ATPAQA A-Level Biology: Revision notes
Section 1
Structure of ATP
ATP (adenosine triphosphate) is a nucleotide derivative. A single molecule is formed from:
- one molecule of ribose
- one molecule of adenine
- three phosphate groups
The ribose and adenine together form adenosine. ADP has two phosphate groups, and Pi is a single inorganic phosphate group.
ATP contains ribose, not deoxyribose, and adenine only. Do not write deoxyribose or thymine.
Section 2
Hydrolysis of ATP
ATP is hydrolysed to ADP and Pi: ATP + H₂O → ADP + Pi. The reaction is catalysed by the enzyme ATP hydrolase and releases energy, about 30 kJ per mole. For example, hydrolysing 0.25 mol releases 0.25 × 30 = 7.5 kJ.
ATP is an ideal immediate energy source because the hydrolysis is a single-step reaction that releases a small, manageable amount of energy, so little is wasted as heat. ATP is small and soluble, so it can be moved to where energy is needed.
Section 3
Coupling and phosphorylation
The hydrolysis of ATP can be coupled to energy-requiring reactions in the cell, so the energy released drives them. Examples include active transport, protein synthesis and muscle contraction.
The inorganic phosphate released can be used to phosphorylate other compounds, which often makes them more reactive. For example, glucose is phosphorylated at the start of glycolysis, and a phosphorylated carrier protein changes shape in active transport.
Two roles of the phosphate: the energy released drives the reaction, and Pi can phosphorylate a compound to make it more reactive.
Section 4
Resynthesis of ATP
ATP is resynthesised by the condensation of ADP and Pi, a reaction that releases water, catalysed by the enzyme ATP synthase. This occurs during photosynthesis (in chloroplasts, using light energy) and during respiration (in mitochondria, using energy from respiratory substrates such as glucose).
Cells store very little ATP, so it is continually cycled: hydrolysed to ADP and Pi, then resynthesised. For example, if muscles hydrolyse 0.80 mol of ATP per minute but hold only 0.10 mol, the store would last 0.10 / 0.80 = 0.125 minutes (7.5 s) if it were not resynthesised.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on ATP
- The hydrolysis of ATP to ADP and inorganic phosphate (Pi) releases about 30 kJ of energy per mole of ATP.Describe the hydrolysis of ATP.2 marks
- In the first step of glycolysis, glucose reacts with ATP to form glucose phosphate and ADP. Glucose phosphate is more reactive than glucose, and the reaction is catalysed by an enzyme.Explain how the hydrolysis of ATP makes the formation of glucose phosphate possible.2 marks
- During intense exercise the muscles of an athlete hydrolyse ATP at a rate of 0.80 mol per minute, but the muscles contain only about 0.10 mol of ATP at any one time. Assume that 30 kJ of energy is released per mole of ATP hydrolysed.Suggest why ATP, rather than glucose, is the immediate source of energy for muscle contraction.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).