What is the primary energy source for most living organisms?

The primary energy source for most living organisms is ATP (adenosine triphosphate). ATP is often referred to as the "energy currency" of the cell because it is the molecule that stores and transports chemical energy within cells. When ATP is hydrolyzed (broken down) to ADP (adenosine diphosphate) and inorganic phosphate, it releases energy that can be harnessed by the cell to perform various functions, such as muscle contraction, enzyme activity, and bio-synthesis of macromolecules.

While glucose serves as a major fuel source that is broken down during cellular respiration to ultimately produce ATP, it is not the direct energy currency used by cells. Instead, glucose is converted into ATP through metabolic processes. Similarly, fatty acids can also be utilized for energy through a process called beta-oxidation, but they must first undergo conversion into acetyl-CoA, which then enters the Krebs cycle leading to the production of ATP. NADH, a reduced form of nicotinamide adenine dinucleotide, is an electron carrier that plays a critical role in the electron transport chain, leading to ATP synthesis, but it is not the immediate energy source itself.

Therefore, ATP is the correct answer, as it is the