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Transport across a cell membrane is a tightly regulated process, because cell function is highly dependent on maintain strict concentrations of various molecules. When a molecule moves down its concentration gradient is it participating in passive transport; moving up the concentration gradient requires energy making it active transport.
Osmosis across cellular membranes relies on the facilitated transport of water by aquaporins. The passive diffusion of water molecules, can be demonstrated with an artificial (e.g., dialysis) membrane. Water will cross such a membrane if solute concentrations are higher on one side of the membrane.
Transport across the membrane. ligand-gated ion channel: nicotinic acetylcholine receptor. The nicotinic acetylcholine receptor is an example of a ligand-gated ion channel. It is composed of five subunits arranged symmetrically around a central conducting pore.
Most cells expend most of their energy, in the form of adenosine triphosphate (ATP), to create and maintain an uneven distribution of ions on the opposite sides of their membranes. The structure of the plasma membrane contributes to these functions.
Transport across the membrane. One of the great wonders of the cell membrane is its ability to regulate the concentration of substances inside the cell.
