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Cell Physiology and Membrane Transport

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Cell Physiology and Membrane Transport — Key Facts for FMGE Core concept: Cells maintain ionic gradients via Na/K ATPase and transport substances across membranes via various mechanisms High-yield point: Primary active transport uses ATP directly; secondary active transport uses ionic gradients; facilitated diffusion and channels are passive ⚡ Exam tip: Understand how the Na/K ATPase creates the basis for secondary active transport and how each transporter type is different

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Cell Physiology and Membrane Transport — FMGE Study Guide

Membrane Structure

Lipid Bilayer

  • Phospholipids: Hydrophilic head (phosphate group) + hydrophobic tail (fatty acids)
  • Forms the basic structural barrier
  • Cholesterol: Inserts between phospholipids; increases rigidity at high temperatures, prevents packing at low temperatures
  • Fluid mosaic model: Proteins float in a fluid lipid matrix; some attached to surface (peripheral), some embedded (integral)

Membrane Proteins

  • Integral proteins: Span the membrane; include channels, receptors, transporters, enzymes
  • Peripheral proteins: Attached to surface; often have enzymatic function or structural role
  • Glycoproteins: Sugar chains attached; cell recognition (MHC), blood groups, adhesion molecules

Types of Membrane Transport

Passive Transport

Simple diffusion:

  • Movement from high to low concentration
  • Through lipid bilayer (O₂, CO₂, N₂, steroids) or pores
  • Rate depends on concentration gradient, lipid solubility, molecular size
  • Non-polar molecules: Diffuse through lipid core
  • Polar molecules: Require channels or transporters

Facilitated diffusion:

  • Carrier-mediated or channel-mediated
  • Specific: Each transporter has specificity
  • Saturation kinetics: Vmax when all carriers occupied (Km reflects affinity)
  • Examples: GLUT transporters (glucose), ion channels (Na, K, Ca channels)

Osmosis:

  • Water moves from low solute concentration to high solute concentration
  • Osmolarity: Total solute concentration per liter
  • Osmolality: Total solute concentration per kg of water (more commonly used)
  • Isotonic: Equal osmolarity (0.9% NaCl, 5% glucose)
  • Hypotonic: Lower osmolarity (water enters cells → hemolysis)
  • Hypertonic: Higher osmolarity (water leaves cells → crenation)

Active Transport

Primary active transport:

  • Direct use of ATP to move substances against concentration gradient
  • Na/K ATPase: Antiport; 3 Na out, 2 K in per ATP; creates Na and K gradients
    • Maintains resting membrane potential (-70mV)
    • Creates Na gradient for secondary active transport
    • Accounts for 60-70% of resting metabolic activity in neurons

Secondary active transport:

  • Uses energy from ionic gradients (created by primary active transport)
  • Symport (cotransport): Both substances in same direction
    • SGLT1/SGLT2 (Na-glucose cotransporter) in intestine and kidney
    • Glucose + Na enters cell together
  • Antiport (exchanger): Substances in opposite directions
    • Na/Ca exchanger (NCX): 3 Na in, 1 Ca out
    • Na/H exchanger: Na in, H out

Vesicular Transport

Exocytosis:

  • Secretory granules fuse with plasma membrane
  • Constitutive: Continuous secretion (plasma proteins)
  • Regulated: Requires signal (hormones, neurotransmitters)
  • Used for: Hormone release, neurotransmitter release, plasma membrane addition

Endocytosis:

  • Phagocytosis: Large particles (bacteria, debris) - “eating”
  • Pinocytosis: Fluid and dissolved solutes - “drinking”
  • Receptor-mediated endocytosis: Specific ligands (LDL, transferrin, insulin)
  • Clathrin-coated pits: Important for receptor-mediated endocytosis
  • Lysosomes: Fuse with endosomes for degradation

Ion Channels

Types of Ion Channels

Voltage-gated channels:

  • Open/close based on membrane potential
  • Na channels: Rapid opening during depolarization; inactivated during repolarization
  • K channels: Repolarization, slower kinetics
  • Ca channels: L-type (long-lasting), N-type (neuronal), T-type (transient)

Ligand-gated channels:

  • Open when ligand (neurotransmitter, hormone) binds
  • Nicotinic ACh receptor: Na channel; opens when ACh binds
  • GABA-A receptor: Cl channel; causes hyperpolarization
  • NMDA/AMPA: Glutamate receptors; Ca and Na entry

Mechanically-gated channels:

  • Open with mechanical deformation
  • Hair cells in ear: Stereocilia bend → K channel opening → depolarization

Leak channels:

  • Always open; allow passive ion movement
  • K leak channels: Maintain resting membrane potential

Resting Membrane Potential

  • Most cells: -70 mV (negative inside)
  • Cause: Unequal distribution of ions, K leak channels
  • K equilibrium potential (Ek): -90 mV (K wants to leave)
  • Na equilibrium potential (Ena): +60 mV (Na wants to enter)
  • Cl equilibrium potential (Ecl): -70 mV (close to resting potential)
  • Equation: Goldman-Hodgkin-Katz (GHK) voltage equation

Cell Membrane Receptors

Types of Receptors

G-protein coupled receptors (GPCR):

  • 7 transmembrane domain structure
  • Associated with G-proteins (heterotrimeric: α, β, γ subunits)
  • Gq pathway: PLC → IP3/DAG → ↑Ca²⁺, ↑PKC
  • Gs pathway: ↑cAMP → PKA
  • Gi pathway: ↓cAMP → ↓PKA

Receptor tyrosine kinases (RTK):

  • Insulin, growth factors
  • Dimerization → autophosphorylation → intracellular signaling cascades

Nuclear receptors:

  • Steroid hormones, thyroid hormone
  • Intracellular/indirect DNA-binding
  • → alter gene transcription

Transport Across Epithelia

Epithelial Transport

Apical membrane (luminal side):

  • Contains transport proteins for absorption or secretion

Basolateral membrane (blood side):

  • Contains Na/K ATPase

Absorption:

  • Glucose from lumen → enters cell via SGLT (apical) → exits via GLUT2 (basolateral) → blood

Secretion:

  • Ion leaves cell on blood side → moves through paracellular pathway to lumen

Tight Junctions

  • Between adjacent epithelial cells
  • Prevent paracellular movement
  • Determine which substances can cross paracellularly

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