Final answer:
L and D sugars are distinguished by the position of the hydroxyl group on the second-last chiral carbon in the Fisher projection: right for D and left for L. These descriptors define the sugars' absolute configuration but do not indicate the optical rotation.
Step-by-step explanation:
We can identify L and D sugars by examining the orientation of the hydroxyl (-OH) group on the bottom-most chiral carbon in the Fisher projection. For monosaccharides, such as glucose and glyceraldehyde, the D and L stereochemical descriptors are assigned based on the position of this -OH group. If the -OH group on the penultimate carbon (the second-last chiral carbon) is on the right, the sugar is given the D-configuration, whereas if it is on the left, it is given the L-configuration. This convention is important in determining the absolute configuration of the sugar's chiral centers.
Natural monosaccharides commonly have D-configuration, while natural amino acids generally have L-configuration, with some exceptions. It's noteworthy that the D/L descriptors do not indicate how the sugar rotates plane-polarized light; that property is indicated by lowercase d- (dextro) or l- (levo). For example, D-glyceraldehyde is an enantiomer of L-glyceraldehyde, but they may rotate plane-polarized light in the same or opposite directions. This rotation is separate from the uppercase D or L configuration, which strictly relates to the spatial arrangement of atoms around the chiral carbon.