Membranes, diffusion and osmosis

A plasma membrane contains a phospholipid bilayer and proteins. Selective permeability helps maintain internal conditions. Diffusion is net movement down a concentration gradient through random motion. Facilitated diffusion uses proteins without a direct energy input. For ions, electrical potential also matters: passive movement follows the electrochemical gradient. Active transport can move a substance against its relevant gradient using cellular energy.

Osmosis is net movement of water across a selectively permeable membrane towards lower water potential. In simple solutions, water moves towards the higher concentration of solutes that cannot cross. Animal cells may swell or shrink; plant walls resist expansion and allow turgor. Percentage mass change equals final minus initial mass, divided by initial mass, multiplied by 100.

Watch out: Channels and carriers can enable passive facilitated diffusion down a relevant gradient; transport direction and energy use distinguish the processes.