Phosphocreatine ATP system
Laura Armstrong
Teacher
Contents
Recall Questions
This topic requires prior knowledge of aerobic and anaerobic respiration. You can test your knowledge on these below.
What is the main source of ATP during long-duration, low-intensity exercise?
Aerobic respiration in mitochondria.
What happens to pyruvate in anaerobic respiration in muscle cells?
It is converted to lactate to regenerate NAD, allowing glycolysis to continue.
Why does anaerobic respiration produce less ATP than aerobic respiration?
Because glucose is only partially broken down, yielding just 2 ATP per molecule.
Topic Explainer Video
ATP as an Energy Source in Muscle Cells
- ATP is the energy source for muscle contraction.
- It is produced by aerobic respiration and anaerobic respiration (when there is insufficient oxygen).
- It is hydrolysed by ATPase to ADP + Pi, releasing energy: ATP → ADP + Pi
- Skeletal muscle cells can also rely on the phosphocreatine (PCr) system, especially during short bursts of high-intensity activity.
- This is because muscles demand a very rapid supply of ATP during contraction, and the PCr system can regenerate ATP almost instantly, even without oxygen.
The Phosphocreatine (PCr) System
- Phosphocreatine (PCr) is a high-energy compound stored in muscles.
- It regenerates ATP very rapidly by donating a phosphate group to ADP: PCr + ADP → ATP + Creatine
- Does not require oxygen (anaerobic).
- Used during short bursts of high-intensity activity (e.g., sprinting, weight lifting).
- This system allows a fast but short-lived supply of energy.
- PCr stores deplete quickly (around 8–10 seconds of activity).
- Creatine is either reused or converted to creatinine and excreted in urine.
Recovery and Replenishment
- PCr is replenished during rest using ATP produced by aerobic respiration.
- ATP + creatine → PCr +ADP.
Comparison with Other Pathways
| Pathway | Oxygen Required | ATP Yield | Duration of Energy Supply | Speed of ATP Production |
| Phosphocreatine (PCr) | X | 1 ATP per PCr. | ~10 sec. | Very fast. |
| Anaerobic glycolysis | X | 2 ATP per glucose. | ~1–2 min. | Fast. |
| Aerobic respiration | ✓ | About 32 ATP glucose. | Long-duration. | Slower. |
Key Terms
- ATP (Adenosine Triphosphate): The immediate energy source for muscle contraction.
- Phosphocreatine (PCr): A molecule that stores phosphate to rapidly regenerate ATP in muscle cells.
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Exam Tip
Students often forget about the PCr system altogether as a way that skeletal muscles can produce ATP. Also, some confuse it with anaerobic respiration. It’s a separate, distinct system.
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What is the role of phosphocreatine (PCr) in providing energy during muscle contraction? (2 marks)
- Phosphocreatine donates a phosphate group to ADP
- forming ATP.
Practice Question 1
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!
Practice Question 2
If you want to try out another one, check this video out and see how you do!