What you need to know
- 5.1Overall reaction: light splits water, hydrogen is stored in glucoseA hexose (six-carbon) monosaccharide, C₆H₁₂O₆. It is the main respiratory substrate. with carbon dioxide, oxygen is released.
- 5.3light-dependent reactionsThe reactions on the thylakoid membranes that use light energy to make ATP and reduced NADP, splitting water and releasing O₂.: excitation of chlorophyllThe green pigment in chloroplasts. It absorbs mainly red and blue light. electrons, ATPAdenosine triphosphate: the molecule that releases energy for cell processes when it is hydrolysed to ADP and Pi., reduced NADPNADP that has accepted hydrogen in the light-dependent reactions. It supplies the hydrogen to reduce GP to GALP., cyclic and non-cyclic photophosphorylationMaking ATP from ADP and Pi using energy from light., photolysisThe splitting of water using light energy, giving H⁺ ions, electrons and O₂. of water.
- 5.4(i–ii)light-independent reactionsThe reactions in the stroma (the Calvin cycle) that use ATP and reduced NADP to fix CO₂ and make GALP.: Calvin cycleThe cycle of light-independent reactions: CO₂ is fixed by RuBP to make GP, GP is reduced to GALP, and RuBP is regenerated., GP, GALPGlyceraldehyde 3-phosphate: a three-carbon sugar made by reducing GP. It is used to make glucose and other organic molecules and to regenerate RuBP., RuBPRibulose bisphosphate: the five-carbon compound that accepts CO₂ in the Calvin cycle., RUBISCOThe enzyme that catalyses carbon fixation, joining CO₂ to RuBP.; sugars used in respirationThe breakdown of respiratory substrates such as glucose in cells to release energy, which is used to make ATP. and synthesis of other biological molecules.
Use the specification names GP, GALP, RuBP and RUBISCO. TP or G3P may appear in other books as the same triose as GALP; do not treat extra synonyms as required. Detailed mitochondrial respiration is Topic 7.
By the end, you should be able to…
- State the overall photosynthetic reaction in the language of 5.1.
- Compare cyclic and non-cyclic photophosphorylation.
- Outline carbon fixationCombining CO₂ with RuBP, catalysed by RUBISCO, to form two molecules of GP. and the fates of GALP.
Build your understanding
What photosynthesis is for
The overall reaction uses energy from light to split the strong bonds in water. Hydrogen from that water is stored in a fuel, glucoseA hexose (six-carbon) monosaccharide, C₆H₁₂O₆. It is the main respiratory substrate., by combining it with carbon dioxide. Oxygen is released to the atmosphere. A balanced summary is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, but 5.1 is the causal story, not just the formula.
Oxygen comes from water, not from carbon dioxide. That is why photolysisThe splitting of water using light energy, giving H⁺ ions, electrons and O₂. matters. Glucose is not conjured as a single step; it is assembled from three-carbon products of 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.. Plants, algae and cyanobacteria all run a version of this process; animals do not, but they depend on the fuel and the oxygen.
Light-dependent reactions
Light is trapped by exciting electrons in chlorophyllThe green pigment in chloroplasts. It absorbs mainly red and blue light. in the thylakoidA flattened membrane sac in a chloroplast, where the light-dependent reactions take place. membrane. In non-cyclic photophosphorylationMaking ATP from ADP and Pi using energy from light., electrons from photosystemA cluster of chlorophyll and other pigments with proteins in the thylakoid membrane that absorbs light and passes energy to a reaction centre. II pass along carriers to photosystem I. The energy of that flow is used to generate ATPAdenosine triphosphate: the molecule that releases energy for cell processes when it is hydrolysed to ADP and Pi.. Photosystem I electrons reduce NADPA coenzyme that accepts hydrogen in the light-dependent reactions, becoming reduced NADP. to reduced NADPNADP that has accepted hydrogen in the light-dependent reactions. It supplies the hydrogen to reduce GP to GALP.. Photolysis of water replaces the electrons taken from photosystem II and releases oxygen: water → protons + electrons + oxygen.
In cyclic photophosphorylationMaking ATP when excited electrons from photosystem I pass along the electron carriers and back to photosystem I. No reduced NADP or O₂ is made., electrons from photosystem I return to the same photosystem via carriers. ATP is still generated; reduced NADP and oxygen are not, because photolysis is not needed to refill a hole in photosystem II. Cyclic flow can supply extra ATP when the Calvin cycle needs more ATP than reduced NADP. Do not write that cyclic photophosphorylation “makes glucose”.
Light-independent reactions
Carbon dioxide is reduced using ATP and reduced NADP from the light-dependent stage. RUBISCOThe enzyme that catalyses carbon fixation, joining CO₂ to RuBP. catalyses carbon fixationCombining CO₂ with RuBP, catalysed by RUBISCO, to form two molecules of GP.: carbon dioxide is added to the five-carbon acceptor RuBPRibulose bisphosphate: the five-carbon compound that accepts CO₂ in the Calvin cycle., producing two molecules of the three-carbon compound GP (glycerate 3-phosphate). GP is reduced to GALPGlyceraldehyde 3-phosphate: a three-carbon sugar made by reducing GP. It is used to make glucose and other organic molecules and to regenerate RuBP. (glyceraldehyde 3-phosphate) using ATP and reduced NADP.
Some GALP leaves the cycle and is used to make simple sugars. Those sugars fuel respirationThe breakdown of respiratory substrates such as glucose in cells to release energy, which is used to make ATP. and the synthesis of polysaccharides, amino acidThe monomer of proteins: a central carbon with an amine group, a carboxyl group, a hydrogen and an R group., proteins, lipids and nucleic acids in plants and, after food chains, in animals and other organisms. Most GALP is rearranged, using ATP, to regenerate RuBP so the acceptor is not used up. The cycle still needs a continuous supply of ATP and reduced NADP, which is why it is light-independent in chemistry but light-dependent in practice.
Follow the process
Non-cyclic: light excites electrons in photosystem II → those electrons pass along carriers to photosystem I, and that flow is coupled to ATP formation → light also excites photosystem I and its electrons reduce NADP → photolysis of water replaces the electrons lost from photosystem II and releases oxygen. The required products are ATP, reduced NADP and oxygen. Cyclic flow is shorter: photosystem I electrons leave, travel along carriers and return to photosystem I, forming ATP only. NADP is not reduced and water is not split, so this route releases no oxygen.
Follow the process (continued)
Calvin cycle: carbon dioxide joins RuBP, catalysed by RUBISCO → the unstable six-carbon product splits into two molecules of GP → ATP and reduced NADP reduce GP to GALP → some GALP leaves to make sugars and other molecules → most GALP is rearranged, using ATP, to regenerate RuBP. Each turn fixes one carbon dioxide. Six turns fix enough carbon for one glucose, because five of every six GALP carbons are used to rebuild RuBP. Do not write that one turn makes one glucose. The proton gradient behind ATP formation is the enrichment already met in 5A 1. You are not required to name ATP synthaseThe enzyme in the inner mitochondrial membrane and thylakoid membrane that makes ATP as H⁺ ions flow through it. unless the question gives it.
Diagram A
NOT: cyclic photophosphorylation does not reduce NADP, does not release oxygen and does not make glucose.
- Non-cyclic products: ATP, reduced NADP and oxygen
- Oxygen comes from water, not from carbon dioxide
- Overall: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
- Reduced NADP is the name used here, not an ADP equation
- ATP synthase is enrichment unless the question names it
Two traps this diagram blocks
- Do not say cyclic photophosphorylation releases oxygen or reduces NADP.
- Do not say the oxygen released in photosynthesis comes from carbon dioxide.
From the diagram alone
Question
Compare the products of cyclic and non-cyclic photophosphorylation. [4]
Show a model answer
Both can produce ATP. Non-cyclic flow also produces reduced NADP, and photolysis of water releases oxygen. Cyclic flow returns electrons to photosystem I, so it does not reduce NADP and does not release oxygen.
Diagram B
NOT: one turn does not make one glucose. The reactions do not use photons directly, but they stop without ATP and reduced NADP.
- Each turn fixes one CO₂
- Six turns fix enough carbon for one glucose
- Five of every six GALP carbons are used to rebuild RuBP
- Sugars are used in respiration and to make other molecules
- GP is glycerate 3-phosphate; GALP is glyceraldehyde 3-phosphate
Two traps this diagram blocks
- Do not write that one turn of the cycle produces one glucose.
- Do not say the light-independent reactions run all night in a sealed plant with no ATP supply.
From the diagram alone
Question
Describe what happens to the GALP made in one turn of the Calvin cycle, and state how many turns fix the carbon in one glucose. [4]
Show a model answer
Some GALP leaves the cycle and is converted into sugars and other molecules. Most GALP is used, with ATP, to regenerate RuBP. One turn fixes one CO₂, so six turns are needed to fix the six carbons of one glucose.
| Feature | Non-cyclic | Cyclic |
|---|---|---|
| Photosystems | II then I | I only |
| ATP | Generated | Generated |
| Reduced NADP | Produced | Not produced |
| Photolysis / oxygen | Yes | No |
| Electrons | End on NADP | Return to chlorophyll |
Make your reasoning visible
Build the explanation
X happens because Y, leading to Z. X: reduced NADPNADP that has accepted hydrogen in the light-dependent reactions. It supplies the hydrogen to reduce GP to GALP. and oxygen form. Y: non-cyclic flow takes electrons from water, through photosystemA cluster of chlorophyll and other pigments with proteins in the thylakoid membrane that absorbs light and passes energy to a reaction centre. II and photosystem I, onto NADPA coenzyme that accepts hydrogen in the light-dependent reactions, becoming reduced NADP.. Z: 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. can reduce GP, and oxygen leaves as the waste from photolysisThe splitting of water using light energy, giving H⁺ ions, electrons and O₂.. Incomplete: “Light makes glucoseA hexose (six-carbon) monosaccharide, C₆H₁₂O₆. It is the main respiratory substrate. in the thylakoidA flattened membrane sac in a chloroplast, where the light-dependent reactions take place..” Improved: “light-dependent reactionsThe reactions on the thylakoid membranes that use light energy to make ATP and reduced NADP, splitting water and releasing O₂. in the thylakoid make ATPAdenosine triphosphate: the molecule that releases energy for cell processes when it is hydrolysed to ADP and Pi. and reduced NADP and release oxygen; the stroma uses those to fix carbon dioxide into GALPGlyceraldehyde 3-phosphate: a three-carbon sugar made by reducing GP. It is used to make glucose and other organic molecules and to regenerate RuBP..” The improvement keeps the two stages apart.
Tell the 5.1 story
Light, water split, hydrogen stored in glucose with carbon dioxide, oxygen released. The formula alone is not the specification sentence.
Keep the names
GP, GALP, RuBPRibulose bisphosphate: the five-carbon compound that accepts CO₂ in the Calvin cycle., RUBISCOThe enzyme that catalyses carbon fixation, joining CO₂ to RuBP.. Mixing in unofficial abbreviations without explaining them loses the 5.4 trail.
Both products of the light stage
GP → GALP needs ATP and reduced NADP. Naming only ATP is incomplete.
Key explanations
- The chemistry does not absorb photons, but it depends on a continuing supply of ATP and reduced NADP.
Avoid this misconception
The claim: The light-independent reactionsThe reactions in the stroma (the Calvin cycle) that use ATP and reduced NADP to fix CO₂ and make GALP. do not need light, so they run equally well at midnight in a sealed cupboard.
The correction: The chemistry does not use photons directly, but it uses ATP and reduced NADP that the light-dependent reactions must keep supplying.
How light energy ends up in sugar
Photosynthesis happens in two linked stages inside a chloroplast. In the thylakoid membranes, light energy splits water and makes two energy carriers, ATP and reduced NADP. In the stroma, the Calvin cycle uses them to turn carbon dioxide into sugar.
What the model shows
Electrons moving through the thylakoid membrane on one side, and the Calvin cycle on the other, with a graph of the three Calvin cycle compounds.
What to look for
What the light stage makes and where the oxygen comes from. How the Calvin cycle compounds change when you turn off the light, or take away carbon dioxide.
What it tells us
The Calvin cycle needs a steady supply of ATP and reduced NADP, so it stops soon after the light does. Taking away carbon dioxide stops the step that makes GP, so RuBP builds up instead. Six turns of the cycle are needed for one glucose.
Be careful what you claim
Amounts are in made-up units and the speeds are simplified. Some details of real chloroplasts are left out.
Try it in the model: press Play, then press Light off and watch GP and RuBP on the graph.
Light reactions and the Calvin cycle
The thylakoid makes ATP and reduced NADP; the Calvin cycle uses them to fix carbon dioxide. Switch the light or the CO₂ off and watch RuBP, GP and GALP respond.
What you see
- Left: a piece of thylakoid membrane with photosystem II, electron carriers and photosystem I. Electrons are dots. Water is split at photosystem II and oxygen leaves.
- Right: the Calvin cycle in the stroma. Circle sizes show how much RuBP, GP and GALP there is, and arrow widths show how fast each step runs.
- Below: a graph of RuBP, GP and GALP over time, with your changes marked on it.
How to use it
- Play, pause and choose the speed. Change the light, set CO₂ to normal, low or none, and allow or block cyclic flow.
- Watch the ATP and reduced NADP bars, the oxygen rate and the turn and glucose counters.
- Reset pools puts every pool back to its steady value.
What it shows
- Non-cyclic photophosphorylation: PSII → carriers → PSI → NADP, making ATP and reduced NADP, with photolysis releasing oxygen.
- Cyclic photophosphorylation: PSI electrons return to PSI, making ATP only.
- The Calvin cycle: RUBISCO fixes CO₂ onto RuBP giving 2 GP; GP is reduced to GALP using ATP and reduced NADP; most GALP rebuilds RuBP; one in six leaves to make sugars.
Limits of the model
- Pool sizes are in arbitrary units and rate constants are illustrative. The stoichiometry is real: 3 ATP and 2 reduced NADP per CO₂ fixed.
- Non-cyclic flow is given about 1.3 ATP per reduced NADP, so cyclic flow is needed to make up the ATP. Blocked for long enough, this model's cycle stalls; real chloroplasts have other ways to balance ATP and slow down instead.
- RUBISCO activity follows the light with a lag, as in real leaves. Proton pumping and ATP synthase are not drawn; they are the enrichment met in 5A 1. Photorespiration is not included.
Guided tour · step 1 of 9
- In Conditions, just below these steps, press Play.
- Watch electrons flow in the thylakoid membrane and carbon go round the Calvin cycle in the stroma.
The light-dependent reactions make ATP and reduced NADP. The Calvin cycle uses them to turn GP into GALP and to rebuild RuBP.
Conditions
Questions
Check your understanding
Try each question before opening the suggested answer.
Check your understanding (continued)
Practice question
Retrieval and explanation · suggested answer
State the overall reaction of photosynthesis as specified in 5.1.
Show suggested answer
Light energy splits water; hydrogen is stored in glucose by combination with carbon dioxide; oxygen is released to the atmosphere.
Practice question
Retrieval and explanation · suggested answer
Compare cyclic and non-cyclic photophosphorylation in terms of products.
Show suggested answer
Both generate ATP. Only non-cyclic photophosphorylation reduces NADP and produces oxygen by photolysis of water. Cyclic flow returns electrons to chlorophyll and does not photolyse water.
Practice question
Retrieval and explanation · suggested answer
Outline how GP is converted to GALP and why RuBP must be regenerated.
Show suggested answer
GP is reduced to GALP using ATP and reduced NADP. Most GALP is used, with ATP, to regenerate RuBP so carbon dioxide can continue to be fixed.
Practice question
Retrieval and explanation · suggested answer
Give two fates of the simple sugars produced from GALP.
Show suggested answer
Respiration in the plant, and synthesis of polysaccharides, amino acids, proteins, lipids or nucleic acids; consumers then use those molecules too.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A researcher withholds carbon dioxide but keeps bright light, water and a functioning electron-transport chain. Predict the short-term changes in GP, GALP and RuBP, stating an assumption.
Show a hint
Ask which step has lost its substrate.
Explore the reasoning
Carbon fixation slows, so GP formation falls. GALP then falls as existing GP is used. RuBP may rise at first because it is still regenerated but is not being carboxylated. Assume ATP and reduced NADP are still available; if the plant is sealed long-term, those too will back up.
Recall this lesson
Lesson recall questions
Question 1 of 5: Describe, in order, the overall reaction of photosynthesis starting from water.
Overall reaction of photosynthesis
Practice question
Retrieval and explanation · suggested answer
Describe, in order, the overall reaction of photosynthesis starting from water.
Lesson: 5A 3: The Biochemistry of Photosynthesis
Build the sequence from memory first. Then reveal the model order.
Tap the steps in the order you recall. Tap a placed step to send it back.
- 01Next step
- 02Waiting
- 03Waiting
Tap to place
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
How the cycle was mapped
5A · The Biochemistry of PhotosynthesisMelvin Calvin received the 1961 Nobel Prize in Chemistry for tracing how carbon dioxide is assimilated in plants, using carbon-14 and chromatography on algae. The prize is for…Specification 5.4(i)Read & use the scienceClose article
Why this matters
Melvin Calvin received the 1961 Nobel Prize in Chemistry for tracing how carbon dioxide is assimilated in plants, using carbon-14 and chromatography on algae. The prize is for that research programme. It is not an official diagram of GP and GALP, and you do not need the full historical intermediate list. It does show why 5.4 names a cycle rather than a single jump from carbon dioxide to glucose. The experiment depended on sampling algae at different times after adding labelled carbon dioxide. An early labelled compound is stronger evidence for an early step than a compound that appears only after the label has travelled through several reactions.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 5.4(i) understand the light-independent reactions as reduction of carbon dioxide using the products of the light-dependent reactions (carbon fixation in the Calvin cycle, the role of GP, GALP, RuBP and RUBISCO)
Use the science
Why did labelling carbon dioxide, rather than labelling water, reveal the Calvin cycle intermediates?
Explore a possible response
The carbon atoms of the sugars come from carbon dioxide. Tracing 14C showed which organic molecules acquired that carbon and in which order. Oxygen released in photosynthesis comes from water, so labelling water would have tracked a different atom.
Explore the source
- Nobel Prize in Chemistry 1961: Melvin CalvinNobel Prize Outreach
Watching the water-splitting machinery work
5A · The Biochemistry of PhotosynthesisBerkeley Lab researchers used X-ray laser images to examine photosystem II, the protein complex that helps split water during the light-dependent reactions. A structural snapshot can show where…Specification 5.3Read & use the scienceClose article
Why this matters
Berkeley Lab researchers used X-ray laser images to examine photosystem II, the protein complex that helps split water during the light-dependent reactions. A structural snapshot can show where atoms lie, but a working mechanism needs evidence from several stages and measurements. The school sequence says light excites chlorophyll electrons, water replaces electrons lost from photosystem II, and oxygen is released. The research goes further into the detailed catalytic machinery; students are not expected to memorise its atom-by-atom structure. This example shows why oxygen release is tied to water, not to carbon dioxide being fixed later.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 5.3 understand the light-dependent reactions of photosynthesis, including how light energy is trapped by exciting electrons in chlorophyll and the role of these electrons in generating ATP, reducing NADP in cyclic and non-cyclic photophosphorylation and producing oxygen through photolysis of water
Use the science
If water splitting stopped while carbon dioxide was still available, what would happen to non-cyclic electron flow and oxygen release?
Explore a possible response
Replacement electrons would be unavailable, so sustained non-cyclic flow and oxygen release would fall; carbon dioxide availability would not restore the missing electrons.