The Calvin cycle and its numbers

What the light reactions made

The light reactions have made ATP and NADPH. They sit in the stroma. Now they are put to work. The biosynthetic phase uses them to turn carbon dioxide into sugar. You may also hear it called the dark reactions. It does not need light itself. It needs the products of light.

How the cycle was found

Melvin Calvin and his team fed algae such as Chlorella and Scenedesmus with carbon dioxide that held radioactive carbon, $^{14}\text{C}$. After a few seconds they killed the cells and looked for the first compound that held the radioactive carbon. It was a compound with three carbon atoms: 3-phosphoglyceric acid, or PGA.

This was the first stable product of carbon fixation. Because it has three carbon atoms, this route is the C3 pathway. The whole route is a cycle, the Calvin cycle. It runs in the stroma of every green plant, whatever its group.

The three stages

1. Carboxylation. Carbon dioxide is added to a 5-carbon compound, ribulose bisphosphate (RuBP). The enzyme is RuBisCO, the most abundant enzyme in the world. One RuBP and one carbon dioxide give two molecules of 3-PGA. The carbon count checks out: $5 + 1 = 6 = 2 \times 3$.
2. Reduction. 3-PGA is turned into triose phosphate (glyceraldehyde 3-phosphate, G3P). This needs ATP and NADPH. For each carbon dioxide fixed, the cycle spends 2 ATP and 2 NADPH here. Triose phosphate is the sugar that leaves the cycle. It goes on to make glucose, sucrose and starch.
3. Regeneration. The cycle must make RuBP again, or it would stop. This needs one more ATP for each carbon dioxide fixed.

Add up one turn. Each carbon dioxide fixed costs 3 ATP and 2 NADPH.

The numbers for one glucose

Glucose has six carbon atoms, so it takes six carbon dioxide molecules and six turns of the cycle.

• ATP: $6 \times 3 = 18$.
• NADPH: $6 \times 2 = 12$.

The carbon also adds up. Six turns take in $6$ RuBP ($30$ carbon atoms) and $6$ carbon dioxide ($6$ carbon atoms). That is $36$ carbon atoms in $12$ molecules of triose phosphate. Two of them, $6$ carbon atoms, leave as glucose. The other ten, $30$ carbon atoms, rebuild the six RuBP.

You should find that $12$ turns fix $12$ carbon dioxide molecules and spend $36$ ATP and $24$ NADPH. They make two glucose molecules. Doubling the turns doubles everything.

Try it in Python

The program counts the ATP and NADPH for different numbers of turns, and checks the carbon of one glucose.

You should see $18$ ATP and $12$ NADPH for $6$ turns, with $1$ glucose. The carbon in is $36$, and the carbon in the $12$ triose phosphates is also $36$. With $24$ turns, you get $4$ glucose molecules.

Practise with new numbers

Common mistakes

• Counting one turn for one glucose. One glucose takes six turns.
• Putting the Calvin cycle in the thylakoid. It runs in the stroma.
• Mixing up the first product and the acceptor. The acceptor is RuBP, with five carbon atoms. The first stable product is PGA, with three.
• Giving the reduction step to carboxylation. ATP and NADPH are spent in reduction. Carboxylation uses RuBisCO.
• Forgetting the ATP of regeneration. The total is $3$ ATP for each carbon dioxide, not $2$.

Remember these

• Calvin used $^{14}\text{C}$ and algae. The first product is PGA, with three carbon atoms.
• Carboxylation (RuBisCO), reduction (ATP and NADPH), regeneration (ATP).
• Each carbon dioxide fixed: $3$ ATP and $2$ NADPH.
• One glucose: $6$ turns, $18$ ATP, $12$ NADPH.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What was the first stable product that Calvin found, and why is the route called C3?

2. Name the enzyme and the acceptor of carboxylation.

3. Which stage of the cycle spends both ATP and NADPH?

4. How many ATP and NADPH are spent for each carbon dioxide fixed?

5. How many turns of the cycle make one glucose, and how many ATP does it cost?