{ Thursday, November 23, 2006 } 10:25 AM

Cambridge may be wrong

This was a MCQ question from my Biology 9283 Paper 1. I'm pretty sure there's something wrong with it, unless someone tells me factually that I am wrong. I don't believe in omission of answers when it came to this question.

In photosynthesis, RUBP (a 5-carbon compound) combines with carbon dioxide to from a 6-carbon compound which is unstable. Therefore, this 6-carbon compound breaks down into 2 molecules of 3-carbon compound. This 3-carbon compound is called 3-Phosphoglycerate. (Ok, I don't know the actual way of spelling it but it is GP.)

Then, 6 ATP molecules are used up to form one molecule of 1,3-Bisphosphoglycerate (BPG) from my one molecule of 3-Phosphoglycerate. And another 6 NADPH molecules are used to convert one molecule of 1,3-Bisphosphoglycerate into one molecule of Glyceraldehyde-3-Phosphate (G3P).

Some Glyceraldehyde-3-Phosphate are used to make other organic compounds, such as starch. The remaining unused Glyceraldehyde-3-Phosphate will each combine with 3 molecules of ATP to form back my RUBP. Then the whole cycle repeats itself.

Then, this question asks me.

In which step (A,B,C or D) are both ATP and NADPH used up?




There was no G3P, so I was stuck between answers C and D. I finally decided to put C, because in step D, only ATP was used.

And after the exam everyone says, answer is B. However, the spontaneous breakdown of the 6-carbon compound gives GP without any other molecules needed!

I'm going crazy. If B is the answer, then it's conceptually, practically, theoretically, physically, mathematically, biologically, completely and TOTALLY wrong!!!

I am frustrated.