Membrane Transport Mechanisms and Body Fluid Compartment Composition

Body Fluid Compartments

  • Intracellular Fluid (ICF): Fluid inside cells, including nucleoplasm and cytosol.
  • Extracellular Fluid (ECF): Fluid outside cells, includes:
    1. Interstitial Fluid (IF): Fluid between cells, not in blood.
    2. Plasma: Fluid portion of blood.
  • Barriers Separating Fluids:
    • Plasma Membrane: Separates intracellular fluid (ICF) from interstitial fluid (IF).
    • Blood Vessel Wall: Separates interstitial fluid (IF) from plasma.

Composition of Body Fluids

  • Organic Substances:
    • Glucose
    • Amino acids
    • Fatty acids
    • Hormones
    • Enzymes
  • Inorganic Substances:
    • Sodium (Na⁺)
    • Potassium (K⁺)
    • Calcium (Ca²⁺)
    • Magnesium (Mg²⁺)
    • Chloride (Cl⁻)
    • Phosphate (PO₄³⁻)
    • Sulfate (SO₄²⁻)
  • General Composition: Body fluids contain water and solids (organic and inorganic substances).

Membrane Transport Mechanisms

  • Barrier: The lipid bilayer’s hydrophobic interior blocks most polar molecules.
  • Selective Transport: Regulates solute concentrations in the cytosol, extracellular fluid, and organelles.
  • Transport Proteins: Facilitate movement across membranes.

Differences in Extracellular vs Intracellular Fluid

  • Extracellular Fluid (ECF):
    • Most abundant cation: Sodium (Na⁺)
    • Functions: muscle contraction, impulse transmission, fluid, and electrolyte balance.
    • Most abundant anion: Chloride (Cl⁻)
    • Functions: regulates osmotic pressure, forms HCl in gastric acid.
  • Intracellular Fluid (ICF):
    • Most abundant cation: Potassium (K⁺)
    • Functions: resting membrane potential, action potentials, maintains intracellular volume, regulation of pH.
    • Most abundant anions: proteins and phosphates (HPO₄²⁻).
  • Na⁺/K⁺ Pumps: Maintain high K⁺ inside cells and high Na⁺ outside.

Factors Affecting Diffusion

  • Hydrophobicity: Major factor—more hydrophobic (nonpolar) molecules diffuse easily.
  • Molecule Size: Minor factor—smaller molecules diffuse faster.
  • Fast Diffusion: O₂ and CO₂ cross easily.
  • Slow Diffusion: Urea and water diffuse much slower.

Lipid Bilayer Permeability

  • High Permeability: Small, nonpolar gases (O₂, CO₂)
  • Moderate Permeability: Small uncharged polar molecules (e.g., water, urea)
  • No Permeability: Large uncharged polar molecules (e.g., glucose), charged ions (Cl⁻, Na⁺, K⁺) require transport proteins.

Membrane Transport Proteins

  • Definition: Specific proteins that facilitate movement across membranes.
  • Transport Types:
    • Transporters (Carriers/Permeases): Bind to specific solutes, change shape to move solute.
    • Channels: Briefly interact and form continuous pores; allow rapid transport of specific solutes.

Transport Mechanisms

  • Passive Transport: Channels & some transporters move solutes passively (downhill, no energy required). Charged solutes movement is based on:
    • Concentration gradient (high to low).
    • Membrane potential (voltage difference across the membrane).
  • Active Transport: Cells pump solutes uphill (against the electrochemical gradient), requires energy (from ion gradients or ATP).

Types of Active Transport

  1. Coupled Transporters: Use the energy from one solute’s downhill movement to drive another solute uphill.
  2. ATP-Driven Pumps: Use ATP hydrolysis to power uphill transport.
  3. Light/Redox-Driven Pumps: Found in bacteria, archaea, mitochondria, and chloroplasts.

Specific Active Transport Mechanisms

  • Na⁺-Driven Glucose Transport:
    • Cooperative binding enhances binding of Na⁺ and glucose.
    • High Na⁺ outside favors glucose binding.
    • Strict coupling of binding.
  • Na⁺-K⁺ Pump Mechanism:
    • Actively transports Na⁺ and K⁺ against gradients (3 Na⁺ out, 2 K⁺ in).
    • ATP provides energy for transport.

ATP Pumps

  • Definition: ATP-driven pumps use energy from ATP hydrolysis.
  • Roles: Maintain ion gradients, pH balance, and cellular homeostasis.
  • Classes:
    1. P-type: Phosphorylate during transport (e.g., Na⁺/K⁺ pump).
    2. ABC Transporters: Move small organic molecules.
    3. V-type: Pump H⁺ to acidify organelles (e.g., lysosomes).
    4. ATP Synthases: Generate ATP using proton gradients.

ABC Transporters and Disease

  • Multidrug Resistance (MDR) Protein: Pumps drugs out of cancer cells; affects chemotherapy success.
  • Cystic Fibrosis Transmembrane Conductance Regulator (CFTR): Mutations cause cystic fibrosis; impacts chloride channel function.

Summary of Key Points

  • Understanding intracellular and extracellular compartments helps grasp solute transport and body fluid regulation.
  • Membrane transport mechanisms (passive, active) and types of transport proteins are crucial for cellular homeostasis.