What macromolecule serves as the primary energy source for cells?

Carbohydrates are the primary energy source for cells due to their structure and role in metabolism. They consist of carbon, hydrogen, and oxygen, usually in a ratio of 1:2:1, which makes them easily usable for energy. The most common simple carbohydrate, glucose, is critical for cellular respiration, where it is broken down to release energy stored in its chemical bonds.

Cells convert glucose through a series of metabolic pathways including glycolysis, the Krebs cycle, and oxidative phosphorylation. This leads to the production of adenosine triphosphate (ATP), the energy currency of the cell. Carbohydrates can be quickly mobilized for energy, making them an efficient source for immediate energy needs during physical activity or metabolic processes.

While proteins can also supply energy and nucleic acids play crucial roles in genetic information and regulation, they are not primary energy sources like carbohydrates. Lipids, though energy-dense, primarily serve as long-term energy storage and insulation rather than immediate energy supply. Therefore, carbohydrates are unequivocally the most immediate and accessible source of energy for cellular functions.