Practice question

Plasma membrane selectively controls solute entry through distinct physico-chemical mechanisms dictated by molecular properties. Small non-polar gases such as O2, CO2, N2 and NO partition readily into hydrophobic core and cross by simple diffusion governed by Fick's law, without assistance, explaining lung alveolar exchange and mitochondrial oxygen supply. However many essential nutrients are hydrophilic or charged and face high energetic barrier. Facilitated diffusion provides protein-assisted route down concentration gradient without direct energy coupling, accelerating equilibration thousandsfold. Carriers such as GLUT family for glucose and channels such as aquaporins for water or ion channels for K+ increase effective permeability by lowering activation energy through transient binding or aqueous pore formation. Both display saturability, specificity, competitive inhibition and regulation by gating or translocation, distinguishing them from simple diffusion which is linear, non-saturable and non-selective beyond partition coefficient. Active transport moves solutes against gradients requiring energy, either primary via ATP hydrolysis as with Na+/K+ ATPase and V-ATPase or secondary using coupled ion gradients such as Na+/glucose symport. Osmosis represents water diffusion across semipermeable membranes, also energy independent. Therefore hallmark of facilitated diffusion is requirement for integral membrane protein while remaining passive and driven by gradient.