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These are complete NCERT Solutions for CBSE Class 11 Biology Chapter 8 — Cell: The Unit of Life. Attempt each question before checking. Use precise organelle names — marks are given for terms like 'mesosome', 'nucleoid', 'F₁ particle', 'tonoplast', etc.
Key Concepts (Quick Recap)
- Cell theory: Schleiden & Schwann (1838-39); Virchow added Omnis cellula e cellula (all cells arise from pre-existing cells).
- Cell size: Smallest cells — mycoplasma (0.3 μm); largest cell — ostrich egg; longest cell — nerve cell.
- Prokaryotic cell: No nuclear membrane, no membrane-bound organelles, 70S ribosomes, nucleoid, plasmids, mesosomes, cell wall with peptidoglycan.
- Eukaryotic cell: Well-defined nucleus, membrane-bound organelles, 80S ribosomes, cytoskeleton.
- Cell wall: Plants (cellulose), Fungi (chitin), Bacteria (peptidoglycan).
- Plasma membrane: Fluid Mosaic Model (Singer & Nicolson, 1972); selectively permeable.
- Endomembrane system: ER → Golgi → Lysosomes → Vacuoles.
- Mitochondria: Powerhouse; double membrane; own DNA, 70S ribosomes; F₁ particles on inner membrane.
- Chloroplasts: Double membrane; thylakoids → grana → stroma; own DNA, 70S ribosomes.
- Centrosome: Two centrioles (9+0 arrangement); forms spindle during cell division (absent in plant cells).
Intext Questions
Page 111
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.
Wait — let me re-evaluate: Statement (a) says "Robert Brown discovered the cell" — this is incorrect. Robert Brown discovered the nucleus, not the cell. The cell was first observed by Robert Hooke (1665).
Answer(a) is not correct. Robert Brown discovered the nucleus, not the cell. The cell was discovered by Robert Hooke.
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. This is Rudolf Virchow's principle — Omnis cellula e cellula — all cells arise from pre-existing cells.
Page 115
Q1. Match the following:
Answer
| Column I | Column II |
|---|
| (a) Cristae | (iii) Infoldings in mitochondria |
| (b) Cisternae | (iv) Disc-shaped sacs in Golgi apparatus |
| (c) Thylakoids | (i) Flat membranous sacs in stroma of chloroplast |
| (d) Chromatin | (ii) DNA + histone |
Q2. Which of the following is not a function of cytoskeleton? (a) Maintenance of cell shape (b) Intracellular transport (c) Cell wall formation (d) Muscle contraction
Answer
Wait — actually, the cytoskeleton IS involved in muscle contraction (actin filaments are part of the cytoskeleton and enable muscle contraction). Let me reconsider. The cytoskeleton does NOT directly form the cell wall — that is done by cellulose synthase and Golgi-derived vesicles. So (c) Cell wall formation is not a direct function of the cytoskeleton.
Answer(c) Cell wall formation is not a function of the cytoskeleton. Cell wall formation involves enzymes and Golgi-derived vesicles, not the cytoskeleton directly.
Chapter-End Exercise
Q1. Which of the following is not correct? (a) Robert Brown discovered the cell.
(See Intext Page 111 Q1 — answered above.)
Q2. New cells generate from: (c) Pre-existing cells
(See Intext Page 111 Q2 — answered above.)
Q3. Match the following:
Answer
(See Intext Page 115 Q1 — answered above.)
Q4. Which of the following is not a function of cytoskeleton? (c) Cell wall formation
(See Intext Page 115 Q2 — answered above.)
Q5. What is a mesosome? What are its functions? Where is it found?
Answer
A mesosome is a specialised membranous structure formed by the infolding of the plasma membrane in prokaryotic cells.
Functions: 1. Cell wall formation — helps in synthesising cell wall material. 2. DNA replication — provides attachment point for DNA during replication. 3. Respiration — increases the surface area for enzymatic reactions involved in respiration. 4. Secretion — helps in secretion of substances.
Where found: Only in prokaryotic cells (bacteria). Mesosomes are absent in eukaryotic cells.
Q6. Differentiate between prokaryotic and eukaryotic cells.
Answer
| Feature | Prokaryotic Cell | Eukaryotic Cell |
|---|
| Size | Small (1-10 μm) | Large (5-100 μm) |
| Nucleus | No nuclear membrane (nucleoid) | Well-defined nucleus with nuclear membrane |
| Nucleolus | Absent | Present |
| Chromosomes | Single circular chromosome | Multiple linear chromosomes |
| Membrane-bound organelles | Absent | Present (ER, Golgi, mitochondria, etc.) |
| Ribosomes | 70S | 80S (cytoplasmic); 70S (in organelles) |
| Cell wall | Present (peptidoglycan in bacteria) | Present in plants (cellulose), absent in animals |
| Plasmids | Present | Absent |
| Mesosomes | Present | Absent |
| Cell division | Amitosis or binary fission | Mitosis/Meiosis |
| Examples | Bacteria, Cyanobacteria, Mycoplasma | Plant cells, Animal cells, Fungi, Protozoa |
Q7. Describe the structure of a typical eukaryotic cell.
Answer
A typical eukaryotic cell has:
1. Cell wall (plant cells only): Made of cellulose, provides structural support and protection. 2. Plasma membrane: Phospholipid bilayer with proteins (fluid mosaic model); selectively permeable. 3. Nucleus: Surrounded by double membrane (nuclear envelope) with nuclear pores; contains chromatin (DNA + histones) and nucleolus. 4. Endoplasmic reticulum (ER): Rough ER (with ribosomes — protein synthesis) and Smooth ER (lipid synthesis, detoxification). 5. Golgi apparatus: Stacked cisternae; modifies, packages, and sorts proteins and lipids. 6. Mitochondria: Double membrane; inner membrane folded into cristae; produces ATP (powerhouse). 7. Lysosomes: Membrane-bound vesicles with hydrolytic enzymes; intracellular digestion. 8. Ribosomes: 80S in cytoplasm; 70S in mitochondria and chloroplasts; protein synthesis. 9. Centrosome: Contains two centrioles (9+0 arrangement); forms spindle during cell division (animal cells only). 10. Vacuoles: Large central vacuole in plant cells (tonoplast membrane); smaller in animal cells. 11. Cytoskeleton: Microfilaments, microtubules, intermediate filaments — provides shape, support, and intracellular transport. 12. Chloroplasts (plant cells only): Double membrane; thylakoids → grana; photosynthesis.
Q8. What is the significance of having nuclear pores?
Answer
Nuclear pores are large protein-lined channels in the nuclear envelope that: 1. Allow the transport of mRNA from the nucleus to the cytoplasm for protein synthesis. 2. Allow the entry of ribosomal subunits and proteins (like histones, transcription factors) from the cytoplasm into the nucleus. 3. Regulate the passage of molecules — small molecules pass freely, but large molecules require active transport with signal sequences. 4. Maintain the distinct nuclear compartment while allowing essential communication between the nucleus and cytoplasm.
Q9. How does an Interphase nucleus differ from a nucleus in the division phase?
Answer
| Feature | Interphase Nucleus | Dividing Nucleus |
|---|
| Nuclear envelope | Intact | Broken down (dissolves) |
| Chromatin | Loosely organised, thread-like | Condensed into distinct chromosomes |
| Nucleolus | Prominent | Disappears |
| DNA replication | Occurs (S phase) | Does not occur during M phase |
| Chromosomes | Not visible (dispersed as chromatin) | Clearly visible under microscope |
| Nuclear pores | Functional | Disrupted as envelope breaks down |
Q10. What is the role of the following cellular organelles? (a) Lysosomes (b) Mitochondria (c) Plastids (d) Ribosomes (e) Golgi apparatus
Answer
(a) Lysosomes: Membrane-bound vesicles containing hydrolytic enzymes (lipase, protease, carbohydrase, nuclease). Functions: intracellular digestion of macromolecules, autolysis (self-digestion of worn-out organelles), phagocytosis, and apoptosis. Called 'suicide bags' of the cell.
(b) Mitochondria: Double-membraned organelles with cristae and F₁ particles (oxysomes). Site of aerobic respiration — Krebs cycle in matrix, ETC on inner membrane. Produce ATP through oxidative phosphorylation. Called 'powerhouses of the cell'. Have own DNA and 70S ribosomes.
(c) Plastids: Found in plant cells. Three types: (1) Chloroplasts (green — photosynthesis), (2) Chromoplasts (coloured — pigment storage, attracts pollinators), (3) Leucoplasts (colourless — storage: amyloplasts=starch, elaioplasts=lipids, aleuroplasts=proteins).
(d) Ribosomes: Small granular particles made of rRNA and proteins. 80S in cytoplasm (eukaryotic), 70S in prokaryotes and organelles. Site of protein synthesis (translation of mRNA into polypeptide chains).
(e) Golgi apparatus: Stacks of flattened disc-shaped cisternae. Functions: modifying and packaging proteins from the ER, forming glycoproteins and glycolipids, producing lysosomes, forming the acrosome of sperm, synthesising cell wall material in plants. Called the 'traffic police' of the cell.
Internal Links
- [Chapter 8 Guide — Cell: The Unit of Life](/school/cbse/class/11/biology/cbse-cell-unit-of-life)
- [All Class 11 Biology Solutions](/school/cbse/class/11/biology/solutions)
- [Class 11 Biology Subject Guide](/school/cbse/class/11/biology)
