RC

Membrane Proteins, Permeability, Transport, and Osmosis - Study Notes

Functions of Membrane Proteins

  • 1) Ion channels (pores)
  • 2) Carriers or Transporters (require energy)
  • 3) Receptors (cell recognition sites)
  • 4) Enzymes
  • 5) Linkers
  • 6) Cell identity markers

Membrane Permeability

  • Plasma membranes are selectively permeable (some things get through, some can't).
  • Very permeable to lipid-soluble, nonpolar molecules.
  • Moderately permeable to H₂O and urea.
  • Impermeable to large uncharged polar molecules (e.g., glucose).
  • The more hydrophobic the molecule, the more permeable the membrane for that molecule.

Membrane Transport

  • Passive transport: down concentration gradient; requires no energy (ATP).
  • Active transport: usually against concentration gradient; requires energy (ATP).
  • Passive movement of molecules down their concentration gradient.
  • Simple diffusion: K⁺, Cl⁻, Na⁺, Ca²⁺ (ions listed as examples in notes).
  • "Diffusion" = passive.
  • Facilitated diffusion: glucose, fructose, vitamins.
  • (Both simple and facilitated diffusion are passive processes.)

Osmolarity, Tonicity, and Water Movement

  • Hypertonic: solute concentration is higher on the outside of a membrane than inside.
  • Hypotonic: solute concentration is lower on the outside than inside.
  • Isotonic: solute concentration is equal on both sides of the membrane.
  • Fluid always moves toward the hypertonic side.
  • Consequences for cells:
    • In hypertonic solutions: cells tend to lose water and shrink (crenate).
    • In hypotonic solutions: cells tend to gain water and swell; may lyse.
    • In isotonic solutions: no net water movement; cell size remains stable.
  • Quick reference:
    • Hypertonic outside: $C{ ext{outside}} > C{ ext{inside}}$ → water moves out → cell crenates.
    • Hypotonic outside: $C{ ext{outside}} < C{ ext{inside}}$ → water moves in → cell swells and may lyse.
    • Isotonic: $C{ ext{outside}} = C{ ext{inside}}$ → no net water movement.

Connections and Practical Significance

  • Understanding membrane permeability explains why certain drugs or toxins enter cells more readily (lipophilicity) and why some nutrients require transporters.
  • Ion transport underpins nerve impulses, muscle contraction, and drug actions targeting ion channels or pumps.
  • Osmolarity concepts are essential in medical contexts (intravenous fluids, dehydration, edema, and cell viability).
  • Disruptions in membrane transporter function can lead to diseases (e.g., transporter deficiencies, receptor signaling issues).
  • Energy considerations (ATP use in active transport) highlight the cost of maintaining ion gradients across membranes, which is fundamental to cellular physiology.

Quick Formulas and Key Notation

  • Isotonic condition: C{ ext{inside}} = C{ ext{outside}}
  • Hypertonic condition: C{ ext{outside}} > C{ ext{inside}}
  • Hypotonic condition: C{ ext{outside}} < C{ ext{inside}}
  • Direction of water movement aligns with gradient toward higher solute concentration (toward hypertonic side).