Important Questions and Answers for Class 9 Science Exploration Chapter 2 – Cell: The Building Block of Life | 2026–27

These Class 9 Chapter 2 Science important questions cover different aspects of the chapter, from estimating the size of an onion cell to understanding how cells control growth and division. The 24 solved questions help students practise scientific concepts through experiments, diagrams and application-based reasoning.

Multiple Choice Questions on Cells and Microscopy

Q1. Scientists studying the hot springs of Puga Valley in Ladakh discovered microorganisms living in water with extremely high temperatures. Which characteristic best explains how these organisms survive?

(a) They are multicellular plants with leaves and roots.
(b) They are thermophiles adapted to high-temperature environments.
(c) They are viruses that require human cells for reproduction.
(d) They can survive only in frozen water.

Answer: (b) They are thermophiles adapted to high-temperature environments.

Explanation: Thermophiles are heat-loving microorganisms. The chapter discusses bacteria living in Puga Valley's hot springs and explains how such environments may help scientists investigate conditions similar to those on early Earth.

Q2. A student uses a microscope with a 10X eyepiece and a 40X objective lens. What is the total magnification?

(a) 50X
(b) 100X
(c) 400X
(d) 4000X

Answer: (c) 400X.

Solution:

Total magnification = Eyepiece magnification × Objective magnification

= 10 × 40

= 400X

Explanation: Total magnification is calculated by multiplying the magnifying powers of the eyepiece and objective lens. However, greater magnification does not always mean greater image clarity.

Q3. Two animal cells are kept in different liquids. Cell A swells in pure water, while Cell B shrinks in concentrated salt solution. Which explanation is correct?

(a) Salt enters Cell B and increases its volume.
(b) Water moves into Cell A and out of Cell B through osmosis.
(c) Cell A grows a cell wall in water.
(d) The nucleus absorbs all the water in Cell B.

Answer: (b) Water moves into Cell A and out of Cell B through osmosis.

Explanation: Water moves across a selectively permeable membrane according to differences in water concentration. In a hypotonic solution, the cell gains water. In a hypertonic solution, it loses water.

Q4. A microscopic cell has genetic material in a nucleoid region but no membrane-bound nucleus or membrane-bound organelles. Which statement is correct?

(a) It must be a typical animal cell.
(b) It is a prokaryotic cell.
(c) It is a plant cell containing chloroplasts.
(d) It cannot perform essential life processes.

Answer: (b) It is a prokaryotic cell.

Explanation: Prokaryotic cells, such as bacteria, do not possess a membrane-bound nucleus. Their genetic material lies in a nucleoid region, and many cellular activities occur in the cytoplasm.

Q5. A cell produces proteins that must be transported outside the cell. Which sequence best represents the usual pathway for a secreted protein?

(a) Ribosomes on rough ER → Golgi apparatus → Transport vesicles → Cell membrane
(b) Cell wall → Nucleus → Chloroplast → Vacuole
(c) Lysosome → Mitochondrion → Nucleoid → Cell wall
(d) Vacuole → Chromosome → Cell wall → Ribosome

Answer: (a) Ribosomes on rough ER → Golgi apparatus → Transport vesicles → Cell membrane.

Explanation: Ribosomes attached to the rough endoplasmic reticulum synthesise many proteins destined for secretion. These proteins pass through the ER and Golgi apparatus before vesicles deliver them to the cell membrane.

Q6. Which statement correctly describes meiosis?

(a) It always produces two genetically identical cells with the same chromosome number.
(b) It produces cells with half the original chromosome number and contributes to genetic variation.
(c) It occurs only in bacterial cells.
(d) It prevents the formation of gametes.

Answer: (b) It produces cells with half the original chromosome number and contributes to genetic variation.

Explanation: Meiosis involves two successive divisions that commonly produce four haploid cells from one diploid parent cell. It helps maintain chromosome number across generations and creates genetic variation.

Very Short Answer Questions on Cell Structure and Functions

Q7. Why did Robert Hooke use the word 'cell' for the structures he observed in cork?

Answer: In 1665, Robert Hooke observed small, box-like compartments in a thin slice of cork using a microscope. The compartments reminded him of small rooms, so he named them cells.

Q8. What is the difference between magnification and resolution?

Answer: Magnification indicates how much larger an image appears than the actual object. Resolution is the ability to distinguish two closely spaced points as separate structures. A highly magnified image may still appear unclear if resolution is poor.

Q9. What is plasmolysis?

Answer: Plasmolysis occurs when a plant cell loses water in a hypertonic solution, causing the cell membrane and inner contents to shrink away from the rigid cell wall.

Q10. What are cristae, and why are they important?

Answer: Cristae are folds of the inner mitochondrial membrane. They increase the surface area available for reactions involved in producing ATP during cellular respiration.

Q11. Why do mature human red blood cells lack a nucleus?

Answer: Mature human red blood cells lose their nucleus during development, providing more space for haemoglobin. This helps them transport oxygen efficiently, although they can no longer divide and have a limited lifespan.

Short Answer Questions on Cell Experiments and Organelles

Q12. A student measures the diameter of a microscope's field of view as 3 mm. Eight onion cells fit in a straight line across this diameter. Estimate the average length of one cell.

Solution:

Step 1: Convert the field diameter into micrometres.

1 mm = 1000 µm

3 mm = 3000 µm

Step 2: Divide by the number of cells.

Average cell length = Field diameter ÷ Number of cells

= 3000 ÷ 8

= 375 µm

Answer: The estimated average cell length is 375 micrometres.

Explanation: This method estimates actual cell size using the microscope's field diameter. It assumes the cells occupy the measured line without significant gaps or overlapping lengths.

Q13. Two equal-sized potato pieces are placed in separate beakers. One is kept in plain water, and the other in concentrated sugar solution. Predict the changes in mass and explain why the initial masses should be recorded.

Answer:

The potato piece kept in plain water is likely to gain mass as water enters its cells.

The potato piece kept in concentrated sugar solution is likely to lose mass as water moves out of its cells.

Both changes occur through osmosis.

Why Record the Initial Masses?

Recording the initial mass allows the student to calculate the change in mass accurately.

Using similar-sized potato pieces also helps make the experiment a fair comparison.

Conclusion: The experiment demonstrates how differences in solution concentration affect water movement across cell membranes.

Q14. A student observes that the outer shape of a plant cell remains almost unchanged in concentrated sugar solution, but its inner contents shrink. Why does this happen?

Answer:

A plant cell has both a cell membrane and a rigid cell wall.

When placed in a concentrated sugar solution, water moves out of the cell by osmosis.

The cytoplasm and vacuole lose water, so the inner contents shrink.

However, the rigid cell wall generally retains its shape.

This separation of the cell membrane from the cell wall is called plasmolysis.

Unlike plant cells, animal cells lack a rigid cell wall and may shrink more noticeably when they lose water.

Q15. Why is the cell membrane called selectively permeable, while the cell wall is generally described as permeable?

Answer:

The cell membrane controls the movement of many substances into and out of the cell.

It allows some substances to cross more readily than others, depending on their properties and the transport proteins present.

The plant cell wall is generally permeable to water and many dissolved substances.

Conclusion: The membrane regulates exchange, while the wall primarily supports and protects the cell.

Q16. Explain why the fluid-mosaic model is used to describe the structure of the plasma membrane.

Answer:

The plasma membrane consists mainly of a lipid bilayer with proteins embedded in or associated with it.

Fluid: Many membrane molecules can move sideways within the membrane. This allows flexibility and helps the membrane perform different functions.

Mosaic: Proteins are distributed among the lipid molecules, producing an arrangement compared with a mosaic.

The membrane also contains proteins involved in transport, signalling and recognition.

Conclusion: The fluid-mosaic model explains how the membrane remains flexible while controlling communication and substance movement.

Q17. Compare rough endoplasmic reticulum and smooth endoplasmic reticulum. Why does a cell need both?

Answer:

Cells require proteins for structure, enzymes and communication.

They also require lipids for membranes and other functions.

Both forms of ER contribute to manufacturing and processing materials needed for cell activities.

Q18. Why would a cell have difficulty surviving if its Golgi apparatus stopped functioning?

Answer:

The Golgi apparatus modifies, sorts and packages materials received from the endoplasmic reticulum.

If it stopped functioning:

• Many proteins and lipids would not be processed correctly.

• Some materials would fail to reach their intended destinations.

• Normal secretion could be disrupted.

• The formation and functioning of certain cellular compartments could be affected.

Example: A cell that produces substances for export may make the proteins but fail to package and deliver them efficiently.

Conclusion: Cell organelles work together, so disruption of one organelle can affect several other cellular processes.

Q19. A green leaf cell and a carrot root cell both belong to plants but contain different types of plastids. Explain why.

Answer:

Plastids perform different functions according to the needs of the cell.

Chloroplasts: Contain chlorophyll and are responsible for photosynthesis in green plant tissues.

Chromoplasts: Contain pigments associated with yellow, orange and red colours.

Leucoplasts: Are generally colourless and are involved in storage functions.

A green leaf cell usually contains chloroplasts because it captures sunlight for photosynthesis.

Carrot root cells contain pigment-bearing plastids that contribute to their orange colour.

Conclusion: Different plastids help cells perform specialised functions, even within the same plant.

Long Answer and Application-Based Questions on Cell Organisation and Division

Q20. Compare bacterial, plant and animal cells using their main structures and functions.

Answer:

Bacterial, plant and animal cells all perform essential life processes, but their internal organisation differs.

Bacterial Cell

A bacterium is prokaryotic.

Its genetic material is not enclosed inside a membrane-bound nucleus.

It does not possess organelles such as mitochondria or Golgi bodies.

Plant Cell

A typical plant cell is eukaryotic.

It has a cell wall, cell membrane, nucleus and several organelles.

Many plant cells also possess a large central vacuole.

Photosynthetic plant cells contain chloroplasts.

Animal Cell

An animal cell is eukaryotic but lacks a rigid cell wall.

It contains a membrane-bound nucleus, mitochondria, ER, Golgi apparatus and other organelles.

Conclusion

The main distinction is between prokaryotic and eukaryotic organisation, while plant and animal cells also differ in structures such as cell walls and plastids.

Q21. Explain how the nucleus, ribosomes, ER, Golgi apparatus and cell membrane work together to produce and release a protein.

Answer:

A cell functions as a coordinated system rather than a collection of independent organelles.

Consider a cell producing a protein that must be released outside the cell.

Step 1: Instructions in the Nucleus

DNA contains instructions for making proteins.

A relevant gene is transcribed into messenger RNA (mRNA).

Step 2: Movement of Information

The mRNA leaves the nucleus through nuclear pores and reaches the cytoplasm.

Step 3: Protein Synthesis

Ribosomes read the mRNA instructions and join amino acids to make a protein.

Many proteins intended for secretion are synthesised on ribosomes attached to the rough ER.

Step 4: Processing in the ER

The newly made protein enters the endoplasmic reticulum, where it begins folding and processing.

Step 5: Transport to the Golgi Apparatus

Transport vesicles carry the protein from the ER to the Golgi apparatus.

Step 6: Sorting and Packaging

The Golgi apparatus further modifies and sorts the protein.

Step 7: Release from the Cell

Vesicles transport the packaged protein to the plasma membrane.

The vesicles may fuse with the membrane and release their contents outside the cell through exocytosis.

Important Sequence:

Nucleus → Ribosomes on RER → ER → Golgi apparatus → Vesicles → Cell membrane

Conclusion

The process demonstrates division of labour and coordination among cell organelles.

Q22. A teacher shows a diagram of a mitochondrion and asks why its inner membrane has numerous folds. Explain its structure and the importance of ATP.

Answer:

Mitochondria are organelles involved in cellular respiration and ATP production.

Structure of a Mitochondrion

A typical mitochondrion has:

• An outer membrane

• An inner membrane

• Folds called cristae

• An inner region called the matrix

• Its own DNA and ribosomes

Function of Cristae

The inner membrane contains proteins involved in ATP production.

The folds increase its surface area, creating more space for reactions associated with energy generation.

Role of Cellular Respiration

Cells break down glucose and other food molecules through chemical processes that release usable energy.

Mitochondria are important sites for these processes in eukaryotic cells.

What Is ATP?

ATP stands for Adenosine Triphosphate.

It is commonly called the energy currency of the cell because it transfers energy for cellular activities.

Cells use ATP for processes such as transport, movement and biosynthesis.

Why Mitochondria Matter

If mitochondrial ATP production is severely disrupted, many energy-dependent activities are affected.

Cells may still produce some ATP through other pathways, such as glycolysis, but their energy supply can become insufficient.

Conclusion

The structure of mitochondria supports ATP production, helping cells meet their energy requirements.

Q23. Explain how mitosis and meiosis differ in their purpose, products and effect on chromosome number.

Answer:

Cells divide for growth, repair, reproduction and the formation of specialised reproductive cells.

Mitosis and meiosis are two different forms of cell division.

Mitosis

During mitosis, duplicated chromosomes are separated into two daughter nuclei.

After the cell divides, two daughter cells are generally produced.

Each retains the same chromosome number as the parent cell.

Meiosis

During meiosis, a diploid cell undergoes two successive divisions.

The resulting cells usually have half the original chromosome number.

The process also produces genetic variation through chromosome recombination and independent assortment.

Example in Humans

Most human body cells contain 46 chromosomes.

Human gametes contain 23 chromosomes.

Fertilisation restores the chromosome number to 46.

Why the Difference Matters

If gametes were formed by ordinary mitosis instead of meiosis, chromosome number would not be halved before fertilisation.

This would disrupt the usual chromosome-number balance across generations.

Conclusion

Mitosis supports growth and maintenance, while meiosis is essential for sexual reproduction and genetic variation.

Q24. What prevents normal cells from dividing indefinitely, and how can disruption of growth control lead to tumours?

Answer:

Normal cells receive signals that help regulate when they grow and divide.

Cell division is coordinated with the needs of the organism.

1. Growth Regulation

Cells respond to chemical signals and interactions with neighbouring cells.

These signals help maintain normal tissue organisation.

2. Contact Inhibition

Many normal cells stop dividing when they come into contact with neighbouring cells, and space becomes limited.

This phenomenon is called contact inhibition.

3. Programmed Cell Death

Cells may undergo programmed cell death when they are damaged, no longer needed or receive particular biological signals.

This process helps maintain healthy tissues.

4. Loss of Growth Control

If mutations or other changes disrupt systems controlling cell division, some cells may continue multiplying when they should stop.

5. Tumour Formation

Uncontrolled cell division can produce a mass of abnormal cells called a tumour.

Not every tumour is cancerous.

Benign tumours remain localised, while malignant tumours can invade surrounding tissues and may spread to other parts of the body.

6. Can Plants Also Develop Tumours?

Yes. Plants can develop abnormal growths, such as galls, due to infections or disruptions in growth regulation.

Conclusion

Normal growth control and programmed cell death help maintain healthy tissues. Their disruption can cause abnormal cell proliferation and tumour formation.

Important Concepts, Diagrams and Preparation Tips for Class 9 Science Chapter 2

The important questions of Class 9 Science Chapter 2 focus on how cells are observed, how substances move through membranes, how organelles perform specialised functions, and how cells grow and divide. Students should revise these concepts carefully to strengthen their understanding and prepare for examinations.

Important Formula:

Total Microscope Magnification = Eyepiece Magnification × Objective Magnification

Estimated Cell Size = Microscope Field Diameter ÷ Number of Cells Across the Diameter

Preparation Tips: Practise labelled diagrams of plant, animal and bacterial cells, revise the potato osmosis experiment, compare organelle functions, and solve microscope magnification and cell-size questions. Give special attention to the processes of mitosis, meiosis and protein transport.

The Class 9 Science Chapter 2 Important Questions PDF, once available, can help students revise solved questions, diagrams, comparisons and experiments from Cell: The Building Block of Life for the 2026–27 academic session.

Additional Study Materials for Class 9 Science Chapter 2 – Cell: The Building Block of Life

CBSE Class 9 Science Chapter-wise Important Questions

These chapter-wise resources cover concepts from the Exploration Science textbook for the 2026–27 academic session. They help students revise important topics, practise answers and connect related scientific ideas.

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