Amino acids, glucose and other large membrane insoluble compounds move through the cell membrane through a process known as facilitated diffusion. This process involves transmembrane proteins, which open up a small water-filled channel through which the molecules can pass into or out of the cell.
Glucose undergoes facilitated diffusion by binding to a transporter protein, which then changes its configuration to release glucose into the cell. This process is controlled by insulin, which stimulates the membrane to increase the number of glucose transporter proteins at the surface. When the cell needs more glucose, the increased number of transporter proteins leads to increased glucose diffusion into the cell.
Each of the various amino acids and other lipid insoluble molecules has its own specific transporter protein that it binds to in order to diffuse into the cell. These transporter proteins are also responsible for removing excess molecules from the cells when necessary, as the diffusion process is always fully reversible and controlled by the same transporter protein. Diffusion is considered passive transport, as it requires no energy, while other molecules must enter the cells through active transport that requires energy. Water is the only compound that can pass across the cell membrane without requiring diffusion or active transport.