Cell Biology & Organisation

**1. Introduction & Core Concept**

*Assalam-o-Alaikum*, students! Ms. Ayesha Siddiqui here.

Every living thing — from the tiniest bacterium in Karachi's Lyari River to the blue whale in the Arabian Sea — is made of cells. The cell is the basic unit of life. Understanding cells means understanding life itself.

When you look at a drop of water from a rice-cooking pot at home, you cannot see anything unusual. But under a microscope, that same drop swarms with single-celled organisms — prokaryotes — that have been alive on Earth for 3.8 billion years. Life, at its simplest and most complex, begins here.

**2. Core Theory**

2.1 — Animal vs Plant Cells

| Feature | Animal Cell | Plant Cell |

|---------|-------------|------------|

| Cell wall | No | Yes (cellulose) |

| Chloroplasts | No | Yes (in green parts) |

| Large central vacuole | No | Yes |

| Nucleus | Yes | Yes |

| Mitochondria | Yes | Yes |

| Cell membrane | Yes | Yes |

2.2 — Key Organelles and Their Functions

• Nucleus: contains DNA (genetic material); controls all cell activities.
• Mitochondria: site of aerobic respiration; produces ATP (energy currency of the cell).
• Ribosomes: tiny structures where proteins are made (translation).
• Cell membrane: partially permeable; controls what enters and exits the cell.
• Chloroplasts: contain chlorophyll; the site of photosynthesis (plant/algae only).
• Vacuole: stores cell sap (sugars, minerals); maintains turgor pressure in plant cells.
• Cell wall: gives rigid support and shape to plant cells; made of cellulose.

2.3 — Prokaryotic vs Eukaryotic Cells

• Eukaryotic: has a membrane-bound nucleus and membrane-bound organelles (e.g., animals, plants, fungi).
• Prokaryotic: NO membrane-bound nucleus; NO membrane-bound organelles (e.g., bacteria).
• Bacteria have: cell wall (peptidoglycan, not cellulose), plasmids, ribosomes (smaller), cell membrane.

2.4 — Cell Division

*Mitosis:* for growth and repair.

• Produces 2 genetically IDENTICAL daughter cells.
• Chromosome number maintained (diploid → diploid).
• Stages: Prophase → Metaphase → Anaphase → Telophase → Cytokinesis.

*Meiosis:* for sexual reproduction (produces gametes).

• Produces 4 genetically DIFFERENT cells.
• Chromosome number halved (diploid → haploid).
• Occurs in: ovaries (eggs) and testes (sperm).

2.5 — Transport Across Membranes

• Diffusion: movement of particles from HIGH to LOW concentration. Passive (no energy needed). e.g., oxygen from lungs into blood.
• Osmosis: movement of WATER from high water potential (dilute) to low water potential (concentrated) through a partially permeable membrane.
• Active transport: movement AGAINST a concentration gradient. Requires energy (ATP). e.g., glucose absorbed in small intestine.

**3. Worked Examples**

Example 1: Explain why plant cells do not burst when placed in pure water.

*Answer:* Water enters by osmosis. But the rigid cellulose cell wall prevents unlimited expansion. The cell becomes *turgid* — firm but not bursting. Animal cells lack cell walls so they burst (lyse) in pure water.

Example 2: A student looks at cheek cells and onion cells under a microscope. List TWO features they would only see in onion cells.

*Answer:* (1) Cell wall. (2) Large central vacuole. (Chloroplasts would not be visible in onion cells — they are non-green.)

Example 3: Why are mitochondria called the "powerhouse of the cell"?

*Answer:* Mitochondria carry out aerobic respiration, producing ATP. ATP provides the energy needed for all cellular processes — active transport, protein synthesis, muscle contraction.

**4. Pakistan Angle**

Pakistan's textile industry depends on cotton — and cotton fibres are the elongated cell walls of seed hair cells of the *Gossypium* plant. Punjab is the world's 4th largest cotton producer. Each cotton fibre is a single dead plant cell, valued precisely because of its cellulose cell wall structure. Understanding cell biology literally underpins Pakistan's largest export industry.

PEMRA-registered pharmaceutical companies in Karachi (like Searle, AGP) produce antibiotics that work by targeting bacterial cell walls — penicillin destroys the peptidoglycan cell wall of bacteria, causing them to burst. Knowing the difference between prokaryotic and eukaryotic cells is what makes this drug effective without harming human cells.

**5. Exam Strategy**

• Mitosis vs Meiosis: examiners love asking you to compare. Key differences: number of cells produced (2 vs 4), genetic diversity (identical vs different), chromosome number (maintained vs halved).
• Osmosis: always say "water potential" not just "concentration" — examiners want the correct term.
• Organelle functions: know at least mitochondria (respiration/ATP), ribosome (protein synthesis), nucleus (DNA/control), chloroplast (photosynthesis).
• Prokaryotes: bacteria do have ribosomes — just smaller ones (70S vs 80S in eukaryotes).
• Draw and label diagrams neatly — 1 mark per correctly labelled structure.