What is one of the four classes of macromolecules that are functional polypeptides?

Proteins are indeed one of the four classes of macromolecules that function as polypeptides. Polypeptides are chains of amino acids linked by peptide bonds, and when these chains fold into specific three-dimensional structures, they become functional proteins. Proteins perform a vast array of functions within biological systems, such as catalyzing biochemical reactions (enzymes), providing structural support (like collagen), facilitating communication between cells (like hormones), and transporting molecules (like hemoglobin).

The distinction between proteins and the other classes of macromolecules is crucial in understanding their role in biological systems. Carbohydrates primarily serve as energy sources and structural components, nucleic acids are involved in storing and transmitting genetic information, and lipids mainly function in long-term energy storage and forming cellular membranes. While these other macromolecules are essential to life, they do not have the same functional role as polypeptides.