Question

multiple choice question how many turns of the calvin cycle are needed to produce one six carbon sugar like glucose? six one two three

Explanation:

In the Calvin cycle, each turn fixes one carbon atom (from \(CO_2\)). Glucose has 6 carbon atoms. So, to produce a 6 - carbon sugar like glucose, the number of turns needed is calculated by considering that each turn contributes 1 carbon. Since glucose has 6 carbons, we need 6 turns? Wait, no, correction: Each turn of the Calvin cycle fixes one \(CO_2\) molecule (1 carbon). But the net synthesis of one G3P (a 3 - carbon molecule) requires 3 turns (fixing 3 \(CO_2\) to make 6 G3P, one of which exits the cycle, and 5 are recycled). To make glucose (a 6 - carbon sugar), we need 2 G3P molecules (since 2 G3P with 3 carbons each make 6 carbons). Since 3 turns give 1 G3P, for 2 G3P, we need \(3\times2 = 6\)? Wait, no, let's recall: The Calvin cycle uses \(CO_2\) to make glyceraldehyde - 3 - phosphate (G3P). Each turn of the cycle (using one \(CO_2\)) produces a 3 - carbon intermediate. To make one molecule of glucose (6 carbons), we need two molecules of G3P. Each G3P requires 3 turns of the Calvin cycle (because 3 turns fix 3 \(CO_2\) to make 6 G3P, one leaves, 5 are recycled). Wait, no, the correct fact is: Each turn of the Calvin cycle fixes 1 \(CO_2\) (1 carbon). Glucose has 6 carbons. But in the cycle, for the synthesis of one glucose, since each turn gives a 3 - carbon gain (after recycling), actually, each turn produces a 3 - carbon unit (G3P). So to get 6 carbons (glucose), we need 2 G3P, and each G3P requires 3 turns? No, I think I messed up. Let's get the correct concept: The Calvin cycle has three phases: carbon fixation, reduction, and regeneration. In carbon fixation, \(CO_2\) combines with RuBP (a 5 - carbon molecule) to form two 3 - carbon molecules (3 - PGA). Each turn of the cycle (processing one \(CO_2\)) results in the net production of 1/6 of a glucose molecule? No, the correct number is that 6 turns of the Calvin cycle are needed to produce one molecule of glucose. Wait, no, actually, each turn fixes one \(CO_2\) (1 C). Glucose is \(C_6H_{12}O_6\), so 6 C atoms are needed. So 6 turns (each fixing 1 C) would provide 6 C atoms to make glucose. Wait, but another way: The net synthesis of one G3P (3 - C) requires 3 turns (fixing 3 \(CO_2\)). Since glucose is 6 - C, we need 2 G3P, so \(3\times2 = 6\) turns. So the correct answer is six? Wait, no, wait the options: the options are six, one, two, three. Wait, maybe I made a mistake. Wait, let's check the correct biology concept: The Calvin cycle must turn 6 times to produce one molecule of glucose. Because each turn incorporates one \(CO_2\) molecule, and glucose has 6 carbon atoms. So 6 turns (each fixing 1 \(CO_2\)) will give 6 carbon atoms to build glucose. So the correct option is "six".

Answer:

A. six