Question

Redraw the cycle in Figure 10.19 using numerals to indicate the numbers of carbons instead of gray balls, multiplying at each step to ensure that you have accounted for all the carbons. In what forms do the carbon atoms enter and leave the cycle?

Short answer

Carbon dioxide is the molecule that enters the cycle one by one. The three different carbon atoms enter the cycle and leave as a single three-carbon molecule of glyceraldehyde-3-phosphates concerning three turns of a cycle.

Step-by-step solution

Reactions in chloroplasts 

Chloroplasts are an important organelle of plants in which the Calvin cycle and light reactions occur. Both these mechanisms are interdependent.

Steps in the Calvin cycle

There are three phases in the Calvin cycle such as carbon fixation, reduction, and regeneration. In carbon fixation, the carbon dioxide entering the reaction gets converted into 3-phosphoglycerate, which is converted into 1, 3 bisphosphoglycerate.

The next step is the reduction phase, in which glyceraldehydes-3-phosphates are formed. After this step, the regeneration of RuBP takes place.

Calculation of carbon atoms involved

Three different carbon atoms present in three molecules of carbon dioxide enter into the Calvin cycle's reaction. It is then converted into a component containing 18 carbon atoms from which a single carbon atom with three carbons is formed and leaves the reaction to form the sugar molecules.

The molecules getting into the regeneration phase has 15 carbon atoms in them. The complete cycle is illustrated below: