ATP
AQA
A-Level
ATP is a phosphorylated nucleotide derivative comprising the organic base adenine, the pentose sugar ribose, and a chain of three phosphate groups. It serves as the immediate energy source for biological processes. The bonds between the phosphate groups are unstable. While breaking the terminal phosphate bond requires a small input of activation energy, the overall hydrolysis reaction to form ADP and inorganic phosphate is highly exergonic, releasing approximately 30.5 kJ mol⁻¹ of usable energy. This provides a small, manageable packet of energy, preventing thermal damage to the cell. ATP hydrolase catalyses this breakdown, while ATP synthase re-forms ATP from ADP and Pi during respiration and photosynthesis, ensuring continuous recycling.
9
Objectives
8
Exam Tips
13
Pitfalls
17
Key Terms
15
Mark Points
Subtopics in this area
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.
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.
ATP Revision Guide
Learning Objectives
What you need to know and understand
- Name the three components of an ATP molecule and identify which one is removed during hydrolysis.
- Write the equation for the hydrolysis of ATP and name the enzyme that catalyses it and the enzyme that reverses it.
- Explain two properties of ATP that make it a suitable immediate source of energy rather than a long-term store.
- Describe the role of ATP hydrolysis in the movement of a myosin head during muscle contraction.
- Explain how the energy released from ATP is coupled to a named energy-requiring process rather than lost as heat.
- Suggest, for an organism that no longer needs to carry out one ATP-consuming process, two other processes the saved ATP could be used for.
- Explain how the phosphate released from ATP makes another molecule more reactive, using glucose in glycolysis as the example.
- Give two properties of ATP, other than the amount of energy released, that make it suitable as an immediate energy source.
- Distinguish phosphorylation of a substrate from the condensation reaction that forms ATP from ADP.
Marking Points
Key points examiners look for in your answers
- Name the three components of ATP: adenine, ribose, and three phosphate groups.
- Identify ATP as a nucleotide derivative and the immediate energy source of the cell.
- State that ATP hydrolase catalyses the hydrolysis of ATP to ADP and inorganic phosphate, releasing energy.
- State that ATP synthase catalyses the condensation reaction that re-forms ATP from ADP and inorganic phosphate.
- Explain that ATP releases energy in small, manageable amounts, making it a suitable immediate energy source.
- one mark for ATP being hydrolysed to ADP and inorganic phosphate by ATP hydrolase, releasing energy
- one mark for that energy being used directly in a named energy-requiring process, such as active transport, muscle contraction or the synthesis of a named molecule
- one mark for hydrolysis of ATP providing energy to 'cock' the myosin head back to its resting position
- one mark for the attachment of a new ATP molecule to each myosin head causing the head to detach from actin
- one mark for ATP releasing a small amount of energy at a time, so little is lost as heat, and being rapidly resynthesised
- one mark for the phosphate released being transferred to another compound, that is phosphorylation
- one mark for phosphorylation often making that compound more reactive
- one mark for a named example, such as the phosphorylation of glucose to glucose-6-phosphate in glycolysis or of glycerate 3-phosphate in the light-independent reaction
- one mark for ATP releasing a relatively small amount of energy, so little is lost as heat
- one mark for ATP being rapidly resynthesised and not being lost from the cell
Examiner Tips
Expert advice for maximising your marks
- 💡Ensure you name the specific enzymes for each direction: ATP hydrolase for breakdown and ATP synthase for synthesis.
- 💡When evaluating ATP's suitability, contrast its small, immediate energy release with the large, slower energy release from oxidising a whole glucose molecule.
- 💡Always follow 'energy is released' with 'used for', then a named process; the second half carries the mark.
- 💡Have three named uses ready from different topics, for instance active transport, muscle contraction and protein synthesis.
- 💡If the question is about an organism saving ATP, say what else that ATP can now be used for, such as growth or synthesis.
- 💡'Give two ways in which ATP is suitable as an energy source' is a common style of question; learn the list, including small energy release, instant release, rapid resynthesis and not leaving the cell. Note that phosphorylation is a role of the released inorganic phosphate, not a property that makes ATP itself suitable as an energy source.
- 💡Use the word 'reactive' explicitly, since that is the term the mark scheme rewards.
- 💡Name the molecule that gets phosphorylated in your example; an unnamed 'other compound' rarely gains the example mark.
Common Mistakes
Pitfalls to avoid in your exam answers
- Naming the base as adenosine; correct by specifying the base is adenine, while adenosine refers to the base plus the ribose sugar.
- Stating that ATP contains deoxyribose; correct by identifying ribose as the pentose sugar in ATP.
- Claiming the phosphate bonds 'store' large amounts of energy; correct by explaining that the bonds are unstable and it is the overall hydrolysis reaction that releases energy.
- writing that ATP is used for respiration, which examiners reject, since respiration is what produces ATP
- saying ATP creates or produces energy rather than releasing it
- stating that hydrolysis of ATP causes the myosin head to bend (the power stroke), when hydrolysis actually re-cocks the head and the power stroke follows ADP and Pi release
- confusing ATP hydrolase with ATP synthase when naming the enzyme
- saying the phosphate is released as waste or excreted
- writing that phosphorylation 'gives the molecule energy' without saying that it makes it more reactive
- confusing phosphorylation of another compound with the phosphorylation of ADP that forms ATP
- answering that ATP is useful because it releases a large amount of energy, when the advantage is a small usable amount
- giving ADP alone as the product of hydrolysis and omitting the inorganic phosphate
- stating that glucose is phosphorylated to fructose-6-phosphate; correct by noting that glucose is phosphorylated to glucose-6-phosphate, which is later isomerised to fructose-6-phosphate