These proteins are substrate specific , suggesting presence of substrate binding sites. Unlike simple diffusion, the proteins facilitating transport can be inhibited by competitive inhibition. The transport proteins can be classified as uniports, symports or antiports. The glucose transporter is a uniport protein transporter. The Band 3 protein in erythrocytes is an ion exchanger, thus as antiporter.
Ion channels permit water soluble molecules and ions to cross the plasma membrane.
Ion channels can be voltage-gated channels whose conformational state depends on the difference in ionic charge on the two sides of the membrane. Absorption of photons of light. Movement of other molecules down their respective electrochemical gradients. These ionic gradients are established in the cell by active transport. The cytoplasm contains a high concentration of organic molecules, including macromolecules, amino acids, sugars, and nucleotides.