Carrier Proteins

• A substance will bind on a binding site on the membrane side where it is at a high concentration and release where it is at a low concentration (e.g. glucose and amino acids); flips between two states alternately opened on opposite sides
• Na^+ / glucose co-transport: Found in the epithelia of the small intestine and proximal tubule of the nephron in the kidney; they use the energy from a downhill sodium gradient to transport glucose against its concentration gradient (still passive process, no ATP breakdown)