Protein molecules differ from carbohydrate or fat molecules by being linked together through peptide bonds between amino acids. Specifically, proteins are polymers formed by amino acids connected via peptide bonds, which are covalent bonds formed between the carboxyl group of one amino acid and the amino group of another

. In contrast:

• Carbohydrates are polymers of monosaccharides (sugars) linked by glycosidic bonds

• Fats (triglycerides) are composed of glycerol linked to three fatty acids by ester bonds , not forming polymers in the strict sense but aggregates of lipid molecules

Thus, the key distinguishing feature of proteins is the peptide linkage of amino acids, which is unique compared to the glycosidic linkages in carbohydrates and ester linkages in fats