Where Mindful Learning Begins
Where Mindful Learning Begins
Complete NCERT Solutions for Chapter 2 of the new Class 9 Science Exploration textbook (CBSE 2026-27) — every Think It Over, Activity, What If, Pause & Ponder, Revise Reflect Refine, and Quest Continues question on this one page, solved with the reasoning behind each answer.
Cell: The Building Block of Life covers everything from the discovery of the cell to the structure and function of every major organelle, osmosis and diffusion through the cell membrane, and how cells divide by mitosis and meiosis. These NCERT solutions work through the potato and carrot osmosis experiments, the prokaryotic vs. eukaryotic comparison, and every Revise Reflect Refine question in full detail, making this one of the most exam-relevant chapters in the biology portion of the Class 9 Science syllabus.
Cell: The Building Block of Life is the first proper content chapter of Class 9 Science, and it's a long one — it moves from how we study cells with microscopes, to the cell membrane and osmosis, to the cell wall, through every major organelle, and finally to how cells grow, divide, and eventually die. Every question woven through the chapter is solved here, section by section, in the exact order the textbook presents them: Think It Over, Activities 2.1–2.5, the "What If" boxes and inline questions, Pause and Ponder, the end-of-chapter Revise Reflect Refine set, and The Quest Continues.
Limit of resolution, magnification, and estimating a real cell's size from what you see through the eyepiece.
Why the potato swells in water and shrinks in salt solution — and what that tells us about selectively permeable membranes.
The nucleus, ER, Golgi, mitochondria and plastids as a coordinated system — plus mitosis, meiosis, and why cells eventually die.
Answer: Cells come from pre-existing cells through the process of cell division. This is one of the three key principles of the Cell Theory, stated by Rudolf Virchow (1855): "All cells arise from pre-existing cells." A parent cell produces new cells either by mitosis (for growth and repair) or meiosis (for reproduction). No cell arises spontaneously from non-living matter.
Answer: The human eye can only resolve objects separated by about 0.1 mm. Since most cells are far smaller than this, they cannot be seen unaided — technological tools transformed our understanding:
Each advance in microscopy revealed a new layer of cell structure — from the cell wall (Hooke) to organelles (light microscope) to molecular detail (electron microscope). Technology directly enabled the Cell Theory and our understanding of organelle function.
Answer: The cell is the smallest unit that is independently alive and can carry out all basic life processes — energy production, waste removal, protein synthesis, reproduction, and response to environment. Whether unicellular (bacteria) or multicellular (humans), all life functions ultimately occur at the cellular level. Even when cells are organised into tissues and organs, each individual cell still carries out its own metabolic activities, and no smaller unit of matter can independently sustain life. This makes the cell both the structural unit — the building block from which all organisms are built — and the functional unit — the site where all life processes occur.
Answer: Cells multiply through cell division, mainly of two kinds:
All cells arise from pre-existing cells (Virchow, 1855). Eukaryotic cell division is regulated by the cell cycle, and errors in division can lead to tumours (mitosis errors) or genetic disorders (meiosis errors).
Answer: Estimated size of cell = Diameter of visible field (in µm) ÷ Number of cells along the diameter.
Unit conversion: 1 mm = 1000 µm. If the field diameter is 5 mm (= 5000 µm) and 25 cells are seen along the diameter, then size = 5000 ÷ 25 = 200 µm.
Answer: Total magnification = eyepiece × objective = 10 × 10 = 100×. The 200 µm cell will appear 100 times larger — i.e., it will appear to be 200 × 100 = 20,000 µm = 20 mm in size under the microscope.
Observation: Beaker A — the potato piece swells. Beaker B — the potato piece shrinks.
Inference: The cell membrane allows water molecules to pass through but not sugar or salt molecules (it's selectively permeable). Water moves from a dilute solution (plain water, more water molecules) into the cell in Beaker A by osmosis, causing swelling. In Beaker B, the external solution is more concentrated than the cell contents, so water moves out of the cell by osmosis, causing shrinking.
This movement of water through a selectively permeable membrane is called osmosis — the diffusion of water from a region of lower solute concentration to higher solute concentration.
Answer: The mung bean seeds will shrink and may shrivel. After soaking in water for 12 hours, the seeds absorb water by osmosis and swell. When placed in a concentrated (hypertonic) solution, the external concentration is higher than inside the seeds, so water moves out of the seed cells by osmosis (exosmosis), causing the cells to lose water and the seeds to shrink and shrivel — the reverse of the initial swelling.
Isotonic solution (external concentration = internal): no net movement of water — cell size unchanged.
Hypotonic solution (external concentration < internal, i.e. more dilute outside): water enters the cell by osmosis (endosmosis). Animal cells swell and may burst (lysis); plant cells become turgid — their rigid cell wall prevents bursting.
Hypertonic solution (external concentration > internal): water leaves the cell by osmosis (exosmosis). Animal cells shrink; plant cells undergo plasmolysis — the cell membrane pulls away from the cell wall.
Onion peel cells: box-shaped, regularly arranged, have a distinct cell wall, with a large vacuole visible.
Cheek cells: irregular in shape and loosely arranged, with no cell wall.
Why: plant cells (onion) have a rigid cellulose cell wall which maintains a fixed box shape and regular arrangement. Animal cells (cheek) lack a cell wall — they are held only by the flexible cell membrane, so they take irregular shapes and arrange loosely.
Rhoeo leaf cells: the outer boundary (cell wall) remains intact, but the inner content (cytoplasm + membrane) shrinks away from the cell wall — the space between the cell wall and the shrunk cytoplasm increases. This is called plasmolysis.
Cheek cells: shrink considerably — they lose water and reduce in size because there is no rigid cell wall to maintain their shape.
Reason: the 20% sugar solution is hypertonic. Water moves out of the cells by osmosis; the plant cell wall prevents outer shrinkage but the inner membrane pulls away, while animal cells (cheek), having no wall, shrink entirely.
| Cell structure | Bacterial | Plant | Animal |
|---|---|---|---|
| Cell membrane | Present | Present | Present |
| Cell wall | Present (not cellulose) | Present (cellulose) | Absent |
| Cytoplasm | Present | Present | Present |
| Well-defined nucleus (membrane-bound) | Absent | Present | Present |
| Primitive nucleus / nucleoid | Present | Absent | Absent |
| Membrane-bound organelles | Absent | Present | Present |
Prokaryotic: the bacterial cell — it lacks a well-defined, membrane-bound nucleus (it has a nucleoid instead) and lacks membrane-bound organelles.
Eukaryotic: the plant cell and the animal cell — both have a well-defined membrane-bound nucleus and membrane-bound organelles (mitochondria, ER, Golgi apparatus, and so on).
Answer: No, the cells are not similar in structure. Different cells show different structural appearances because the cells of a growing root tip divide continuously, so at any given moment, different cells are at different stages of cell division (mitosis). A cell's appearance changes at each stage — chromosomes condense, align, separate, and eventually two daughter cells form — so we see cells with different chromosomal arrangements, shapes, and nuclear appearances, each corresponding to a different stage of the cell cycle.
Answer: The first stage is Interphase — the resting/preparation phase where DNA is replicated — followed by Prophase (chromosomes condense and become visible), Metaphase (chromosomes align at the centre), Anaphase (chromatids separate to opposite poles), and finally Telophase/Cytokinesis (two new daughter cells form). On a microscope slide, interphase cells (with diffuse chromatin and a clear nuclear membrane) are the most numerous, since most time in the cell cycle is spent in this phase.
Answer: The cell membrane of alveolar cells is selectively permeable, made of a lipid bilayer with embedded proteins. The lipid bilayer restricts the passage of large, charged, or polar molecules. Oxygen (O₂) and carbon dioxide (CO₂) are small, non-polar molecules that dissolve in the lipid bilayer and diffuse freely across the membrane down their concentration gradients. Proteins embedded in the membrane act as channels or carriers for specific ions and larger molecules. This structural specificity ensures only the right substances (O₂ in, CO₂ out) cross during gas exchange.
Answer: The previously swollen seeds will shrink and may wrinkle. During soaking, the cells absorbed water by osmosis. Now in a concentrated solution, the external medium is hypertonic, so water moves out of the cells by osmosis (exosmosis). The cells lose turgor, the seed coat wrinkles, and the seeds shrink in size — at very high concentrations, plasmolysis may occur in the seed cells.
Isotonic: no net water movement — cell size unchanged; the cell is in equilibrium with its surroundings.
Hypotonic: the external solution is more dilute, so water enters the cell by osmosis. Animal cells swell (and may burst); plant cells become turgid but don't burst, due to the cell wall.
Hypertonic: the external solution is more concentrated, so water leaves the cell by osmosis. Animal cells shrink (crenation); plant cells undergo plasmolysis — the membrane pulls away from the cell wall.
Answer: The cell wall provides structural rigidity, mechanical support, and protection. Plants cannot move to avoid environmental stresses (wind, rain, gravity), so the rigid wall is essential to keep them upright, maintain leaf and flower shape, and withstand osmotic pressure (preventing bursting in hypotonic conditions). In bacteria and fungi, the cell wall similarly protects the cell from the external environment and gives it a characteristic shape — without it, these cells would be vulnerable to osmotic lysis and physical damage.
Answer: Yes. Chromoplasts contain non-green pigments — carotenoids (yellow, orange, red) — present in flower petals, ripe fruits, and some vegetables.
Flowers and fruits get their bright colours through chromoplasts containing pigments like carotenoids (yellow, orange) and anthocyanins (red, purple, blue — stored in vacuoles, not plastids). These colours serve real biological roles: bright flower colours attract pollinators such as bees and butterflies, and bright fruit colours attract fruit-eating animals that help disperse seeds. The red of tomatoes and the yellow of marigolds, for instance, both come from chromoplast pigments.
Answer: No, cells do not live forever — every cell has a definite lifespan. Human red blood cells, for example, live for about 120 days, and skin cells are continuously replaced. Programmed Cell Death (PCD), also called apoptosis, is a genetically regulated, organised process of selective cell destruction essential for normal development and tissue maintenance — for instance, PCD helps form fingers in an embryo by eliminating the cells between digits. When cells fail to die when they should, or divide uncontrollably, tumours and cancer can result.
Answer: the bacterial cell is much smaller than the plant and animal cells shown in the figure — bacterial (prokaryotic) cells are typically 1–10 µm in diameter, while the plant and animal (eukaryotic) cells are typically 10–100 µm, roughly 10 times larger.
Why: prokaryotic cells lack the internal membrane-bound organelles (nucleus, ER, Golgi apparatus, mitochondria, and so on) that eukaryotic cells use to organise and compartmentalise their much larger volume of internal machinery. Without needing to house these extra membrane-bound structures, a bacterial cell can function efficiently at a much smaller size, relying on simple diffusion to move materials around its cytoplasm — something that becomes far less efficient once a cell grows beyond a certain size, which is one reason eukaryotic cells evolved internal compartments in the first place.
Answer: Plants are stationary and constantly exposed to wind, rain, gravity, and osmotic pressure from soil water. Without a rigid cell wall, plant cells would collapse, burst in dilute soil water, and be unable to maintain an upright structure — the cell wall provides mechanical support and rigidity, helps withstand turgor pressure, and keeps the plant erect. Animals, on the other hand, need to move — muscles contract, cells change shape during movement, white blood cells squeeze through capillary walls — and a rigid cell wall would prevent this flexibility. Animal cells have only the flexible cell membrane, which supports the mobility and function of animal tissues.
Answer: This is necessary to make the experiment a fair test (a controlled experiment):
Without these controls, the results would be unreliable and the experiment could not prove that osmosis caused the observed changes.
Answer: Yes, white flowers contain pigments, but not coloured ones. White flowers reflect all wavelengths of visible light, giving them their white appearance. They may contain:
White in flowers is also partly due to air spaces between cells that scatter light — similar to why snow appears white despite being made of colourless water.
Answer: This is a drawing activity. Key structures to include for a plant cell, using the clues given (nucleus as a dark round body, ER spreading like a network from the nuclear envelope, mitochondria and chloroplasts rod-shaped):
Answer: A single giant mitochondrion would have a much smaller surface-area-to-volume ratio than many small ones of the same total volume. Energy production happens on the inner membrane (cristae), so greater inner membrane surface area means more reaction sites and more ATP produced per unit time — many small mitochondria together provide far more total inner membrane surface area than one large one of the same volume. Small mitochondria can also be distributed throughout the cell wherever energy is needed most, whereas a single large one would be less mobile and efficient. This is the same principle behind why lungs have millions of tiny alveoli rather than one large air sac.
Answer: Skin normally heals by mitosis, producing daughter cells genetically identical to the parent with the same diploid chromosome number (46 in humans). If skin cells divided by meiosis instead:
Meiosis also introduces genetic variation through crossing over, so even if cells survived, they would be genetically different from the rest of the body — similar to having foreign cells in the wound.
(i) Cell membrane vs cell wall — permeability: the cell membrane is selectively permeable, allowing only certain substances to pass and tightly controlling what enters and exits. The cell wall is freely permeable, letting water and dissolved minerals pass through without selection — it provides structure, not filtration.
(ii) RER vs SER — structure: RER (Rough Endoplasmic Reticulum) has ribosomes attached to its surface, giving it a rough/granular appearance under the electron microscope, and is involved in protein synthesis and secretion. SER (Smooth ER) has no ribosomes on its surface, appears smooth, and is involved in lipid and hormone synthesis and storage.
(iii) Chloroplasts vs chromoplasts — pigments: chloroplasts contain the green pigment chlorophyll and are the site of photosynthesis, found in green plant parts. Chromoplasts contain non-green pigments (yellow, orange, red — carotenoids), found in flower petals and ripe fruits, attracting pollinators and seed-dispersing animals.
Answer: The correct statement is (iii) Water moved into Cell X and moved out of Cell Y through the cell membrane. Pure water is hypotonic relative to the cell contents, so water moves into Cell X by osmosis, causing swelling. The concentrated salt solution is hypertonic, so water moves out of Cell Y by osmosis, causing shrinking. The cell membrane is selectively permeable — it allows water but not salt to move freely.
Option (i) is wrong — salt molecules do not move in to cause shrinking. Option (ii) is only partially and misleadingly worded. Option (iv) is wrong — solute movement does not cause osmosis; water movement does.
Answer (based on a typical plant cell diagram):
| Label | Organelle | Function |
|---|---|---|
| (a) | Chloroplast | Helps in manufacturing food (photosynthesis) |
| (b) | Nucleus | Controlling all the activities of a cell |
| (c) | Endoplasmic Reticulum | Transport and synthesis network linking nucleus to membrane |
| (d) | Mitochondria | Site of cellular respiration (energy/ATP production) |
| (e) | Vacuole | Storage organelle that also provides rigidity to the cell |
| (f) | Cell membrane | Separates the cell contents from surroundings |
| (g) | Cell wall | Provides structural rigidity to the cell |
Golgi apparatus ("packs and stores materials received from ER") should be matched to whichever label points to the stacked, flattened sacs in your specific diagram.
Answer: The correct option is (i) Leucoplast (present in plant cells) — Cell wall (absent in animal cells). Leucoplasts are plastids found only in plant cells, storing starch, oils or proteins. Cell walls are present in plant, fungal and bacterial cells but not in animal cells. The other options are wrong — mitochondria and ribosomes exist in both, and so do the Golgi apparatus and ER.
Answer: Renu is correct. Plastids are present in plant roots, but they are not chloroplasts (which perform photosynthesis) — root cells contain leucoplasts, colourless plastids that store food materials such as starch. Potato tubers (modified stems) and carrot roots, for example, are rich in leucoplasts. Rohit is correct that roots don't photosynthesise, but wrong to conclude that plastids are absent — plastids come in three types (chloroplasts, chromoplasts, and leucoplasts), and roots contain the leucoplast type.
Similarities:
Differences:
| Feature | Mitochondria | Chloroplasts |
|---|---|---|
| Found in | All eukaryotic cells (plant + animal) | Plant cells only |
| Function | Cellular respiration — produces ATP | Photosynthesis — produces glucose |
| Inner membrane | Folded into cristae (increases surface area) | Contains thylakoids/grana with chlorophyll |
| Semi-fluid inside | Matrix | Stroma |
| Pigment | None | Chlorophyll (green) |
| Energy conversion | Chemical (glucose) → ATP | Light energy → chemical energy (glucose) |
Answer: The correct pair is (ii) Mitochondria and Nucleus. The nucleus contains chromosomal DNA, the main genetic material. Mitochondria have their own small circular DNA (and ribosomes), allowing them to produce some of their own proteins. Chloroplasts also contain DNA, but that option isn't listed here. Ribosomes, Golgi bodies, and lysosomes do not contain DNA.
She is testing whether the concentration of the surrounding solution affects the movement of water in and out of plant cells through osmosis — specifically, whether plant cells gain water in a dilute (hypotonic) solution and lose water in a concentrated (hypertonic) one.
(ii) How could the experiment be improved?Plain water is hypotonic relative to the carrot cells, so water enters by osmosis, making the cells turgid (full of water and firm) — the rigid cell wall prevents bursting, so the carrot stays stiff and crunchy. In concentrated salt solution, the external solution is hypertonic, so water moves out of the cells by osmosis. The cells lose turgor pressure, and with no water to keep them firm, the carrot becomes rubbery and limp.
| Structure | Bacterial cell | Animal cell |
|---|---|---|
| Chromosome | Present (single, circular, no membrane) | Present (multiple, linear, membrane-bound in nucleus) |
| Nucleus | Absent (nucleoid instead, no membrane) | Present (membrane-bound true nucleus) |
| Mitochondria | Absent | Present |
| Golgi complex | Absent | Present |
| Chromoplasts | Absent | Absent (found only in plant cells) |
The sugar (Cup B) and salt (Cup C) create a concentrated (hypertonic) solution inside the hollow — higher than in the surrounding potato cells. Water moves by osmosis from the potato cells (lower solute concentration) into the hollow (higher solute concentration), while water from the beaker also moves into the potato through its walls. The net result is water accumulating in the hollow.
(ii) Why is Cup A (empty) necessary?Cup A is the control. With no sugar or salt, there is no concentration gradient to drive osmosis into the hollow. Since water gathers only in Cups B and C and not A, we can confidently attribute the accumulation to the osmotic effect of the dissolved substances, not to leaking or other factors — the control validates the experiment.
(iii) Why does water not gather in Cups A and D (boiled potato with sugar)?Cup A has no dissolved substance, so there's no concentration gradient and no osmosis. In Cup D, the potato is boiled, which kills the cells and destroys the cell membranes — dead cells cannot carry out osmosis, which requires a functional, selectively permeable living membrane. Even with sugar present, no osmosis occurs without that membrane, so no water accumulates.
Answer: The incorrect pair is (ii) SER — Lipid and cellulose synthesis. SER (Smooth Endoplasmic Reticulum) is involved in the synthesis and storage of lipids (fats) and hormones, not cellulose — cellulose (which makes up the plant cell wall) is synthesised by the Golgi apparatus and specialised enzymes at the cell membrane surface. (i) Ribosome — protein synthesis: correct. (iii) Lysosome — digestion of foreign agents: correct.
Answer: Mitochondria are the primary sites of ATP production through cellular respiration. Removing all mitochondria would mean:
Some unicellular organisms (such as certain parasites) lack mitochondria and rely entirely on anaerobic pathways — but complex eukaryotic cells cannot survive without mitochondria.
Answer: Contact inhibition is the phenomenon that prevents tumour formation in animal cells — when normal cells come in contact with neighbouring cells, cell division automatically stops. Cancer cells lose this mechanism and keep dividing uncontrollably, forming tumours (benign or malignant); malignant tumours can invade surrounding tissues and spread (metastasis).
Yes, plants can develop tumours. Plant cells have rigid cell walls and don't show contact inhibition in the same way animal cells do. Plant tumours can form due to:
Plant cells show contact inhibition differently — the rigid cell wall naturally limits some types of uncontrolled growth, but plant tumours do occur.
Answer: The organelles involved are ribosomes (on RER) for proteins, SER for lipids, and the Golgi apparatus for processing and packaging.
Step 1 — Protein synthesisRibosomes on the RER synthesise membrane proteins.
Step 2 — Lipid synthesisSER synthesises phospholipids — the components of the lipid bilayer.
Step 3 — Transport to GolgiBoth proteins and lipids are packaged into vesicles that bud off from the ER and travel to the Golgi apparatus.
Step 4 — ProcessingThe Golgi apparatus modifies, sorts, and packages them into secretory vesicles.
Step 5 — Fusion with membraneSecretory vesicles travel to and fuse with the cell membrane, adding new lipid and protein components.
Ribosomes (on RER) → proteins, and SER → lipids, both feed into the Golgi apparatus → vesicles → cell membrane.
Answer: Normally, gametes (sperm and eggs) are produced by meiosis, which halves the chromosome number (diploid → haploid). If gametes were formed by mitosis instead:
This is exactly why meiosis evolved specifically for sexual reproduction — it maintains a constant chromosome number across generations and creates genetic diversity.
Farmer Deepa has applied the principle of osmosis. High concentrations of salt or sugar create a hypertonic environment outside microbial cells, drawing water out by osmosis — dehydrating and killing bacteria and fungi, and thereby preserving the food.
(ii) How high salt/sugar prevents microbial growthWhen food is placed in a highly concentrated salt or sugar solution, the external environment is hypertonic relative to bacterial and fungal cells. Water moves out of the microbial cells by osmosis (exosmosis), causing plasmolysis — the cell contents shrink and the membrane pulls away from the wall. Without adequate water, microbes cannot carry out their metabolic processes (enzyme reactions require water), cannot grow, and cannot reproduce — at very high concentrations, the microbial cells die.
(iii) A healthy preservation recipeAmla Murabba (Indian Gooseberry Preserve): wash and prick fresh amla fruits, then briefly boil to soften. Prepare a syrup using jaggery (unrefined natural sugar) and water, with cardamom and a pinch of saffron. Immerse the amla in the warm jaggery syrup — the high sugar concentration creates a hypertonic environment — and let it soak for 48 hours, then store in sterilised glass jars. The high sugar concentration preserves the fruit by osmosis while retaining Vitamin C and other nutrients; using jaggery instead of refined sugar also retains minerals like iron and calcium.
(iv) Scientific values addressedAnswer: Synthetic biology aims to build cells from non-living chemical components. Current progress (J. Craig Venter, 2010) chemically synthesised a bacterial cell's DNA and transplanted it into a living cell shell, producing a "synthetic" bacterium. Future directions include:
Scientists believe a fully synthetic cell may be achievable in the coming decades, but enormous technical challenges remain — especially in getting all molecular components to work together spontaneously.
Answer: Creating synthetic life raises major ethical concerns:
The cell is the basic structural and functional unit of every living thing — a coordinated system of organelles, bounded by a selectively permeable membrane, that grows, divides by mitosis or meiosis, and eventually dies, all governed by the same three principles of Cell Theory across bacteria, plants, animals and humans alike.
Here are extra practice questions for Class 9 Chapter 2, Cell: The Building Block of Life. It is a mix of MCQs, Assertion-Reason, Very Short, Short, and Long Answer Questions, and a Case Study — apart from the NCERT questions — to make you confident in your understanding of the chapter. Attempt the questions yourself first, and then cross-check your answers below.
Answer: (b) Mitochondria — they release energy by breaking down glucose and other molecules during cellular respiration, storing it in the form of ATP, which is used to power most cellular activities.
Answer: (b) Osmosis — this is specifically the diffusion of water (not other particles) through a selectively permeable membrane like the cell membrane, moving from a dilute solution (more water) to a concentrated solution (less water) until the concentrations equalise.
Answer: (c) Nuclear membrane — prokaryotic cells lack a well-defined, membrane-bound nucleus; their genetic material lies free in the cytoplasm in a region called the nucleoid. Cytoplasm, ribosomes, and a cell membrane are all present in prokaryotic cells too.
Each question below has an Assertion (A) and a Reason (R). Choose the correct option:
(A) Both A and R are correct, and R is the correct explanation of A.
(B) Both A and R are correct, but R is not the correct explanation of A.
(C) A is true but R is false.
(D) Both A and R are false.
Answer: (A) — both Assertion and Reason are correct, and the Reason correctly explains the Assertion.
The cell membrane follows the fluid-mosaic model: it consists of a lipid bilayer with proteins embedded in it, and these lipid and protein molecules can move sideways, flip, and rotate within the membrane. This constant movement of its own molecules is exactly what gives the membrane its fluid nature — so the Reason directly and correctly explains the Assertion.
Answer: The Assertion is actually false — the cell wall is a non-living, freely permeable structure secreted by the cell; the living part of a plant cell is the protoplasm (cell membrane, cytoplasm, nucleus and organelles) that it surrounds. The Reason, on its own, is true — the cell wall does provide protection, rigidity and support.
Strictly, this combination (Assertion false, Reason true) doesn't match any of the four options as given — that combination is usually covered by a fifth option ("A is false but R is true") in standard assertion-reason format, which appears to be missing here. Worth flagging with your teacher if this appears in a test as written. If you must pick from only A–D, none is fully accurate, though (D) is the closest since it at least correctly rejects the Assertion.
Answer: (A) — both Assertion and Reason are correct, and the Reason correctly explains the Assertion.
Gases like CO₂ and O₂ are small enough to diffuse freely across the cell membrane, moving from a region of higher concentration to lower concentration. This is possible precisely because the plasma membrane is selectively permeable — it permits certain materials to pass in and out — so the Reason correctly explains why this diffusion of gases can occur.
Answer: (A) — both Assertion and Reason are correct, and the Reason correctly explains the Assertion.
A bacterial (prokaryotic) cell's genetic material lies free in the cytoplasm as a region called the nucleoid, without any membrane enclosing it. It is precisely this absence of a nuclear membrane that means bacteria lack a well-defined, true nucleus — so the Reason directly explains the Assertion.
Answer: Ribosomes are found either lying freely in the cytoplasm, or attached to the outer surface of the rough endoplasmic reticulum (RER).
Answer: Leucoplasts are colourless plastids specialised for storing food material such as starch, oils, or proteins. In storage tissues like a potato tuber, leucoplasts store large amounts of starch, which the plant later draws on as an energy reserve.
Answer: The vacuole's cell sap stores water and dissolved minerals (it also stores sugars and waste materials).
Answer: the Golgi apparatus (Golgi body) — it modifies, sorts and packages proteins and lipids received from the ER into vesicles for transport, secretion, or lysosome formation, much like a post office sorting and dispatching parcels.
Answer: Robert Hooke discovered the cell in 1665, using a self-designed microscope (capable of about 200–300X magnification). While examining a thin slice of cork, he observed small box-like compartments and named them 'cells'.
Answer: DNA stands for Deoxyribonucleic Acid.
Answer: A plant cell wall is primarily made of cellulose, a carbohydrate formed by many glucose units linked together. (Cell walls in other organisms differ — fungal cell walls are made of chitin, and bacterial cell walls are made of peptidoglycan.)
Answer: Both mitochondria and chloroplasts share several features with bacteria:
These similarities support the idea that mitochondria and chloroplasts were once free-living bacteria that were engulfed by a larger ancestral cell long ago, and instead of being digested, formed a permanent, mutually beneficial relationship with it — eventually becoming the organelles we see in eukaryotic cells today.
Answer: Contact inhibition is the process by which normal animal cells stop dividing once they come into contact with neighbouring cells.
It is important because it regulates and limits cell division, ensuring tissues and organs grow to the correct size and stop growing once available space is filled — preventing overcrowding and disorganised growth. Cancer cells lose this property and keep dividing uncontrollably even when surrounded by other cells, which is what leads to the formation of tumours. Contact inhibition therefore acts as a natural safeguard against this kind of uncontrolled growth in healthy tissue.
Answer: Meiosis is important for two main reasons:
Answer: Errors in mitosis can lead to uncontrolled cell division, resulting in the formation of tumours, and can also produce daughter cells with an abnormal number of chromosomes. Both consequences can disrupt normal cell function, and in some cases contribute to diseases such as cancer.
Answer: In a hypotonic solution, water enters both plant and animal cells by osmosis, since the water concentration outside is higher than inside the cell.
Answer: The nucleus is called the brain (or control centre) of the cell. This is because it contains the cell's DNA — the coded genetic instructions for making proteins and carrying out every cellular activity — and so it directs and controls all the structural and functional processes of the cell.
(i) Muscle cells require large, continuous amounts of energy to power repeated contraction and movement. Since mitochondria are the "powerhouses" of the cell — producing energy through cellular respiration — muscle cells contain many more mitochondria to meet this high energy demand, compared to skin cells, which are far less energy-intensive and don't need as much ATP production.
(ii) Cellular respiration — the breakdown of glucose and other molecules to release energy — takes place in the mitochondria.
(iii) ATP (Adenosine Triphosphate) — it acts as the cell's energy currency and is used to power most cellular activities.
Pair these solutions with our free Class 9 Science Notes PDF — quick, chapter-wise revision notes covering every unit, perfect for last-minute recall before a test.
📘 Get Class 9 Science Notes →Next up is Chapter 3 — Tissues in Action. Or explore the full chapter list, browse the Class 9 Science hub, or book a free demo class for personalised coaching.
Expert CBSE Coaching · Class 9–12