IB Biology HLPhotosynthesisPaper 1 & 2~13 min read
Light-Independent Reactions
Six turns of this cycle build one glucose molecule — but ten of every twelve products go straight back into the cycle to keep it running. The Calvin cycle spends most of its effort rebuilding its own starting material.
📚 What you need to know
The light-independent reactions, or Calvin cycle, take place in the stroma.
They do not use light directly but depend on ATP and reduced NADP from the light-dependent reactions.
Fixation: CO2 joins the 5C acceptor RuBP, catalysed by the enzyme rubisco, giving an unstable 6C intermediate that immediately splits into two 3C molecules of GP.
Reduction: GP is reduced to TP (triose phosphate) using ATP and reduced NADP.
Regeneration: most of the TP is used, with more ATP, to regenerate RuBP.
Per turn: 1 CO2 fixed, 3 ATP and 2 reduced NADP used.
Per glucose: 6 turns, 18 ATP and 12 reduced NADP, producing 12 TP of which only 2 leave the cycle.
TP is the useful product: it is used to make glucose, starch, cellulose, lipids, amino acids and nucleotides.
The three stages
The cycle is mostly maintenance. Five sixths of the product is ploughed straight back in so the acceptor never runs out.
Stage 1 — Fixation
Carbon dioxide diffuses into the stroma and is joined to ribulose bisphosphate (RuBP), a 5C acceptor molecule. The enzyme is rubisco, short for ribulose bisphosphate carboxylase — probably the most abundant enzyme on Earth.
The 6C molecule formed is unstable and immediately breaks into two molecules of glycerate-3-phosphate (GP), each with three carbons.
Fixation
CO2 (1C) + RuBP (5C) → unstable 6C intermediate → 2 × GP (3C)
Stage 2 — Reduction
GP is reduced to triose phosphate (TP). This is where the products of the light-dependent reactions get spent:
ATP supplies the energy.
Reduced NADP supplies the hydrogen, and is reoxidised to NADP, which returns to the thylakoid.
Stage 3 — Regeneration
TP is the useful product, but if all of it left, the cycle would run out of RuBP and stop. So most of it is used — along with more ATP — to regenerate the RuBP acceptor.
For every 12 TP made from six turns, 10 are recycled into 6 RuBP and only 2 leave to be built into glucose.
The carbon bookkeeping is worth doing yourself once, because it makes the whole cycle click. Ten TP is 10 × 3 = 30 carbons. Six RuBP is 6 × 5 = 30 carbons. It balances exactly. Nothing is lost and nothing is invented.
The full account per glucose
Quantity
Per turn
Per glucose (6 turns)
CO2 fixed
1
6
RuBP used
1
6
GP formed
2
12
TP formed
2
12
ATP used
3
18
Reduced NADP used
2
12
TP leaving the cycle
—
2 (which make 1 glucose)
🧠
Three, two, one
Per turn: 3 ATP, 2 reduced NADP, 1 CO2. Multiply all three by six for a glucose. That single line of numbers answers most calculation questions on this page.
What TP becomes
Triose phosphate is a genuine crossroads molecule. Depending on what the plant needs, it can be converted into:
Glucose, and from there starch for storage or cellulose for cell walls
Lipids, by conversion to fatty acids and glycerol
Amino acids, once nitrogen is added
Nucleotides, with nitrogen and phosphorus
Why the plant needs minerals. Photosynthesis only supplies carbon, hydrogen and oxygen. To turn TP into proteins the plant must also take up nitrate from the soil, and nucleotides need phosphate too. That is why a plant in perfect light with plenty of CO2 can still be stunted by poor soil.
Worked examples
WE 1
Describe the Calvin cycle
Describe the three stages of the light-independent reactions. (5 marks)
Location
The stroma of the chloroplast.
Stage 1: fixation
CO2 combines with the 5C acceptor RuBP, catalysed by rubisco, forming an unstable 6C compound that splits into two 3C molecules of GP.
Stage 2: reduction
GP is reduced to TP using ATP and reduced NADP from the light-dependent reactions.
Stage 3: regeneration
Most of the TP is converted back into RuBP using further ATP, so the cycle can continue.
The product
The remaining TP leaves the cycle and is used to make glucose and other organic molecules.
Fix, reduce, regenerate — with a little product siphoned offname RuBP, GP and TP with their carbon numbers; the numbers themselves earn credit
WE 2
Calculate the cost of a glucose
Calculate how many molecules of ATP and reduced NADP are needed to produce two molecules of glucose. (3 marks)
Step 1: turns needed
One glucose needs 6 turns, so two need 12 turns.
Step 2: ATP
3 ATP per turn, so 12 × 3 = 36 ATPStep 3: reduced NADP
2 per turn, so 12 × 2 = 24 reduced NADP36 ATP and 24 reduced NADPwork in turns first — going straight from glucose to ATP is where errors creep in
WE 3
Explain the recycling
Of the 12 TP produced by six turns of the Calvin cycle, only 2 leave. Explain why, and show that the carbon balances. (3 marks)
Point 1: why recycle
RuBP is the acceptor for CO2. If it were not regenerated, no more carbon could be fixed and the cycle would stop.
Point 2: the arithmetic
10 TP × 3 carbons = 30 carbons, which is exactly 6 RuBP × 5 carbons = 30 carbons.
Point 3: the product
The remaining 2 TP carry 6 carbons, exactly enough for one molecule of glucose.
10 TP rebuild the acceptor; 2 TP become the glucoseshowing the carbon arithmetic is usually worth a mark on its own
💡 Exam tips
Learn the three stage names: fixation, reduction, regeneration.
Name rubisco and say what it catalyses.
Carbon numbers earn marks: RuBP 5C, GP 3C, TP 3C, and an unstable 6C intermediate.
Note that ATP is used twice — in reduction and again in regeneration.
Reduced NADP is used only in the reduction step.
Remember the ratio: 3 ATP : 2 reduced NADP : 1 CO2 per turn.
⚠ Common mistakes
Saying one turn makes a glucose. Six turns are needed.
Saying the 6C intermediate is a product. It is unstable and splits immediately.
Forgetting the ATP used in regeneration. That is why it is 3 per turn, not 2.
Saying all the TP becomes glucose. Most of it rebuilds RuBP.
Writing NADPH2 or reduced NAD. It is reduced NADP, or NADPH.
Putting the Calvin cycle in the thylakoid. It is in the stroma.
Up next: Interdependent Photosynthetic Reactions. Both halves of photosynthesis are now on the table. The last page of the topic pulls them together and shows what happens to the intermediates the moment one half is disturbed.
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