The Calvin Cycle, C4 Plants and Limiting Factors: Quick Revision

A brief note on The Calvin Cycle, C4 Plants and Limiting Factors. Read it before a test. Try each question at the end before you open its answer.

The Calvin cycle (C3 pathway)

• In the stroma. First stable product PGA (3C). Acceptor RuBP (5C). Enzyme RuBisCO.
• Carboxylation: RuBP + carbon dioxide give $2$ PGA.
• Reduction: PGA to triose phosphate, with ATP and NADPH.
• Regeneration: RuBP is made again, with ATP.
• Each carbon dioxide: $3$ ATP and $2$ NADPH. One glucose: $6$ turns, $18$ ATP, $12$ NADPH.

C4 plants

• Maize, sugarcane, sorghum. First stable product OAA (4C). Kranz anatomy.
• Mesophyll: PEP (3C) + carbon dioxide, with PEP carboxylase, gives OAA, then malic acid.
• Bundle sheath: carbon dioxide is set free, RuBisCO and the Calvin cycle run. Pyruvate goes back and becomes PEP with ATP.
• No photorespiration, higher temperature optimum, more biomass.

Photorespiration and limiting factors

• RuBisCO binds oxygen when carbon dioxide is low and oxygen is high. Chloroplast, peroxisome, mitochondrion.
• No sugar, no ATP, no NADPH. Carbon dioxide is released. Wasteful.
• Blackman: the factor nearest its minimum sets the rate.
• Light saturates near $10$ percent of full sunlight. Carbon dioxide is the main limit: C4 saturates near $360\ \mu\text{L/L}$, C3 near $450\ \mu\text{L/L}$. Temperature and water also matter.

A worked example

A plant makes $3$ glucose molecules in the Calvin cycle. How many turns, ATP and NADPH?

1. Turns. $3 \times 6 = 18$.
2. ATP. $18 \times 3 = 54$.
3. NADPH. $18 \times 2 = 36$.

Practice with new numbers

A program to try

The program counts the turns, ATP and NADPH for a number of glucose molecules.

Mistakes to avoid

• One glucose is six turns.
• C4 plants do have the Calvin cycle, in the bundle sheath cells.
• RuBisCO is absent from the mesophyll of C4 plants.
• Photorespiration makes no ATP.
• The limiting factor is the one nearest its minimum.

Check yourself

1. What are the three stages of the Calvin cycle?

2. How many ATP and NADPH does one glucose cost?

3. What is the first stable product in a C4 plant, and where is it made?

4. Why do C4 plants lack photorespiration?

5. State Blackman's law.