Final answer:
A hydrophilic amino acid is water soluble and interacts favorably with water molecules due to its polar, acidic, or basic side chains which can form hydrogen bonds or ionize, thus appearing to 'love' water.
Step-by-step explanation:
A hydrophilic amino acid is one that is soluble in water due to its ability to form hydrogen bonds or electrostatic interactions with water molecules. These amino acids typically possess side chains that are either polar and uncharged, acidic, or basic, which can ionize and interact favorably with water.
Specifically, hydrophilic amino acids like Glycine (Gly), Serine (Ser), Threonine (Thr), Cysteine (Cys), Tyrosine (Tyr), Aspartic acid (Asp), Asparagine (Asn), Glutamic acid (Glu), Glutamine (Gln), Lysine (Lys), Arginine (Arg), and Histidine (His) behave in a specific way around water. They are attracted to water molecules and tend to dissolve, or be 'wetted' by water, thus they are described as "water-loving" or hydrophilic. Amino acids like cysteine and glycine, which have polar neutral side chains containing groups such as -OH or -CONH₂, do not ionize but can still hydrogen bond with water. Acidic amino acids with carboxylic acid groups ionize under physiological conditions to -COO-, further increasing their hydrophilicity. Basic amino acids have primary or secondary amine groups in their side chains that ionize to cations like -NH₃ or -NRH₂ at physiological pH, contributing to their solubility in water.
In contrast, hydrophobic amino acids such as Alanine (Ala), Valine (Val), Leucine (Leu), Isoleucine (Ile), Proline (Pro), Methionine (Met), Phenylalanine (Phe), and Tryptophan (Trp) are "water-fearing" and tend to avoid water, aggregating together away from aqueous environments.