hydrophilic

Meaning: A hydrophilic species is polar or charged, so it can form hydrogen bonds or ion–dipole interactions with water and dissolves readily. Examples include ions, sugars, the phosphate head group of a phospholipid, and polar or charged amino acid side chains (Ser, Thr, Asn, Gln, Asp, Glu, Lys, Arg, His). Dissolving is favorable when the new solute–water interactions make up for the solute–solute and water–water interactions that are broken.

On the MCAT: Expect protein structure questions. In a soluble protein, hydrophilic residues tend to sit on the surface in contact with water, while hydrophobic residues cluster in the core. A mutation that swaps a surface Glu for Val lowers solubility and may promote aggregation. In a membrane-spanning region the pattern flips, with hydrophobic residues facing the lipid bilayer. Phospholipids are amphipathic: the hydrophilic head faces water and the hydrophobic tails face each other inside a bilayer or micelle. Hydrophilic molecules such as ions and glucose generally need channels or transporters to cross a membrane.

Common confusion: Hydrophilic does not mean charged, because polar uncharged groups such as hydroxyl and amide groups also count. Hydrophobic groups are not truly repelled by water. They cluster mainly because this raises the entropy of the surrounding water, which is the hydrophobic effect.