Cell Biology — The Fundamental Unit of Life

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Cell biology is a foundational topic for INI CET — questions appear regularly and a thorough understanding of cell structure and function is essential for all medical specialities. Focus on the differences between prokaryotic and eukaryotic cells, cell organelles, and the fluid mosaic model of the plasma membrane.

High-Yield Facts for INI CET:

• Mitochondria: Double membrane, ATP production via oxidative phosphorylation; own DNA (maternal inheritance)
• Endoplasmic reticulum: Rough (ribosome-studded, protein synthesis) vs Smooth (lipid synthesis, detoxification)
• Golgi apparatus: Modification, packaging, and transport of proteins
• Nucleus: Contains genetic material (DNA), nucleolus (rRNA synthesis), nuclear envelope with pores

⚡ Exam tip: INI CET frequently asks about the fluid mosaic model of membrane structure — be ready to identify which components are amphipathic (hydrophilic heads + hydrophobic tails). Also know the differences between cell organelles found in prokaryotes vs eukaryotes.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

The Cell — Structure and Function:

Prokaryotes vs Eukaryotes

Prokaryotic cells (bacteria, archaea):

• No membrane-bound nucleus — genetic material in nucleoid region (circular DNA)
• No membrane-bound organelles
• Smaller (0.2-2 μm diameter)
• 70S ribosomes (30S + 50S subunits)
• Cell wall present (peptidoglycan in bacteria)
• Binary fission for reproduction

Eukaryotic cells (animals, plants, fungi, protists):

• Membrane-bound nucleus containing DNA
• Membrane-bound organelles (mitochondria, ER, Golgi, lysosomes, peroxisomes)
• Larger (10-100 μm diameter)
• 80S ribosomes in cytoplasm (70S in organelles — mitochondria and chloroplasts have their own)
• May have cell wall (plants, fungi)

The Plasma Membrane

Fluid Mosaic Model (Singer & Nicolson, 1972):

• The membrane is a dynamic structure — phospholipid bilayer with proteins embedded or attached
• Phospholipids: Amphipathic molecules — hydrophilic (water-loving) phosphate heads face outward, hydrophobic (water-fearing) fatty acid tails face inward
• Cholesterol: Inserts between phospholipids — increases rigidity at high temperatures, prevents excessive fluidity at low temperatures
• Proteins: Integral proteins span the membrane; peripheral proteins attach to surface. Functions: receptors, transporters, enzymes, cell adhesion

Membrane Transport:

• Simple diffusion: Non-polar molecules (O₂, CO₂) cross without assistance
• Facilitated diffusion: Channel proteins or carrier proteins allow polar molecules (glucose, ions) to cross; no energy required
• Active transport: Pumps (e.g., Na⁺/K⁺ ATPase) use ATP to move substances against concentration gradient
• Vesicular transport: Exocytosis (secretion out) and endocytosis (phagocytosis, pinocytosis — substances enter cell)

Cell Junctions:

• Tight junctions: Seal adjacent cells — prevent paracellular movement (e.g., intestinal epithelial cells)
• Desmosomes: Spot welds — provide mechanical strength (e.g., cardiac muscle)
• Gap junctions: Allow direct cytoplasmic communication between cells (connexins form channels)

Key Organelles

Nucleus:

• Nuclear envelope: Double membrane with nuclear pores
• Nucleoplasm: Gel-like matrix containing chromatin
• Chromatin: DNA + histone proteins (forms chromosomes when condensed)
• Nucleolus: Site of rRNA synthesis; disappears during cell division

Mitochondria:

• Double membrane: Outer smooth membrane, inner folded into cristae (increases surface area)
• Matrix: Contains mitochondrial DNA (circular), ribosomes (70S), Krebs cycle enzymes
• Cristae: Site of electron transport chain and oxidative phosphorylation
• Function: ATP production — “powerhouse of the cell”
• Maternal inheritance (mtDNA transmitted from mother)

Endoplasmic Reticulum (ER):

• Rough ER: Ribosome-studded; protein synthesis and modification; continuous with nuclear envelope
• Smooth ER: No ribosomes; lipid synthesis, steroid hormone synthesis, Ca²⁺ storage, detoxification (liver cells)
• Protein synthesis → rough ER → modification → Golgi apparatus → secretion

Golgi Apparatus:

• Stack of flattened membrane sacs (cisternae)
• Receives proteins from ER at cis face (forming face)
• Modifies, sorts, and packages proteins at trans face (maturing face)
• Forms lysosomes, secretory vesicles, and transport vesicles

Lysosomes:

• Membrane-bound vesicles containing hydrolytic enzymes (acid hydrolases)
• pH ~5 inside (acidic)
• Functions: Intracellular digestion (phagocytosis, autophagy), macromolecule recycling

Cytoskeleton:

• Microfilaments (actin): 7 nm diameter; cell movement, muscle contraction, cell shape
• Intermediate filaments: 10 nm diameter; mechanical strength (keratin, vimentin, desmin)
• Microtubules: 25 nm diameter; hollow tubes of tubulin; cell division (spindle fibres), intracellular transport (via motor proteins dynein and kinesin)

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Detailed Cell Biology for INI CET:

The Cell Membrane — Beyond the Basics

Membrane Protein Functions:

1. Transport proteins: Channels (ion channels, aquaporins for water), carriers (facilitated diffusion or active transport)
2. Enzymatic activity: Membrane-bound enzymes (e.g., peptidases in brush border)
3. Receptors: Bind signalling molecules (hormones, neurotransmitters) — triggers intracellular response
4. Cell adhesion: CAMs (cell adhesion molecules) — selectins, integrins, Ig superfamily
5. Identity markers: Glycoproteins and glycolipids on cell surface (glycocalyx) — important for immune recognition

Types of Membrane Transport:

• Osmosis: Diffusion of water across a semipermeable membrane
• Tonicity: Isotonic (equal solute concentration), hypertonic (higher solute), hypotonic (lower solute)
  • Animal cells in hypertonic solution → crenation (shrinkage)
  • Plant cells in hypertonic solution → plasmolysis (cell membrane pulls away from cell wall)
  • Plant cells in hypotonic solution → turgidity (full rigidity); animal cells in hypotonic → lysis
• Bulk transport: Vesicle-mediated — endocytosis (phagocytosis, pinocytosis, receptor-mediated) and exocytosis

The Cytoskeleton in Detail

Motor Proteins:

• Dynein: Moves cargo toward minus end of microtubules (retrograde transport); also forms axonemal dynein in cilia/flagella
• Kinesin: Moves cargo toward plus end of microtubules (anterograde transport)
• Myosin: Works with actin filaments; muscle contraction (thick filament); also involved in cytokinesis and cell migration

Centrosome and Centrioles:

• Centrosome: Main microtubule-organising centre (MTOC) in animal cells; contains pair of centrioles
• Centrioles: Barrel-shaped structures; form basal bodies of cilia and flagella; participate in organisation of spindle apparatus during cell division

Cell Division:

• Mitosis: Somatic cell division — produces two identical daughter cells
  • Stages: Prophase → Metaphase → Anaphase → Telophase → Cytokinesis
  • Chromosomes align at metaphase plate during metaphase
  • Separated sister chromatids move to opposite poles during anaphase
• Meiosis: Germ cell division — produces four genetically unique haploid gametes
  • Two rounds of division: Meiosis I (homologous chromosome pairs separate) + Meiosis II (sister chromatids separate)

Apoptosis vs Necrosis:

• Apoptosis: Programmed cell death — energy-dependent, cell shrinks, nuclear condensation, membrane blebbing, phagocytosis (no inflammation)
  • Triggered by: DNA damage, growth factor withdrawal, Fas ligand, mitochondrial pathway (cytochrome c release)
  • Caspase cascade (cysteine-aspartic proteases) — initiator caspases (8, 9) → executioner caspases (3, 6, 7)
• Necrosis: Uncontrolled cell death — energy-independent, cell swells, membrane rupture, inflammation

Cell Cycle Checkpoints:

• G1 checkpoint (Restriction point): Checks cell size, nutrients, DNA integrity before S phase
• G2/M checkpoint: Ensures DNA replication is complete before mitosis
• Spindle assembly checkpoint: Ensures all chromosomes are properly attached to spindle before anaphase

Cancer and Cell Cycle:

• Carcinogenesis involves loss of cell cycle control
• Oncogenes (e.g., RAS, MYC) — promote cell proliferation when activated
• Tumour suppressor genes (e.g., p53, RB) — normally inhibit proliferation; loss of function leads to uncontrolled growth
• p53: “Guardian of the genome” — arrests cell cycle at G1/S checkpoint, promotes DNA repair, triggers apoptosis if DNA damage is irreparable

Special Cell Types:

• Stem cells: Self-renewal + differentiation potential; totipotent (zygote), pluripotent (ESC), multipotent (adult stem cells)
• Neurons: Highly specialised; amitotic (cannot divide in adults); longest cells can be up to 1 metre
• Red blood cells: Enucleated (no nucleus); biconcave disc shape; contain haemoglobin; survive ~120 days; produced in bone marrow