2.3 Transport across cell membranes

Explain the role of cholesterol in cell membranes

It restricts the movement of other molecules that make up the membrane so decreases fluidity and permeability/increases rigidity

Describe how movement across membranes occurs by simple diffusion

Lipid soluble (non-polar) or very small substances eg oxygen move from an area of higher concentration to an area of lower concentration down a concentration gradient across a phospholipid bilayer. This is a passive process so doesn’t require energy from ATP/respiration

Describe the movement of molecules across membranes by facilitated diffusion

Water-soluble (polar) or slightly larger molecules move down a concentration gradient through specific channel or carrier proteins. Passive process so doesn’t require energy from ATP/respiration

Describe the role of channel proteins in facilitated diffusion

Channel proteins facilitate diffusion of water-soluble substances (may be gated so can open and close)

Describe the role of carrier proteins in facilitated diffusion

Carrier proteins facilitate diffusion of slightly larger substances

Complimentary substance attaches to binding site and protein changes shape to transport substance