This MCQ module is based on: NCERT Exercises and Solutions: Cell: The Unit of Life
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NCERT Exercises and Solutions: Cell: The Unit of Life
🎓 Class 11
Biology
CBSE
Theory
Ch 8 – Cell: The Unit of Life
⏱ ~8 min
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NCERT Exercises and Solutions: Cell: The Unit of Life
8.20 Chapter Summary
- Cell theory (Schleiden & Schwann, modified by Virchow): all living organisms are made of cells and products of cells; all cells arise from pre-existing cells.
- Cells differ in size — Mycoplasma (0.3 µm) is smallest; ostrich egg is the largest single cell.
- Prokaryotic cells (bacteria, cyanobacteria, mycoplasma, PPLO): no membrane-bound nucleus; DNA in nucleoid; 70S ribosomes; cell envelope (glycocalyx + cell wall + plasma membrane); may have plasmids, mesosomes, flagella, pili, fimbriae; uricotelic by-products handled by mesosome.
- Eukaryotic cells (protists, fungi, plants, animals): true nucleus, membrane-bound organelles, 80S ribosomes in cytoplasm.
- Fluid mosaic model (Singer-Nicolson, 1972): phospholipid bilayer with proteins (integral + peripheral); quasi-fluid, allows lateral movement.
- Plant cell wall — non-living; cellulose, hemicellulose, pectins, proteins; middle lamella (Ca-pectate) glues adjacent cells; plasmodesmata connect cytoplasm.
- Endomembrane system = ER (RER protein, SER lipid) + Golgi (modify, sort, package) + lysosomes (digest) + vacuoles (storage, turgor).
- Mitochondria — power-house; double membrane; inner folded into cristae; matrix has mtDNA + 70S ribosomes; Krebs cycle there.
- Plastids — chloroplast (photosynthesis: grana of stacked thylakoids in stroma); chromoplast (coloured pigments); leucoplast (storage).
- Cytoskeleton — microfilaments, intermediate filaments, microtubules.
- Cilia & flagella — covered by plasma membrane; axoneme has 9 + 2 microtubule arrangement; basal body anchors them.
- Centrosome — two perpendicular centrioles (9 triplets of microtubules each); MTOC in animal cells.
- Nucleus — double envelope with pores; nucleoplasm with chromatin and nucleolus (rRNA synthesis).
- Chromosomes — primary constriction = centromere; kinetochore on centromere; four types by centromere position: metacentric, sub-metacentric, acrocentric, telocentric.
- Karyotype = ordered display of chromosomes; human 2n = 46.
8.21 Key Terms
| Term | One-line meaning |
|---|---|
| Cell theory | All life = cells; cells from cells. |
| Mesosome | Plasma membrane infoldings in bacteria. |
| Plasmid | Accessory DNA outside the main genome. |
| Fluid mosaic model | Modern model of cell membrane. |
| Cristae | Folds of inner mitochondrial membrane. |
| Grana | Thylakoid stacks in chloroplast. |
| Centromere | Primary constriction on chromosome. |
| Karyotype | Ordered chromosome display. |
| Nucleolus | rRNA factory inside nucleus. |
| Endomembrane system | Coordinated membranous organelles. |
8.22 NCERT Exercises
Q1. Which of the following is not correct?
(a) Robert Brown discovered the cell.
(b) Schleiden and Schwann formulated the cell theory.
(c) Virchow explained that cells are formed from pre-existing cells.
(d) A unicellular organism carries out its life activities within a single cell.
Answer: (a) Robert Brown did NOT discover the cell — he discovered the nucleus in 1831. The discovery of the cell is credited to Robert Hooke (1665) who first described 'cells' in cork. So statement (a) is incorrect.
Q2. New cells generate from
(a) bacterial fermentation
(b) regeneration of old cells
(c) pre-existing cells
(d) abiotic materials
Answer: (c) Pre-existing cells. Stated by Rudolf Virchow (1855) — Omnis cellula-e cellula. This is the second postulate of the cell theory.
Q3. Match the following.
| Column I | Column II |
|---|---|
| (a) Cristae | (i) Flat membranous sacs in stroma |
| (b) Cisternae | (ii) Infoldings in mitochondria |
| (c) Thylakoids | (iii) Disc-shaped sacs in Golgi apparatus |
(a) — (ii) Cristae → Infoldings in mitochondria.
(b) — (iii) Cisternae → Disc-shaped sacs in Golgi apparatus.
(c) — (i) Thylakoids → Flat membranous sacs in stroma of chloroplast.
(b) — (iii) Cisternae → Disc-shaped sacs in Golgi apparatus.
(c) — (i) Thylakoids → Flat membranous sacs in stroma of chloroplast.
Q4. Which of the following is correct?
(a) Cells of all living organisms have a nucleus.
(b) Both animal and plant cells have a well defined cell wall.
(c) In prokaryotes, there are no membrane-bound organelles.
(d) Cells are formed de novo from abiotic materials.
Answer: (c) In prokaryotes, there are no membrane-bound organelles — true (no ER, no Golgi, no nucleus). (a) is wrong (prokaryotes have no nucleus; mature RBCs and sieve cells lack a nucleus). (b) is wrong (animal cells have no cell wall). (d) is wrong (Virchow showed cells come from pre-existing cells).
Q5. What is a mesosome in a prokaryotic cell? Mention the functions that it performs.
Mesosome is a special, infolded extension of the plasma membrane in a bacterial cell, appearing as vesicles, tubules and lamellae projecting into the cytoplasm.
Functions:
Functions:
- Cell wall formation.
- DNA replication and distribution to daughter cells during binary fission.
- Increases the surface area of the plasma membrane (and so increases enzyme content).
- Site of respiration (electron transport chain).
- Secretion of substances.
Q6. How do neutral solutes move across the plasma membrane? Can the polar molecules also move across it in the same way? If not, then how are these transported across the membrane?
Neutral solutes: Move across the plasma membrane by simple diffusion, going from higher to lower concentration, without using ATP. Small non-polar molecules (O₂, CO₂) cross easily through the lipid bilayer.
Polar molecules: CANNOT cross the hydrophobic lipid interior of the bilayer freely. Their transport requires carrier proteins:
Polar molecules: CANNOT cross the hydrophobic lipid interior of the bilayer freely. Their transport requires carrier proteins:
- Facilitated diffusion: Down concentration gradient via specific transport proteins; no ATP needed (e.g., glucose into RBC).
- Active transport: Against concentration gradient using ATP-driven pumps (e.g., Na⁺/K⁺ pump).
Q7. Name two cell-organelles that are double membrane-bound. What are the characteristics of these two organelles? State their functions and draw labelled diagrams of both.
Mitochondria and chloroplasts are the two double-membrane organelles.
Characteristics and functions:
Characteristics and functions:
| Feature | Mitochondrion | Chloroplast |
|---|---|---|
| Inner membrane | Folded into cristae | Smooth; thylakoids are separate |
| Internal sacs | None | Thylakoids in stacks called grana |
| Own DNA? | Yes, circular mtDNA | Yes, circular cpDNA |
| Ribosomes | 70S (in matrix) | 70S (in stroma) |
| Main function | ATP production (aerobic respiration) | Photosynthesis |
| Location | All eukaryotes | Plant cells, some protists |
Q8. What are the characteristics of prokaryotic cells?
- They lack a true nuclear membrane; DNA lies in the cytoplasm as a nucleoid.
- Genetic material is a single, circular, double-stranded, naked DNA; may also have plasmids.
- They lack membrane-bound organelles (no ER, no Golgi, no mitochondria, no chloroplast).
- Ribosomes are 70S (50S + 30S subunits).
- Cell envelope = glycocalyx + cell wall (peptidoglycan in bacteria) + plasma membrane.
- Mesosomes are present (multifunctional plasma membrane infoldings).
- Cell division is by binary fission (no mitosis or meiosis).
- Flagella, if present, are made of flagellin and structurally different from eukaryotic flagella.
- Sex pili and fimbriae may be present.
- Generally smaller (1-10 µm) and reproduce rapidly.
Q9. Multicellular organisms have division of labour. Explain.
In a multicellular organism, different cells specialise in different tasks. For example, in humans:
- Nerve cells conduct impulses for coordination.
- Muscle cells contract for movement.
- Red blood cells transport oxygen.
- Acinar cells of pancreas secrete digestive enzymes.
- Bone cells (osteocytes) form skeletal support.
Q10. Cell is the basic unit of life. Discuss in brief.
A cell is the basic unit of life because every characteristic of life is performed at the cellular level:
- Structural unit: The bodies of all organisms are made of one or more cells. Even the most complex animal (e.g., human) is built from ~37 trillion cells.
- Functional unit: All life processes — metabolism, response to stimuli, reproduction, growth, energy capture — occur inside cells.
- Genetic unit: DNA is housed and replicated inside cells; new cells inherit the same genetic information.
- Independence: Many single cells (bacteria, protists) live independently and fulfil all functions of life — proving the cell is a self-sufficient unit.
- Origin of new cells: All cells arise from pre-existing cells (Virchow), so the cell is the unit of continuity of life.
Q11. What are nuclear pores? State their function.
Nuclear pores are minute openings in the nuclear envelope formed by the fusion of the inner and outer nuclear membranes. They appear as proteinaceous channels (~100 nm wide overall, with a much narrower selective channel).
Functions:
Functions:
- Allow regulated, two-way passage of materials between the nucleus and the cytoplasm.
- Export of mRNA, rRNA, tRNA and ribosomal subunits to the cytoplasm.
- Import of proteins (transcription factors, histones, polymerases) into the nucleus.
- Maintain the chemical environment of the nucleus by selective permeability.
Q12. Both lysosomes and vacuoles are endomembrane structures, yet they differ in terms of their functions. Comment.
| Feature | Lysosome | Vacuole |
|---|---|---|
| Found in | Mainly animal cells | Mainly plant cells (large central) and protists |
| Origin | Pinched off from Golgi | From ER or formed by coalescence of small vesicles |
| Membrane | Single, lysosomal membrane | Single, called tonoplast |
| Contents | Hydrolytic enzymes (active at acidic pH) | Sap, water, ions, pigments, sugars, wastes |
| Main function | Intracellular digestion (autophagy, heterophagy) | Storage; turgor; osmoregulation (Amoeba); food digestion (protists) |
| pH | Acidic (~5) | Variable (often slightly acidic) |
Q13. Describe the structure of the following with the help of labelled diagrams: (i) Nucleus, (ii) Centrosome.
(i) Nucleus: Double-membrane organelle (outer continuous with rough ER) with perinuclear space, nuclear pores for selective transport, nucleoplasm containing chromatin (DNA + histone) and one or more nucleoli (rRNA synthesis).
(ii) Centrosome: Contains two centrioles oriented perpendicular to each other, surrounded by pericentriolar material. Each centriole is a cylinder of nine triplets of microtubules arranged in a cartwheel pattern.
Q14. What is a centromere? How does the position of centromere form the basis of classification of chromosomes? Support your answer with a diagram showing the position of centromere on different types of chromosomes.
Centromere is the primary constriction on a chromosome where the two sister chromatids are joined. The kinetochore (disc-shaped protein structure) forms on each side of the centromere and is the attachment point for spindle fibres during cell division.
Classification by centromere position:
Classification by centromere position:
- Metacentric: Centromere in the middle; two equal arms.
- Sub-metacentric: Centromere slightly off middle; one shorter and one longer arm.
- Acrocentric: Centromere near one end; one very short and one very long arm.
- Telocentric: Centromere at the very end; only one arm (NOT found in normal humans).
Competency-Based Questions — Chapter Recap
Q1. Which scientist proposed 'Omnis cellula-e cellula'? L1 Remember
Answer: (c) Rudolf Virchow. 1855 — modified Schleiden-Schwann theory by stating that all cells arise from pre-existing cells.
Q2. Match: cristae : mitochondria :: ___ : chloroplast. L2 Understand
Thylakoids (grana). Both increase surface area for membrane-embedded energy machinery — ETC in mitochondria, photosystems in chloroplast.
Q3. Apply: A pathogenic bacterium is found to have a glycocalyx capsule. How might this help it survive the host's immune system? L3 Apply
The capsule:
1. Masks bacterial surface antigens — so host antibodies cannot easily recognise the bacterium.
2. Prevents phagocytosis — the slippery capsule makes it hard for macrophages and neutrophils to engulf the bacterium.
3. Resists desiccation — helps survive between hosts.
4. Adhesion — helps the bacterium stick to host tissues.
Examples: Streptococcus pneumoniae (causes pneumonia) is dangerous because of its capsule; vaccines like the pneumococcal vaccine target capsule antigens.
1. Masks bacterial surface antigens — so host antibodies cannot easily recognise the bacterium.
2. Prevents phagocytosis — the slippery capsule makes it hard for macrophages and neutrophils to engulf the bacterium.
3. Resists desiccation — helps survive between hosts.
4. Adhesion — helps the bacterium stick to host tissues.
Examples: Streptococcus pneumoniae (causes pneumonia) is dangerous because of its capsule; vaccines like the pneumococcal vaccine target capsule antigens.
Q4. Analyse: Compare the size of a typical bacterium (~3 µm) with a typical eukaryotic cell (~30 µm). What internal differences are forced by this size difference? L4 Analyse
A eukaryotic cell is ~10x bigger in linear dimension → ~1000x larger in volume. Consequences:
1. Surface-area to volume ratio drops sharply — a single plasma membrane can't supply enough material for the cell. Solution: internal membranes (ER, mitochondria, Golgi) increase total membrane area dramatically.
2. Diffusion alone is too slow across large distances. Solution: cytoplasmic streaming, vesicular transport, motor proteins on cytoskeleton tracks.
3. Genome is bigger → must be organised on histones and packed into chromosomes; needs a compartment (nucleus) for regulation.
4. Energy demand → many specialised mitochondria.
This is why eukaryotic complexity is, in part, a response to size.
1. Surface-area to volume ratio drops sharply — a single plasma membrane can't supply enough material for the cell. Solution: internal membranes (ER, mitochondria, Golgi) increase total membrane area dramatically.
2. Diffusion alone is too slow across large distances. Solution: cytoplasmic streaming, vesicular transport, motor proteins on cytoskeleton tracks.
3. Genome is bigger → must be organised on histones and packed into chromosomes; needs a compartment (nucleus) for regulation.
4. Energy demand → many specialised mitochondria.
This is why eukaryotic complexity is, in part, a response to size.
Q5. Create: Imagine you have to teach the concept of 'cell' to a Class 5 student in one minute. Write the script. L6 Create
"Hold up a brick. Now imagine a house — schools, hospitals, homes — all built from bricks. Just like that, every living thing is built from tiny living bricks called cells. A tomato is made of cells. A tiger is made of cells. You and I are made of cells — about 37 trillion of them!
A cell is so small you need a microscope to see it. Inside, each cell has a 'brain' (nucleus) that tells it what to do, a 'kitchen' (mitochondria) that makes energy, and a 'wall' (plasma membrane) that decides what comes in and goes out. Some cells, like in plants, even have a 'hard outer wall' (cell wall) like a brick layer outside.
The amazing part? All cells come from other cells — never from non-living matter. So the cell you start with as a baby keeps splitting and splitting to grow you up. That's why we say: the cell is the unit of life."
A cell is so small you need a microscope to see it. Inside, each cell has a 'brain' (nucleus) that tells it what to do, a 'kitchen' (mitochondria) that makes energy, and a 'wall' (plasma membrane) that decides what comes in and goes out. Some cells, like in plants, even have a 'hard outer wall' (cell wall) like a brick layer outside.
The amazing part? All cells come from other cells — never from non-living matter. So the cell you start with as a baby keeps splitting and splitting to grow you up. That's why we say: the cell is the unit of life."
Assertion–Reason Questions
Choose: (A) Both true, R explains A. (B) Both true, R doesn't explain A. (C) A true, R false. (D) A false, R true.
A: Plant cells have a large central vacuole.
R: The vacuole helps maintain turgidity through osmotic pressure.
Answer: (A). Both true; R explains A. Filled vacuole presses against the wall, keeping the plant erect.
A: Endomembrane system does not include mitochondria.
R: Mitochondrial functions are not coordinated with ER and Golgi.
Answer: (A). Both true; R explains A. Endomembrane = coordinated organelles (ER, Golgi, lysosomes, vacuoles); mitochondria are independent power plants.
A: Human chromosome 21 is acrocentric.
R: Acrocentric chromosomes have their centromere at the middle.
Answer: (C). Assertion is true. Reason is false — acrocentric chromosomes have their centromere NEAR ONE END (forming a tiny short arm and a long arm), NOT in the middle. Middle = metacentric.
Frequently Asked Questions - NCERT Exercises and Solutions: Cell: The Unit of Life
What are the most-asked NCERT exercise questions in Chapter 8 Cell: The Unit of Life?
NCERT Class 11 Biology Chapter 8 Cell: The Unit of Life exercises cover definitions, classification, structure-function relationships, labelled diagrams, and application-based questions. The MyAiSchool exercise set provides full step-by-step solutions for every NCERT question, aligned with the CBSE board exam pattern. Students should master scientific terminology, diagram labelling, and concept comparison tables to score full marks.
How should students approach labelled diagram questions in Cell: The Unit of Life?
For labelled diagram questions in NCERT Class 11 Biology Chapter 8 Cell: The Unit of Life: (1) draw a clean, large, proportional diagram with sharp pencil lines, (2) label each part with horizontal lines on the right or left side, (3) use scientific terminology (Latin/Greek names where applicable), (4) write a 1-2 line description below if asked. The MyAiSchool solutions provide editable reference diagrams aligned with NCERT textbook figures.
What types of CBSE board questions come from Chapter 8?
CBSE Class 11 Biology board questions from Chapter 8 (Cell: The Unit of Life) typically include: (1) 1-mark MCQs on definitions and classification, (2) 2-mark short-answers on structure-function or comparisons, (3) 3-mark labelled-diagram questions, (4) 5-mark long-answer essays combining diagram + description + significance. The MyAiSchool exercise set tags each question by mark weight and Bloom level (L1-L6).
How do I compare two biological concepts in 5-mark questions?
For 5-mark comparison questions in NCERT Class 11 Biology Chapter 8: (1) use a two-column table with feature headings down the left side, (2) compare on at least 5-6 features (structure, function, location, examples, significance), (3) include one labelled diagram if relevant, (4) end with one line on biological significance. The MyAiSchool solutions follow this CBSE-aligned tabular format consistently for full marks.
What are common mistakes in Chapter 8 exercises?
Common mistakes in NCERT Class 11 Biology Chapter 8 (Cell: The Unit of Life) include: (1) confusing similar scientific names or terminology, (2) skipping diagram labels or drawing too small, (3) missing examples in classification questions, (4) writing essay-style answers when a table is expected, (5) forgetting to mention biological significance. The MyAiSchool solutions highlight these traps so students avoid losing marks unnecessarily.
How does the MyAiSchool solution differ from other NCERT solution sets?
MyAiSchool Class 11 Biology Chapter 8 Cell: The Unit of Life solutions use NEP 2024-aligned pedagogy with Bloom Taxonomy tagged questions (L1 Remember to L6 Create), step-by-step working with biological reasoning, fully labelled SVG diagrams, comparison tables, interactive simulations, and Competency-Based Questions (CBQs) for board exam practice. Each solution is verified against NCERT textbook and CBSE marking schemes.
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