What occurs if the Calvin cycle runs through six turns?

• A. Six turns produce six glucose molecules.
• B. Six turns produce no fixed carbon.
• C. Enough carbon will be fixed to make one glucose, but the carbons will be distributed among molecules.
• D. It will require ten turns to complete one glucose.

Answer: (C)

The Calvin cycle is building carbon into small, three-carbon units. Each turn fixes one CO2 and, after the cycle’s steps, yields a triose phosphate that exits the cycle. Six turns fix six CO2 and produce two molecules of glyceraldehyde-3-phosphate (G3P), each with three carbons. Those two G3P molecules together hold six carbons and can be combined to make one glucose molecule. But the carbons aren’t all bottled into a single molecule right away; they’re distributed across those two G3P molecules as they exit the cycle and are later assembled into glucose. So six turns provide enough carbon to make one glucose, while the carbon is distributed among molecules rather than all in one.