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 (O2, CO2, steroids) diffuse freely; ions and large polar molecules like glucose need channels or carriers.
Diffusion, active transport, and membrane dynamics
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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.
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: Eion=z61log[ion]in[ion]out with EK≈−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 EK because of potassium leak channels.