A-Level · Biology · AQA · Mark scheme decoded
AQA A-Level Biology: ATP Structure and Function — mark scheme explained
The short answer
Adenosine triphosphate (ATP) is a crucial molecule in biological systems, serving as the primary energy currency of cells. Understanding its structure and function is essential for grasping how energy is transferred and utilized within living organisms.
The question
Write the equation for the hydrolysis of ATP to ADP and inorganic phosphate (P i ).
[Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
How the examiner actually awards the marks on this topic.
Gradora's own decode of the marking approach — not the exam board's published mark scheme.
How marks are awarded
For identification questions, marks are typically awarded for correctly naming the components or structures. For explanation questions, marks are given for clear and detailed descriptions that demonstrate a deep understanding of the topic. For writing equations, marks are awarded for correct reactants, products, and balancing.
What the command words demand
- Identify
- Recognize and name specific components or structures.
- Explain
- Provide a detailed description of the concept, including relevant principles and relationships.
- Write
- Formulate a balanced chemical equation for a given reaction.
- Describe
- Give a clear and concise account of a process or mechanism.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate approximately 5-7 minutes per question to ensure you have enough time to carefully read the problem, provide detailed answers, and check your work.
- Identify the reactants and products.1 markReactant: ATP (adenosine triphosphate)Products: ADP (adenosine diphosphate), P i (inorganic phosphate), Energy
- Write the balanced equation.2 marksATP + H 2 O → ADP + P i + Energy
Final answer: ATP + H 2 O → ADP + P i + Energy
Work through every step correctly and you earn all 3 marks.
Another worked example
Explain how the energy released during the hydrolysis of ATP can be coupled to an energy-requiring reaction, such as active transport.
- Describe the hydrolysis of ATP.2 marksThe hydrolysis of ATP releases a large amount of energy when one phosphate group is removed from ATP, forming ADP and inorganic phosphate (P i ).
- Explain how this energy can be coupled to active transport.2 marksThis released energy can be used to drive the movement of ions or molecules against their concentration gradient. For example, in the sodium-potassium pump, the energy from ATP hydrolysis is used to move sodium ions out of the cell and potassium ions into the cell.
Final answer: The hydrolysis of ATP releases a large amount of energy when one phosphate group is removed from ATP, forming ADP and inorganic phosphate (P i ). This released energy can be coupled to active transport by driving the movement of ions or molecules against their concentration gradient. For example, in the sodium-potassium pump, the energy from ATP hydrolysis is used to move sodium ions out of the cell and potassium ions into the cell.
Work through every step correctly and you earn all 4 marks.
Common mistakes
Misidentifying the components of ATP.
Why it happens: Students may confuse the components of ATP, such as mistaking adenine for a different nitrogenous base or forgetting the number of phosphate groups.
Fix: Always remember that ATP consists of ribose (a five-carbon sugar), adenine (a purine base), and three phosphate groups.
Forgetting the role of high-energy bonds in ATP.
Why it happens: Students may overlook the importance of the high-energy bonds between the phosphate groups, which are crucial for energy release during hydrolysis.
Fix: Understand that the high-energy bonds between the phosphate groups in ATP are responsible for the large amount of energy released during hydrolysis.
Misunderstanding the role of ATP hydrolase.
Why it happens: Students may not recognize that ATP hydrolase is the enzyme responsible for catalyzing the hydrolysis of ATP.
Fix: Remember that ATP hydrolase facilitates the breaking of the high-energy bond between the phosphate groups, releasing energy and forming ADP and P i .
Failing to explain how energy from ATP is coupled to cellular processes.
Why it happens: Students may struggle to articulate the concept of coupling, leading to vague or incorrect explanations.
Fix: Practice explaining how the energy released during the hydrolysis of ATP can be used to drive various energy-requiring reactions in cells, such as active transport and muscle contraction.
Confusing phosphorylation with other processes.
Why it happens: Students may mix up the concept of phosphorylation with other cellular processes, leading to incorrect answers.
Fix: Understand that phosphorylation involves adding a phosphate group to a molecule, often making it more reactive. This process is crucial for enzyme activation and metabolic pathways.
Forgetting the role of ATP synthase in resynthesis.
Why it happens: Students may not recognize that ATP synthase is the enzyme responsible for catalyzing the resynthesis of ATP from ADP and P i .
Fix: Remember that ATP synthase facilitates the synthesis of ATP during both photosynthesis and cellular respiration by using energy to combine ADP and P i .
Where the marks go
The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.
| Question type | What you’re asked to do | Marks |
|---|---|---|
| Write Equation | Write the balanced equation for ATP hydrolysis into ADP and inorganic phosphate. | 3 |
| Explain ATP Coupling | Explain how energy from ATP hydrolysis is coupled to drive active transport. | 4 |
| Explain Enzyme Activation | Explain how phosphorylation changes enzyme shape to activate it for catalytic function. | 3 |
| Write Equation | Write the balanced word equation showing ATP being resynthesised from ADP and inorganic phosphate. | 3 |
| Explain ATP Synthesis | Describe how ATP is resynthesized in both photosynthesis and cellular respiration. | 4 |
| Total across these question types | 17 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.