Cell Physiology & Homeostasis – Quick Review

Introduction to Physiology & Homeostasis

  • Claude Bernard ➔ internal milieu must stay stable despite external change (homeostasis)
  • Homeostasis = “same-standing”; maintenance of nearly constant internal environment
    • Disease = altered homeostasis
  • Normal set-points: fasting glucose 80!–!100 mg/dL80!\text{–}!100\,\text{mg/dL}; blood pH 7.35!–!7.457.35!\text{–}!7.45; tight electrolyte ranges

Control Mechanisms

  • Negative feedback (major): response opposes stimulus (e.g., thyroid axis thermoregulation, baroreflex)
  • Positive feedback: amplifies stimulus; stops on endpoint (e.g., oxytocin → labor)
  • Feed-forward: anticipatory, adaptive (e.g., salivation before meal)
  • Elements: Receptor → Control Center → Effector; performance depends on sensor sensitivity & response speed

Body-Fluid Compartments

  • Total body water ≈ 60%60\% BW (newborn ∼75%\sim75\%)
    • 23\frac{2}{3} intracellular, 13\frac{1}{3} extracellular

Cell Structure & Composition

  • Regions: Nucleus | Cytoplasm | Plasma membrane
  • Protoplasm: Water 70!–!85%70!–!85\% | Proteins 10!–!20%10!–!20\% | Lipids ∼2%\sim2\% | Carbohydrates ∼1%\sim1\% | Ions

Major Organelles (key function/clinical)

  • rER: protein synthesis/modification
  • sER: lipid & steroid synthesis; detox; sarcoplasmic reticulum ⇢ Ca2+^{2+} store
  • Golgi: modify-package proteins (cis→trans)
  • Mitochondria: ATP via oxidative phosphorylation; own DNA
  • Lysosomes: hydrolytic digestion; Tay-Sachs = Hexosaminidase ↓
  • Peroxisomes: oxidative enzymes; Zellweger syndrome = absent peroxisomes

Cytoskeleton & Projections

  • Microfilaments (actin, 7 nm7\,\text{nm}): motility, microvilli core
  • Intermediate filaments (10 nm10\,\text{nm}): tensile strength (e.g., keratin)
  • Microtubules (25 nm25\,\text{nm}): vesicle/chromosome movement; motors kinesin ↔ dynein
  • Cilia (motile vs primary); Kartagener = dynein defect
  • Microvilli: ↑surface (intestine, PCT)
  • Stereocilia: long, non-motile (epididymis, inner ear)

Plasma Membrane

  • Composition: Protein 55%55\% | Phospholipid 25%25\% | Cholesterol 13%13\% | Other lipids 4%4\% | Carbohydrate 3%3\%
  • Fluidity ↑ with ↑T, ↑unsaturated FA; cholesterol buffers (↑fluidity low T, ↓fluidity high T)
Junctions (function)
  • Tight: seal, fence; regulate paracellular flow
  • Desmosome/Adherens: mechanical adhesion cell-cell; hemidesmosome cell-ECM
  • Gap junction: connexon channels for ions, low-resistance (e.g., heart)

Membrane Transport Proteins

  • Aquaporins: main water route (AQP2 regulated by ADH)
  • Ion channels: selective; gated (voltage, ligand, mechanical) vs leak
  • Solute carriers: Uniporter (GLUT1-4) | Symporter (Na-K-2Cl, SGLT) | Antiporter (Na⁺/H⁺, Na⁺/Ca²⁺)
  • ATP-dependent pumps: P-type (Na⁺/K⁺-ATPase 3 Na⁺ out : 2 K⁺ in), V-type (H⁺ pump), F-type (mitochondrial ATP synthase), ABC transporters

Transport Processes

  • Passive: Diffusion, Osmosis, Filtration
    • Fick: J=−D A ΔCΔxJ = -D\,A\,\frac{\Delta C}{\Delta x}
    • Van’t Hoff: π=nCRT\pi = nCRT (osmotic pressure)
    • Osmolarity (mOsm/L) vs Osmolality (mOsm/kg H₂O, T-independent)
  • Active: Primary (ATP pump) | Secondary (symport/antiport driven by ion gradient)

Vesicular & Epithelial Transport

  • Endocytosis: Phagocytosis (large), Pinocytosis (fluid), Receptor-mediated (clathrin coat)
  • Exocytosis: constitutive vs regulated (SNARE-dependent)
  • Epithelial pathways:
    • Paracellular: between cells, limited by claudins
    • Transcellular: across apical→basolateral via channels/carriers, Na⁺/K⁺-ATPase key driver

Cellular Communication

  • Gap junctions: direct ionic/chemical coupling
  • Signal transduction: ligand → receptor → intracellular cascade → response
  • Signaling modes: Juxtacrine | Paracrine | Synaptic | Endocrine | Autocrine
Receptor Classes (location & example output)
  • Ion-channel linked: Ach-nicotinic (Na⁺/K⁺ influx)
  • GPCR: β-adrenergic → cAMP pathway
  • Enzyme-linked: Insulin → tyrosine-kinase
  • Nuclear: steroid/thyroid hormones → gene transcription modulation

Key Clinical Correlations

  • Hyper/Hypokalemia ➔ arrhythmia
  • Na⁺/K⁺-ATPase maintains resting Vm≈−70 mVV_m \approx -70\,\text{mV}
  • Tight junction defects → leaky barriers (e.g., BBB compromise)
  • Aquaporin-2 dysfx → diabetes insipidus
  • Gap junction loss (connexin 43) → cardiac conduction block