Cell Membrane Proteins and Water Diffusion

Protein-Mediated Transport

• Channel proteins facilitate massive diffusion in plant and animal cells.
• Ligand-gated channel proteins open or close in response to a stimulus.

Aquaporins and Urine Production

• Aquaporins are responsible for transporting water molecules across the cell membrane.
• Without a high number of aquaporins in certain kidney cells, urine production would decrease.

Carrier Proteins

• Carrier proteins seem to change shape to translocate the solute-binding site across the membrane.
• Carrier proteins bring about a tremendous increase in the rate of solute transport across the membrane.

Transport Mechanisms

• Transport proteins are needed to move water molecules rapidly and in large quantities across a membrane because the lipid bilayer is impermeable to water.
• Some transport proteins hold onto their passengers and change shape to shuttle them across the membrane.

Selectivity of Carrier Proteins

• The selectivity of a carrier protein in transporting substances across the membrane is determined by the shape and chemistry of the binding site.
• Some aquaporins exclude hydronium ions but allow glycerol and water molecules to pass due to the basis of their selectivity.

Diffusion and Dynamic Equilibrium

• In the process of diffusion, when two solutions reach dynamic equilibrium, the movement of molecules from one solution to another becomes equal in both directions.
• Small molecules, such as oxygen and carbon dioxide, can cross a lipid bilayer without the assistance of membrane proteins.