'Cell: Structure and Function'

Cell Theory

Key Proponents

• Robert Hooke (1665): Discovered cells in cork (dead plant cells).
• Anton van Leeuwenhoek: First observed living cells.
• Schleiden and Schwann (1838-39): Proposed cell theory.

Postulates of Cell Theory

1. All living organisms are composed of cells.
2. The cell is the basic structural and functional unit of life.
3. All cells arise from pre-existing cells (Rudolf Virchow, 1855).

Exceptions

• Viruses are acellular.
• Some algae have coenocytic (multinucleate) condition.

Prokaryotic vs Eukaryotic Cells

Cell Membrane (Plasma Membrane)

Fluid Mosaic Model (Singer and Nicolson, 1972)

• Lipid bilayer (phospholipids) with embedded and peripheral proteins.
• 'Fluid' because lipids and proteins can move laterally.
• 'Mosaic' because proteins are scattered throughout.

Functions

• Selective permeability.
• Cell-to-cell recognition (glycoproteins).
• Cell signalling (receptor proteins).

Transport Across Membrane

Passive transport (no energy):

• Simple diffusion: Small, non-polar molecules (O2, CO2).
• Facilitated diffusion: Via channel or carrier proteins. Down concentration gradient.
• Osmosis: Movement of water across semipermeable membrane.
  • Hypotonic: Water enters cell (swells).
  • Hypertonic: Water exits cell (shrinks).
  • Isotonic: No net water movement.

Active transport (requires ATP):

• Movement against concentration gradient.
• Through carrier proteins (pumps).
• Example: Na+/K+ pump.

Bulk transport:

• Endocytosis: Cell engulfs external material (phagocytosis = solid, pinocytosis = liquid).
• Exocytosis: Vesicle fuses with membrane to release contents.

Cell Wall

• Present in plants, fungi, bacteria (not in animals).
• Plants: Primary wall (cellulose, hemicellulose, pectin) + Middle lamella + Secondary wall.
• Functions: Structural support, protection, prevents bursting.

Endomembrane System

A group of organelles that work together in synthesis, packaging, and transport.

Endoplasmic Reticulum (ER):

• Rough ER: Ribosomes attached, protein synthesis.
• Smooth ER: Lipid synthesis, detoxification, carbohydrate metabolism.

Golgi Apparatus (Dictyosomes):

• Modifies, sorts, and packages proteins from ER.
• Forms lysosomes, secretory vesicles.
• Discovered by Camillo Golgi.

Lysosomes:

• 'Suicide bags of the cell' (de Duve).
• Contain hydrolytic enzymes (acid hydrolases).
• Digest worn-out organelles (autophagy), foreign material.

Vacuoles:

• Plant cells: Large central vacuole (tonoplast). Stores water, ions, pigments.
• Animal cells: Small vacuoles. Contractile vacuole in Amoeba (osmoregulation).

Mitochondria

• 'Powerhouse of the cell' (Kingsbury).
• Double membrane: outer + inner (cristae).
• Matrix contains DNA, ribosomes (70S), enzymes for Krebs cycle.
• Site of aerobic respiration (ATP synthesis).

Plastids

• Found only in plant cells.
• Chloroplasts: Photosynthesis. Double membrane + thylakoids (grana) + stroma.
• Chromoplasts: Coloured (carotenoids) - give colour to flowers, fruits.
• Leucoplasts: Colourless (amyloplasts store starch).

Ribosomes

• Site of protein synthesis.
• 70S (prokaryotes, mitochondria, chloroplasts) and 80S (eukaryotic cytoplasm).
• Composition: rRNA + proteins (two subunits: large and small).

Cytoskeleton

• Network of protein fibres: microfilaments, microtubules, intermediate filaments.
• Functions: Cell shape, cell division, intracellular transport, cell motility.

Nucleus

• Contains genetic material (DNA + histones = chromatin).
• Nuclear envelope (double membrane with nuclear pores).
• Nucleolus: rRNA synthesis.
• Chromosomes condense during cell division.

Microbodies

• Peroxisomes: Contain catalase. Break down H2O2 (toxic to water and oxygen).
• Glyoxysomes: In plant seeds, convert fats to carbohydrates.

Cilia and Flagella

• Cilia: Short, numerous (movement of particles).
• Flagella: Long, few (cell motility).
• Structure: 9+2 arrangement of microtubules (9 outer doublets + 2 central).

Worked Examples

Example 1: Why are mitochondria called the powerhouse of the cell? Solution: They produce ATP through aerobic respiration (Krebs cycle + oxidative phosphorylation), providing energy for cellular activities.

Example 2: What would happen if a plant cell is placed in a hypertonic solution? Solution: Water leaves the cell, protoplast shrinks (plasmolysis). The cell becomes flaccid.

Common Mistakes

1. 70S vs 80S: S = Svedberg unit (sedimentation coefficient), not size.
2. Prokaryotic ribosomes are 70S: Even though they are 'smaller' than 80S.
3. Lysosomes are membrane-bound: They are not free enzymes in the cytoplasm.
4. All cells have cell membrane but not all have cell wall: Animal cells lack cell walls.

ISC Exam Focus

• Theory (70%): Cell theory, membrane structure, organelle structure and functions, endomembrane system.
• Application (30%): Identifying organelles, explaining processes, comparison tables.
• ISC frequently asks: "Draw and label the structure of ..." and "Explain the fluid mosaic model."

Self-Test Questions

Q1: State the cell theory. Answer: All organisms are composed of cells; cell is the basic unit; all cells arise from pre-existing cells.

Q2: Differentiate between prokaryotic and eukaryotic cells. Answer: Prokaryotic: no nucleus, no membrane organelles, 70S ribosomes. Eukaryotic: nucleus present, membrane organelles, 80S ribosomes.

Q3: Describe the fluid mosaic model of the cell membrane. Answer: Lipid bilayer with embedded proteins. Fluid because components move laterally. Mosaic because proteins are scattered.

Q4: What are the functions of Rough ER and Smooth ER? Answer: RER: protein synthesis. SER: lipid synthesis, detoxification.

Q5: Why are lysosomes called 'suicide bags'? Answer: They contain hydrolytic enzymes that can digest the cell itself if ruptured.

Q6: Draw a labelled diagram of a chloroplast. Answer: Label: outer membrane, inner membrane, thylakoid, granum, stroma, lamella.