in the photosynthetic dark reaction there are two types of reactions :light dependent and independent. in the 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 u 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?