses water and shrinks. This movement of water through a selectively permeable membrane is called osmosis. So the cell membrane controls the movement of water in response to the surrounding solution.

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4What do you think is the necessity of the cell wall in these cells?Show solution

The cell wall is necessary in plant cells because plants are usually fixed in one place and must withstand environmental stresses like wind and rain. The rigid cell wall gives shape, firmness, and support and helps the plant stay upright. It also helps protect the cell and, along with the cell membrane, allows water and some dissolved minerals to pass through for absorption.

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Activity 2.2: Let us experiment

1With the help of a kitchen knife, carefully cut a potato into two pieces of roughly equal size (Fig. 2.5).Show solution

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2Measure and record the initial weight of both the pieces using a weighing balance.Show solution

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3Put one piece of the potato in Beaker A with plain water.Show solution

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4Put the other piece of the potato in Beaker B with 20 per cent salt or sugar solution.Show solution

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5Leave them undisturbed for about an hour or until you notice a visible change in the size of the pieces.Show solution

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6Measure and record the final weight of each piece.Show solution

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7Calculate the difference between the initial and their final weights.Show solution

To find the difference between the initial and final weights, subtract one from the other:

- Difference = final weight − initial weight if the weight increased
- Difference = initial weight − final weight if the weight decreased

The chapter says to calculate the difference between the initial and final weights of each potato piece.

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Activity 2.3: Let us investigate

1Prepare temporary slides of a thin peel of an onion leaf or a Rhoeo (Cradle lily) leaf and mount it with safranin using cover slip to observe plant cells under a microscope.Show solution

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2Similarly, prepare a temporary slide of cheek cells by gently scraping the inner side of your cheek with a cotton swab or the blunt end of a toothpick.Show solution

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3Spread the cheek cells on a clean glass slide.Show solution

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4Add a drop of water followed by a few drops of methylene blue stain and carefully place a coverslip.Show solution

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5Observe both the slides under a microscope.Show solution

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6What do you observe?Show solution

The observation is that the potato piece in plain water swells and the potato piece in 20% salt or sugar solution shrinks. The increase in weight in plain water and decrease in weight in concentrated solution happen because of osmosis.

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7Why do you think this difference exists?Show solution

This difference exists because of osmosis. The cell membrane allows water to move in and out, but not salt or sugar molecules. In plain water, water enters the potato cells and the piece swells. In concentrated solution, water leaves the cells and the piece shrinks.

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Pause and Ponder

1What 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?Show solution

Plant cells need a cell wall because plants are usually fixed in one place and must remain upright and resist external forces like wind and rain. In animals, cells need more flexibility because the body and tissues often move and change shape. So the rigid cell wall is useful in plants for support and protection, while animals function without it and their cells can change shape more easily.

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

If a plant cell’s cell wall became as flexible as the cell membrane, the cell would lose its rigid shape and support. It would not be able to stay firm, withstand wind and rain, or maintain the plant’s upright form. The chapter states that the rigid wall keeps plant cells firm and in their original shape, so without it the plant would likely become weak and wilt easily.

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3Why is it important to cut the two potato pieces in roughly equal size and measure their initial weight before placing them in different liquids?Show solution

It is important to cut the two potato pieces in roughly equal size and measure their initial weight so that the experiment is fair. Then any change in weight can be compared properly and attributed to the liquid they were placed in, not to differences in size or mass of the potato pieces.

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4Do white flowers contain any pigment? Give reasons.Show solution

White flowers may still contain pigment, but usually they do not contain green chlorophyll. The chapter explains that plastids in petals and fruits can have pigments like yellow, orange, or red, and some flowers may appear white because they lack visible coloured pigments or because pigments are absent in the petals. So white flowers are not necessarily pigment-free, but they do not show coloured pigments strongly.

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5Draw a well-labelled schematic diagram of a plant or an animal cell using these clues —Show solution

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6Instead of many small ones, why does a cell not have a single giant mitochondrion? How does this relate to the concept of surface area?Show solution

A cell does not have a single giant mitochondrion because many small mitochondria provide a much larger surface area for reactions and energy production. The inner membrane is folded into cristae to increase surface area. More surface area means more room for the chemical reactions of cellular respiration, so many smaller mitochondria are more efficient than one giant one.

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

8What happens if meiosis and mitosis do not happen properly? If there is any error in these processes, it can lead to various problems in the body of an organism.

Activity 2.4: Let us study

1Study the given diagrams of a bacterial cell, a plant cell, and an animal cell (Fig. 2.10a, b and c).

2Observe the different structures present in each of them.

3Record your observations in Table 2.1.

4Which of the cells given in Fig. 2.10 fall under the categories of prokaryotic and eukaryotic cells?

Threads of Curiosity

1Do you know any other cells without nucleus?

2What is the future of the development of synthetic cells using non-living chemicals? If a synthetic cell is developed, what may be the related ethical issues?

3Some cells are specialised to perform specific functions. For example, mature Red Blood Cells (RBCs) in humans do not have a nucleus (enucleate). The absence of a nucleus provides more space for haemoglobin, allowing it to transport a larger amount of oxygen to all cells of the body. Since they lack a nucleus, they cannot repair or divide themselves. As a result, their lifespan is short and they survive approximately for 120 days. Do you know any other cells without nucleus?

4How do cancer cells grow and spread?

Activity 2.5: Let us enhance our skills

1Take a jar and fill it up with plain water.

2Now, place an onion bulb over the jar in such a way that its base bearing roots, is immersed in the water.

3Leave the setup for 5-6 days and observe. Do you observe the roots growing? Cut

2 - 3\mathrm{cm}

of the freshly grown roots and transfer them to freshly prepared aceto-alcohol (glacial acetic acid:ethanol :: 1:3). Keep the root tips in aceto-alcohol for 24 hours and then transfer them to 70 per cent ethanol (for preservation).

4Take one or two preserved roots, wash them in water and then, place them on a clean slide.

5Put one drop of dilute Hydrochloric acid (HCl) on the root tips to soften the tissue. Rinse the roots after 10-15 minutes. Then add 2-3 drops of aceto-carmine stain on them.

6Leave the slide for 5-10 minutes and then, gently warm it (with caution) over a spirit lamp.

7Cut the tip portion of the root on the slide and put a coverslip. Gently squash the coverslip with your thumb to spread the cells on the slide.

8Observe the slide under a microscope.

9Are they similar in structure?

10Do you find any structural differences in these cells?

11If yes, why is it so?

12Can you identify which stage comes first during cell division?

Revise, Reflect, Refine

1Differentiate between the following pairs of terms based on the clues given in parentheses:

2Two similar animal cells are placed in two different solutions:

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

5Two 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.

6Mitochondria 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.

7Which of the following pairs of cell organelles contains DNA?

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

10Carry out the following experiment:

11Identify the pair that incorrectly matches the cell organelle with its function.

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

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

14The 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.

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

16A 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:

1(i)Cell membrane and cell wall

1(ii)RER and SER

1(iii)Chloroplasts and chromoplasts

2Two 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?

8A 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?

10Carry 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.

2Which statement provides the correct explanation for the above observations?

3Identify the parts labelled from (a) to (g) and correctly match them with their functions given below:

8What hypothesis does she want to test through this experiment?

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

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

2.3.1 Why do eukaryotic cells need these organelles?

1Eukaryotic cells carry out various life processes in different cell organelles independently at the same time.

2.5 Cell Theory — The Unifying Principle of Biology

1An important observation about living organisms is that all organisms are made up of cells. In 1838, a German botanist named Matthias Schleiden reported that all plants are made up of cells. In 1839, German zoologist Theodor Schwann, found that all animals are also made up of cells. Later, in 1855, a German scientist named Rudolf Virchow, further expanded the Cell Theory by stating that new cells are formed only from pre-existing cells. Together, their work led to the formulation of the Cell Theory.

2.5.1 Do cells grow and reproduce forever?

1Cells grow and divide in a controlled way, stay in the right place, carry out their functions, and eventually die when they are no longer needed. Dead cells are replaced by new cells that carry out the same function. Thus, every cell has a definite life span. If cells do not die when they should or if they die too early problems can arise in the body. In many animal cells, cell division usually stops when cells come in contact with neighbouring cells. This process is called contact inhibition. However, cancer cells lose this control and keep dividing uncontrollably, leading to the formation of tumours. Plant cells grow differently. Due to their rigid cell walls plant cells do not show contact inhibition and follow a different pattern of growth.

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Frequently Asked Questions

What are the important topics in Cell :The Building Block of Life for CBSE Class 9 Science?

Cell :The Building Block of Life covers several key topics that are frequently asked in CBSE Class 9 board exams. Focus on the core concepts listed on this page and practise related questions to build confidence.

How to score full marks in Cell :The Building Block of Life — CBSE Class 9 Science?

Understand the core concepts first, then work through the 84 practice questions available for this chapter. Revise formulas and definitions regularly, and use flashcards for quick recall before the exam.

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Cell :The Building Block of Life

Studying Cell :The Building Block of Life? Get the full interactive chapter.

Think It Over

Ready to Go Beyond

Activity 2.1: Let us estimate the size of a cell

What if...

Activity 2.2: Let us experiment

Activity 2.3: Let us investigate

Pause and Ponder

Activity 2.4: Let us study

Threads of Curiosity

Activity 2.5: Let us enhance our skills

Revise, Reflect, Refine

2.3.1 Why do eukaryotic cells need these organelles?

2.5 Cell Theory — The Unifying Principle of Biology

2.5.1 Do cells grow and reproduce forever?

Frequently Asked Questions

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