The glucose molecule is small enough to be absorbed directly through the walls of the digestive system but starch is a polymer of glucose. Starch must be broken down first by carbohydrase enzymes called amylase into maltose and then into glucose molecules by another carbohydrase.
Proteins are big molecules that are too large to pass through the gut wall. They must first be broken down into amino acids by protease enzymes. These amino acids can then be used to make proteins in the body.
In a lipid, there may be 1, 2 or 3 fatty acids combined with glycerol - fatty acid 1, 2 and 3 may be the same or different.
Lipid molecules are too large to pass through the gut wall and must be digested first by lipase enzymes.