Navigating through the newly revised CBSE Class 9 Science curriculum (Exploration) requires a thorough understanding of cellular organization, microscopic resolution, membrane transport mechanisms (osmosis and diffusion), organelle specialization, the endomembrane system, and cell division cycles (mitosis and meiosis). Chapter 2, “Cell: The Building Block of Life”, establishes the fundamental structural and functional basis of all living organisms. It investigates how historical technological advancements—from Robert Hooke’s primitive compound microscope to modern scanning electron microscopes—revealed sub-cellular architectures; explains the fluid-mosaic model of the selectively permeable plasma membrane; compares prokaryotic and eukaryotic compartmentalization; details the bioenergetics of mitochondria and plastids; and explores how regulated cell division maintains life while unregulated proliferation leads to oncological disorders. To help students master every aspect of this high-weightage foundational chapter, this comprehensive solutions guide offers textbook-accurate, highly structured, and pedagogically sound responses strictly aligned with the latest CBSE Class 9 evaluation standards.
Every question presented in the official NCERT textbook—ranging from all in-text exploratory activities and “Pause and Ponder” prompts to the complete end-of-chapter “Revise, Reflect, Refine” exercises (Questions 1 to 16 on Pages 24–27)—has been solved with exhaustive detail. Short-answer conceptual responses follow the official examination word limits, while multi-step physiological deductions, organelle pathways, and transport calculations use structured comparison tables and step-by-step text blocks. Key scoring terms, official CBSE exam tags, and an expanded set of 15 high-yield FAQs have been integrated to ensure students secure top marks in their examinations.
Master Concept & Comparative Summary Tables
1. Structural Comparison: Prokaryotic Cell vs. Eukaryotic Cell
| Feature / Characteristic | Prokaryotic Cell (e.g., Bacteria, Blue-green Algae) | Eukaryotic Cell (e.g., Plant and Animal Cells) |
|---|---|---|
| Size of Cell | Typically small: 1 to 10 µm (1\text{ µm} = 10^{-6}\text{ m}). | Comparatively larger: 10 to 100 µm. |
| Nuclear Region | Poorly defined; lacks a nuclear membrane. Genetic material lies naked in the cytoplasm as a Nucleoid. | Well-defined; true nucleus enclosed by a double-layered Nuclear Membrane. |
| Chromosome Number | Single, circular DNA molecule. | Multiple linear chromosomes composed of DNA and histone proteins. |
| Membrane-Bound Organelles | Absent (no ER, Golgi apparatus, mitochondria, plastids, or lysosomes). | Present (Mitochondria, Golgi apparatus, ER, plastids, vacuoles, etc.). |
| Ribosomes | Smaller 70S ribosomes scattered freely in cytoplasm. | Larger 80S ribosomes (in cytoplasm/RER) and 70S (inside mitochondria/chloroplasts). |
| Cell Division Mode | Simple binary fission or budding (Amitosis). | Controlled Mitosis (for somatic growth) and Meiosis (for gamete formation). |
2. Comparison: Plant Cell vs. Animal Cell
| Cellular Component / Organelle | Plant Cell (e.g., Onion Peel Cell, Leaf Cell) | Animal Cell (e.g., Human Cheek Cell) |
|---|---|---|
| Outer Boundary | Surrounded by a rigid Cellulose Cell Wall outside the cell membrane. | Enclosed only by a flexible Plasma Membrane; cell wall is completely absent. |
| Plastids | Present (Chloroplasts for photosynthesis, Chromoplasts, Leucoplasts). | Absent in all animal cells. |
| Vacuoles | Has a single, large central permanent vacuole maintaining cell turgidity. | Vacuoles are small, temporary, and multiple (or completely absent). |
| Shape & Rigidity | Fixed, rigid, rectangular or box-like shape. | Flexible, irregular, spherical, or oval shape. |
| Centrosome / Centrioles | Absent in most higher plant cells. | Present near the nucleus to organize spindle fibers during cell division. |
3. Behavior of Cells in Different Osmotic Solutions
| Solution Type | Relative Solute Concentration | Direction of Net Water Movement | Effect on Animal Cell (Cheek Cell/RBC) | Effect on Plant Cell (Onion/Rhoeo Cell) |
|---|---|---|---|---|
| Hypotonic Solution (Dilute / Pure Water) | Medium has lower solute (higher water potential) than cell interior. | Water enters the cell (Endosmosis). | Cell swells up and may burst (lyse) due to excessive internal pressure. | Cell swells and becomes turgid; rigid cell wall prevents bursting. |
| Isotonic Solution (Equal Concentration) | Medium has identical solute concentration to cell cytoplasm. | No net movement of water (Dynamic equilibrium). | Cell size and shape remain unchanged. | Cell size and shape remain unchanged (Flaccid state). |
| Hypertonic Solution (Concentrated Salt/Sugar) | Medium has higher solute (lower water potential) than cell interior. | Water moves out of the cell (Exosmosis). | Cell loses water and shrinks (crenates) completely. | Cytoplasm shrinks away from the cell wall (Plasmolysis); outer wall stays rigid. |
NCERT In-Text Questions & “Pause and Ponder”
Page No. 12: In-Text “What if…”
Question What if mung bean seeds are kept in a concentrated solution after soaking in water for 12 hours? What will happen to them? [Exam Favorite]
Answer: When mung bean seeds that have been soaked in water for 12 hours (and have absorbed water becoming swollen and fully turgid) are transferred into a concentrated salt or sugar solution:
- The concentrated solution acts as a hypertonic medium relative to the internal cell sap of the seed cells.
- Water molecules move out of the seed cells into the surrounding concentrated solution through the selectively permeable cell membrane via Exosmosis.
- Consequently, the seed cells lose internal water, the seeds lose weight, become soft, flaccid, and visibly shrink.
Page No. 14: Pause and Ponder (Questions 1 to 3)
Question 1 What argument would you give for the necessity of a cell wall in plants usually fixed in one place versus in animals usually moving from one place to the other? [Exam Favorite]
Answer:
- Necessity in Plants: Plants are stationary (sessile) organisms that cannot move to seek shelter from harsh atmospheric changes. They must withstand severe environmental stresses such as high wind speeds, heavy rain, mechanical pressure, and osmotic variations. The rigid cellulose cell wall provides structural strength, prevents cells from bursting in hypotonic rainwater, and allows plants to stand upright.
- Absence in Animals: Animals are motile and actively move from one place to another for food, shelter, and self-defense. They require cellular flexibility and muscular elasticity for locomotion and tissue deformation, which would be severely hindered by a rigid, inflexible outer cell wall.
Question 2 What consequences would you predict for a plant cell if its cell wall were to become as flexible as a cell membrane? [Exam Favorite]
Answer: If a plant cell wall became as flexible as a plasma membrane:
- Loss of Mechanical Support: Plants would lose their structural rigidity and upright posture, causing stems, branches, and leaves to droop and collapse under their own weight.
- Cell Lysis (Bursting) in Hypotonic Conditions: When plant roots absorb excess water from the soil during rains, endosmosis would build up high internal turgor pressure. Without a rigid cell wall to exert equal counter-wall pressure, the plant cells would swell excessively and burst, leading to tissue destruction and plant death.
Question 3 Why is it important to cut the two potato pieces in roughly equal size and measure their initial weight before placing them in different liquids?
Answer:
- Control of Variables: Cutting the potato pieces into equal sizes ensures that both experimental samples have approximately the same surface area exposed to the liquids for osmotic exchange, preventing surface-area bias.
- Establishing Baseline Data: Measuring and recording the initial weight before immersion establishes a definitive quantitative baseline, enabling researchers to calculate the exact percentage change in mass (weight gain due to endosmosis versus weight loss due to exosmosis) accurately.
Page No. 16: In-Text “Threads of Curiosity”
Question Do you know any other cells without a nucleus? [Exam Favorite]
Answer: Yes. In addition to mature mammalian Red Blood Cells (Erythrocytes), other cells that function without a nucleus include:
- Sieve Tube Elements in Plants: Mature phloem sieve tube cells lose their nuclei at maturity to create an unobstructed longitudinal channel for the translocation of food (sucrose), controlled by adjacent companion cells.
- Blood Platelets (Thrombocytes) in Mammals: These cell fragments circulate in the blood without nuclei to assist in blood clotting.
- All Bacterial / Prokaryotic Cells: Lack a true, membrane-bound nucleus (genetic material exists as a naked nucleoid).
Page No. 19: Pause and Ponder (Questions 4 & 5)
Question 4 Do white flowers contain any pigment? Give reasons. [Exam Favorite]
Answer: No, white flowers do not contain colored chromoplast pigments (such as carotenoids or anthocyanins).
- Scientific Reason: The petals of white flowers contain colorless plastids called Leucoplasts or flavonoids that absorb UV light (visible to pollinating insects like bees). The white appearance to human eyes is caused by the presence of microscopic air spaces between petal cells that reflect and scatter all wavelengths of incident white light equally, making the petals look pure white.
Question 5 Draw a well-labelled schematic diagram of a plant or an animal cell using these clues: (i) Nucleus appears as a dark and round body inside the cell. (ii) ER spreads like a network of extended nuclear envelope. (iii) Mitochondria and chloroplasts are rod shaped. (Refer to Fig. 2.10).
Answer:
================================================================================ SCHEMATIC STRUCTURAL MAP OF A GENERALIZED PLANT CELL: -------------------------------------------------------------------------------- Outer Boundary: [CELL WALL] (Outer rigid protective layer of cellulose) └── [PLASMA MEMBRANE] (Inner selectively permeable boundary) Cytoplasmic Interior & Organelle Layout: ├── [NUCLEUS]: Centrally/peripherally located dense spherical body │ ├── Nuclear Envelope with Nuclear Pores │ ├── Nucleolus (Ribosome synthesis factory) │ └── Chromatin Network (DNA + Proteins) │ ├── [ENDOPLASMIC RETICULUM (ER)]: Network extending from nuclear membrane │ ├── Rough ER (RER): Studded with dark granular Ribosomes (Protein synthesis) │ └── Smooth ER (SER): Tubular smooth network (Lipid & steroid synthesis) │ ├── [GOLGI APPARATUS]: Stacks of parallel flattened membrane cisternae (Packaging) ├── [CHLOROPLASTS]: Green, double-membrane rod-shaped plastids (Photosynthesis) ├── [MITOCHONDRIA]: Cylindrical double-membrane rod-shaped powerhouses (ATP) ├── [LARGE CENTRAL VACUOLE]: Massive fluid-filled central sac (Cell sap & turgor) └── [CYTOPLASM]: Jelly-like semi-fluid ground substance bathing all organelles ================================================================================
Page No. 22: Pause and Ponder (Questions 6 & 7)
Question 6 Instead of many small ones, why does a cell not have a single giant mitochondrion? How does this relate to the concept of surface area? [Exam Favorite]
Answer: A cell contains numerous small mitochondria rather than a single giant mitochondrion due to the Surface Area to Volume Ratio principle:
- Maximizing Surface Area: Multiple small mitochondria provide a substantially larger total outer and inner membrane surface area per unit volume than a single large structure of equivalent mass.
- Higher Rate of ATP Production: Cellular respiration enzymes and ATP-synthase complexes are embedded in the inner mitochondrial folded membranes (cristae). A larger total surface area maximizes the rate of ATP generation.
- Rapid Intracellular Diffusion: Having mitochondria dispersed throughout the cytoplasm ensures that ATP is generated locally near high-energy-demanding sites without long, slow diffusion delays.
Question 7 If the skin cells start dividing by meiosis instead of mitosis, what do you think will happen to a cut on the skin? [Exam Favorite]
Answer: If skin cells divided by meiosis instead of mitosis:
- Halving of Chromosome Number: Meiosis is a reduction division that halves the chromosome number (2n \rightarrow n). With every subsequent round of skin cell division, the chromosome count would be reduced further (46 \rightarrow 23 \rightarrow 11.5), leading to catastrophic genetic instability, cell malfunction, and cell death.
- Genetic Non-Identical Cells: Meiosis introduces crossing over and genetic variation. The newly formed replacement cells would not be identical somatic skin cells.
- Failure of Wound Healing: The wound would fail to heal properly, leading to open ulcerations, loss of epidermal barrier protection, severe fluid loss, and fatal bacterial infections.
NCERT Chapter-End Exercises: “Revise, Reflect, Refine” (Pages 24–27)
Question 1 Differentiate between the following pairs of terms based on the clues given in parentheses: [Exam Favorite] (i) Cell membrane and cell wall (permeability) (ii) RER and SER (structure) (iii) Chloroplasts and chromoplasts (pigments)
Answer:
================================================================================ DIFFERENTIATION BASED ON SPECIFIED CLUES: -------------------------------------------------------------------------------- (i) Cell Membrane vs. Cell Wall (Basis: Permeability) • Cell Membrane: It is SELECTIVELY PERMEABLE; it regulates the entry and exit of specific solutes while blocking others. • Cell Wall: It is FREELY PERMEABLE; it allows water, dissolved mineral salts, and small molecules to pass through without selective restriction. (ii) RER vs. SER (Basis: Structure) • Rough Endoplasmic Reticulum (RER): Has tiny granular RIBOSOMES attached to its outer membrane surface, giving it a rough, studded appearance under a microscope. • Smooth Endoplasmic Reticulum (SER): LACKS ribosomes on its surface, appearing as a smooth network of interconnected tubules and vesicles. (iii) Chloroplasts vs. Chromoplasts (Basis: Pigments) • Chloroplasts: Contain the green photosynthetic pigment CHLOROPHYLL (along with yellow/orange carotenoids) for trapping solar energy. • Chromoplasts: Contain non-green, brightly colored pigments like CAROTENE (orange/red) and XANTHOPHYLL (yellow) to impart vibrant colors to flowers and fruits. ================================================================================
Question 2 Two similar animal cells are placed in two different solutions: • Cell X is placed in pure water. • Cell Y is placed in a concentrated salt solution. Cells are observed after some time. Cell X swells, and Cell Y shrinks. Which statement provides the correct explanation for the above observations? (i) Salt molecules moved into Cell Y, causing it to shrink. (ii) Water moved into Cell X and more water moved out of Cell Y than the salt solution entered in it. (iii) Water moved into Cell X and moved out of Cell Y through the cell membrane. (iv) Solute movement caused osmosis in both cells.
Answer: (iii) Water moved into Cell X and moved out of Cell Y through the cell membrane. Explanation:
- Pure water is a hypotonic medium relative to Cell X, causing water to enter the cell via endosmosis, making it swell.
- Concentrated salt solution is a hypertonic medium relative to Cell Y, causing water to leave the cell via exosmosis, making it shrink.
- The cell membrane is selectively permeable only to water molecules; solute (salt) molecules cannot freely cross it.
Question 3 Look at the diagram of a cell in Fig. 2.20. Identify the parts labelled from (a) to (g) and correctly match them with their functions given below: [Exam Favorite] (i) Controlling all the activities of a cell. (ii) Site of cellular respiration. (iii) Storage organelle that also provides rigidity to the cell. (iv) Separates the cell contents from surroundings. (v) Provides structural rigidity to the cell. (vi) Packs and stores materials received from ER. (vii) Helps in manufacturing food.
Answer:
| Labelled Part in Fig. 2.20 | Name of Organelle / Structure | Matching Function from the Given List |
|---|---|---|
| (a) | Mitochondrion | (ii) Site of cellular respiration (Generates ATP). |
| (b) | Nucleus | (i) Controlling all the activities of a cell (Master control center). |
| (c) | Golgi Apparatus | (vi) Packs and stores materials received from ER into secretory vesicles. |
| (d) | Chloroplast | (vii) Helps in manufacturing food through photosynthesis. |
| (e) | Cell Wall | (v) Provides structural rigidity to the cell and maintains shape. |
| (f) | Cell Membrane (Plasma Membrane) | (iv) Separates the cell contents from surroundings and regulates transport. |
| (g) | Vacuole | (iii) Storage organelle that also provides rigidity to the cell via cell sap turgor. |
Question 4 Which of the following option(s) of the pairs of cell organelles are correctly placed under the given categories?
| Option | Present in the plant cells | Absent in the animal cells |
|---|---|---|
| (i) | Leucoplast | Cell wall |
| (ii) | Mitochondria | Ribosome |
| (iii) | Cell wall | Golgi apparatus |
| (iv) | Lysosome | Endoplasmic reticulum |
Answer: Option (i) is correctly placed. Explanation:
- Leucoplasts are storage plastids strictly present in plant cells (e.g., in potato tubers, roots).
- Cell walls are completely absent in animal cells (animal cells have only a plasma membrane).
- Options (ii), (iii), and (iv) incorrectly list organelles like ribosomes, Golgi apparatus, and ER as absent in animal cells, whereas they are present in both plant and animal cells.
Question 5 Two students, Renu and Rohit, were having a discussion on the plastids. Renu emphasised that all parts of the plants, even roots, contain plastids. However, Rohit did not agree with the statement and told her that plastids are absent in plant roots since the roots are underground and do not need to perform photosynthesis. Who is correct? Justify your answer. [Exam Favorite]
Answer: Renu is correct.
================================================================================ SCIENTIFIC JUSTIFICATION: -------------------------------------------------------------------------------- 1. Classification of Plastids: Plastids are not limited to green, photosynthetic chloroplasts. They exist in three distinct functional forms: • Chloroplasts (Green - Photosynthetic) • Chromoplasts (Colored - Petals and fruits) • Leucoplasts (Colorless - Storage organs) 2. Plastids in Roots: Underground plant roots do not perform photosynthesis and therefore lack green chloroplasts. However, root cells contain abundant LEUCOPLASTS (colorless plastids) whose primary function is the storage of nutrients such as starch (amyloplasts), oils/lipids (elaioplasts), and proteins (aleuroplasts). 3. Example: Leucoplasts are densely packed in the root storage tissues of sweet potato, carrot, taro (Colocasia), and radish. ================================================================================
Question 6 Mitochondria 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. [Exam Favorite]
Answer:
1. Structural and Functional Similarities:
- Double Membrane Envelope: Both are enclosed by two distinct lipid membranes (an outer membrane and an inner membrane).
- Semi-Autonomous Nature: Both contain their own circular DNA and 70S ribosomes, allowing them to synthesize some of their own structural proteins and self-replicate through binary fission.
- Energy Transduction: Both organelles are specialized for cellular bioenergetics and ATP synthesis through electron transport mechanisms.
2. Structural and Functional Differences:
| Parameter | Mitochondria | Chloroplasts |
|---|---|---|
| Inner Membrane Structure | Inner membrane is folded into finger-like Cristae to increase surface area. | Inner membrane encloses a fluid Stroma containing disc-like membranous sacs called Thylakoids (Grana). |
| Pigments | No pigments present. | Contain the green photosynthetic pigment Chlorophyll (plus carotenoids). |
| Primary Biochemical Process | Cellular Respiration: Breaks down glucose in the presence of O_2 to release ATP energy. | Photosynthesis: Traps solar light energy to synthesize glucose from CO_2 and H_2O. |
| Gas Exchange Role | Consumes Oxygen (O_2) and releases Carbon Dioxide (CO_2). | Consumes Carbon Dioxide (CO_2) and releases Oxygen (O_2). |
| Occurrence | Present in nearly all eukaryotic cells (plants, animals, fungi). | Present only in photosynthetic plant cells and green algae. |
Question 7 Which of the following pairs of cell organelles contains DNA? (i) Chloroplasts, Ribosomes (ii) Mitochondria, Nucleus (iii) Golgi bodies, Ribosomes (iv) Nucleus, Lysosomes
Answer: (ii) Mitochondria, Nucleus (Chloroplasts also contain DNA). Explanation: The nucleus contains genomic nuclear DNA in chromosomes. Mitochondria (and chloroplasts) are semi-autonomous organelles possessing their own extranuclear circular DNA. Ribosomes, Golgi bodies, and lysosomes do not contain DNA.
Question 8 A 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? [Exam Favorite]
Answer:
- (i) Tested Hypothesis: Plant tissues gain or lose water depending on the relative solute concentration (osmotic potential) of the surrounding external medium via osmosis across their cell membranes.
- (ii) Suggestions for Improvement:
- Measure and record the initial mass, length, and diameter of both carrots before immersion, and measure final values to obtain quantitative percentage changes.
- Use multiple carrot replicates to calculate average values and minimize biological variation.
- Maintain a controlled temperature and cover the beakers to prevent water evaporation from altering salt concentration.
- (iii) Reason for Texture Differences:
- In Plain Water (Hypotonic Medium): Water moves into the carrot cells by endosmosis. The vacuoles fill with water, exerting high outward turgor pressure against the rigid cell walls, making the carrot firm, stiff, and crunchy (turgid).
- In Concentrated Salt Solution (Hypertonic Medium): Water moves out of the carrot cells into the salt solution by exosmosis. The cells lose turgor pressure and undergo plasmolysis, causing the carrot to lose structural firmness and become soft, rubbery, and limp (flaccid).
Question 9 Indicate the presence or absence of following structures in bacterial and animal cells: [Exam Favorite]
| Structures in a cell | Bacterial cell | Animal cell |
|---|---|---|
| Chromosome | Present (Single circular DNA molecule / Nucleoid) | Present (Multiple linear chromatin chromosomes) |
| Nucleus | Absent (Lacks nuclear membrane) | Present (True, double-membrane enclosed nucleus) |
| Mitochondria | Absent (Lacks membrane-bound organelles) | Present (Powerhouses of the cell) |
| Golgi complex | Absent | Present (For protein sorting & packaging) |
| Chromoplasts | Absent | Absent (Plastids exist only in plants/algae) |
Question 10 Carry out the following experiment: Take four peeled potato halves and scoop each one out to make potato cups. One of these potato cups should be made from a boiled potato. Place each of the potato cups in a beaker containing water (Fig. 2.22). Now, set up the experiment as follows: (a) Keep Cup A empty. (b) Add one teaspoon sugar in Cup B. (c) Add one teaspoon salt in Cup C. (d) Add one teaspoon sugar in the boiled potato in Cup D. Observe the four potato cups after two hours and answer the following questions: [Exam Favorite] (i) Explain why water gathers in the hollowed portion of Cup B and Cup C. (ii) Why is Cup A necessary for this experiment? (iii) Explain why water does not gather in the hollowed portions of Cups A and D.
Answer:
================================================================================ EXPERIMENTAL ANALYSIS OF POTATO OSMOSCOPE: -------------------------------------------------------------------------------- (i) Water Accumulation in Cup B (Sugar) and Cup C (Salt): • In Cups B and C, the presence of sugar/salt inside the cavity creates a highly concentrated HYPERTONIC environment with low water potential. • The surrounding water in the beaker is HYPOTONIC (high water potential). • Living potato cells act as a selectively permeable membrane. Water moves from the beaker through living potato cells into the cavity by ENDOSMOSIS, causing water to gather in the cavity. (ii) Necessity of Cup A (Empty Potato Cup): • Cup A serves as the EXPERIMENTAL CONTROL. • It proves that water movement into the cavity is driven specifically by the presence of a solute concentration gradient (sugar/salt) and does not occur spontaneously in an empty cavity. (iii) Why Water Does Not Gather in Cup A and Cup D: • In Cup A: There is NO solute concentration gradient (cavity is empty); hence no osmosis occurs. • In Cup D (Boiled Potato): Boiling KILLS the living potato cells and denatures the proteins of the plasma membrane, destroying its SELECTIVE PERMEABILITY. Dead cell membranes cannot carry out osmosis; hence, no water enters the cavity. ================================================================================
Question 11 Identify the pair that incorrectly matches the cell organelle with its function: (i) Ribosome – Protein synthesis (ii) SER – Lipid and cellulose synthesis (iii) Lysosome – Digestion of foreign agents
Answer: Pair (ii) is incorrectly matched. Explanation:
- Smooth Endoplasmic Reticulum (SER) is responsible for the synthesis of lipids, phospholipids, and steroid hormones, as well as drug detoxification in liver cells.
- Cellulose is NOT synthesized by SER. In plant cells, cellulose is synthesized at the cell surface by Cellulose Synthase enzyme complexes located directly within the plasma membrane.
Question 12 What outcome do you expect, if all the mitochondria are removed from a eukaryotic cell? [Exam Favorite]
Answer: If all mitochondria are surgically or enzymatically removed from a eukaryotic cell:
- Arrest of Aerobic Respiration: The cell will be completely unable to carry out the Krebs cycle and oxidative phosphorylation.
- Severe Energy (ATP) Crisis: The cell will be restricted to anaerobic glycolysis in the cytoplasm, which yields only a meager 2 ATP molecules per glucose molecule (compared to 36–38 ATP in mitochondria).
- Shutdown of Cellular Functions: Active transport across the cell membrane, protein packaging, DNA replication, and motility will halt due to lack of ATP.
- Cellular Death: The cell will accumulate metabolic wastes, lose homeostatic balance, and die.
Question 13 Which phenomenon inhibits the formation of tumors in the human body? Can plants also develop tumors? Explain. [Exam Favorite]
Answer:
- Inhibition of Tumors in Humans: In normal animal and human tissues, tumor formation is inhibited by a regulatory mechanism called Contact Inhibition. When normal dividing cells come into physical contact with neighboring cells, cell-surface signals halt further mitosis. In cancer cells, this contact inhibition mechanism is lost, leading to uncontrolled proliferation and malignant tumor formation.
- Tumor Formation in Plants:Yes, plants can also develop tumors (galls).
- Explanation: Although plant cells do not exhibit animal-like contact inhibition due to their rigid cell walls, they can develop abnormal cellular growths or tumors called Crown Galls. These are induced by plant pathogens such as the soil bacterium Agrobacterium tumefaciens, which transfers its T-DNA plasmid into plant genome cells, forcing them to overproduce auxins and cytokinins, leading to uncontrolled cell division.
Question 14 The cell membrane of a cell is made up of proteins and lipids. Which cell organelles help in the synthesis of cell membrane? Write the path of these compounds from their site of synthesis to the cell membrane and show this through a labelled diagram. [Exam Favorite]
Answer: The process of synthesizing and assembling the plasma membrane using lipids and proteins is termed Membrane Biogenesis.
- Organelles Involved:
- Rough Endoplasmic Reticulum (RER): Synthesizes membrane proteins via its attached ribosomes.
- Smooth Endoplasmic Reticulum (SER): Synthesizes membrane lipids (phospholipids) and cholesterol.
- Golgi Apparatus: Modifies, sorts, glycosylates, and packages proteins and lipids into transport vesicles.
================================================================================
BIOSYNTHETIC PATHWAY OF CELL MEMBRANE COMPONENTS (MEMBRANE BIOGENESIS):
--------------------------------------------------------------------------------
[RER] (Synthesizes Proteins) & [SER] (Synthesizes Lipids/Phospholipids)
│ │
└─────────────┬──────────────┘
│
▼
(Transport Vesicles bud off)
│
▼
[GOLGI APPARATUS]
(Modifies, sorts, and packages into glycoproteins & glycolipids)
│
▼
(Secretory Transport Vesicles)
│
▼
[PLASMA MEMBRANE]
(Vesicles fuse with cell membrane, renewing its lipid bilayer & proteins)
================================================================================
Question 15 What would happen if gametes are formed by mitotic divisions? [Exam Favorite]
Answer: If reproductive gametes (sperms and ova) were produced by mitosis instead of meiosis:
- Doubling of Chromosome Number: Mitosis is an equational division that preserves the full diploid (2n = 46) chromosome number in daughter cells.
- Exponential Chromosomal Doubling: Upon fertilization, the fusion of two diploid gametes (2n + 2n) would produce a tetraploid zygote (4n = 92\text{ chromosomes}) in the first generation.
- Generational Catastrophe: In the next generation, fusion would result in 8n = 184\text{ chromosomes}, doubling exponentially in every generation.
- Loss of Species Identity: Such severe chromosomal abnormalities would disrupt normal genetic balance, causing developmental arrest, lethal gene dosage imbalances, and total extinction of the species.
Question 16 A 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: [Exam Favorite] (i) Which scientific concept has the farmer applied in the preservation of the farm produce? (ii) How does the addition of high concentrations of salt and sugar create an environment that prevents the growth of spoilage-causing bacteria and fungi? (iii) Suggest a healthy recipe of this kind for food preservation. (iv) What are the scientific values addressed in this case?
Answer:
- (i) Scientific Concept Applied: The farmer applied the biological principle of Osmosis (specifically Exosmosis and Plasmolysis) to dehydrate and preserve food products.
- (ii) Mechanism of Microbial Inhibition: High concentrations of salt (in pickles) and sugar/jaggery (in murabbas and sharbats) create a strongly hypertonic external environment. When spoilage-causing bacteria or fungal spores land on the preserved food, water is drawn out from their microbial cells via exosmosis. This causes their cytoplasm to shrink (plasmolysis), which halts their metabolic enzyme activities, inhibits cellular reproduction, and kills the microbes.
- (iii) Healthy Recipe Suggestion: Traditional Amla-Honey Murabba (or Salted Sun-Dried Lemon Pickle): Steam fresh amla pieces to soften them without destroying heat-sensitive Vitamin C. Immerse the pricked amla in pure natural honey or jaggery syrup along with a pinch of black pepper and roasted cumin. The concentrated honey syrup acts as a natural hypertonic preservative while retaining high antioxidant value.
- (iv) Scientific Values Addressed:
- Application of Scientific Knowledge: Using osmotic food preservation principles to solve real-world agricultural problems.
- Waste Reduction & Sustainability: Minimizing post-harvest food loss through value-added agro-processing.
- Economic Empowerment & Innovation: Transitioning from primary farming to sustainable rural food entrepreneurship.
Frequently Asked Questions (FAQs) – Class 9 Science Chapter 2
Question 1: Who discovered the cell, and what material was used? [Exam Favorite] Answer: Robert Hooke discovered the cell in 1665 while examining a thin slice of bottle cork under his self-designed compound microscope. He observed empty, box-like compartments resembling monastery rooms and coined the term cell (Latin cellula = small room).
Question 2: What is the limit of resolution of the naked human eye? [Exam Favorite] Answer: The limit of resolution of the unaided human eye at the standard near point (25 cm) is 0.1 mm (or 100 µm). Objects or separation distances smaller than 0.1 mm appear as a single merged dot without magnification.
Question 3: What is the Cell Theory, and who formulated it? [Exam Favorite] Answer: The Cell Theory states: (i) All living organisms are composed of one or more cells, (ii) The cell is the basic structural and functional unit of life, and (iii) All cells arise from pre-existing cells (Omnis cellula-e-cellula). It was formulated by Matthias Schleiden (1838), Theodor Schwann (1839), and expanded by Rudolf Virchow (1855).
Question 4: Why is the plasma membrane called a selectively permeable membrane? [Exam Favorite] Answer: The plasma membrane is called selectively permeable because it allows the regulated passage of specific molecules (like water, oxygen, glucose) across its lipid bilayer while actively preventing the entry or exit of unwanted solutes and large macromolecules.
Question 5: What is the Fluid Mosaic Model of the cell membrane? [Exam Favorite] Answer: Proposed by Singer and Nicolson, the Fluid Mosaic Model describes the cell membrane as a quasi-fluid phospholipid bilayer in which globular proteins are embedded like tiles in a mosaic. Lipids provide fluidity and flexibility, while embedded proteins function as selective channels and transport pumps.
Question 6: Why are Lysosomes known as the “suicide bags” of a cell? [Exam Favorite] Answer: Lysosomes contain powerful hydrolytic digestive enzymes capable of breaking down all organic materials. When a cell gets damaged or undergoes metabolic disruption, lysosomes may burst, releasing their digestive enzymes into the cytoplasm, which digest and destroy their own cell.
Question 7: Why are Mitochondria referred to as the “powerhouses of the cell”? [Exam Favorite] Answer: Mitochondria carry out the aerobic oxidation of nutrients during cellular respiration to generate usable chemical energy stored in the form of Adenosine Triphosphate (ATP) molecules, which fuel all metabolic cellular activities.
Question 8: Why are Mitochondria and Plastids called semi-autonomous organelles? [Exam Favorite] Answer: Mitochondria and plastids contain their own independent circular DNA molecules and 70S ribosomes, allowing them to transcribe RNA, synthesize some of their own structural proteins, and self-replicate by binary fission without relying entirely on nuclear DNA.
Question 9: What is Plasmolysis? [Exam Favorite] Answer: Plasmolysis is the biological phenomenon where a living plant cell loses water when placed in a hypertonic (concentrated) solution, causing its cytoplasm, plasma membrane, and central vacuole to shrink and pull away from the rigid outer cell wall.
Question 10: What are the primary functions of Leucoplasts, Chromoplasts, and Chloroplasts? [Exam Favorite] Answer:
- Chloroplasts: Green plastids containing chlorophyll that capture solar energy for photosynthesis.
- Chromoplasts: Colored plastids containing carotenoids that impart yellow, orange, and red colors to flowers and fruits to attract pollinators.
- Leucoplasts: Colorless plastids dedicated to the storage of starch, oils, and proteins in roots and non-photosynthetic plant tissues.
Question 11: What is the function of the Nucleolus? [Exam Favorite] Answer: The nucleolus is a dense, non-membrane-bound region inside the nucleus responsible for transcribing ribosomal RNA (rRNA) and assembling the large and small subunits of Ribosomes, which later move to the cytoplasm for protein synthesis.
Question 12: What is the structural difference between Chromatin and Chromosomes? [Exam Favorite] Answer: In a non-dividing cell, DNA exists as an uncoiled, entangled thread-like network called Chromatin. When the cell prepares to divide, this chromatin material condenses and coils tightly into distinct, rod-shaped structures called Chromosomes.
Question 13: What is the role of the central vacuole in mature plant cells? [Exam Favorite] Answer: The central vacuole is filled with watery cell sap containing amino acids, sugars, minerals, and organic wastes. By storing water, it exerts outward osmotic pressure against the cell wall, providing turgidity, mechanical support, and rigidity to plant tissues.
Question 14: What is Totipotency, and who proposed it? [Exam Favorite] Answer: Totipotency is the biological capacity of a single mature plant cell to divide, differentiate, and regenerate into an entire, complete plant when cultured in a sterile nutrient medium with appropriate hormones. It was proposed by Gottlieb Haberlandt in 1902, founding Plant Tissue Culture technology.
Question 15: What is Programmed Cell Death (PCD) / Apoptosis? [Exam Favorite] Answer: Programmed Cell Death (PCD) is a genetically controlled, organized cellular process that selectively eliminates old, damaged, or unneeded cells. For example, during human embryonic development, PCD selectively removes webbing cells between digits to form separated fingers and toes.
Mastering the NCERT Solutions for Class 9 Science Chapter 2 (Exploration), “Cell: The Building Block of Life”, equips students with the cytological concepts, organelle pathways, and membrane transport principles required for high performance in CBSE examinations. Review the potato osmometer experiments, organelle comparative tables, and the 15 high-yield FAQs above to secure full marks in your biological evaluations.