Biology – Carbohydrates and lipids | e-Consult

The selective permeability of biological membranes arises directly from the phospholipid bilayer structure. The hydrophobic core of the bilayer presents a significant barrier to the passage of most molecules, particularly polar and charged molecules. These molecules cannot easily cross the lipid bilayer because they are repelled by the non-polar tails.

Small, non-polar molecules, such as oxygen (O2) and carbon dioxide (CO2), can readily diffuse across the membrane, as they can dissolve in the lipid bilayer. This is because they are compatible with the hydrophobic environment.

Water (H2O) can also permeate the membrane, although its movement is facilitated by the presence of aquaporins – protein channels embedded within the bilayer. The hydrophobic core restricts the free movement of water molecules.

Polar molecules, such as ions (e.g., Na+, K+), glucose, and amino acids, have difficulty crossing the membrane. They require the assistance of membrane transport proteins (either channel proteins or carrier proteins) to facilitate their movement. These proteins provide a pathway for these molecules to cross the hydrophobic barrier. The specific types of transport proteins present determine which polar molecules can cross the membrane and the rate at which they do so.