Question

For every $C{O}_2$ molecule entering the Calvin cycle, the number of ATP and NADPH required is

• A. 2 ATP+2 NADPH
• B. 2 ATP+3 NADPH
• C. 3 ATP+2 NADPH
• D. 3 ATP+3 NADPH

Answer 1

Answer: (C)

The immediate products of one turn of the Calvin cycle are two glyceraldehyde-$3$ phosphate$(G_{3}P)$ molecules, three ADP and two $NAD{P}^{+}$(ADP and $NAD{P}^{+}$are regenerated in the light-dependent reactions). Each G3P molecule is composed of three carbons. In order for the Calvin cycle to continue, $RuBP$ (ribulose $1,5$-bisphosphate) must be regenerated. So, $5/6$ carbon from the two $G_{3}P$ molecules are used for this purpose. Therefore, there is only $1$ net carbon produced to play with for each turn. To create 1 surplus $G_{3}P$, three carbons are required and therefore three turns of the Calvin cycle. To make one glucose molecule (which can be created from two $G_{3}P$ molecules) would require six turns of the Calvin cycle. Surplus $G_{3}P$ can also be used to form other carbohydrates such as starch, sucrose and cellulose depending on what the plant needs.