When 1 mole of ATP is hydrolyzed in a test tube without an enzyme, about twice as much heat is given off as when 1 mole of ATP is hydrolyzed in a cell. Which of the following best explains these observations?

• A. Cells are less efficient at energy metabolism than reactions that are optimized in a test tube.
• B. The amount of heat released by a reaction has nothing to do with the free energy change of the reaction.
• C. In cells, ATP is hydrolyzed to ADP and P_i, but in the test tube it is hydrolyzed to carbon dioxide and water.
• D. Cells have the ability to store heat; this cannot happen in a test tube.
• E. In the cell, the hydrolysis of ATP is coupled to other endergonic reactions.

Answer: (E)

Verified step by step guidance

1. Understand the concept of ATP hydrolysis: ATP (adenosine triphosphate) is a molecule that releases energy when it is hydrolyzed to ADP (adenosine diphosphate) and inorganic phosphate (Pi). This reaction is exergonic, meaning it releases energy.
2. Recognize the difference between reactions in a test tube and in a cell: In a test tube, ATP hydrolysis occurs in isolation, releasing energy primarily as heat. In a cell, ATP hydrolysis is often coupled with other reactions, meaning the energy released is used to drive other processes rather than being lost as heat.
3. Consider the role of coupling in cellular reactions: Cells use the energy from ATP hydrolysis to perform work, such as muscle contraction, active transport, or biosynthesis. This coupling makes the process more efficient in terms of energy utilization, as less energy is lost as heat.
4. Evaluate the options given: The correct explanation involves understanding that in cells, ATP hydrolysis is coupled to endergonic (energy-requiring) reactions, which means less energy is lost as heat compared to when ATP is hydrolyzed in a test tube.
5. Conclude with the correct explanation: The observation that less heat is released in cellular ATP hydrolysis compared to a test tube is best explained by the coupling of ATP hydrolysis to other endergonic reactions in the cell, making the process more efficient in terms of energy use.