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In the photosynthetic dark reaction there are two types of reactions: light-dependent and light-independent. In the light-independent reactions during reduction 3-phosphoglycerate is converted to glyceraldehyde-3-phosphate. I don't find any difference between them because both have a 3 carbon chain attached to a phosphate group. Can you please clear it up for me because my book says that the latter one is a sugar. Why isn't the first one sugar too?

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The difference between the two molecules is highlighted in red.

enter image description here

The functional group in 3-phosphoglycerate is a carboxylic acid. That in glyceraldehyde-3-phosphate is an aldehyde.

Sugars have the general formula $\ce{C_$n$H_{$2n$}O_$n$}$. To accomplish this specific ratio of carbon to hydrogen to oxygen, the requirement is that one of the carbons must have a carbonyl group (i.e. forms an aldehyde or ketone), whereas the remainder of the carbons have one hydroxyl group each. For example, the open-chain structure of glucose is:

enter image description here

The carbonyl group is in red and the hydroxyl groups are in blue.

Therefore, glyceraldehyde-3-phosphate is simply a phosphorylated sugar: C-1 is the aldehydic carbon and C-2 and C-3 have hydroxyl groups, the latter of which is phosphorylated.

3-Phosphoglycerate is one oxidation state too high.

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    $\begingroup$ Which makes 3-phosphoglycerate an aldonic acid ;) $\endgroup$ – Jan Nov 5 '16 at 19:43

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