Which of the following best represents the correct sequence of events in the ATP cycle during cellular respiration?

• A. ATP is hydrolyzed to ADP + P_i, releasing energy; ADP is then phosphorylated to regenerate ATP using energy from cellular respiration.
• B. ADP and P_i combine spontaneously to form ATP without the need for energy input.
• C. ADP is hydrolyzed to ATP, releasing energy; ATP is then phosphorylated to regenerate ADP using energy from cellular respiration.
• D. ATP is synthesized from glucose directly without any intermediate steps.

Answer: (A)

Verified step by step guidance

1. Understand the ATP cycle: ATP (adenosine triphosphate) is the primary energy currency of the cell. It is used to power various cellular processes by releasing energy when it is hydrolyzed into ADP (adenosine diphosphate) and an inorganic phosphate (P_i).
2. Step 1: ATP hydrolysis occurs when ATP is broken down into ADP and P_i. This reaction releases energy that can be used by the cell for various functions. The reaction can be represented as: ${\mathrm{ATP}}_{}\to {\mathrm{ADP}}_{}+P_{}$_i + Energy.
3. Step 2: ADP and P_i are then phosphorylated (recombined) to regenerate ATP. This process requires an input of energy, which is provided by cellular respiration. The reaction can be represented as: ${\mathrm{ADP}}_{}+P_{}$_i + Energy → ATP.
4. Step 3: Cellular respiration provides the energy needed for the phosphorylation of ADP to ATP. This energy is derived from the breakdown of glucose and other molecules during processes such as glycolysis, the citric acid cycle, and oxidative phosphorylation.
5. Step 4: The ATP cycle is a continuous process where ATP is hydrolyzed to release energy, and ADP is phosphorylated to regenerate ATP using energy from cellular respiration. This ensures a constant supply of ATP for cellular activities.