Cell Membrane Physiology & Cellular Communication – Session 1 Comprehensive Notes

Cell Membrane – Structure & Composition

• Lipid bilayer forms fundamental barrier between extracellular fluid (ECF) & intracellular fluid (ICF)
  • Amphipathic phospholipids (~25 % of membrane mass)
  • Polar hydrophilic heads face aqueous media
  • Non-polar hydrophobic tails orient inward → create impermeable core for water–soluble solutes
  • Determine basic membrane permeability
  • Cholesterol (~13 %)
  • Typical 4-ring steroid structure
  • Inserts between phospholipid tails → ↓ membrane water-solubility, stabilises membrane, modulates fluidity (↓ at high T°, ↑ at low T°)
  • Other lipids (~4 %): glycolipids, sphingolipids, etc.
• Proteins (~55 %)
  • Integral proteins (span bilayer) & peripheral proteins (cytosolic / outer surface)
  • Functional categories (see dedicated section)
• Carbohydrates (~3 %)
  • Present only on outer surface → glycocalyx (glycolipids + glycoproteins)
  • “Identity molecules” – cell recognition, immunity, blood typing, receptor sites, water retention, inter-cell attachment (negative charge repels other negative surfaces)

Fluid-Mosaic Model Visualization

• Electron micrographs show dark–light–dark trilaminar appearance (hydrophilic–hydrophobic–hydrophilic)
• Short carbohydrate chains attach only on ECF side
• Dynamic: lipids & many proteins can laterally diffuse; cholesterol & temperature alter fluidity

Membrane Junctions

• Three mechanical contributors to tissue cohesion
  • Glycoprotein CAMs & wavy contours (minor)
  • 1. Tight junctions
  • Strands of claudin proteins fuse adjacent lateral membranes near apical border
  • Form impermeable “kiss sites” → no paracellular passage; forces selective trans-cellular transport (e.g., intestinal epithelium)
  • 2. Desmosomes (adhering / spot-rivets)
  • Plaques linked by cadherins; tie into keratin intermediate filaments → resist mechanical stress (skin, heart)
  • 3. Gap junctions (communicating)
  • Connexon hemichannels from each cell align → tunnels (Ø ≈ $1.5\,\text{nm}$)
  • Permit ions & small molecules; electrical coupling (cardiac & smooth muscle, embryo)

Functional Categories of Membrane Proteins

• Structural: cytoskeleton anchors, junctional proteins
• Transporters (detailed below)
• Enzymes (metabolic & signaling)
• Receptors (signal transduction, second-messenger generation, receptor-mediated endocytosis)
• Cell-adhesion molecules
• Communication (gap junction connexins)

Membrane Transport – Master Overview

• Passive (no ATP, no carrier energy input)
  • Simple diffusion
  • Facilitated diffusion via channels or carriers
• Active (energy-requiring)
  • Primary active transport – hydrolyses ATP directly
  • Secondary active transport – uses ion gradient (usually $Na^+$) created by primary pump

Passive Transport: Simple Diffusion

• Down electro-chemical gradient (↑→↓ concentration)
• Lipid-soluble &/or very small uncharged, non-polar molecules
  • $O<em>2$, $CO</em>2$, $N_2$, alcohols, steroid hormones
• Rate ∝ lipid solubility & concentration gradient

Passive Transport: Facilitated Diffusion

• Still passive (no ATP), but requires protein assistance
• Two subclasses
  • Channel proteins (fast, less selective)
  • Aquaporins (osmosis)
  • Leak channels (e.g., $K^+$ leak sets RMP)
  • Gated channels
    • Voltage-gated: $Na^+$, $K^+$, $Ca^{2+}$ (AP)
    • Ligand-gated: ACh nicotinic receptor, GABA, glutamate
    • Mechanical: stretch-activated nociceptor $Na^+$, hair cell channels
    • Thermal & light gated (thermoreceptors, photoreceptors)
  • Selectivity filter example: tetrameric $K^+$ channel – carbonyl oxygens strip hydration shell to fit dehydrated $K^+$
  • Carrier proteins (slower, highly specific; conformational saturation)
  • Example: GLUT-4 transports glucose into adipocytes & muscle in response to insulin
• Saturation kinetics: transport maximal at $T_{max}$; unlike linear simple diffusion

Active Transport

Primary Active Transport

• Carrier is an ATPase; moves solute uphill (↓→↑ concentration)
• Examples & stoichiometries
  • $\text{Na}^+/\text{K}^+$-ATPase: $3\,Na<em>{in}^{+}\rightarrow out$ & $2\,K</em>{out}^{+}\rightarrow in$ per ATP hydrolysed
  • $Ca^{2+}$-ATPase: pumps $Ca^{2+}$ out of cytosol into ECF (plasma mem.) or SR/ER (SERCA)
  • $H^+$-ATPase: parietal cells (stomach acid), renal intercalated cells (acid-base balance)
  • $H^+/K^+$-ATPase: gastric proton pump inhibited by PPIs

Detailed Anatomy of Na⁺/K⁺-ATPase

• Heterodimer $\alpha$ (catalytic; ATPase & binding sites) + $\beta$ (glycoprotein stabiliser)
• Cycle:
  1. Intracellular face: high affinity for 3 $Na^+$; ATP binding → phosphorylation
  2. Conformational flip releases $Na^+$ to ECF
  3. Exposed high-affinity sites bind 2 $K^+$ → de-phosphorylation
  4. Return to original state, releasing $K^+$ inside
• Functions
  • Maintains steep Na⁺ (low inside) & K⁺ (high inside) gradients
  • Contributes small electrogenic hyperpolarization (≈ $-5\,\text{mV}$ of RMP)
  • Controls osmotic equilibrium & cell volume
  • Essential for recovery of resting membrane potential after action potentials
• Regulatory factors
  • ↑ activity: high intracellular $Na^+$, insulin, thyroid hormone, aldosterone (↑ pump number), cAMP, DAG, arachidonic derivatives
  • ↓ activity: dopamine in kidney (via phosphorylation)
  • Pharmacologic inhibition: cardiac glycosides (digoxin) → ↑ intracellular $Na^+$ → ↓ $Na^+/Ca^{2+}$ exchange → ↑ $Ca^{2+}$ inside → positive inotropy

Secondary Active Transport

• Harnesses potential energy of Na⁺ gradient (created by Na⁺/K⁺-ATPase)
• Carrier simultaneously binds driving ion & solute
  • Symport (co-transport): both move same direction
  • SGLT-1/2 (Na⁺-glucose): intestinal & renal reabsorption (SGLT-2 inhibitors for diabetes)
  • Na⁺/amino-acid, Na⁺/K⁺/2Cl⁻ (loop of Henle; target of loop diuretics)
  • Antiport (counter-transport): opposite directions
  • Na⁺/Ca²⁺ exchanger (NCX) in most membranes; vital for cardiac Ca²⁺ extrusion
  • Na⁺/H⁺ exchanger in proximal tubule & many cells (stimulated by aldosterone)

Vectorial Transport Across Epithelia (Cellular Sheets)

• Example: intestinal glucose absorption
  1. Basolateral Na⁺/K⁺-ATPase creates Na⁺ gradient (primary active)
  2. Luminal SGLT imports Na⁺ (downhill) + glucose (uphill) (secondary active)
  3. Basolateral GLUT facilitates glucose exit (passive)
• Same vectorial principles in renal tubules, gall-bladder, choroid plexus, exocrine glands

Membrane Potentials

Definitions & Units

• Membrane potential $V_m$ = difference in charge across membrane (ICF relative to ECF) measured in millivolts (mV)
• All cells possess RMP; excitable tissues (neurons, muscles, gland, ciliated cells, macrophages) modulate $V_m$ for signalling

Resting Membrane Potential (RMP)

• Typical range: $-70\,\text{mV}$ (neurons) to $-90\,\text{mV}$ (skeletal muscle)
• Establishing forces
  1. Large $K^+$ leak outward via passive channels (membrane more permeable to $K^+$ than $Na^+$)
  2. Small $Na^+$ leakage inward (opposite direction)
  3. Electrogenic contribution of $Na^+/K^+$-ATPase (≈ $-5\,\text{mV}$)
  4. Fixed intracellular anions (proteins, phosphates) maintain electroneutrality
• Nernst equation (equilibrium potential $E<em>{ion}$): $E</em>{ion}=\frac{RT}{zF}\ln\left(\frac{[ion]<em>{out}}{[ion]</em>{in}}\right)$
• Goldman-Hodgkin-Katz equation combines permeabilities to predict $V_m$

Graded Potentials

• Local, decremental changes (depolarization or hyperpolarization)
• Magnitude proportional to stimulus; can summate (spatial & temporal)
• If threshold (~$-55\,\text{mV}$ at axon hillock) reached → triggers action potential

Action Potentials (AP)

• Definition: rapid (~1 ms), transient reversal of $V_m$ (~$-70 \rightarrow +30 \rightarrow -90 \rightarrow -70\,\text{mV}$)
• All-or-none, non-decremental; propagate along axon bidirectionally in theory but physiologically forward due to refractory period
• Ion channel choreography
  • Depolarization: voltage-gated $Na^+$ channels open fast (positive feedback)
  • Peak: $Na^+$ channels inactivate; voltage-gated $K^+$ channels open
  • Repolarization & slight hyperpolarization (after-potential): $K^+$ efflux
  • Return to RMP: $Na^+/K^+$-ATPase restores gradients
• Refractory periods
  • Absolute: another AP impossible (Na⁺ channels inactivated)
  • Relative: stronger-than-normal stimulus can elicit AP (some Na⁺ channels reset, K⁺ still elevated)
• Conduction velocity ↑ by myelin (saltatory conduction between nodes of Ranvier)

Initiation Triggers for APs

1. Mechanical disturbance (e.g., pressure on skin receptor)
2. Chemical neurotransmitters (synaptic input)
3. Direct electrical current (gap junctions in cardiac & visceral smooth muscle)

• Requirement: inward $Na^+$ current > outward $K^+$ current until threshold reached (≈ 15–30 mV depolarization)

Roles of Other Ions & Modulators

• $Ca^{2+}$ outside stabilises membrane; hypocalcaemia lowers threshold → tetany
• Impermeant intracellular anions help set RMP

Clinical / Pharmacological Connections

• Local anaesthetics (e.g., lidocaine) block voltage-gated $Na^+$ channels from cytoplasmic side → prevent AP initiation & pain transmission
• SGLT-2 inhibitors (dapagliflozin et al.) block renal Na⁺-glucose symport → glycosuria to treat diabetes
• Loop diuretics inhibit Na⁺/K⁺/2Cl⁻ symport in loop of Henle → natriuresis
• Cardiac glycosides (digoxin) inhibit Na⁺/K⁺-ATPase → ↑ cardiac contractility via Ca²⁺ handling
• Proton-pump inhibitors inhibit gastric $H^+/K^+$-ATPase → ↓ acid secretion

Numerical & Formula Highlights

• Na⁺/K⁺ pump stoichiometry: $\displaystyle 3Na^+<em>{in}\; + \; 2K^+</em>{out} \; + \; ATP \rightarrow 3Na^+<em>{out} \; + \; 2K^+</em>{in} \; + \; ADP \; + \; P_i$
• Resting neuron potentials: $V_{rest}\approx -70\,\text{mV}$; skeletal muscle $\approx -90\,\text{mV}$
• Threshold potential: $\approx -55\,\text{mV}$
• AP amplitude: $\approx 100\,\text{mV}$ (from $-70$ to $+30$)
• Channel diameter of gap junction connexon: $1.5\,\text{nm}$

Ethical & Practical Implications

• Understanding membrane transport underpins drug design (e.g., channel blockers, transporter inhibitors)
• Ion pump defects → disease (e.g., CFTR in cystic fibrosis, hereditary spherocytosis from cytoskeletal protein defects)
• Energy cost: $\approx 20–40\%$ of basal ATP consumption fuels Na⁺/K⁺-ATPase—relevant to hypoxia sensitivity in neurons

Connections to Previous & Future Topics

• Foundational for subsequent lectures on neurotransmission, muscle contraction, renal physiology, acid–base balance, endocrinology
• Upcoming sessions: detailed study of graded potentials & full kinetics of action potentials, plus synaptic physiology