Functions of Proteins and Phospholipids

Cell membranes are described as selectively permeable. This is because the hydrophobic layer in the middle of the bilayer is impermeable to polar and charged particles. Non-polar molecules like oxygen, carbon dioxide and fat-soluble vitamins (A, D, E and K) dissolve in the hydrophobic layer and can cross the membrane by simple diffusion.

Channel proteins have hydrophilic R groups lining the space down the middle of the protein. These channels are specific to particular charged or polar particles and allow them to diffuse across the membrane in facilitated diffusion. Some proteins are involved in active transport and facilitated diffusion across the membrane; these are specific to the polar or charged molecules carried and are termed protein carriers.

Some proteins are specific receptors, e.g., for hormones or neurotransmitters. Some are enzymes, e.g., ATP synthetase is an intrinsic protein embedded into the cristae of mitochondria.