Dark Reactions of Photosynthesis

(Take place in the Stroma)

These reactions can occur in the light, but light is not essential for them to take place. They are often called Light Independant Reactions (LIR). They involve a series of reactions that form a cycle of events. The cycle was first discovered by a scientist called Calvin and so was thus named the Calvin Cycle.

Calvin and his team of scientists used unicellular algae to work out what the order of reactions were in this stage of photosynthesis. These algae could be killed instantly by dipping them into boiling alcohol and their chemical contents could then be analysed. Calvin was able to build up a picture of the reactions that took place from one second to the next. After about 15 seconds the cycle was repeated.

The Calvin Cycle

1. The plant already has a 5-carbon compound present in its leaves. This is called Ribulose bisphosphate (RuBP).

2. The plant takes in CO₂ to form a 6-carbon compound which is unstable and so splits.

3. The 6-carbon compound splits into two 3C compounds. Each of theseis called Glycerate-3-phosphate.

4. Each 3C becomes a 4C compound by taking in CO₂. From this stage on Whenever the plant takes in CO₂ it also picks up hydrogen from NADPH₂. The energy for this comes from the ATP made in the LDR.

5. Each 4C compound becomes a 5C compound, again by taking up CO₂ and also H₂ from NADPH₂ using the energy from ATP. Someof the 5C compounds will be Ribulose bisphosphates and wiil be recycled back to stage 1.

6. Other 5C compounds go on to form 6C compounds in the same ways as previously described. This compound is Glucose and is converted to Starch for storage in the chloroplasts.

The plant has now successfully generated its food supply for survival.