What is ATP and why is it important?

• A. Adenosine triphosphate; the cell’s main energy currency.
• B. Adenosine triphosphate; a hormone that regulates metabolism.
• C. Adenosine diphosphate; a structural component of membranes.
• D. Oxygen-binding molecule used in respiration.

Answer: (A)

ATP is the cell’s main energy currency because it stores energy in the bonds between its phosphate groups and can release that energy quickly where it’s needed. It’s made of adenosine (adenine plus ribose) attached to three phosphate groups. When the cell needs energy, the bond on the last phosphate is split, producing ADP and inorganic phosphate and releasing usable energy to power processes like muscle contraction, active transport across membranes, and biosynthesis. The cell continuously regenerates ATP from ADP using energy from nutrients, mainly through cellular respiration or fermentation, so energy is always available for immediate needs.

This isn’t a hormone, so it doesn’t regulate metabolism in the way a signaling molecule would. It’s not adenosine diphosphate, which has one fewer phosphate and stores less energy per molecule. And it’s not an oxygen-binding molecule used in respiration (that’s hemoglobin or myoglobin).