Why does the fifth step of glycolysis convert dihydroxyacetone-phosphate into glyceraldehyde-3-phosphate?
ASo the pathway continues with one isomer only
BSo carbon dioxide can be released at once
CSo the sugar can re-enter the bloodstream
DSo the cell can store the sugar as glycogen
Answer & Solution
Correct answer: A. So the pathway continues with one isomer only
1. Cleaving the six-carbon sugar gives two different three-carbon isomers.
2. An isomerase transforms dihydroxyacetone-phosphate into its isomer glyceraldehyde-3-phosphate.
3. The pathway therefore continues with two molecules of a single isomer.
4. One shared intermediate means the same enzymes can process both halves of the original sugar.
5. No carbon dioxide leaves during glycolysis, so a decarboxylation answer misplaces a later event.
6. Glycogen storage happens elsewhere and needs glucose-1-phosphate rather than this triose.
_Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 7 "Cellular Respiration", section 7.2 Glycolysis_
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