Complete question-answer bank for Class 9 Science Chapter 2: Cell — The Building Block of Life. Covers 1-mark, 2-mark, 3-mark, and 5-mark questions with detailed answers, along with competency-based questions and case studies. Topics include cell theory, cell organelles, osmosis, diffusion, plasmolysis, prokaryotic vs eukaryotic cells, and cell division. Aligned with CBSE and NCERT syllabus 2026–27.
Complete Question Bank | 1-Mark to 5-Mark Questions | Competency-Based | Case Studies | Reading Time: 20 minutes
Master the Cell chapter with this comprehensive question bank covering all important topics from NCERT Class 9 Science Chapter 2. Each answer includes detailed explanations with links to concept pages for deeper understanding. Questions are organized by marks for effective exam preparation.
Each question carries 1 mark. Answer in one word or one sentence.
Answer: Robert Hooke discovered the cell in 1665 while observing a thin slice of cork under a microscope. Read more about cell discovery.
Answer: The cell is the basic structural and functional unit of all living organisms.
Answer: Anton van Leeuwenhoek (1674) first observed living cells in pond water using an improved microscope.
Answer: Robert Brown discovered the nucleus in orchid cells in 1831.
Answer: Mitochondria are called the powerhouse of the cell because they generate ATP through cellular respiration. Learn about cell organelles.
Answer: Cellulose is the main component of plant cell walls.
Answer: Ribosomes are responsible for protein synthesis in cells.
Answer: Osmosis is the movement of water across a selectively permeable membrane from a region of high water concentration to low water concentration. See osmosis.
Answer: Lysosomes are known as "suicide bags" because they contain digestive enzymes that can break down cellular waste and damaged organelles.
Answer: Chloroplasts (green plastids for photosynthesis) are found only in plant cells and some protists.
Answer: ATP stands for Adenosine Triphosphate, the energy currency of cells.
Answer: Bacteria and blue-green algae (cyanobacteria) are prokaryotic organisms that lack a well-defined nucleus.
Answer: The membrane surrounding a vacuole is called the tonoplast.
Answer: Meiosis produces gametes (sex cells) with half the chromosome number.
Answer: Plasmolysis is the shrinkage of cell cytoplasm away from the cell wall when placed in a hypertonic solution. Observe this in Activity 2.2.
Answer: The Golgi apparatus (Golgi complex) packages, modifies, and dispatches proteins and lipids.
Answer: Chromoplasts are colored plastids that provide red, yellow, or orange color to flowers and fruits.
Answer: Chromosomes align at the equator during metaphase of mitosis.
Answer: The cell membrane is composed of lipids (phospholipids) and proteins.
Answer: Meiosis produces four haploid daughter cells from one diploid parent cell.
Each question carries 2 marks. Answer in 2-3 sentences (30-40 words).
Answer: The three postulates of cell theory are:
Answer: The main differences are:
| Prokaryotic Cells | Eukaryotic Cells |
|---|---|
| Lack nuclear membrane | Have a well-defined nucleus with nuclear membrane |
| No membrane-bound organelles | Contain membrane-bound organelles |
| Example: Bacteria | Example: Plant and animal cells |
Learn more about prokaryotic vs eukaryotic cells.
Answer: The plasma membrane is called selectively permeable because it allows only certain substances to pass through while blocking others. Small molecules like water, oxygen, and carbon dioxide can pass easily, while larger molecules require special transport mechanisms. This selective nature helps maintain the cell's internal environment.
Answer: If the plasma membrane ruptures, the cell will lose its ability to regulate what enters and exits. Essential molecules and ions will leak out, while harmful substances will enter uncontrollably. This will lead to the death of the cell as it cannot maintain its internal environment or carry out metabolic processes.
Answer:
| Diffusion | Osmosis |
|---|---|
| Movement of any molecules from high to low concentration | Movement of water molecules only |
| Does not require a membrane | Requires a selectively permeable membrane |
| Example: Perfume spreading in air | Example: Water absorption by roots |
Read more about osmosis and diffusion.
Answer: Plant cells do not burst in hypotonic solutions because they have a rigid cell wall made of cellulose. When water enters the cell by osmosis, the cell swells but the cell wall prevents it from bursting. The cell becomes turgid, which actually provides structural support to the plant.
Answer: Mitochondria perform cellular respiration, breaking down glucose to produce ATP (energy). They are called the "powerhouse of the cell" because they generate the energy needed for all cellular activities. Mitochondria have their own DNA and can self-replicate. Learn more about cell organelles.
Answer: The nucleus is the control center of the cell. It contains chromosomes (DNA) that carry genetic information for inheritance and cell functioning. The nucleus regulates all cellular activities including growth, metabolism, and reproduction by controlling protein synthesis through gene expression.
Answer: Lysosomes contain powerful digestive enzymes that break down worn-out cell organelles, foreign materials, and waste products. If the lysosome membrane ruptures, these enzymes can digest the cell's own components, leading to cell death. Hence they are called "suicide bags" of the cell.
Answer:
| Plant Cell | Animal Cell |
|---|---|
| Has cell wall | No cell wall (only cell membrane) |
| Has chloroplasts | No chloroplasts |
| Large central vacuole | Small or no vacuoles |
See this difference in Activity 2.3.
Answer: Chloroplasts are the sites of photosynthesis in plant cells. They contain the green pigment chlorophyll which traps sunlight energy and converts it into chemical energy (glucose) using carbon dioxide and water. This process also releases oxygen as a by-product.
Answer: Large vacuoles in plant cells store water, maintaining turgor pressure that keeps plants upright and rigid. They also store nutrients, ions, waste products, and pigments. The vacuole helps regulate the cell's internal environment and can occupy up to 90% of the cell volume in mature plant cells.
Answer: Mitochondria and chloroplasts have their own DNA because they are believed to have originated from free-living bacteria that were engulfed by primitive eukaryotic cells (endosymbiotic theory). Their own DNA allows them to self-replicate and produce some of their own proteins independently of the nucleus.
Answer: The Golgi apparatus modifies, packages, and sorts proteins and lipids received from the endoplasmic reticulum. It prepares these molecules for secretion or delivery to other parts of the cell. It also forms lysosomes by packaging digestive enzymes.
Each question carries 3 marks. Answer in 50-60 words with examples where needed.
Answer: In 1665, Robert Hooke examined a thin slice of cork (dead plant tissue) under a primitive microscope he had improved. He observed a honeycomb-like structure consisting of tiny compartments separated by walls. He called these box-like structures "cells" because they reminded him of the small rooms (cells) that monks lived in at monasteries. However, Hooke only saw the cell walls of dead cells, not the living contents. His discovery marked the beginning of cell biology. Learn more about cell discovery.
Answer: Osmosis is the movement of water molecules through a selectively permeable membrane from a region of higher water concentration (dilute solution) to a region of lower water concentration (concentrated solution).
Example: When raisins are placed in water, they swell because water enters them by osmosis. Try Activity 2.2.
Types of solutions:
Answer: Plasmolysis is the shrinkage of the cytoplasm of a plant cell away from the cell wall due to loss of water when placed in a hypertonic solution.
Process: When a plant cell is placed in a concentrated salt or sugar solution (hypertonic), water moves out of the cell by osmosis. The cell membrane shrinks and pulls away from the rigid cell wall. The cell becomes flaccid and wilts. This process is reversible if the cell is placed back in water.
Example: When salt is added to cut vegetables, they release water and become limp due to plasmolysis.
Answer: The endoplasmic reticulum is a network of membrane-bound tubules and flattened sacs that extends throughout the cytoplasm, connected to the nuclear envelope.
Two types:
Functions: Protein synthesis (RER), lipid synthesis (SER), transport of materials, and formation of skeletal framework of the cell. Learn about cell organelles.
Answer: Plastids are double-membrane-bound organelles found only in plant cells and some protists. They contain their own DNA and ribosomes.
Types of plastids:
Answer: Chromosomes are thread-like structures present in the nucleus that become visible during cell division. They are made of DNA (deoxyribonucleic acid) tightly coiled around histone proteins.
Structure: Each chromosome has a constriction point called the centromere that divides it into two sections (arms). During cell division, sister chromatids are joined at the centromere.
Significance: Chromosomes carry genes (hereditary units) that transmit information from parents to offspring. They contain instructions for making proteins and controlling all cellular activities. Each organism has a specific number of chromosomes (humans have 46). See chromosomes in Activity 2.5.
Answer: Mitosis has four main stages:
Answer: The cell wall is a rigid outer covering made of cellulose that provides several important functions:
Without cell wall: Plant cells would burst in hypotonic solutions like animal cells do. Plants would lose their rigid structure, wilt, and could not stand upright. They would be vulnerable to mechanical damage.
Answer: The endosymbiotic theory proposes that mitochondria and chloroplasts originated from free-living bacteria that were engulfed by primitive eukaryotic cells billions of years ago. Instead of being digested, these bacteria formed a symbiotic relationship with the host cell.
Evidence supporting this theory:
Answer: [Students should draw a labeled diagram showing: cell wall, cell membrane, cytoplasm, nucleus, mitochondria, chloroplasts, vacuole, endoplasmic reticulum, Golgi apparatus, ribosomes]
Three organelles:
Compare with animal cell in Activity 2.3.
Answer: Active transport is the movement of substances across the cell membrane against their concentration gradient (from low to high concentration) using energy (ATP).
Differences:
| Active Transport | Passive Transport |
|---|---|
| Requires energy (ATP) | No energy required |
| Against concentration gradient | Along concentration gradient |
| Example: Absorption of minerals by roots | Example: Diffusion, osmosis |
Answer: Cells are generally small (typically 1-100 micrometers) due to the surface area to volume ratio.
Limitations on cell size:
This is why large organisms are multicellular with many small cells rather than having a few giant cells.
Each question carries 5 marks. Answer in 80-100 words with detailed explanations, diagrams, and examples.
Answer: The cell theory is one of the fundamental principles of biology that explains the role of cells in living organisms.
Proposers:
Three postulates of cell theory:
Exceptions to cell theory:
Answer: Cells are classified into two types based on the presence of a well-defined nucleus.
Detailed comparison:
| Feature | Prokaryotic | Eukaryotic |
|---|---|---|
| Nucleus | No nuclear membrane; DNA in nucleoid | Well-defined with nuclear membrane |
| Size | Small (1-10 μm) | Large (10-100 μm) |
| Organelles | No membrane-bound organelles | Membrane-bound organelles present |
| DNA | Circular, not associated with histones | Linear, associated with histone proteins |
| Ribosomes | 70S type (smaller) | 80S type (larger) |
| Cell division | Binary fission | Mitosis and meiosis |
| Examples | Bacteria, blue-green algae | Plants, animals, fungi, protists |
Learn more about prokaryotic vs eukaryotic cells.
Answer: Mitochondria are double membrane-bound organelles found in most eukaryotic cells.
Structure:
Functions:
Why "powerhouse"? Mitochondria are called the powerhouse because they generate most of the cell's supply of ATP through aerobic respiration. One glucose molecule produces approximately 36-38 ATP molecules in mitochondria, providing energy for all cellular activities including movement, growth, and synthesis.
Special features: Mitochondria have their own DNA and ribosomes, suggesting they were once independent organisms (endosymbiotic theory). They can self-replicate.
Answer: Mitosis is a type of cell division that produces two identical daughter cells from one parent cell, maintaining the same chromosome number.
Stages of mitosis:
Significance of mitosis:
Observe mitosis stages in Activity 2.5.
Answer: The plasma membrane (cell membrane) is a thin, flexible barrier that separates the cell's internal environment from the external environment.
Chemical composition:
Fluid Mosaic Model (Singer and Nicolson, 1972):
Functions of plasma membrane:
Answer: [Students should draw a labeled diagram of animal cell showing all major organelles]
Major cell organelles and functions:
Compare with plant cells in Activity 2.3.
Answer: Photosynthesis is the process by which plants convert light energy into chemical energy (glucose) using carbon dioxide and water.
Equation:
6CO₂ + 6H₂O + Light energy → C₆H₁₂O₆ (glucose) + 6O₂
Structure of chloroplasts:
Role of chloroplasts in photosynthesis:
Why plants are autotrophs: Plants are called autotrophs (self-feeders) because they can synthesize their own food (glucose) from simple inorganic substances (CO₂ and H₂O) using sunlight. They don't depend on other organisms for nutrition. This makes them primary producers in food chains, supporting all other life forms.
Chloroplasts are found only in plant cells, which you can observe in Activity 2.3.
Answer: Both are types of cell division but serve different purposes in organisms.
Detailed comparison:
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | One division | Two divisions (Meiosis I & II) |
| Daughter cells | 2 cells produced | 4 cells produced |
| Chromosome number | Diploid (2n) → 2n + 2n | Diploid (2n) → n + n + n + n |
| Genetic variation | Identical to parent (no variation) | Genetically different (variation) |
| Crossing over | No crossing over | Crossing over occurs |
| Where occurs | Somatic (body) cells | Reproductive organs only |
| Purpose | Growth, repair, asexual reproduction | Sexual reproduction (gamete formation) |
Significance of meiosis:
Answer: This experiment demonstrates osmosis using potato or raisins. See Activity 2.2.
Materials needed: Fresh potato, two beakers, distilled water, concentrated salt solution, knife
Procedure:
Observations:
Explanation:
Conclusion: This proves that osmosis occurs in plant cells through the selectively permeable cell membrane, and the direction of water movement depends on the concentration gradient.
Answer: [Students should draw two labeled diagrams side by side showing plant and animal cells with all major organelles]
Five main differences:
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| 1. Cell wall | Present (made of cellulose) | Absent |
| 2. Chloroplasts | Present (for photosynthesis) | Absent |
| 3. Vacuoles | Large central vacuole (up to 90% of cell volume) | Small, temporary vacuoles or absent |
| 4. Centrosomes | Absent (except in lower plants) | Present (with centrioles) |
| 5. Shape | Fixed, rectangular shape due to cell wall | Irregular or round shape |
Additional differences: Plant cells store food as starch, while animal cells store as glycogen. Plants cells have plasmodesmata for communication, animal cells have gap junctions. Observe these differences in Activity 2.3.
These questions test your ability to apply concepts, analyze situations, and solve problems.
Answer: The plants wilted because of plasmolysis caused by the hypertonic seawater solution.
Explanation: Seawater contains high concentrations of salts (sodium chloride, magnesium chloride, etc.), making it a hypertonic solution compared to the plant cell sap. When seawater was used for watering, water moved out of the plant cells by osmosis (exosmosis) from a region of higher water concentration (inside cells) to lower water concentration (seawater). This caused the cells to lose turgor pressure, shrink, and the plant to wilt.
Remedy:
Answer: Patients with mitochondrial diseases experience muscle weakness and fatigue because their mitochondria cannot produce sufficient ATP (energy).
Detailed explanation:
Conclusion: The severity of symptoms depends on how many mitochondria are affected and which tissues are involved.
Answer: The nucleus couldn't be seen because there was insufficient contrast between the nucleus and the surrounding cytoplasm in the unstained specimen.
Explanation:
Lesson: Staining is essential for observing cell structures under a light microscope. Different stains can be used to highlight different organelles — for example, methylene blue stains the nucleus blue in animal cells. Try Activity 2.3 to observe stained cells.
Answer: This observation is explained by changes in the potato's density due to osmosis.
In water (hypotonic solution):
In concentrated salt solution (hypertonic solution):
Key concept: An object floats if its density is less than the liquid's density. The potato's density decreases due to water loss, while the salt solution's density is high, causing flotation. This is similar to how it's easier to float in the Dead Sea (high salt concentration) than in a freshwater lake.
Answer: Cancer results from failures in the normal control mechanisms of mitosis and the cell cycle.
What goes wrong in cancer cells:
Why chemotherapy targets rapidly dividing cells:
Answer: The difference in behavior is due to the presence of a rigid cell wall in plant cells.
In distilled water (hypotonic solution):
In concentrated salt solution (hypertonic solution):
Conclusion: The cell wall in plant cells provides protection from bursting and maintains structure even when cells lose water. This is why plants wilt in salty soil but recover when watered properly.
Read each case study carefully and answer the MCQs that follow.
In 1665, Robert Hooke used a primitive compound microscope to examine a thin slice of cork. He observed tiny, box-like compartments that reminded him of the small rooms where monks lived in monasteries. He called these structures "cells." However, what Hooke observed were actually dead cell walls of plant tissue, not living cells. Nearly a decade later, in 1674, Anton van Leeuwenhoek improved the microscope and became the first person to observe living cells in pond water, including bacteria and protozoa. These pioneering observations laid the foundation for cell theory, which was formally proposed by Schleiden and Schwann in 1838-1839.
Answer: (c) Dead cell walls
Explanation: Cork is dead plant tissue. Hooke saw the empty cell walls (made of cellulose) but not the living
contents which had dried out.
Answer: (b) Anton van Leeuwenhoek
Explanation: Leeuwenhoek (1674) used an improved microscope to observe living microorganisms in pond water,
including bacteria and protists.
Answer: (a) Cells, because they resembled small rooms
Explanation: The box-like structures reminded Hooke of the cells (small rooms) in a monastery where monks lived.
Answer: (c) Schleiden and Schwann
Explanation: Matthias Schleiden (1838) proposed that all plants are made of cells, and Theodor Schwann (1839)
extended this to animals, forming the basis of cell theory.
Meena's mother was making pickles. She cut fresh mangoes and mixed them with salt. After a few hours, Meena noticed that water had accumulated at the bottom of the container, and the mango pieces had shrunk. Her mother explained that this happens due to osmosis. The high salt concentration outside the mango cells created a hypertonic solution. Water moved out of the mango cells through the selectively permeable cell membrane, causing the cells to lose water and shrink. This is why vegetables release water and become soft when salt is added to them. The same principle is used in food preservation — salt or sugar creates conditions where bacteria cannot survive because they lose water through osmosis.
Answer: (c) Hypertonic solution
Explanation: The salt solution had a higher solute concentration than the mango cell sap, making it hypertonic
relative to the cells.
Answer: (b) Osmosis (exosmosis)
Explanation: Osmosis is the movement of water through a selectively permeable membrane. Here, water moved from
inside the cells (higher water concentration) to outside (lower water concentration due to salt).
Answer: (d) They lose water through osmosis and become dehydrated
Explanation: High salt or sugar concentrations create a hypertonic environment. Bacterial cells lose water by
osmosis, shrivel, and cannot carry out metabolic functions, leading to their death.
Answer: (b) They would swell as water enters the cells
Explanation: Distilled water is hypotonic compared to the now-salty mango cells. Water would move into the cells
by osmosis (endosmosis), causing them to swell and regain some firmness. Try this in
Activity 2.2.
Rahul, a 12-year-old boy, frequently complained of severe muscle weakness and fatigue, especially after physical activities. Medical tests revealed he had a mitochondrial disorder. Doctors explained that his mitochondria were not functioning properly, which affected ATP (energy) production. Since muscles require large amounts of ATP for contraction, Rahul's muscles couldn't work efficiently. Mitochondrial diseases are unique because mitochondria have their own DNA (separate from nuclear DNA), and mutations in this DNA can be inherited only from the mother. These diseases particularly affect organs with high energy needs: muscles, brain, heart, and nerves. There is no cure currently, but treatments focus on managing symptoms and supporting energy production through dietary supplements and exercise therapy.
Answer: (c) Production of ATP through cellular respiration
Explanation: Mitochondria are called the powerhouse of the cell because they generate ATP, the energy currency
needed for all cellular activities.
Answer: (b) Muscles require large amounts of ATP for contraction
Explanation: Muscle cells are packed with mitochondria because they need constant energy for contraction. When
mitochondria are defective, muscles can't generate enough ATP.
Answer: (c) Only from the mother (maternal inheritance)
Explanation: Mitochondria have their own DNA (mtDNA). During fertilization, the egg contributes all the
mitochondria to the embryo, while sperm mitochondria are destroyed. Therefore, mitochondrial DNA is inherited
only from the mother.
Answer: (d) Brain, heart, and nerves
Explanation: These organs have very high energy demands and rely heavily on mitochondrial ATP production. When
mitochondria malfunction, these tissues are most severely affected.
Priya's biology teacher demonstrated cell division by showing prepared slides of onion root tip cells under a microscope. The root tip was chosen because it's a region of active growth where cells are constantly dividing by mitosis. Students observed cells in different stages: some cells showed condensed chromosomes, some had chromosomes aligned at the center, and others had chromosomes moving to opposite ends. The teacher explained that this type of division produces two identical daughter cells with the same number of chromosomes as the parent cell (2n = 16 in onions). This process is essential for growth, where a single cell (zygote) develops into a multicellular organism. See Activity 2.5. In contrast, reproductive cells undergo meiosis, producing four genetically different cells with half the chromosome number, which is essential for sexual reproduction.
Answer: (b) It is a region of active cell division
Explanation: Root tips contain meristematic tissue where cells are actively dividing to enable root growth.
This makes it easy to observe different stages of mitosis.
Answer: (b) 16 (diploid)
Explanation: Mitosis maintains the chromosome number. Both daughter cells receive the same number of chromosomes
as the parent cell (2n = 16).
Answer: (b) Metaphase
Explanation: During metaphase, chromosomes align at the equatorial plate (center) of the cell, and spindle
fibers attach to their centromeres.
Answer: (d) Meiosis produces 4 haploid cells; mitosis produces 2 diploid cells
Explanation: Meiosis involves two divisions,
producing four cells with half the chromosome number (n), while mitosis produces two cells with the same
chromosome number (2n).