ATP metabolism involves several key small molecules, including adenosine triphosphate (ATP), adenosine diphosphate (ADP), adenosine monophosphate (AMP), oxidized nicotinamide adenine dinucleotide (NAD+), reduced nicotinamide adenine dinucleotide (NADH), coenzyme A (CoA), acetyl coenzyme A (acetyl-CoA), and inorganic phosphate (Pi). ATP is the primary unit of energy transfer within cells, crucial for storing and transmitting chemical energy.