1. Cell Structure and Organelles

•    Nucleus

o    Contains DNA, site of transcription (mRNA synthesis).

o    Nuclear envelope: double membrane with nuclear pores.

•    Mitochondria

o    ATP production via oxidative phosphorylation.

o    Contains its own (maternal) DNA → basis for mitochondrial inheritance.

o    Role in apoptosis (cytochrome c release).

•    Endoplasmic Reticulum (ER)

o    Rough ER: ribosome-studded; protein synthesis for secretion or membrane.

o    Smooth ER: lipid synthesis, detoxification (especially in liver).

•    Golgi apparatus

o    Protein modification, sorting, packaging (glycosylation).

o    Produces lysosomes.

•    Lysosomes

o    Acidic environment (pH ~5).

o    Contain hydrolytic enzymes for degradation (e.g., in autophagy).

•    Other:

o    Peroxisomes: β-oxidation of very long-chain fatty acids, H₂O₂ breakdown.

2. Cell Cycle and Apoptosis

•    Phases

o    G₁: cell growth, pre-DNA synthesis.

o    S: DNA replication.

o    G₂: preparation for mitosis.

o    M: mitosis (prophase, metaphase, anaphase, telophase).

o    G₀: quiescent phase.

•    Checkpoints

o    G₁/S and G₂/M critical for DNA integrity.

o    p53 ("guardian of the genome"): halts cell cycle if DNA damaged; induces apoptosis if irreparable.

•    Apoptosis (programmed cell death)

o    Intrinsic (mitochondrial) and extrinsic (death receptor) pathways.

o    Caspase activation → cell dismantling without inflammation.

3. Membrane Structure and Transport

•    Phospholipid bilayer

o    Hydrophilic heads, hydrophobic tails.

o    Cholesterol: modulates fluidity.

•    Membrane proteins

o    Integral proteins: span membrane (e.g., channels, transporters).

o    Peripheral proteins: attach to membrane surface.

•    Transport mechanisms

o    Passive diffusion: small, nonpolar molecules (e.g., O₂, CO₂).

o    Facilitated diffusion: via channels/carriers; no energy (e.g., GLUT transporters for glucose).

o    Active transport: against gradient, requires ATP (e.g., Na⁺/K⁺ ATPase).

o    Secondary active transport: uses gradient (e.g., Na⁺-glucose cotransporter).

4. Ion Channels and Membrane Potentials

•    Resting membrane potential

o    ~–70 mV (neurons); maintained by Na⁺/K⁺ ATPase (3 Na⁺ out, 2 K⁺ in).

•    Action potential (AP)

o    Depolarisation: rapid Na⁺ influx (voltage-gated channels).

o    Repolarisation: K⁺ efflux.

o    Hyperpolarisation: transient overshoot due to continued K⁺ outflow.

•    Ca²⁺ channels

o    Important in muscle contraction, neurotransmitter release.

5. Signal Transduction

•    Receptors

o    G-protein coupled receptors (GPCRs): large family, involve heterotrimeric G proteins.

    E.g., β-adrenergic receptors (activate adenylate cyclase → ↑cAMP).

o    Tyrosine kinase receptors: e.g., insulin receptor.

•    Second messengers

o    cAMP: activates protein kinase A.

o    IP₃: releases Ca²⁺ from endoplasmic reticulum.

o    DAG: activates protein kinase C.

•    Examples of downstream effects

o    Smooth muscle relaxation: β₂ activation → ↑cAMP → MLCK inhibition.

o    Platelet activation: thromboxane A₂ and ADP → Ca²⁺ mobilization.

Extra Revision Pearls

•    Apoptosis vs necrosis: apoptosis = no inflammation; necrosis = cell lysis, inflammatory response.

•    Oncogenes often activate growth signals (e.g., RAS), tumour suppressors inhibit them (e.g., p53, RB).

•    GLUT4 translocation to cell surface (insulin-stimulated) is via vesicle fusion in muscle/adipose.

•    Inhibition of Na⁺/K⁺ ATPase by digoxin increases intracellular Ca²⁺ 

                                                        (via Na⁺/Ca²⁺ exchanger), increasing cardiac contractility.