Exercise Solutions Available
Step-by-step answers for all textbook questions in this chapter
Chapter Introduction
The cell is the fundamental structural and functional unit of all living organisms. Chapter 8 of NCERT Class 11 Biology traces the discovery of the cell, presents cell theory, and then dives deep into the detailed structure of prokaryotic and eukaryotic cells. You study the plasma membrane, cell wall, endomembrane system (ER, Golgi, lysosomes, vacuoles), mitochondria, plastids, ribosomes, cytoskeleton, centrosome, cilia, and flagella. This chapter is the foundation for understanding cell division (Chapter 10), physiology, and genetics.
Key Definitions
| Term | Definition |
|---|---|
| Cell | Fundamental structural and functional unit of life |
| Cell theory | All living organisms are composed of cells; cell is the basic unit; all cells arise from pre-existing cells |
| Prokaryote | Organism whose cell lacks a true nucleus and membrane-bound organelles |
| Eukaryote | Organism whose cell has a well-defined nucleus and membrane-bound organelles |
| Nucleoid | Undifferentiated, naked, circular DNA region in prokaryotes (no nuclear envelope) |
| Plasmid | Small, circular, extrachromosomal DNA in prokaryotes; carries accessory genes |
| Mesosome | Infolding of plasma membrane in prokaryotes; aids respiration, replication, septum formation |
| Fluid mosaic model | Plasma membrane model: phospholipid bilayer with embedded proteins (Singer and Nicolson, 1972) |
| Endomembrane system | Functionally connected membrane organelles: ER, Golgi, lysosomes, vacuoles |
| Cristae | Foldings of inner mitochondrial membrane; increase surface area; bear F₁ particles |
| F₁ particle | ATP synthase complex on mitochondrial cristae; synthesises ATP |
| Thylakoid | Disc-shaped sac in chloroplast; contains chlorophyll; stacks form grana |
| Centriole | Cylindrical structure of 9+0 microtubule triplets; forms centrosome and basal bodies |
| Cytoskeleton | Network of protein filaments (microtubules, microfilaments, intermediate filaments) |
| Glycocalyx | Outer coating of polysaccharides on plasma membrane; cell recognition and adhesion |
Cell Theory
| Scientist | Year | Contribution |
|---|---|---|
| Robert Hooke | 1665 | First to observe cells (cork cells under microscope); coined the term "cell" |
| Anton van Leeuwenhoek | 1674 | First to observe and describe living cells (bacteria, protozoa, RBCs, sperm) |
| Matthias Schleiden | 1838 | All plants are made of cells (plant cytologist) |
| Theodor Schwann | 1839 | All animals are made of cells (animal cytologist); proposed cell theory with Schleiden |
| Rudolf Virchow | 1855 | "Omnis cellula e cellula" — all cells arise from pre-existing cells |
Cell theory postulates: 1. All living organisms are composed of cells and products of cells. 2. All cells arise from pre-existing cells. 3. Cell is the basic structural and functional unit of life.
Exceptions to cell theory: Viruses (no cellular organisation), prions (protein only, no nucleic acid), Viroids (RNA only, no protein coat), mycoplasma (smallest cell, no cell wall).
Cell Size and Shape
- Smallest cell: Mycoplasma (0.1–0.3 micrometres)
- Largest cell: Ostrich egg (170 mm x 130 mm)
- Longest cell: Nerve cell (neuron, up to 1 metre)
- Human RBC: 7 micrometres; disc-shaped, biconcave
- Bacteria: 1–2 micrometres
- Shape varies: round (RBC), elongated (nerve), branched (cardiac muscle), disc-like (WBC), rectangular (plant parenchyma)
Prokaryotic Cell
Prokaryotes include bacteria, cyanobacteria (blue-green algae), mycoplasma, and archaebacteria.
Cell Envelope
| Layer | Description |
|---|---|
| Glycocalyx | Outermost; loose sheath (slime layer) or thick tough coat (capsule); protection and adhesion |
| Cell wall | Rigid; made of peptidoglycan (murein) in bacteria; determines shape; prevents bursting |
| Plasma membrane | Innermost; phospholipid bilayer; selectively permeable; mesosomes arise as infoldings |
Internal Structure
| Structure | Description |
|---|---|
| Nucleoid | Undifferentiated region; circular, naked DNA; no nuclear envelope, no nucleolus |
| Plasmids | Small, circular, extrachromosomal DNA; carry antibiotic resistance genes; used in genetic engineering |
| Mesosomes | Extensions of plasma membrane; central mesosome (septum formation) and lateral mesosome (respiration, DNA replication) |
| Ribosomes (70S) | 50S + 30S subunits; site of protein synthesis; polyribosomes form during active translation |
| Inclusion bodies | Reserve material: granules (glycogen/volutin), gas vacuoles (cyanobacteria for buoyancy), lipid droplets |
| Flagella | Long, whip-like; 3 parts — basal body, hook, filament; for locomotion |
| Pili and Fimbriae | Short, hair-like surface structures; pili (conjugation in bacteria), fimbriae (attachment) |
Key Prokaryotic Features
- No membrane-bound organelles (no ER, Golgi, mitochondria, lysosomes, plastids)
- Single, circular, naked chromosome (no histones)
- 70S ribosomes (50S large + 30S small subunit)
- Cell division by fission (no mitosis/meiosis)
- Cell wall of peptidoglycan (except mycoplasma which lacks cell wall)
Eukaryotic Cell
Cell Wall
- Present in plant cells, fungi, and some protists; absent in animal cells
- Plant cell wall: Cellulose (fibres) + hemicellulose + pectin + lignin (in some)
- Middle lamella: First layer formed; pectin-rich; cements adjacent cells
- Primary wall: Thin, flexible, capable of growth; cellulose microfibrils
- Secondary wall: Thick, rigid; formed inside primary wall after growth stops; may contain lignin
- Plasmodesmata: Cytoplasmic strands through cell wall; connect cytoplasm of adjacent cells; allow transport
- Fungal cell wall: Chitin
- Functions: Shape, protection from mechanical injury, prevention of osmotic bursting
Plasma Membrane (Fluid Mosaic Model)
Singer and Nicolson (1972) proposed the fluid mosaic model.
- Phospholipid bilayer: Hydrophilic heads face outward; hydrophobic tails face inward
- Proteins: Two types:
- Integral (intrinsic) proteins: Partially or totally buried in lipid bilayer; transmembrane; difficult to remove
- Peripheral (extrinsic) proteins: On the surface; loosely attached; easily removed
- Fluid nature: Lipids and integral proteins can move laterally within the membrane
- Quasi-fluid: Allows cell functions like endocytosis, cell division, cell growth
- Cholesterol: In animal cells; stabilises membrane fluidity
- Glycocalyx: Sugar chains on membrane proteins/lipids; cell recognition and adhesion
Functions: Selective permeability, transport (passive: diffusion, osmosis; active: Na⁺/K⁺ pump), cell signalling, cell adhesion
Endomembrane System
The endomembrane system includes organelles whose membranes are functionally continuous. Mitochondria, plastids, and peroxisomes are NOT part of this system.
Endoplasmic Reticulum (ER)
| Feature | Rough ER (RER) | Smooth ER (SER) |
|---|---|---|
| Ribosomes | Present on surface (gives rough appearance) | Absent |
| Function | Protein synthesis; transports proteins to Golgi | Lipid synthesis; detoxification; steroid hormone synthesis |
| Location | Abundant in protein-secreting cells (plasma cells, pancreatic cells) | Abundant in lipid-synthesising cells (adipocytes, steroidogenic cells) |
| Appearance | Flat, stacked sacs (cisternae) | Tubular, vesicular |
- Lumen: Internal space of ER; connected to nuclear envelope outer membrane
- ER membrane: Continues with outer nuclear membrane
- ER forms the skeletal framework of the cell and provides surface for biochemical activities
Golgi Complex (Golgi Apparatus)
- Discovered by: Camillo Golgi (1898)
- Structure: Stacked cisternae (flattened, disc-shaped sacs) arranged parallel; convex cis face (forming face, near ER) and concave trans face (maturing face, towards plasma membrane)
- Functions:
- Packaging and modification of proteins/lipids from ER
- Glycosylation (adding carbohydrate tags)
- Formation of lysosomes
- Production of secretory vesicles (for exocytosis)
- Formation of plasma membrane components
- Acrosome of sperm (modified Golgi)
- Transport: Vesicles from ER fuse at cis face; modified products leave from trans face
Lysosomes
- Structure: Membrane-bound vesicles; contain hydrolytic enzymes (acid hydrolases — lipases, proteases, carbohydrases, nucleases); active at pH 5 (acidic)
- Formation: Produced from Golgi apparatus
- Types of digestion:
- Autophagy: Lysosome digests the cell's own worn-out organelles
- Heterophagy: Digestion of external material (after phagocytosis)
- Suicide bags: If lysosome ruptures, enzymes digest the cell itself (autolysis)
- Functions: Intracellular digestion, defence (WBCs digest bacteria), metamorphosis (frog tail digestion), osteoclast activity
Vacuoles
- Structure: Membrane-bound sacs; tonoplast (single membrane)
- Plant cells: Large central vacuole (90% of volume); provides turgor pressure; stores sap, pigments, waste, nutrients
- Animal cells: Small, numerous vacuoles
- Contractile vacuole: In freshwater protozoa (Amoeba, Paramecium); osmoregulation — expels excess water
- Food vacuole: Formed by phagocytosis; digested by lysosomes
Mitochondria
- Powerhouse of the cell: Produce ATP through aerobic respiration
- Structure: Double membrane-bound; outer membrane smooth, inner membrane folded into cristae
- Matrix: Inner compartment; contains its own circular DNA, 70S ribosomes, enzymes of Krebs cycle
- Cristae: Increase surface area for electron transport chain; bear F₁ particles (ATP synthase) — oxysomes
- Semi-autonomous: Own DNA + ribosomes → can synthesise some of their own proteins
- Number: Varies — 1 in yeast, many in muscle cells; more in active cells
- Endosymbiotic theory: Mitochondria evolved from free-living aerobic bacteria engulfed by ancestral eukaryotic cell (supported by: double membrane, own DNA, 70S ribosomes, binary fission)
Plastids
| Type | Description | Pigment | Function | Example |
|---|---|---|---|---|
| Chloroplasts | Green; double membrane; contain thylakoids, grana, stroma | Chlorophyll a, b; carotenoids | Photosynthesis | Leaves, green stems |
| Chromoplasts | Yellow, orange, red; carotenoid-rich | Carotenes, xanthophylls | Attract pollinators; fruit colour | Tomato, carrot, rose |
| Leucoplasts | Colourless; no pigments | None | Storage | Root, seed, tuber cells |
Sub-types of leucoplasts:
- Amyloplasts: Store starch (potato)
- Elaioplasts: Store lipids/oil (castor seed)
- Aleuroplasts (proteinoplasts): Store proteins (aleuron grains in cereal seeds)
Chloroplast Structure
- Double membrane: Outer and inner; both smooth
- Stroma: Matrix inside inner membrane; contains enzymes for dark reaction (Calvin cycle); own circular DNA, 70S ribosomes, starch granules
- Thylakoids: Disc-shaped sacs; membrane contains chlorophyll, carotenoids, electron carriers; site of light reaction
- Grana: Stacks of thylakoids (10–100 per granum); connected by stroma lamellae (inter-granal thylakoids)
- Lumen: Inner space of thylakoid; protons accumulate here during light reaction
Ribosomes
| Feature | 70S Ribosome | 80S Ribosome |
|---|---|---|
| Size | Smaller | Larger |
| Subunits | 50S (large) + 30S (small) | 60S (large) + 40S (small) |
| Found in | Prokaryotes, mitochondria, chloroplasts | Eukaryotic cytoplasm |
| Composition | rRNA + proteins | rRNA + proteins |
| Sedimentation coefficient | 70 Svedberg | 80 Svedberg |
- Polyribosome (polysome): Multiple ribosomes attached to a single mRNA; active during protein synthesis
- Function: Site of protein synthesis (translation)
- Ribosomes are not membrane-bound — they are present in both prokaryotes and eukaryotes
Cytoskeleton
Network of protein filaments in the cytoplasm:
| Component | Description | Function |
|---|---|---|
| Microtubules | Hollow tubes of tubulin protein; 25 nm diameter | Shape, transport (tracks for vesicles), spindle fibres, cilia/flagella |
| Microfilaments | Solid rods of actin protein; 6 nm diameter | Cell motility, cytokinesis (cleavage furrow), amoeboid movement |
| Intermediate filaments | Fibrous proteins; 10 nm diameter | Mechanical strength, anchor organelles, nuclear lamina |
Centrosome and Centrioles
- Centrosome: Contains two centrioles; found in animal cells (absent in most plant cells)
- Centriole structure: Cylindrical; 9 peripheral triplets of microtubules (9+0 arrangement); no central microtubules
- Centrosome function: Forms spindle fibres during cell division; organises microtubules
- Basal bodies: Centrioles at the base of cilia and flagella; give rise to their microtubule structure (9+2)
Cilia and Flagella
| Feature | Cilia | Flagella |
|---|---|---|
| Length | Short (5–10 micrometres) | Long (100–200 micrometres) |
| Number | Many per cell | 1–4 per cell |
| Movement | Oar-like (back and forth) | Whip-like (undulating) |
| Function | Move particles (respiratory tract), move fluid (fallopian tube) | Locomotion (sperm, Euglena) |
Structure (both): 9+2 arrangement of microtubules — 9 peripheral doublets + 2 central singlets; covered by plasma membrane; basal body (9+0) at the base; dynein arms provide sliding force
Prokaryotic vs Eukaryotic Cell
| Feature | Prokaryotic Cell | Eukaryotic Cell |
|---|---|---|
| Nucleus | Nucleoid (no nuclear envelope, no nucleolus) | True nucleus with nuclear envelope and nucleolus |
| DNA | Circular, naked (no histones) | Linear, associated with histones → chromosomes |
| Cell wall | Peptidoglycan (murein) | Cellulose (plants), chitin (fungi), absent (animals) |
| Membrane-bound organelles | Absent (no ER, Golgi, mitochondria, lysosomes) | Present |
| Ribosomes | 70S (50S + 30S) | 80S in cytoplasm; 70S in mitochondria/chloroplasts |
| Plasmids | Present | Absent |
| Mesosomes | Present | Absent |
| Flagella | Simple (no 9+2); single protein (flagellin) | Complex (9+2 microtubules; tubulin) |
| Cell size | Small (1–5 micrometres) | Larger (5–100 micrometres) |
| Cell division | Fission (no mitosis) | Mitosis and meiosis |
| Examples | Bacteria, cyanobacteria, mycoplasma | Plants, animals, fungi, protists |
Plant Cell vs Animal Cell
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell wall | Present (cellulose) | Absent |
| Plastids | Present (chloroplasts, chromoplasts, leucoplasts) | Absent |
| Vacuole | Large, central | Small, numerous |
| Centrosome/Centrioles | Absent (most plants) | Present |
| Glyoxysomes | Present (germinating seeds) | Absent |
| Plasmodesmata | Present | Absent |
| Lysosomes | Rare | Abundant |
| Tonicity | Cannot burst (cell wall) | Can burst in hypotonic solution |
| Storage | Starch granules | Glycogen granules |
| Shape | Fixed (cell wall) | Variable (plasma membrane) |
Organelles and Their Functions — Summary Table
| Organelle | Membrane | Key Function |
|---|---|---|
| ER (Rough) | Single | Protein synthesis and transport |
| ER (Smooth) | Single | Lipid synthesis, detoxification |
| Golgi apparatus | Single | Packaging, modification, lysosome formation |
| Lysosomes | Single | Intracellular digestion (acid hydrolases) |
| Vacuoles | Single (tonoplast) | Storage, turgor, osmoregulation |
| Mitochondria | Double | ATP production (aerobic respiration) |
| Chloroplast | Double | Photosynthesis (light + dark reactions) |
| Ribosomes | None | Protein synthesis |
| Centrosome | None | Spindle formation, microtubule organisation |
| Nucleus | Double (nuclear envelope) | DNA storage, RNA transcription, cell control |
Important Diagrams to Practise
1. Prokaryotic cell (bacterial cell) — label: glycocalyx, cell wall, plasma membrane, nucleoid, plasmid, mesosome, 70S ribosomes, flagellum, pili, inclusion bodies 2. Animal cell (general) — label: plasma membrane, nucleus, nucleolus, ER (RER and SER), Golgi, mitochondria, lysosomes, ribosomes, centrosome, cytoskeleton 3. Plant cell (general) — label: cell wall, plasma membrane, nucleus, chloroplast, mitochondria, ER, Golgi, vacuole, ribosomes, plasmodesmata 4. Mitochondrion (L.S.) — label: outer membrane, inner membrane, cristae, F₁ particles, matrix, circular DNA, 70S ribosomes 5. Chloroplast (L.S.) — label: outer membrane, inner membrane, stroma, grana, thylakoids, stroma lamellae, starch granule, circular DNA 6. Fluid mosaic model — label: phospholipid bilayer, integral protein, peripheral protein, cholesterol, glycocalyx, hydrophilic head, hydrophobic tail 7. Golgi apparatus — label: cis face, trans face, cisternae, vesicles
Common Mistakes to Avoid
1. Confusing 70S and 80S ribosomes — 70S is in prokaryotes AND eukaryotic organelles (mitochondria, chloroplasts); 80S is in eukaryotic cytoplasm only. The "S" refers to Svedberg units, not size — subunits do NOT add up arithmetically (50S + 30S = 70S, not 80S). 2. Including mitochondria in the endomembrane system — the endomembrane system includes only ER, Golgi, lysosomes, and vacuoles. Mitochondria and plastids have their own DNA and are NOT derived from ER. 3. Saying plant cells have centrioles — most plant cells lack centrioles. The centrosome/centriole is typically found only in animal cells. 4. Confusing mesosomes with mesophyll — mesosomes are infoldings of the prokaryotic plasma membrane; mesophyll is plant leaf tissue. Completely different structures. 5. Forgetting that mitochondria and chloroplasts have 70S ribosomes — they are prokaryotic-type ribosomes, which supports the endosymbiotic theory. Do not write 80S for these organelles. 6. Mixing up cis and trans faces of Golgi — cis (forming face) is near the ER; trans (maturing face) is towards the plasma membrane. Think: cis = close to ER; trans = towards transport. 7. Calling leucoplasts colourless chloroplasts — leucoplasts are a separate type of plastid for storage, not immature chloroplasts. Amyloplasts, elaioplasts, and aleuroplasts are all sub-types of leucoplasts.
Exam Tips
- 5-mark questions often ask: "Describe the structure of a mitochondrion" or "Describe the fluid mosaic model of plasma membrane." Always draw a labelled diagram alongside your description.
- 3-mark questions frequently ask: "Differentiate between prokaryotic and eukaryotic cells" or "Differentiate between plant and animal cells." Use a table.
- 1-mark questions target definitions: cell theory, mesosome, plasmid, F₁ particle, glycocalyx, polyribosome, 70S vs 80S, cis vs trans face, nucleoid.
- Diagram questions: mitochondrion, chloroplast, and prokaryotic cell are the most frequently asked. Label precisely.
- For endomembrane system questions, explicitly name all four components (ER, Golgi, lysosomes, vacuoles) and state what is NOT included (mitochondria, plastids, peroxisomes).
- For the fluid mosaic model, always mention both "fluid" (lateral movement of lipids/proteins) and "mosaic" (proteins embedded like a mosaic pattern).
Quick Revision Notes
- Cell theory: Schleiden (plants), Schwann (animals), Virchow (all cells from pre-existing cells)
- Smallest cell: Mycoplasma (0.1–0.3 micrometre); Largest: Ostrich egg
- Prokaryotic: nucleoid, plasmids, 70S ribosomes, mesosomes, peptidoglycan wall, no membrane organelles
- Eukaryotic: true nucleus, 80S ribosomes, membrane-bound organelles, linear DNA with histones
- Plasma membrane: fluid mosaic model — phospholipid bilayer + integral/peripheral proteins
- Cell wall: plants (cellulose), fungi (chitin), bacteria (peptidoglycan), animals (absent)
- ER: RER (ribosomes, protein synthesis), SER (no ribosomes, lipid synthesis, detox)
- Golgi: cis face (near ER), trans face (towards membrane); packaging, modification, lysosome formation
- Lysosomes: suicide bags; acid hydrolases; autophagy, heterophagy; from Golgi
- Vacuoles: tonoplast; large in plants (turgor); contractile in protozoa (osmoregulation)
- Mitochondria: double membrane, cristae, F₁ particles, own DNA, 70S ribosomes; powerhouse
- Plastids: chloroplasts (photosynthesis), chromoplasts (colour), leucoplasts (storage — amyloplast, elaioplast, aleuroplast)
- Chloroplast: double membrane, thylakoids, grana, stroma lamellae, stroma; own DNA, 70S ribosomes
- Ribosomes: 70S (prokaryotes + mitochondria + chloroplasts), 80S (eukaryotic cytoplasm)
- Cytoskeleton: microtubules (shape, spindle), microfilaments (motility), intermediate filaments (strength)
- Centrosome: 2 centrioles (9+0); animal cells; spindle formation
- Cilia/flagella: 9+2 microtubules; cilia (short, many), flagella (long, few)
- Endomembrane system: ER + Golgi + lysosomes + vacuoles (NOT mitochondria/plastids/peroxisomes)