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
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Step 1: Identify the reactants and products.
- S2
Reactant: ATP (adenosine triphosphate)
- S3
Products: ADP (adenosine diphosphate), P i (inorganic phosphate), Energy
- S4
Step 2: Write the balanced equation.
- S5
ATP + H 2 O → ADP + P i + Energy
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Step 1: Identify the reactants and products.
- S2
Reactant: ATP (adenosine triphosphate)
- S3
Products: ADP (adenosine diphosphate), P i (inorganic phosphate), Energy
- S4
Step 2: Write the balanced equation.
- S5
ATP + H 2 O → ADP + P i + Energy
Final answer: ATP + H 2 O → ADP + P i + Energy
Common mistakes
- Misidentifying the components of ATP. — 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. — 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. — 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. — 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. — 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. — 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
- Full worked solution (all marking points)3 marks