Energy carriers function by:

• A. They permanently store energy for long-term use
• B. They temporarily store and transfer energy from energy-producing reactions to energy-requiring reactions
• C. They are only molecules like NADH
• D. They are lipid molecules

Answer: (B)

Energy carriers act as temporary couriers in metabolism, capturing energy produced in breaking down nutrients and delivering it to reactions that need energy to proceed. ATP stores energy in its phosphate bonds and can release it quickly to drive endergonic steps; other carriers like NADH and NADPH carry electrons (reducing equivalents) to power further reactions or generate a usable energy currency in the cell. They are regenerated after delivering their energy, so they’re not meant for long-term storage. This shuttle role—brief storage plus transfer of energy from energy-producing to energy-requiring processes—best matches how cells couple reactions. Choices that imply permanent storage, that limit carriers to NADH alone, or that identify carriers as lipids don’t fit this broad, functional view.