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?