Explain why ATP is a suitable energy currency
Relate ATP's properties and hydrolysis to its role in transferring usable energy between cellular reactions.
A Level; Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; practical investigations need supervised work and human subject-teacher review remains pending.
Before you begin
- ATP transfers energy for cellular work, and enzymes control metabolic reactions.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is formed when ATP loses its terminal phosphate by hydrolysis?
What you will learn
- Describe ATP as a small, soluble molecule used in cells
- Explain energy transfer by ATP hydrolysis and phosphorylation
Build the explanation
ATP is a small, water-soluble molecule that can move within cells and transfer energy to cellular processes. Hydrolysis removes a terminal phosphate group, forming ADP and inorganic phosphate and transferring energy that can be coupled to cellular work. Phosphate transfer can phosphorylate another molecule, making it more reactive or helping drive a reaction. ATP can be resynthesised from ADP and phosphate, so it acts as a reusable, short-term energy currency.
Apply it carefully
Avoid describing a phosphate bond as a packet that simply contains energy. The energy transfer comes from the overall hydrolysis reaction and how it is coupled to other reactions. ATP is not a long-term energy store comparable to fats or glycogen; it is continually broken down and regenerated.
Pause and explain
Why is ATP described as a currency rather than a long-term store?
Worked example
What products form when ATP is hydrolysed?
Show the worked solution
- Hydrolysis removes the terminal phosphate group.
- Name the molecule remaining and the released phosphate.
Answer ATP + water → ADP + inorganic phosphate; energy is transferred and can be coupled to cellular work.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain how ATP hydrolysis can help power an energy-requiring reaction.
Give me a hint
Include the products and the idea of coupling.
Compare my reasoning
- ATP is hydrolysed to ADP and inorganic phosphate.
- Energy transfer and/or phosphate transfer is coupled to the cellular reaction.
ATP hydrolysis forms ADP and inorganic phosphate. The energy transfer can be coupled to an energy-requiring reaction, and phosphate transfer can make a reactant more reactive.
Look for these in your work
- Products correct
- Coupling explained
- Phosphate transfer distinguished
A learner writes, 'The terminal phosphate bond breaks and releases a packet of energy.' Refine this account.
Give me a hint
Describe the overall reaction and avoid treating a bond as stored fuel.
Compare my reasoning
- ATP hydrolysis forms ADP and inorganic phosphate.
- Energy transfer depends on the overall reaction and its coupling to cellular work.
ATP is hydrolysed to ADP and inorganic phosphate. Energy is transferred by the overall hydrolysis reaction and can be coupled to cellular work; it is misleading to treat the bond alone as a packet of energy.
Look for these in your work
- Hydrolysis products named
- Overall reaction emphasized
- Bond-energy misconception corrected
A cell's ATP supply is used rapidly. Explain how the ATP pool can continue to support cellular work.
Give me a hint
Describe recycling between ATP and ADP.
Compare my reasoning
- Hydrolysis converts ATP to ADP and phosphate.
- Respiration and photosynthesis can drive ATP resynthesis by phosphorylating ADP.
ATP can continue to support work because ADP and inorganic phosphate are resynthesised into ATP using energy transferred from processes such as respiration, and in photosynthetic cells also photosynthesis. ATP is recycled rather than stockpiled as the main long-term store.
Look for these in your work
- ADP and phosphate identified
- Resynthesis described
- Currency/store distinction maintained
Common mistakes
- Calling a phosphate bond a packet of energy
- Treating ATP as the main long-term energy store
- Omitting coupling to cellular work
What can you explain now?
What is formed when ATP loses its terminal phosphate by hydrolysis?
Compare with the explanation
ADP and inorganic phosphate
ATP hydrolysis can transfer energy to coupled cellular processes.
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Make it stick
How does ATP hydrolysis transfer energy, and why can ATP be reused?
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