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5.2.2Topic 5 · On the Wild Side

ATP as a Supply of Energy — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

5 min read 8 parts Lesson 14 of 34 1 quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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5.2.2 ATP as a Supply of Energy — Edexcel A Level Biology A (SNAB) 9BI0 notes: Describe the structure of ATP, explain how it is made and used, and why it is a suitable immediate source of energy for cells.Adenosine triphosphate (ATP) is a nucleotide derivative used as the univers…

  • three phosphate groups
  • ATP hydrolase
  • ATP synthase
  • phosphorylation
  • single step
  • active transport

Part 1 of 6Structure of ATP

Adenosine triphosphate (ATP) is a nucleotide derivative used as the universal energy currency in all living cells.

Structure of ATP
Adenine
It consists of the nitrogenous base adenine, the pentose sugar ribose (together forming adenosine) and three phosphate groups joined in a chain by phosphoanhydride bonds.
The bonds between the phosphates are unstable because the negatively charged phosphate groups repel each other; the products of hydrolysis are more stable, so energy is released when the terminal bond is broken.

ATP is hydrolysed to ADP (adenosine diphosphate) and inorganic phosphate (Pi), in a reaction catalysed by ATP hydrolase (ATPase). Roughly 30.6 kJ mol-1 is released. ATP is reformed from ADP and Pi by a condensation reaction catalysed by ATP synthase, which requires an input of energy.

Part 2 of 6How ATP is made

Cells make ATP by phosphorylation, and in photosynthesis and respiration this occurs by chemiosmosis:

Photophosphorylation
Photophosphorylation in the light-dependent reactions, energy from excited electrons moving along the chain in the thylakoid membrane is used to pump protons into the thylakoid space.
  • The protons then flow back through ATP synthase down their concentration gradient, driving the synthesis of ATP from ADP and Pi.
Oxidative phosphorylation
Oxidative phosphorylation in the mitochondrion works in the same way using the energy released as electrons pass along the chain from reduced coenzymes.
Substrate-level phosphorylation
Substrate-level phosphorylation occurs in glycolysis and the Krebs cycle, where a phosphate group is transferred directly from a substrate to ADP.

Because the pathways are the same in principle in plants and animals, ATP is a good example of unity in biochemistry.

Part 3 of 6Why ATP is a good energy currency

1
The energy released by hydrolysis is a small and manageable amount, so little energy is wasted as heat and it can be used in a controlled way for cellular reactions.
2Single step
It is hydrolysed in a single step reaction that is fast, providing energy immediately, whereas glucose needs many steps.
3Soluble
It is soluble and can move easily around the cell.
4
It is small, stable at cellular pH, and does not leave the cell, so it is not lost.
5
It can be rapidly re-made from ADP and Pi, and is regenerated at a rate of thousands of times in each cell every day.
  • The total amount of ATP at any time is small, but the turnover is great, with a typical person turning over about their own body mass of ATP each day.

It cannot be stored in large quantities, so energy stores such as glucose, starch and lipids are used for long-term storage, and ATP for immediate needs.

Part 4 of 6Uses of ATP in plants

In plants ATP supplies energy for active transport, for example uptake of mineral ions by root hair cells and loading of sucrose into phloem; for synthesis of macromolecules such as proteins, cellulose, starch and DNA (anabolic reactions); for cell division; for the movement of chromosomes and organelles; and for the Calvin cycle itself, where ATP is needed to convert glycerate 3-phosphate into triose phosphate and to regenerate RuBP.

Uses of ATP in plants
ATP is also used to activate molecules by phosphorylation, making them more reactive.
The ATP produced in chloroplasts is used there, whereas ATP for other parts of the cell is produced by mitochondria, which continue to operate in the light and the dark.

This topic connects directly to the light-dependent reactions (5.2.4), and to respiration studied in the Edexcel topic on energy for biological processes.

Part 5 of 6Worked links between ATP and exam questions

When asked to explain why a process needs ATP, name the process, state what it uses the energy for, and say where the ATP comes from.

For example: active transport of nitrate into a root hair cell requires ATP to change the shape of the carrier protein; ATP is made by oxidative phosphorylation in mitochondria.
For photosynthesis, state that the ATP in the Calvin cycle is made in the light-dependent reactions, so it is used up in the chloroplast and not exported.
Photophosphorylation
Also note the difference between photophosphorylation (light energy drives ATP production) and oxidative phosphorylation (energy from oxidation of reduced coenzymes).
Be able to describe ATP synthase as a channel protein and enzyme combined: protons pass through it, and this movement drives the rotation of part of the enzyme which changes its shape so that it joins ADP and phosphate.

An average cell contains only a few seconds worth of ATP, so the supply must be continuous. In muscle, creatine phosphate acts as a short-term reserve that can regenerate ATP rapidly, an example of how cells cope with sudden demand.

Part 6 of 6Exam-style questions

  1. Describe the structure of ATP. [3]3 marks

  2. Explain why ATP is described as the energy currency of the cell. [3]3 marks

  3. Name the enzyme that catalyses the formation of ATP and describe how its activity is driven in the thylakoid membrane. [3]3 marks

Key terms

TermMeaning
ATPAdenosine triphosphate, the immediate energy currency of cells.
ADPAdenosine diphosphate, formed when ATP loses a phosphate group.
PhosphorylationAddition of a phosphate group to a molecule.
PhotophosphorylationMaking of ATP using light energy in chloroplasts.
ATP synthaseEnzyme that makes ATP using the flow of protons across a membrane.
HydrolysisBreaking of a bond using water.
ChemiosmosisMaking ATP using a proton gradient across a membrane.
Energy currencyMolecule that transfers energy for many reactions.

Avoid these mistakes

1
Saying ATP stores energy in the long term: it is for immediate use.
2
Stating that energy is released in making the bond between phosphates: it is released in breaking it (more exactly in forming more stable products).
3
Confusing ATP synthase (makes ATP) with ATP hydrolase (breaks it).
4
Describing ATP as a protein or an enzyme.
5
Forgetting that ATP is produced in respiration as well as in photosynthesis.

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