Membrane Transport

The Fluid Mosaic Membrane

• A phospholipid bilayer studded with integral proteins (channels, receptors, transporters) and peripheral proteins; cholesterol buffers fluidity across temperatures; glycoproteins on the extracellular face mediate cell recognition.
• Small nonpolar molecules ($O_2$, $CO_2$, steroids) diffuse freely; ions and large polar molecules like glucose need channels or carriers.

Transport Mechanisms

• Simple and facilitated diffusion run down gradients without ATP; primary active transport uses ATP directly; secondary active transport (SGLT1 sodium-glucose symport) rides the $Na^+$ gradient.
• The $Na^+/K^+$ ATPase moves 3 $Na^+$ out and 2 $K^+$ in per ATP — electrogenic, and the engine behind secondary active transport and the resting potential near $-70$ mV.
• Receptor-mediated endocytosis uses clathrin-coated pits; defective LDL receptors cause familial hypercholesterolemia.

Osmosis, Tonicity, and Membrane Potential

• Hypertonic solutions shrink cells (crenation); hypotonic solutions swell or lyse them. Only non-penetrating solutes set tonicity — urea crosses membranes, so a pure urea solution is effectively hypotonic.
• Each ion's equilibrium potential follows the Nernst equation: $E_{ion} = \frac{61}{z} \log \frac{[ion]_{out}}{[ion]_{in}}$ with $E_K \approx -90$ mV dominating the resting potential.

Key Takeaways

• Crossing rules: small and nonpolar diffuse; charged or large polar species need transport proteins.
• The 3-out/2-in $Na^+/K^+$ ATPase powers gradients and secondary transport.
• Osmolarity and tonicity differ — penetrating solutes like urea do not affect cell volume.
• Resting membrane potential sits near $E_K$ because of potassium leak channels.