B2.1.6

• Large and polar molecules can enter the cell by facilitated diffusion down their concentration gradient with the aid of proteins.
• Channel proteins are integral transmembrane proteins with a pore that connects the cytoplasm to the solution outside of the cell. The diameter and chemical properties ensure that only one type of particle passes through.
• No energy is required because the particles are allowed to pass through either direction -> passive.
• Cells can select which substances diffuse in and out by the types of channel that are synthesised and placed in the plasma membrane. Some channels can be opened and closed when necessary -> permeability can be temporarily changed.