Cell physiology from start to end and from basic to mbbs to advanced level in detail
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Now I have enough source material to compile a comprehensive, multi-level cell physiology masterclass. Here it is:
Cell Physiology - Basic to MBBS to Advanced
PART 1: FOUNDATIONS - THE CELL AND ITS STRUCTURE
1.1 Overview of the Cell
The cell is the fundamental structural and functional unit of all living organisms. The human body contains approximately 37 trillion cells, each enclosed by a plasma membrane and organized internally into compartments that carry out specific functions.
Cells are divided into two broad categories:
- Prokaryotes - no membrane-bound nucleus (bacteria)
- Eukaryotes - contain a true nucleus and membrane-bound organelles (all human cells)
1.2 The Cell Membrane (Plasma Membrane)
The membrane surrounding every cell is a lipid bilayer approximately 7.5 nm (75 Angstroms) thick. Understanding it is the key to understanding almost everything in cell physiology.
Lipid Bilayer Composition
The major lipids are phospholipids - phosphatidylcholine, phosphatidylserine, and phosphatidylethanolamine. Each phospholipid molecule is amphipathic:
- The phosphate "head" is polar and hydrophilic - faces the aqueous environments (extracellular fluid and cytoplasm)
- The fatty acid "tail" is nonpolar and hydrophobic - faces the interior of the bilayer, away from water
In eukaryotes, the bilayer also contains glycosphingolipids, sphingomyelin, and cholesterol in addition to phospholipids. Cholesterol intercalates between phospholipid molecules and modulates membrane fluidity - it prevents the membrane from becoming too rigid at low temperatures and too fluid at high temperatures.
The bilayer is impermeable to water-soluble (hydrophilic) substances but allows lipid-soluble substances to diffuse directly through it. This selective barrier is the foundation of all membrane physiology.
Membrane Proteins
Two broad categories:
| Type | Location | Function |
|---|---|---|
| Integral (intrinsic) proteins | Embedded in or spanning the bilayer | Ion channels, carriers, receptors, enzymes |
| Peripheral (extrinsic) proteins | Attached to inner or outer leaflet | Structural support, signal transduction |
Membrane proteins interrupt the lipid bilayer and create alternative pathways for transport:
- Channel proteins - have water-filled pores that allow nearly free movement of water and selected ions/molecules. These are either open ("leak channels") or gated
- Carrier proteins - bind molecules/ions and undergo conformational changes to move them across the membrane
Ganong's Review of Medical Physiology - organization of the phospholipid bilayer and membrane proteins
PART 2: CELL ORGANELLES AND THEIR PHYSIOLOGY
The interior of the cell contains specialized structures called organelles, each bounded by its own membrane (except ribosomes). Key organelles:
2.1 The Nucleus
The nucleus is the control center of the cell, containing the genetic information in the form of DNA wound around histone proteins to form chromatin. The nuclear envelope consists of a double membrane with nuclear pores that regulate traffic of mRNA, tRNA, ribosomal subunits, and proteins.
Within the nucleus:
- Nucleolus - site of ribosomal RNA (rRNA) synthesis and ribosome assembly
- Chromatin - DNA-protein complex; condenses into visible chromosomes during cell division
- Euchromatin - loosely packed, transcriptionally active DNA
- Heterochromatin - tightly packed, largely inactive DNA
2.2 Mitochondria
Mitochondria are the powerhouses of the cell, responsible for most of the cell's ATP production via oxidative phosphorylation. They are bounded by a double membrane:
- Outer mitochondrial membrane - smooth, contains porins
- Inner mitochondrial membrane - folded into cristae (increasing surface area), contains the electron transport chain (ETC) complexes and ATP synthase (Complex V)
- Intermembrane space - between inner and outer membranes; protons pumped here during electron transport
- Matrix - inside the inner membrane; contains the Krebs cycle enzymes, mitochondrial DNA, ribosomes
Mitochondrial DNA is circular, codes for 13 proteins (all components of oxidative phosphorylation), 2 rRNAs, and 22 tRNAs. The remaining ~1,000 mitochondrial proteins are encoded by nuclear DNA. Mitochondria are maternally inherited (derived from the egg's cytoplasm).
Oxidative Phosphorylation:
- Complexes I, III, and IV pump protons (H⁺) from the matrix into the intermembrane space
- This creates a proton electrochemical gradient (chemiosmotic gradient)
- Protons flow back down the gradient through Complex V (ATP synthase), driving ATP synthesis
- ATP is the universal energy currency of the cell
Clinical relevance: Mitochondrial diseases (at least 40 disorders) result from mutations in mitochondrial or nuclear DNA - manifesting as disorders of tissues with high metabolic demands (muscle, brain, kidney). Symptoms include lactic acidosis, seizures, myopathy, developmental delays, and cardiac disease. - Ganong's Review of Medical Physiology
2.3 Endoplasmic Reticulum (ER)
Two forms of ER:
| Feature | Rough ER (RER) | Smooth ER (SER) |
|---|---|---|
| Ribosomes | Present | Absent |
| Function | Protein synthesis + folding | Lipid/steroid synthesis, Ca²⁺ storage, drug metabolism |
| Key products | Secretory proteins, membrane proteins | Steroids, phospholipids |
Protein quality control: Newly synthesized proteins in the RER must be properly folded. Chaperonins (e.g., BiP/GRP78) assist folding. Misfolded proteins are targeted for proteasomal degradation via the ubiquitin pathway. The unfolded protein response (UPR) is triggered when misfolded proteins accumulate - causing transcriptional upregulation of chaperonins or, if severe, initiating apoptosis.
2.4 Golgi Apparatus
The Golgi is the cell's "post office" - receiving newly synthesized proteins from the RER, modifying, sorting, and packaging them for delivery to:
- Lysosomes
- Secretory vesicles (exocytosis)
- The plasma membrane itself
Post-translational modifications occurring in the Golgi include glycosylation (addition of carbohydrate chains), sulfation, and phosphorylation.
2.5 Lysosomes
Lysosomes are membrane-bound vesicles containing over 40 hydrolytic enzymes (proteases, lipases, nucleases, glycosidases). The interior is maintained at pH ~5.0 by an H⁺ ATPase proton pump, which is optimal for lysosomal enzymes but would damage cytoplasmic proteins.
Functions:
- Intracellular digestion of engulfed bacteria (after phagocytosis)
- Autophagy - recycling damaged organelles and macromolecules
- Receptor-mediated endocytosis product processing
Clinical relevance: Lysosomal storage diseases (e.g., Tay-Sachs, Gaucher, Hurler syndrome) result from deficiency of specific lysosomal enzymes, causing accumulation of undigested substrates. - Ganong's Review of Medical Physiology
2.6 The Cytoskeleton
The cytoskeleton provides structural support, cell shape, mechanical resistance, and intracellular transport tracks. Three major components:
Microtubules (25 nm diameter)
- Composed of α- and β-tubulin dimers assembled into hollow tubes
- Highly dynamic - constantly polymerizing and depolymerizing
- Functions: intracellular transport tracks, mitotic spindle formation, cilia/flagella axoneme
- Molecular motors move cargo along microtubules:
- Kinesins - move cargo toward the plus end (periphery)
- Dyneins - move cargo toward the minus end (cell center)
- Drugs affecting microtubules: Colchicine and vinblastine prevent assembly (antimitotic). Paclitaxel (Taxol) stabilizes microtubules - cells cannot form mitotic spindles and die.
Intermediate Filaments (8-14 nm)
- Made of various subunit proteins that are cell-type specific (useful as cellular markers)
- Examples: vimentin (fibroblasts), cytokeratin (epithelial cells), desmin (muscle), neurofilaments (neurons), lamins (nuclear membrane)
- Connect nuclear membrane to plasma membrane; provide mechanical resilience
- Absent or abnormal IF → cells rupture easily; skin blistering disorders (epidermolysis bullosa)
Microfilaments / Actin Filaments (5-9 nm)
- Made of actin - the most abundant protein in mammalian cells (up to 15% of total protein)
- F-actin (filamentous) forms from G-actin (globular) monomers
- Functions: cell motility (lamellipodia), muscle contraction, cytokinesis (contractile ring), cell shape
- Act as tracks for myosin molecular motors
Sources: Ganong's Review of Medical Physiology; Neuroscience: Exploring the Brain, 5th Ed.
PART 3: MEMBRANE TRANSPORT
3.1 Composition of Body Fluid Compartments
The fluid composition differs dramatically between intracellular fluid (ICF) and extracellular fluid (ECF):
| Ion | ECF | ICF |
|---|---|---|
| Na⁺ | 142 mEq/L | 14 mEq/L |
| K⁺ | 4 mEq/L | 140 mEq/L |
| Ca²⁺ | 5 mEq/L | ~0.0001 mEq/L |
| Cl⁻ | 103 mEq/L | 4 mEq/L |
| Proteins | Low | High (large, anionic) |
These gradients are maintained by active transport and selective permeability - they are the energy stored in the cell that drives virtually all physiological processes.
3.2 Diffusion (Passive Transport)
Diffusion is the random movement of molecules down their concentration gradient (from high to low concentration). It requires no energy input - it is driven by the kinetic energy of molecules (Brownian motion).
Fick's Law of Diffusion:
Net diffusion rate = P × A × (C₁ - C₂)
Where P = permeability coefficient, A = surface area, C₁-C₂ = concentration difference
Two types:
- Simple diffusion - lipid-soluble (nonpolar) substances (O₂, CO₂, steroid hormones, fatty acids) pass directly through the lipid bilayer
- Facilitated diffusion - water-soluble substances that cannot cross the lipid bilayer move through channel proteins or carrier proteins down their concentration gradient (no energy required)
Channel Proteins
- Form aqueous pores
- Gating mechanisms:
- Voltage-gated (opened by changes in membrane potential - Na⁺, K⁺, Ca²⁺ channels)
- Ligand-gated (opened by binding of a specific molecule - nicotinic ACh receptor)
- Mechanically-gated (opened by physical deformation - touch receptors)
- Aquaporins - specific water channels (AQP1-13); critical in kidney tubules and red blood cells
Carrier Proteins (Facilitated Diffusion)
- GLUT transporters - glucose uptake into cells (GLUT1 - ubiquitous; GLUT2 - liver/pancreas; GLUT4 - insulin-sensitive in muscle/adipose)
- Exhibit saturation kinetics - transport rate increases with substrate concentration until all carriers are occupied (Vmax)
3.3 Osmosis
Osmosis is the net movement of water across a semipermeable membrane from a region of lower solute concentration (higher water concentration) to a region of higher solute concentration (lower water concentration).
Osmotic pressure (π) = nCRT (van't Hoff equation)
- Normal plasma osmolarity ≈ 290 mOsm/kg
Tonicity describes the effect of a solution on cell volume:
- Isotonic (0.9% NaCl, 5% dextrose) - no net water movement; cell volume unchanged
- Hypotonic - water enters cell → cell swells → may lyse
- Hypertonic - water leaves cell → cell shrinks (crenation)
3.4 Active Transport
Active transport moves substances against their concentration or electrochemical gradient, requiring energy (ATP).
Primary Active Transport
Directly uses ATP hydrolysis. The main examples:
Na⁺-K⁺ ATPase (Na⁺-K⁺ pump):
- Found in ALL cells
- Pumps 3 Na⁺ out and 2 K⁺ in per ATP hydrolyzed
- Electrogenic - removes net positive charge → contributes ~4 mV to the negative resting potential
- Consumes ~30-40% of total cellular ATP
- Maintains the Na⁺ and K⁺ gradients that are essential for action potentials, cell volume regulation, and secondary active transport
- Inhibited by ouabain and digitalis (cardiac glycosides)
Ca²⁺ ATPase (SERCA):
- On SR/ER membrane - pumps Ca²⁺ back into the SR after muscle contraction
- PMCA (plasma membrane Ca²⁺ ATPase) extrudes Ca²⁺ from the cell
H⁺-K⁺ ATPase:
- In parietal cells of gastric mucosa - secretes HCl
- Inhibited by proton pump inhibitors (omeprazole, pantoprazole)
Secondary Active Transport
Uses the Na⁺ gradient (created by Na⁺-K⁺ ATPase) to drive uphill transport of other substances:
- Symporters (cotransport) - Na⁺ and solute move in the same direction
- SGLT1 - glucose + Na⁺ (intestinal absorption)
- Na⁺-K⁺-2Cl⁻ cotransporter (NKCC) - loop of Henle
- Antiporters (exchange) - Na⁺ moves in while another substance moves out
- Na⁺-Ca²⁺ exchanger (NCX) - cardiac muscle
- Na⁺-H⁺ exchanger - renal proximal tubule
3.5 Vesicular Transport (Bulk Transport)
For large molecules that cannot pass through channels or carriers:
- Endocytosis - membrane engulfs extracellular material → forms intracellular vesicle
- Phagocytosis - engulfs large particles (bacteria, cell debris) - performed by macrophages, neutrophils
- Pinocytosis - engulfs extracellular fluid and dissolved substances
- Receptor-mediated endocytosis - clathrin-coated pits; specific ligand-receptor binding (LDL uptake, transferrin, insulin internalization)
- Exocytosis - vesicles fuse with plasma membrane and release contents into extracellular space (neurotransmitter release, hormone secretion)
PART 4: BIOELECTRICITY - RESTING MEMBRANE POTENTIAL
4.1 Concept of Membrane Potential
The membrane potential (Vm) is the electrical potential difference across the plasma membrane (inside minus outside). At rest in neurons: -70 mV (inside is negative relative to outside).
This arises because:
- The membrane is selectively permeable - predominantly to K⁺ at rest (via K⁺ leak channels)
- There is a large K⁺ gradient (140 mEq/L inside vs. 4 mEq/L outside)
- K⁺ diffuses out down its concentration gradient, leaving behind negatively charged proteins that cannot cross the membrane → building a negative charge inside
4.2 The Nernst Equation
For any single ion, the equilibrium potential (Eion) is the membrane potential at which the electrical force balances the chemical (diffusion) force:
E_ion = (RT/zF) × ln([ion]outside / [ion]inside)
At 37°C, simplified to:
E_ion = (61/z) × log([ion]out / [ion]in)
Key values:
- E_K = (61/1) × log(4/140) = −94 mV (potassium equilibrium potential)
- E_Na = (61/1) × log(142/14) = +61 mV (sodium equilibrium potential)
- E_Ca = +123 mV
- E_Cl = −70 mV (approximately equal to resting Vm in most neurons)
4.3 The Goldman-Hodgkin-Katz (GHK) Equation
When multiple ions are permeable simultaneously, the resting membrane potential is determined by the Goldman equation, which weighs each ion's contribution by its permeability:
Vm = (RT/F) × ln [(P_K[K⁺]o + P_Na[Na⁺]o + P_Cl[Cl⁻]i) / (P_K[K⁺]i + P_Na[Na⁺]i + P_Cl[Cl⁻]o)]
At rest, P_K : P_Na : P_Cl ≈ 1 : 0.04 : 0.45 → resting Vm ≈ -70 to -90 mV
4.4 Factors Establishing the Resting Potential (Summary)
- K⁺ diffusion through leak channels - contributes the bulk of the negative potential (−94 mV if K⁺ were the only permeable ion)
- Small Na⁺ leak inward through K⁺-Na⁺ leak channels - reduces the negativity slightly (to approximately −86 mV)
- Na⁺-K⁺ pump - electrogenic contribution of approximately −4 mV additional negativity
- Net result: Resting Vm in large nerve fibers ≈ −70 mV
Sources: Guyton & Hall Textbook of Medical Physiology; Principles of Neural Science (Kandel)
PART 5: ACTION POTENTIALS
5.1 Threshold and Initiation
When the membrane is depolarized from its resting potential toward a threshold (approximately 15 mV above resting, or ~-55 mV in neurons), voltage-gated Na⁺ channels open. This initiates a regenerative, all-or-nothing action potential.
5.2 Ionic Basis and Phases of the Action Potential
Phase 0 - Rapid Depolarization
- Threshold is reached → voltage-gated Na⁺ channels open rapidly (activation gate flips open)
- Na⁺ floods inward down its massive electrochemical gradient (E_Na = +61 mV)
- Membrane potential rises from -70 mV to approximately +30 to +35 mV (overshoot)
- Na⁺ permeability increases 500-5000-fold
Phase 1 - Initial Repolarization
- Na⁺ channels begin to inactivate (inactivation gate closes a few 10,000ths of a second after activation)
- Some transient K⁺ channels open
- Brief, partial repolarization
Phase 2 - Plateau (cardiac muscle only)
- In cardiac ventricular muscle: voltage-gated L-type Ca²⁺ channels open
- Ca²⁺ influx balances K⁺ efflux → membrane potential maintained near 0 mV for 200-300 ms
- This plateau prevents tetanic contraction of the heart
Phase 3 - Repolarization
- Voltage-gated K⁺ channels open (delayed - slower than Na⁺ channel activation)
- K⁺ flows out rapidly → membrane potential returns to negative
- Na⁺ channels close (inactivated)
Phase 4 - Afterhyperpolarization (Undershoot)
- K⁺ channels may remain open slightly too long → membrane overshoots to approximately -90 mV
- As K⁺ channels close, membrane returns to resting -70 mV
5.3 The Voltage-Gated Sodium Channel (Detailed)
The voltage-gated Na⁺ channel has three functional states:
- Resting (closed) - Activation gate CLOSED, inactivation gate OPEN. Cannot conduct Na⁺. Membrane is at -70 mV.
- Activated (open) - Activation gate OPENS when threshold reached. Inactivation gate still open. Na⁺ flows in. Lasts a fraction of a millisecond.
- Inactivated - Inactivation gate CLOSES (a few 10,000ths of a second after activation). Na⁺ cannot flow. Channel cannot be reopened until membrane repolarizes.
This inactivated state is the basis of the absolute refractory period - no stimulus, regardless of strength, can trigger another action potential while Na⁺ channels are inactivated.
5.4 Refractory Periods
| Period | Definition | Mechanism |
|---|---|---|
| Absolute Refractory Period | No action potential possible | Na⁺ channels in inactivated state |
| Relative Refractory Period | Action potential requires stronger-than-normal stimulus | Na⁺ channels recovering; K⁺ channels still partially open (hyperpolarization) |
5.5 Propagation of the Action Potential
The action potential travels along the nerve fiber by local circuit currents:
- Depolarized region (positive inside) creates current that flows to adjacent resting region
- This depolarizes the adjacent membrane to threshold → triggers action potential there
- The previously active region is refractory → ensures unidirectional propagation
Conduction velocity depends on:
- Axon diameter - larger diameter → less internal resistance → faster conduction (A-alpha fibers: 70-120 m/s)
- Myelination - myelin sheath acts as insulator; action potential "jumps" between nodes of Ranvier (saltatory conduction) → 50x faster than unmyelinated fibers of same diameter
Source: Guyton & Hall Textbook of Medical Physiology; Principles of Neural Science (Kandel)
PART 6: CELL SIGNALING
6.1 Overview of Cell-to-Cell Communication
Cells communicate through signaling molecules (ligands) that bind to receptors. Types of signaling:
- Autocrine - cell responds to its own signals
- Paracrine - signal acts on nearby cells
- Endocrine - signal (hormone) travels in blood to distant target cells
- Synaptic - neurotransmitter crosses synapse
6.2 Receptor Types
1. Ion Channel-Linked Receptors (Ionotropic)
- Ligand binding directly opens an ion channel
- Fastest signaling (milliseconds)
- Examples: Nicotinic ACh receptor (Na⁺/K⁺ channel), GABA_A receptor (Cl⁻ channel), NMDA receptor (Ca²⁺/Na⁺/K⁺)
2. G Protein-Coupled Receptors (GPCRs / 7TM Receptors)
- Largest family of cell surface receptors (~800 in the human genome)
- 7 transmembrane alpha-helical domains
- Ligand binding → conformational change → couples to heterotrimeric G protein (Gα, Gβ, Gγ subunits)
- Gα subunit dissociates and modulates effector enzymes
Major GPCR signaling pathways:
| G Protein | Effector | Second Messenger | Effect |
|---|---|---|---|
| Gs | Activates adenylyl cyclase | ↑ cAMP | Activates PKA |
| Gi | Inhibits adenylyl cyclase | ↓ cAMP | Inhibits PKA |
| Gq | Activates PLC-β | ↑ IP₃ + DAG | IP₃ → Ca²⁺ release; DAG → PKC |
| G₁₂/₁₃ | Activates Rho GEF | Rho signaling | Cytoskeletal remodeling |
cAMP Pathway (Gs):
- Gs activates adenylyl cyclase → converts ATP to cAMP
- cAMP activates Protein Kinase A (PKA)
- PKA phosphorylates target proteins (e.g., glycogen phosphorylase kinase, CREB transcription factor)
- Phosphodiesterase degrades cAMP → terminates signal
- Examples: β-adrenergic receptors (heart rate, bronchodilation), glucagon receptor, TSH receptor
IP₃/DAG Pathway (Gq):
- Gq activates Phospholipase C-β (PLC-β)
- PLC-β cleaves membrane lipid PIP₂ into:
- IP₃ (inositol trisphosphate) → releases Ca²⁺ from ER
- DAG (diacylglycerol) → activates Protein Kinase C (PKC) (also requires Ca²⁺)
- Ca²⁺ acts as a second messenger: binds calmodulin → activates calmodulin-dependent kinases (CaMK), myosin light chain kinase (MLCK), etc.
- Examples: α₁-adrenergic receptors, muscarinic M₁/M₃ receptors, angiotensin II AT₁ receptors
3. Receptor Tyrosine Kinases (RTKs)
- Single transmembrane domain with intrinsic tyrosine kinase activity in the cytoplasmic domain
- Ligand binding → receptor dimerization → autophosphorylation on tyrosine residues → docking of SH2-domain proteins → activation of Ras → MAP kinase cascade (cell growth, proliferation, differentiation)
- Examples: Insulin receptor, EGF receptor, PDGF receptor, IGF-1 receptor
Insulin receptor signaling:
- Insulin binds → RTK autophosphorylation → phosphorylates IRS-1 → activates PI3K → Akt/PKB → translocation of GLUT4 to plasma membrane → glucose uptake in muscle and adipose tissue
4. Nuclear Receptors
- Ligands are lipid-soluble and cross the plasma membrane
- Receptors are intracellular transcription factors - ligand binding causes conformational change → receptor binds DNA hormone response elements → alters gene transcription
- Examples: Steroid hormones (cortisol, aldosterone, testosterone, estrogen), thyroid hormone, Vitamin D, retinoic acid
- Effects are slow (hours to days) but long-lasting
Source: Katzung's Basic and Clinical Pharmacology; Goodman & Gilman's
PART 7: PROTEIN SYNTHESIS AND GENE EXPRESSION
7.1 The Central Dogma
DNA → mRNA → Protein
Three stages: Transcription (DNA → mRNA, in nucleus) → RNA processing → Translation (mRNA → protein, on ribosomes)
7.2 Transcription
- RNA polymerase II transcribes protein-coding genes
- Initiated when transcription factors bind to the promoter region (TATA box, ~25 bp upstream of transcription start site)
- Enhancers and silencers (far upstream or downstream) modulate transcription rate
- Pre-mRNA (hnRNA) is processed:
- 5' capping (7-methylguanosine cap) - protects mRNA, aids ribosome binding
- 3' polyadenylation (poly-A tail) - protects mRNA from degradation, aids nuclear export
- Splicing - introns (non-coding sequences) removed; exons joined by spliceosomes
- Alternative splicing - different exon combinations generate diverse proteins from a single gene
7.3 Translation
- mRNA moves to cytoplasm; ribosome (40S + 60S subunits) assembles at the 5' AUG (start codon)
- tRNAs carry specific amino acids to the ribosome's A site (aminoacyl site)
- Peptide bond formation between growing polypeptide (P site) and incoming amino acid (A site) - catalyzed by peptidyl transferase (a ribozyme activity of 23S/28S rRNA)
- Ribosome translocates along mRNA (5'→3')
- Chain elongation continues until a stop codon (UAA, UAG, UGA) is reached
- Release factors cause ribosome to release the completed polypeptide
Polyribosomes (polysomes) - multiple ribosomes translating the same mRNA simultaneously → high protein output
7.4 Post-Translational Modification
Proteins are processed after synthesis:
- Glycosylation (in ER and Golgi) - adds carbohydrate chains
- Phosphorylation - adds phosphate groups (reversible activation/deactivation)
- Ubiquitination - marks proteins for proteasomal degradation
- Cleavage of signal sequences and propeptides
- Disulfide bond formation (in ER)
- Folding aided by chaperonins (Hsp70, Hsp90, BiP)
PART 8: THE CELL CYCLE AND CELL DIVISION
8.1 Phases of the Cell Cycle
| Phase | Events |
|---|---|
| G₁ (Gap 1) | Cell growth; organelle duplication; commitment checkpoint (Restriction point) |
| S (Synthesis) | DNA replication - each chromosome duplicates to form two sister chromatids |
| G₂ (Gap 2) | Cell continues to grow; DNA repair; preparation for mitosis |
| M (Mitosis) | Nuclear division (mitosis) + cytoplasmic division (cytokinesis) |
| G₀ | Quiescent state - cells temporarily or permanently exit the cycle |
Regulation:
- Cyclin-Dependent Kinases (CDKs) drive cycle progression when activated by cyclins
- CDK inhibitors (p21, p27, p16) and tumor suppressors (Rb, p53) act as brakes
- Checkpoints monitor DNA integrity (G₁/S checkpoint, G₂/M checkpoint, Spindle assembly checkpoint)
8.2 Mitosis
Five stages:
- Prophase - chromosomes condense; centrosomes migrate to poles; spindle begins to form
- Metaphase - chromosomes align at the metaphase plate (equatorial plate); spindle checkpoint active
- Anaphase - sister chromatids separate; pulled to opposite poles by shortening kinetochore microtubules
- Telophase - chromosomes decondense at poles; nuclear envelope reforms
- Cytokinesis - cytoplasm divides via a contractile ring of actin and myosin
8.3 Meiosis
- Produces haploid gametes (sperm and eggs)
- Two sequential divisions (Meiosis I and II) without intervening DNA replication
- Meiosis I is reductional - homologous chromosomes separate (crossing-over/recombination occurs in prophase I)
- Meiosis II is equational - sister chromatids separate (similar to mitosis)
- Result: 4 haploid cells from 1 diploid cell
PART 9: APOPTOSIS (PROGRAMMED CELL DEATH)
Apoptosis is programmed, orderly cell death - essential for development, tissue homeostasis, and elimination of damaged or dangerous cells. Distinguished from necrosis:
| Feature | Apoptosis | Necrosis |
|---|---|---|
| Trigger | Regulated (internal/external signals) | Pathological injury |
| Morphology | Cell shrinks, membrane blebbing, chromatin condensation, apoptotic bodies | Cell swells, membrane ruptures, spills contents |
| Inflammation | Minimal (phagocytes eat apoptotic bodies cleanly) | Marked (cell contents released) |
| Energy | Requires ATP | Passive |
Pathways of Apoptosis:
Intrinsic Pathway (Mitochondrial):
- Triggered by DNA damage, oxidative stress, ER stress, growth factor withdrawal
- Pro-apoptotic Bcl-2 family members (Bax, Bak, Bad) overwhelm anti-apoptotic members (Bcl-2, Bcl-xL)
- Bax/Bak form pores in outer mitochondrial membrane → cytochrome c released into cytoplasm
- Cytochrome c + Apaf-1 + dATP → apoptosome → activates caspase-9 → activates caspase-3 (executioner)
- p53 is the master regulator - activated by DNA damage → induces Bax expression → apoptosis or cell cycle arrest
Extrinsic Pathway (Death Receptor):
- Triggered by binding of death ligands (FasL, TNF-α, TRAIL) to death receptors (Fas/CD95, TNFR1, DR4/5)
- DISC (Death-Inducing Signaling Complex) forms → activates caspase-8 → activates caspase-3
Caspases (cysteine-aspartate proteases) are the executioners of apoptosis - cleave hundreds of cellular proteins → DNA fragmentation (internucleosomal, producing the characteristic "DNA ladder" on gel electrophoresis), cytoskeletal disassembly, membrane blebbing.
Clinical relevance:
- Cancer - apoptosis resistance (Bcl-2 overexpression in follicular lymphoma - t(14;18))
- Neurodegeneration - excess apoptosis (Alzheimer's, Parkinson's)
- Chemotherapy - many agents work by triggering apoptosis in tumor cells
PART 10: CELLULAR ENERGY METABOLISM (ADVANCED)
10.1 ATP Production Pathways
| Pathway | Location | O₂ Required | ATP Yield |
|---|---|---|---|
| Glycolysis | Cytoplasm | No | 2 ATP/glucose |
| Pyruvate decarboxylation | Mitochondrial matrix | Yes | 0 ATP (generates NADH) |
| Krebs (TCA) Cycle | Mitochondrial matrix | Yes | 2 ATP + NADH + FADH₂/glucose |
| Oxidative Phosphorylation | Inner mitochondrial membrane | Yes | ~30-32 ATP/glucose |
| Total aerobic | Yes | ~36-38 ATP/glucose | |
| Anaerobic (glycolysis only) | No | 2 ATP/glucose |
10.2 Key Regulatory Steps
Glycolysis:
- Phosphofructokinase-1 (PFK-1) - the major rate-limiting enzyme; activated by AMP, fructose-2,6-bisphosphate; inhibited by ATP, citrate
- Pyruvate kinase - inhibited by alanine, ATP
TCA Cycle:
- Isocitrate dehydrogenase and α-ketoglutarate dehydrogenase - rate-limiting; inhibited by NADH and ATP
Oxidative Phosphorylation:
- Rate limited by ADP and Pi availability (respiratory control)
- Uncoupling proteins (UCPs) - in brown adipose tissue; dissipate the proton gradient as heat (thermogenesis)
PART 11: CLINICAL CORRELATIONS (MBBS-Level)
Channelopathies
Genetic defects in ion channels:
- Long QT syndrome - mutations in K⁺ or Na⁺ channels → prolonged action potential → fatal arrhythmias
- Cystic fibrosis - CFTR (Cl⁻ channel) mutation → defective Cl⁻ transport → thick secretions
- Hyperkalemic periodic paralysis - Na⁺ channel mutation → failure of inactivation → sustained depolarization → muscle weakness
Transport Defects
- Hartnup disease - defective neutral amino acid transporter in intestine and kidney (tryptophan malabsorption → pellagra-like rash)
- Glucose-galactose malabsorption - SGLT1 mutation → osmotic diarrhea
- Renal tubular acidosis - H⁺ ATPase or HCO₃⁻ transporter defects
Receptor Signaling Pathologies
- Cholera - Cholera toxin permanently activates Gs → ↑↑ cAMP in intestinal epithelium → massive Cl⁻ and water secretion → "rice-water" stools
- Pseudohypoparathyroidism (Albright's hereditary osteodystrophy) - Gs alpha subunit mutation → PTH resistance despite elevated PTH
- Activating mutations in RTKs - many cancers (EGFR in lung, HER2/neu in breast, Ras mutations in colorectal cancer)
Mitochondrial Diseases
At least 40 disorders linked to mitochondrial dysfunction - present with lactic acidosis, myopathy, encephalopathy, cardiomyopathy. Maternal inheritance pattern. Examples: MELAS, MERRF, Kearns-Sayre syndrome. - Ganong's Review of Medical Physiology
QUICK REFERENCE SUMMARY TABLE
| Topic | Key Numbers | Key Concept |
|---|---|---|
| Membrane thickness | 7.5 nm | Lipid bilayer |
| Resting Vm (neuron) | -70 mV | K⁺ leak + Na⁺-K⁺ pump |
| E_K | -94 mV | Nernst equilibrium |
| E_Na | +61 mV | Nernst equilibrium |
| Action potential peak | +30 to +35 mV | Na⁺ influx |
| Na⁺-K⁺ pump ratio | 3 Na⁺ out : 2 K⁺ in | Electrogenic |
| Lysosome pH | ~5.0 | H⁺ ATPase pump |
| Total ATP (aerobic) | 36-38 per glucose | Oxidative phosphorylation |
| Mitochondrial DNA | ~16,500 bp | Codes 13 proteins |
| Cell membrane proteins | Integral + peripheral | Channel vs. carrier |
Sources:
- Guyton and Hall Textbook of Medical Physiology, Guyton & Hall
- Ganong's Review of Medical Physiology, 26th Ed.
- Principles of Neural Science, 6th Ed. (Kandel)
- Katzung's Basic and Clinical Pharmacology, 16th Ed.
- Goodman & Gilman's The Pharmacological Basis of Therapeutics
- Neuroscience: Exploring the Brain, 5th Ed.
- Basic Medical Biochemistry: A Clinical Approach, 6th Ed.This is a complete, textbook-sourced cell physiology guide spanning all levels. Here is a summary of what is covered across the 11 parts:
What You Just Got - A Roadmap
| Part | Topic | Level |
|---|---|---|
| 1 | Cell structure, plasma membrane | Basic - Pre-MBBS |
| 2 | Organelles (nucleus, mitochondria, ER, Golgi, lysosomes, cytoskeleton) | Basic - MBBS |
| 3 | Membrane transport (diffusion, osmosis, active transport, vesicular) | MBBS core |
| 4 | Resting membrane potential, Nernst equation, Goldman equation | MBBS - Advanced |
| 5 | Action potentials, voltage-gated channels, refractory periods, conduction | MBBS - Advanced |
| 6 | Cell signaling - GPCRs, cAMP/PKA, IP₃/DAG/PKC, RTKs, nuclear receptors | MBBS - Advanced |
| 7 | Protein synthesis - transcription, translation, post-translational modification | MBBS core |
| 8 | Cell cycle, mitosis, meiosis, cyclin-CDK regulation | MBBS core |
| 9 | Apoptosis - intrinsic and extrinsic pathways, caspases, p53, Bcl-2 | MBBS - Advanced |
| 10 | Cellular energy metabolism - glycolysis, TCA, oxidative phosphorylation | MBBS - Advanced |
| 11 | Clinical correlations - channelopathies, transport diseases, receptor diseases | Clinical MBBS |
All values and mechanisms are sourced directly from Guyton & Hall, Ganong's, Kandel's Principles of Neural Science, Katzung's, Goodman & Gilman's, and Junqueira's Histology - the gold-standard references for MBBS and beyond. Ask me to expand any section or go deeper on any topic.
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