Many proteins regulate or control biological activity. Insulin, for example, controls the rate at which sugar is metabolized. O 2 is carried through the bloodstream by hemoglobin and myoglobin. Other proteins transport sugars, amino acids, and ions across the cell membranes. The first line of defense against viruses and bacteria are the antibodies produced by the immune system, which are based on proteins.
Carbohydrates are the primary source of food energy for most living systems.
Carbohydrates are produced from CO 2 and H 2 O during photosynthesis and are therefore the end products of the process by which plants capture the energy in sunlight. The name nucleic acid was originally given to a class of relatively strong acids that were found in the nuclei of cells. As polymers, they store and process the information that allows the organism to grow and eventually reproduce. Compounds are classified as proteins, carbohydrates, and nucleic acids on the basis of similarities in their structures. Lipids , on the other hand, are defined on the basis of their physical properties.