What is the primary energy source used by cells for most of their work?

The primary energy source used by cells for most of their work is ATP, or adenosine triphosphate. ATP is often referred to as the "molecular unit of currency" of intracellular energy transfer. It provides energy for various cellular processes, such as muscle contraction, nerve impulse propagation, biosynthesis, and active transport across cell membranes.

When cells require energy, ATP undergoes hydrolysis, breaking down into ADP (adenosine diphosphate) and an inorganic phosphate, releasing energy that can be harnessed for immediate use. This conversion process and the ability to rapidly regenerate ATP from ADP using cellular respiration makes ATP the most vital energy molecule within living organisms.

While other molecules listed, such as GTP and cAMP, are also involved in cellular processes and signaling, ATP remains the primary energy source that directly powers the majority of metabolic reactions in the cell.