Simple diffusion is when a small, non-polar molecule passes through a lipid bilayer. It is classified as a means of passive transport. In simple diffusion, a hydrophobic molecule can move into the hydrophobic region of the membrane without getting rejected. Simple diffusion does not involve a protein. An example of simple diffusion is osmosis. Hydrophilic molecules cannot participate in simple diffusion because they would move into the hydrophobic region of the membrane and be rejected.
Facilitated diffusion is a type of passive transport that is dependent on single transport protein carriers. These protein carriers operate on a bind, flip, and release mechanism. Facilitated diffusion is non-diffusional because the molecule moves along with the carrier. Saturation occurs in facilitated diffusion because not enough carriers may be available to handle all the free solute molecules. The rate of movement may reach a maximum.
Active Transport There are numerous situations in living organisms when molecules move across cell membranes from an area of lower concentration toward an area of higher concentration. This is counter to what would be expected and is labeled "active transport".
Many crucial processes in the life of cells depend upon active transport. Active transport mechanisms may draw their energy from the hydrolysis of ATP, the absorbance of light, the transport of electrons, or coupling with other processes that are moving particles down their concentration gradients. A vital active transport process that occurs in the electron transport process in the membranes of both mitochondria and chloroplasts is the transport of protons to produce a proton gradient. This proton gradient powers the phosphorylation of ATP associated with ATP synthase.
Osmosis is a special example of diffusion. It is the diffusion of water through a partially permeable membrane from a more dilute solution to a...