Adenosine Triphosphate (ATP) is often referred to as the cell’s main energy currency. It provides energy for living cells so they can carry out all their important life processes. This could include: DNA replication, muscle contraction, cell division, protein synthesis, there are many different examples!
But ATP doesn’t instantly provide energy for cells, it needs to be broken down through hydrolysis in order to release energy for cells.
Tip
Be careful NOT to say energy is produced. The Law of Conservation of Energy (if you remember back to GCSE Physics) states that energy cannot be created or destroyed, only transferred or transformed. Chemical energy is stored in the bonds of ATP, and is released through hydrolysis. This chemical energy can be transformed into various energy types to support life processes, e.g. kinetic, heat and mechanical.
ATP Structure
What you need to know (from the AQA specification)
What you need to know (from the AQA specification)
A single molecule of adenosine triphosphate (ATP) is a nucleotide derivative and is formed from a molecule of ribose, a molecule of adenine and three phosphate groups.
ATP is made of three components:
- Adenine - Nitrogenous base
- Ribose - a pentose (5-carbon) sugar
- Phosphate - 3 phosphates (‘tri’ as the name suggests)
ATP is a nucleotide derivative. It has a similar structure to an RNA nucleotide (same ribose sugar and adenine base, see Structure of DNA & RNA) but with three phosphate groups instead of one.
All the 3 phosphate groups are negatively charged, which causes them to repel each other. This makes ATP unstable.
The two bonds that link the 3 phosphate groups are high energy bonds (see image below). This means when they are broken in a hydrolysis reaction they release a significant amount of energy.
Hydrolysis & Synthesis of ATP
What you need to know (from the AQA specification)
What you need to know (from the AQA specification)
Hydrolysis of ATP to adenosine diphosphate (ADP) and an inorganic phosphate group (Pi ) is catalysed by the enzyme ATP hydrolase.
- The hydrolysis of ATP can be coupled to energy-requiring reactions within cells.
- The inorganic phosphate released during the hydrolysis of ATP can be used to phosphorylate other compounds, often making them more reactive.
You can think of ATP as a power pack (or a battery is a common analogy for it).
When it’s fully charged you can use it to ‘release energy’ to power other devices (your phone, iPods, etc.) - this is ATP hydrolysis.
But, when it’s run out of charge, it needs to be plugged back in and recharged again - this is ATP synthesis.
Hydrolysis
- The hydrolysis of ATP is catalysed by the enzyme ATP hydrolase (ATPase)
- As this is a hydrolysis reaction it requires water
- A high energy phosphate bond is broken and energy is released, producing ADP and an inorganic phosphate (You don’t need to worry about ‘Inorganic’ phosphate, it just refers to a phosphate that is free and not attached to a carbon.)
ATP hydrolysis
- ATP hydrolysis can be ‘coupled’ to other energy-requiring reactions within the cell (called energy coupling).
- This ensures that the energy released from ATP hydrolysis can be used by other reactions that need it, so it isn’t wasted as heat.
- A few examples of these energy-coupling reactions include (there are many of these!)
- Nerve cells: Sodium-potassium pump uses ATP to actively transport sodium out of the cells and potassium into the cells, helping to maintain the cell’s resting membrane potential.
- Muscle Contraction: ATP is used to move the myosin heads (power stroke) and detach them from the actin-myosin bridge
- What happens to the released phosphate from ATP hydrolysis?
- This phosphate can be added to another compound, known as phosphorylation.
- This can make the other compound more reactive (adding a negatively charged phosphate to the compound).
- An example of this is the phosphorylation of glucose to glucose phosphate, which is a more reactive intermediate in Glycolysis
Synthesis
What you need to know (from the AQA specification)
What you need to know (from the AQA specification)
ATP is resynthesised by the condensation of ADP and Pi . This reaction is catalysed by the enzyme ATP synthase during photosynthesis, or during respiration.
ATP regeneration
- ATP can be resynthesised by the condensation reaction of ADP and Pi, and requires energy (water is released).
- This is catalysed by ATP synthase and happens in:
How this topic is tested
This analysis is based on past paper data from 2017 to 2025. It is intended for interest only and is not predictive of what will appear in future papers.
- Tested in 5 of 9 years (2017–2025): 5 question parts worth 10 marks.
- 47th most-examined topic overall by marks, 6th in Unit 1.
Marks by year
Most-tested spec points
- ATP Hydrolysis: tested in 3 parts (8 marks)
- ATP Synthesis: tested in 3 parts (6 marks)
- ATP Structure: tested in 1 part (4 marks)
Also links to: Active Transport, Oxidative Phosphorylation.
Tips from examiner reports
What students commonly get wrong
- Energy is released, not producedWatch out: ATP hydrolysis releases energy. Never write that energy is "produced" or "made": answers that did lost the mark. 2025 P1 Q5.1 · 2025 P3 Q1.3 · 2017 P3 Q1.3
- Describing ATP's structureWhen describing ATP's structure, say ribose, adenine and three phosphate groups. Adenine and ribose together are called adenosine, so "adenosine and three phosphates" is also correct. But "adenine and three phosphates" (no ribose), "ATP is made of ADP and Pi" and "pentose sugar" weren't credited. 2024 P1 Q10.3
- ATP hydrolase vs ATP synthaseWatch out: ATP hydrolase breaks ATP down by hydrolysis, which uses water. ATP synthase makes ATP by condensation, which releases water. 2024 P1 Q10.3
Exam Question Practice
A few exam-style questions on this topic to try. Have a go at each one before opening the answer.
ATP is the immediate source of energy for cells.
Describe the structure of ATP, and outline how named enzymes break down and resynthesise it.
(4 marks)
Hint
What three components make up ATP? What happens to water in the reactions that make and break ATP?
Mark Scheme
- ATP is made of ribose, adenine and three phosphate groups (1 mark)
- ATP hydrolase breaks ATP down into ADP + Pi in a hydrolysis reaction (1 mark)
- ATP synthase makes ATP from ADP + Pi (1 mark)
- In a condensation reaction (1 mark)
Also accepted / rejected
Tips
- ATP structure = adenosine (adenine + ribose) + 3 phosphate groups: don’t just say “ADP + Pi”
- Say ribose specifically, not just “pentose sugar”
- Hydrolysis of ATP uses water and is catalysed by ATP hydrolase: don’t confuse hydrolysis with condensation
- Don’t spend time describing ATP as an “immediate energy source” when the question asks about structure and reactions
Root hair cells are specialised for the absorption of mineral ions from the soil.
A carrier protein in the cell-surface membrane of these cells transports nitrate ions into the cell.
The action of this carrier protein is linked to a membrane-bound ATP hydrolase enzyme.
Explain the function of this ATP hydrolase.
(2 marks)
Hint
Think about what the carrier protein needs in order to carry out its role.
Mark Scheme
- (ATP → ADP + Pi) releases energy (1 mark)
- (The energy) allows ions to be moved against a concentration gradient OR (the energy) allows active transport of ions (1 mark)
Also accepted / rejected
- For point 1: reject “produces/makes/creates energy”
- For point 2: accept nitrate ions for “ions”
- For point 2: the movement must be of ions
Glucagon stimulates the phosphorylation of the enzyme glycogen phosphorylase, which activates the enzyme.
Describe how an enzyme can be phosphorylated.
(2 marks)
Hint
Where does the phosphate come from, and what happens to it when the enzyme is phosphorylated?
Mark Scheme
- Attachment/association of (inorganic) phosphate (to the enzyme) (1 mark)
- (Released from the) hydrolysis of ATP
OR (Released when) ATP → ADP + Pi (1 mark)
Also accepted / rejected
1 and 2. For “phosphate/Pi” accept PO₄³⁻ or P in a circle.
1. Accept “phosphate goes to the enzyme”.
1. Ignore named bonds or where the phosphate attaches, but reject the formation of an enzyme-substrate complex.
2. Accept adenosine triphosphate for ATP and adenosine diphosphate for ADP.
Tips
- Expand on the word in the question: ‘phosphate is used to phosphorylate the enzyme’ isn’t enough; say the phosphate attaches to the enzyme
- Say where the phosphate comes from: hydrolysis of ATP to ADP + Pi
- This isn’t an enzyme-substrate complex
Scientists investigated the action of ATP synthase taken from spinach chloroplasts. They made reaction mixtures each containing:
- ATP synthase
- buffer (to control pH)
- substrates
One of the substrates required in these reaction mixtures is adenosine diphosphate (ADP).
Select one box to show which other substrate the scientists must add to the reaction mixtures to produce ATP.
A fungus causes a disease in wheat crops. Scientists are developing a new fungicide that inhibits the ATP synthase enzyme of this fungus.
Select the appropriate box next to the equation which represents the reaction catalysed by ATP synthase.
Also accepted