what is the difference between diffusion of hydrophobic substances in comparison to hydrophilic ones across plasma membrane?
The difference between the diffusion of hydrophobic substances and hydrophilic substances across the plasma membrane lies in the nature of their interactions with the lipid bilayer that comprises the membrane.
Hydrophobic substances are nonpolar molecules that do not readily dissolve or interact with water. These molecules are typically lipid-soluble and can easily pass through the hydrophobic (nonpolar) region of the lipid bilayer. The lipid bilayer consists of two layers of phospholipids, with their hydrophilic heads facing outward and their hydrophobic tails facing inward. Hydrophobic substances can efficiently diffuse through this hydrophobic region of the membrane by simple diffusion, moving from an area of high concentration to an area of low concentration until equilibrium is reached.
On the other hand, hydrophilic substances are polar or charged molecules that interact strongly with water molecules due to their ability to form hydrogen bonds. Because the interior region of the lipid bilayer is hydrophobic, hydrophilic substances face difficulty in crossing the plasma membrane via simple diffusion. Instead, they require specific transport proteins, such as channel proteins or carrier proteins, to facilitate their passage. Channel proteins form small pores or channels in the membrane, allowing hydrophilic substances to cross by facilitated diffusion. Carrier proteins, on the other hand, undergo a conformational change that transports the hydrophilic substance across the membrane.
In summary, hydrophobic substances can diffuse directly through the hydrophobic region of the lipid bilayer by simple diffusion, while hydrophilic substances require the assistance of specialized transport proteins for their passage across the plasma membrane.