According to their functions, there are two main groups of transport proteins: channel proteins and carrier proteins. There are many different proteins located in biomembranes with the main function of effectively transporting certain solutes across the membrane. Polar and Charged Molecules (Protein-Mediated Transfer)īiological membranes are permeable not only to gases and small lipophilic molecules (by passive diffusion processes), but also to many polar and charged molecules, including water, but through a different path. Moreover, almost all polar and charged molecules such as sugars, amino acids, and ions completely fail to cross the pure phospholipid membranes. In contrast to small lipophilic molecules, in absence of protein channels, it is difficult for water to pass through the pure phospholipid membranes via diffusion. ![]() Usually, a compound with log( K o/w) > 5 is too hydrophobic to passively diffuse through the biological membranes. The underlying reason is that there are two counter-effect parameters: besides being lipid-soluble (to cross the membrane), the solute should also possess enough water solubility to dissolve in the body fluids. However, larger lipophilicity values should not be always interpreted as better passive diffusion. Logarithm of octanol/water partition coefficient of the solute ( K o/w) can be a measure of lipophilicity (higher the log( K o/w), higher the lipophilicity of the solute will be). Examples of chemicals that passively diffuse across the cell membranes are gases, like O 2 and CO 2, and small relatively hydrophobic molecules, such as fatty acids and alcohols. Small Lipophilic Molecules (Passive Diffusion)Ĭertain substances easily pass through the membrane by passive diffusion. ![]() It is worth noting that the mediated transport can be used in drug delivery and defects in transport are the causes of many diseases.
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