What substance provides energy to cells?

Adenosine triphosphate (ATP) is known as the primary energy currency in cells. It plays a crucial role in storing and transferring energy within cells. ATP consists of a nucleotide with three phosphate groups, and its high-energy phosphate bonds, especially the terminal phosphate bond, release energy when hydrolyzed. This energy is then used for various cellular processes including muscle contraction, nerve impulse propagation, and biosynthetic reactions.

While glucose and glycogen are important energy sources, they primarily serve as substrates to be metabolized in a series of biochemical pathways to ultimately produce ATP. Protein can also provide energy, but its main roles are typically in structural support and as enzymes or signaling molecules rather than being the primary energy source. Therefore, ATP is uniquely positioned as the direct provider of energy that cells utilize for immediate needs.