Final answer:
Carbohydrates and lipids are the two macromolecules that are composed solely of Carbon, Hydrogen, and Oxygen. Carbohydrates are vital for energy and structural support, while lipids serve as key components of cell membranes and energy storage.
Step-by-step explanation:
The two macromolecules that contain only Carbon, Hydrogen, and Oxygen as their elements are carbohydrates and lipids. Carbohydrates are organic compounds that are essential for providing energy to cells and giving structural support to certain organisms. They consist of carbon, hydrogen, and oxygen, and are classified into monosaccharides, disaccharides, and polysaccharides. Lipids, including fats, oils, and waxes, are also composed only of carbon, hydrogen, and oxygen. These macromolecules are important for cell membranes and energy storage. Both carbohydrates and lipids are formed by dehydration synthesis, which is a process of joining monomers into polymers that involves the loss of water molecules.