ATP: Structure, Hydrolysis and Resynthesis
The structure of ATP, how its hydrolysis releases energy for energy-requiring reactions, how phosphorylation works, and how ATP is resynthesised. Includes the 5-mark exam question, a worked model answer and the examiner trap on mixing up ATP hydrolase and ATP synthase.
Chapters (9)
Exam question [5 marks]
Describe the structure of ATP. Explain how the hydrolysis of ATP allows a cell to carry out an energy-requiring reaction, and how ATP is resynthesised.
Try it on paper first, then check the mark scheme below. The question is original and was written for this lesson.
Mark scheme
| Mark | Mark point | Also accept | Do not accept |
|---|---|---|---|
| 1 | ATP is a nucleotide derivative made of ribose, adenine and three phosphate groups | "adenosine plus three phosphates", "ribose, adenine, 3 phosphates" | "deoxyribose"; "adenine, thymine and phosphates"; two phosphates |
| 2 | ATP is hydrolysed to ADP and inorganic phosphate (Pi) by ATP hydrolase, releasing energy | "water used to remove a phosphate", "energy released"; mark awarded for ADP + Pi + energy released (the enzyme name is not required, it only matters to reject "synthase") | "ATP synthase" for hydrolysis; "energy is made / produced / created" |
| 3 | Energy released is coupled to / used in an energy-requiring reaction | "drives an endergonic reaction", "used for active transport, muscle contraction, synthesis" | "ATP stores energy for a long time"; "energy used" with no link to a reaction |
| 4 | Pi can phosphorylate another compound, making it more reactive | "adds phosphate to a molecule", "lowers activation energy of the compound" | "Pi gives energy" alone |
| 5 | ATP is resynthesised from ADP and Pi by condensation, catalysed by ATP synthase (in respiration or photosynthesis) | "condensation reaction", "water is released"; condensation of ADP and Pi credited without naming the enzyme | "hydrolysis" for resynthesis; "ATP hydrolase" for resynthesis |
The two enzyme names are the commonest lost marks: hydrolase breaks, synthase builds. Energy is released, never made. ATP is an immediate energy source with a rapid turnover, not a long-term store; a statement that it is a long-term store is rejected. Where this video goes beyond the spec: examples of energy-requiring processes and the short comparison with starch, glycogen and fat as long-term stores (no mark).
Model answer (5/5)
ATP is a nucleotide derivative made of ribose, adenine and three phosphate groups. It is hydrolysed to ADP and inorganic phosphate by ATP hydrolase, and this releases energy. The energy released is coupled to an energy-requiring reaction in the cell, such as active transport. The inorganic phosphate can phosphorylate another compound, making it more reactive. ATP is resynthesised from ADP and inorganic phosphate in a condensation reaction catalysed by ATP synthase, during respiration or photosynthesis.
Examiner trap
- Saying ATP 'stores' energy for long term (it is an immediate energy source)
- Saying energy is made when ATP is hydrolysed rather than released
- Saying ATP is hydrolysed by ATP synthase
- Writing that ADP + Pi forms ATP by hydrolysis
Key terms
- ATP
- ADP
- inorganic phosphate
- ATP hydrolase
- ATP synthase
- phosphorylation
- coupled reaction
- condensation
- hydrolysis
Lesson notes
A written version of the video, chapter by chapter, for revision. Times link to the video.
0:26What ATP is made of
ATP stands for adenosine triphosphate. It is built from three parts. A sugar called ribose, a base called adenine, and a chain of three phosphate groups. Ribose plus adenine is called adenosine. Add three phosphates and you get adenosine triphosphate. Because it is a nucleotide with extra phosphates, we call it a nucleotide derivative.
0:52Hydrolysis releases energy
Now the clever bit. A cell breaks off the end phosphate using water. That is hydrolysis. The enzyme that catalyses it is ATP hydrolase. The products are ADP, adenosine diphosphate, and a free phosphate group, which we call inorganic phosphate, or P i. And this reaction releases energy.
Released, not made. Energy is never created.
1:21Energy for cell reactions
That energy is used by cells in reactions that need it, like making proteins, active transport, or muscle contraction. The cell couples the two reactions. The energy released from ATP hydrolysis drives the reaction that needs energy. One reaction gives, the other takes. That is called coupling. In the exam, always say the energy from hydrolysis is used in a named reaction, not just that it is used.
1:50Phosphorylation
There is a second way ATP helps. The phosphate it releases can attach to another compound. That is called phosphorylation. Adding a phosphate often makes a compound more reactive, so it is easier for it to take part in the next reaction.
2:05Resynthesis by condensation
ATP is not used up for good. It is rebuilt. ADP and P i are joined together by a condensation reaction. A water molecule is lost, and ATP forms. The enzyme is ATP synthase. It does this during respiration, and during photosynthesis. So you get a cycle. Hydrolysis breaks it down, condensation builds it up.
2:32ATP is turned over constantly
Cells keep only a small amount of ATP, and it is turned over all the time. Each molecule is hydrolysed and rebuilt again and again, so it works like a coin that keeps being spent and re-minted. It is an immediate source of energy, not a long-term store. For long-term storage, cells use things like starch, glycogen and fat.
3:37Examiner trap and recap
The trap. Mixing up the two enzymes. Hydrolase breaks ATP down. Synthase builds it up. Hydrolysis breaks, condensation builds.
Recap. ATP is ribose, adenine and three phosphates. Hydrolysis releases energy, condensation rebuilds it. Next up, water, and the five properties that keep living things alive.
Watch first
This lesson builds on:
- DNA and RNA Nucleotides: Structure Compared · AQA 7402 · 3.1.5.1
Keep going
More from 3.1 Biological molecules
- Condensation and Hydrolysis Reactions · AQA 7402 · 3.1.1
- Maltose, Sucrose, Lactose: Glycosidic Bonds · AQA 7402 · 3.1.2
- Alpha vs Beta Glucose: Starch vs Cellulose · AQA 7402 · 3.1.2
- Benedict's and Iodine Tests Explained · AQA 7402 · 3.1.2
All 21 3.1 Biological molecules lessons Whole playlist on YouTube
Independent revision resource, not affiliated with or endorsed by AQA. The exam question is original and was written for this lesson; it is not an AQA question. Spotted an error? Email info@medservatory.com with the lesson code AB-019.