Amino acids diffuse across cell membranes primarily via facilitated diffusion.
While simple diffusion is the movement of molecules across a membrane down their concentration gradient without the assistance of membrane proteins, facilitated diffusion requires the help of such proteins. Amino acids, being polar and often charged molecules, generally cannot efficiently cross the hydrophobic core of a cell membrane through simple diffusion. Therefore, they rely on facilitated diffusion.
Facilitated diffusion involves two main classes of proteins:
- Channel proteins: These proteins form pores or channels in the membrane, allowing specific molecules like amino acids to pass through. The channel provides a hydrophilic pathway, shielding the charged or polar amino acids from the hydrophobic interior of the lipid bilayer. These channels are often highly selective for particular amino acids or groups of amino acids based on size, charge, and other chemical properties.
- Carrier proteins: These proteins bind to the amino acid on one side of the membrane, undergo a conformational change, and then release the amino acid on the other side. Carrier proteins are also highly specific, typically transporting only one or a few closely related amino acids. They are also known as permeases.
Here's a summary table:
| Diffusion Type | Membrane Protein Required? | Concentration Gradient? | Molecule Type |
|---|---|---|---|
| Simple Diffusion | No | Yes | Small, nonpolar molecules (e.g., O2, CO2) |
| Facilitated Diffusion | Yes | Yes | Large, polar/charged molecules (e.g., amino acids) |
In Summary: Amino acids utilize facilitated diffusion, mediated by channel and carrier proteins, to cross cell membranes down their concentration gradient. This is essential because their polarity and charge hinder their direct passage through the lipid bilayer.
