Facilitated diffusion

Facilitated diffusion is the passive movement of molecules across a membrane through specific channel or carrier proteins, down their concentration or electrochemical gradient. Unlike simple diffusion, it depends on membrane proteins and can show selectivity and saturation.

Ion channel: A membrane protein that forms a pore through which selected ions can cross. Ion channels provide a direct pore route for facilitated diffusion.
Cell membrane: A selectively permeable boundary made mainly of a lipid bilayer and proteins that separates a cell from its surroundings. Facilitated diffusion moves substances across this boundary.
Simple diffusion: The passive net movement of particles down a concentration gradient without transport proteins. It crosses membranes without the protein dependence that defines facilitated diffusion.
GLUT1: A glucose transporter that enables facilitated glucose uptake across many cell membranes, including the blood–brain barrier. It carries glucose down its concentration gradient without directly using ATP.
Transport maximum: The upper rate of membrane transport reached when available transport proteins are saturated. Finite carrier numbers place a ceiling on facilitated transport rates.
Carrier protein: A membrane protein that binds specific molecules and changes shape to transport them across a membrane. Carriers move substrates by alternating access rather than opening a continuous pore.
Concentration gradient: A spatial difference in the concentration of a substance. Neutral solutes diffuse in the direction set by this gradient.
Active transport: The movement of substances across a membrane against a gradient, coupled to an energy source. Unlike facilitated diffusion, active transport can move substances uphill.
GLUT4: An insulin-responsive glucose transporter found mainly in skeletal muscle and adipose tissue. Insulin increases its presence at the cell surface, enabling more facilitated glucose uptake.
Competition inhibition: A reduction in substrate transport when competing molecules vie for the same binding site. Related substrates can compete for carrier proteins and alter uptake.

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