What you need to know
- 5.2photophosphorylationMaking ATP from ADP and Pi using energy from light. of ADPAdenosine diphosphate. It is formed when ATP is hydrolysed, and is phosphorylated to make ATP again. requires energy; hydrolysisA reaction that breaks a covalent bond using a molecule of water. of ATPAdenosine triphosphate: the molecule that releases energy for cell processes when it is hydrolysed to ADP and Pi. provides an immediate energy supply for biological processes.
ATP structure is taught so 5.2 can be explained, not as extra specification codes. The light-dependent electron path is 5A 3. Respiration’s ATP yield is Topic 7, not required recall here.
By the end, you should be able to…
- Describe ATP as adenosine with three phosphate groups.
- State that photophosphorylation of ADP requires energy.
- Explain why hydrolysis of ATP is an immediate energy supply.
Build your understanding
A phosphorylated nucleotide
ATPAdenosine triphosphate: the molecule that releases energy for cell processes when it is hydrolysed to ADP and Pi. is adenosine triphosphate. adenineA purine base. It pairs with thymine in DNA and with uracil in RNA. is attached to riboseThe pentose sugar in RNA nucleotides and in ATP.; three phosphate groups are attached in a chain. Remove one phosphate and you have ADPAdenosine diphosphate. It is formed when ATP is hydrolysed, and is phosphorylated to make ATP again., adenosine diphosphate. Inorganic phosphate is written Pi. The names encode the phosphate count; they are not interchangeable with AMP unless a question has removed two phosphates.
A phosphorylated nucleotide (continued)
Cells do not hydrolyse glucoseA hexose (six-carbon) monosaccharide, C₆H₁₂O₆. It is the main respiratory substrate. every time a pump or an enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction. needs a small packet of energy. Glucose is a fuel store. ATP is the transferable packet. Calling ATP a universal energy currency is a useful summary, not a claim that ATP is the only phosphorylated molecule in a cell.
Hydrolysis supplies energy now
hydrolysisA reaction that breaks a covalent bond using a molecule of water. of ATP uses water to remove the terminal phosphate: ATP + H₂O → ADP + Pi. The reaction is catalysed by ATPaseAn enzyme that hydrolyses ATP to ADP and Pi, releasing energy. The myosin head is an ATPase.. A small, usable quantity of energy is made available to the process that is coupled to that hydrolysis: active transportMovement of molecules or ions against their concentration gradient through carrier proteins, using energy from ATP., muscle contraction, polymer synthesis, or the Calvin cycleThe cycle of light-independent reactions: CO₂ is fixed by RuBP to make GP, GP is reduced to GALP, and RuBP is regenerated. in the next lessons.
Immediate matters. ATP is not stored in bulk like starchThe storage polysaccharide in plants, made of amylose and amylopectin. It is compact and insoluble, so it does not affect water potential.. It is made as it is used. That is why a cell that cannot phosphorylate ADP quickly soon stops doing work, even if glucose is still present. Do not write that ATP “stores all the plant’s energy”; it transfers it.
Phosphorylation has to be paid for
Reforming ATP is ADP + Pi → ATP. That condensationA reaction that joins two molecules with a covalent bond and releases a molecule of water. requires energy. In photosynthesisThe process in which plants use light energy to make glucose from carbon dioxide and water, releasing oxygen., the energy originates in light. Phosphorylation of ADP using light is photophosphorylationMaking ATP from ADP and Pi using energy from light.. It happens in chloroplastThe organelle where photosynthesis takes place. It contains thylakoids and stroma.. mitochondrionThe organelle where the link reaction, Krebs cycle and oxidative phosphorylation of aerobic respiration take place. phosphorylate ADP using energy from respiratory substrateAn organic molecule that is broken down in respiration to release energy, such as glucose, fatty acids or amino acids.; that is not photophosphorylation and is not the 5.2 focus.
Electron flow in the thylakoidA flattened membrane sac in a chloroplast, where the light-dependent reactions take place. membrane drives the phosphorylation. The biochemistry of those electrons is 5A 3. For this lesson, hold one causal chain: light energy is required to phosphorylate ADP; hydrolysis of the ATP so formed can then drive processes that would not run on their own.
NOT: ATP is not a nucleic acid. Hydrolysis does not create energy from nothing.
- ATP + H₂O → ADP + Pi, catalysed by ATPase
- Removing two phosphates would make AMP, which this loop does not
- Uses include transport, contraction, synthesis and the Calvin cycle
- Mitochondria phosphorylate ADP, but that is not photophosphorylation
- ATP is made as it is used, unlike a starch store
Two traps this diagram blocks
- Do not call ATP a nucleic acid or the cell energy store.
- Do not say hydrolysis creates energy that was never supplied.
From the diagram alone
Question
Describe the hydrolysis of ATP and explain why the cell must keep phosphorylating ADP. [3]
Show a model answer
ATPase catalyses ATP + H₂O → ADP + Pi, supplying energy to a coupled process. ATP is not stored in bulk, so ADP must be phosphorylated again as it is used. In a chloroplast that phosphorylation uses light energy.
| Process | Reaction | Energy |
|---|---|---|
| Hydrolysis | ATP + water → ADP + Pi | Energy made available to a coupled process |
| Phosphorylation | ADP + Pi → ATP | Energy required; photophosphorylation uses light |
| Photophosphorylation | ADP + Pi → ATP in a chloroplast | Light-driven; not the same as respiratory phosphorylation |
Make your reasoning visible
Name both molecules
“ATPAdenosine triphosphate: the molecule that releases energy for cell processes when it is hydrolysed to ADP and Pi. is made” is weaker than “ADPAdenosine diphosphate. It is formed when ATP is hydrolysed, and is phosphorylated to make ATP again. is phosphorylated to ATP”.
Keep hydrolysis chemical
Water is a reactant. ATP does not “split into energy”.
Immediate is the point
5.2 is about a usable packet now, not about kilojoules you must memorise.
Avoid this misconception
The claim: ATP is energy, so destroying ATP creates energy from nothing.
The correction: hydrolysisA reaction that breaks a covalent bond using a molecule of water. releases energy that was transferred into ATP when ADP was phosphorylated. Light, or another source, paid for that phosphorylation.
If ATP is the energy currency, why does a cell keep so little of it?
Every active process in a cell is paid for with ATP. Yet a cell holds only a small amount, and it rebuilds each molecule over and over.
What the model shows
A small pool of ATP being hydrolysed by a cell process and rebuilt by a condensation reaction in a chloroplast or a mitochondrion.
What to look for
Where water goes in and comes out. Where the energy for rebuilding ATP comes from. What happens to the pool when the energy source is cut.
What it tells us
ATP hydrolysis gives an immediate supply of energy. Rebuilding ATP needs energy from light or respiration, so ATP is a carrier, not a store.
Be careful what you claim
Ten molecules and a fixed rate are model numbers, and no energy value per ATP is given.
Try it in the model: press Hydrolysis, then Condensation reaction, and compare the two equations under the molecule.
The ATP cycle
Hydrolysis removes the terminal phosphate and makes a small amount of energy available at once. Putting that phosphate back is a condensation reaction, and it requires energy. ATP is not a fuel store. The pool of ten molecules is a model number.
What you see
- The cycle on the left: the ATP pool at the top and ADP + Pi at the bottom, with hydrolysis on one side and the condensation reaction on the other.
- The energy source feeding the condensation reaction, and the cell process using the energy from hydrolysis.
- On the right, one ATP molecule close up, and a graph of the ATP in the pool while the model runs.
How to use it
- Press Hydrolysis and Condensation reaction, or click the arrows in the cycle.
- Choose the organelle and turn its energy source on or off. Press Run to keep the cycle going.
What it shows
- Hydrolysis of ATP to ADP and Pi uses water and provides an immediate supply of energy for biological processes (5.2).
- Phosphorylation of ADP is a condensation reaction that requires energy, from light in photophosphorylation or from respiration (5.2).
- A small pool of ATP is recycled rapidly; it is not a long-term energy store.
Limits of the model
- Ten molecules and a fixed rate are model numbers. A real cell has far more ATP and adjusts its rate of resynthesis.
- No energy value per ATP is given; it is not a specification number.
- Both organelles are shown making the same ATP. How the energy is captured (chemiosmosis) is covered in other lessons.
Guided tour · step 1 of 9
- In Cycle, just below these steps, press Hydrolysis (or click the hydrolysis arrow in the simulator).
- Watch water go in and Pi come off.
Hydrolysis of ATP to ADP and Pi uses water and makes a small amount of energy available at once for a cell process.
Cycle
A teaching model, not a measured cell and not a kilojoule value to memorise. The condensation reaction is the same chemistry in either organelle. Only the source of the energy changes.
Questions
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Write the hydrolysis of ATP and state why it is described as an immediate energy supply.
Show suggested answer
ATP + water → ADP + Pi. The energy transferred in that reaction can be used at once by a coupled process; ATP is not a long-term store.
Practice question
Retrieval and explanation · suggested answer
Why does photophosphorylation of ADP require energy?
Show suggested answer
Forming ATP from ADP and Pi is the reverse of hydrolysis and is not spontaneous in the cell without an energy input. In photosynthesis that input originates in light.
Practice question
Retrieval and explanation · suggested answer
A student writes “plants store energy as ATP instead of glucose”. Correct the statement.
Show suggested answer
Glucose (and starch) are the fuel stores. ATP is made and hydrolysed continuously as an immediate transfer molecule. Plants still store carbohydrate.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A chloroplast can still absorb light but cannot phosphorylate ADP. Predict two immediate effects on the light-independent reactions, stating an assumption.
Show a hint
The Calvin cycle uses ATP at more than one step.
Explore the reasoning
GP would not be reduced to GALP at the usual rate, and RuBP regeneration would slow, so carbon fixation would stall even if reduced NADP were present. Assume membranes still make some reduced NADP; if electron flow itself has failed, both products are missing.
Recall this lesson
Lesson recall questions
Question 1 of 5: Name the process that phosphorylates ADP using energy from light-driven reactions.
ATP: photophosphorylation and hydrolysis
Practice question
Retrieval and explanation · suggested answer
Name the process that phosphorylates ADP using energy from light-driven reactions.
Lesson: 5A 1: The Importance of ATP
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
The enzyme that harvests a gradient
5A · The Importance of ATPThe 1997 Nobel Prize in Chemistry recognised work on ATP synthase: a membrane enzyme that phosphorylates ADP when protons flow through it. Photosynthetic membranes use a related idea.…Specification 5.2Read & use the scienceClose article
Why this matters
The 1997 Nobel Prize in Chemistry recognised work on ATP synthase: a membrane enzyme that phosphorylates ADP when protons flow through it. Photosynthetic membranes use a related idea. The prize details are enrichment. You are not required to name ATP synthase for 5.2, but it shows why “electrons make ATP” is really “electron flow builds a gradient that pays for phosphorylation”. This is a physical coupling mechanism, not energy appearing when a phosphate bond breaks. A scientist can ask whether blocking proton flow would also block ATP formation even while chlorophyll still absorbs light.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 5.2 understand how photophosphorylation of ADP requires energy and that hydrolysis of ATP provides an immediate supply of energy for biological processes
Use the science
Why is “ATP synthase” not a substitute for writing photophosphorylation in an exam answer?
Explore a possible response
The specification asks you to link light-driven phosphorylation of ADP with hydrolysis as an energy supply. The enzyme name is extra unless the question gives it.
Explore the source
- Nobel Prize in Chemistry 1997, press releaseNobel Prize Outreach
Can sunlight be turned into a stored fuel?
5A · The Importance of ATPThe US Department of Energy studies solar fuels made with light, water and carbon dioxide. That ambition is inspired by photosynthesis, but an artificial device is not a…Specification 5.2Read & use the scienceClose article
Why this matters
The US Department of Energy studies solar fuels made with light, water and carbon dioxide. That ambition is inspired by photosynthesis, but an artificial device is not a chloroplast and it need not make glucose. The difficult step is storing intermittent light energy in chemical bonds rather than merely collecting electricity while the sun shines. In a leaf, light-driven reactions help form ATP and reduced NADP; the Calvin cycle then uses their energy and reducing power to fix carbon dioxide. Comparing the two systems reveals why an energy input is necessary before a stable fuel can be made.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 5.2 understand how photophosphorylation of ADP requires energy and that hydrolysis of ATP provides an immediate supply of energy for biological processes
Use the science
If a device splits water in sunlight but never reduces carbon dioxide, which important part of the overall photosynthesis comparison is missing?
Explore a possible response
It has harvested light and may release oxygen, but it has not fixed carbon dioxide into an organic fuel. Water splitting alone is not the full photosynthesis reaction.
Explore the source
- DOE Explains: Solar FuelsUS Department of Energy