The glucose molecule is small enough to be absorbed directly through the walls of the digestive system.
Cellulose is also made up of glucose molecules. It makes up plant cell walls. It is therefore a fundamental part of our diet. It cannot be broken down by the digestive system, so is egested from the gut.
Once absorbed by the body, glucose molecules are transported to cells and:
In humans and animals glucose is stored in glycogen. It is not converted into starch.
Proteins are big molecules that are too large to pass through the gut wall. They must first be broken down into amino acids.
Once inside the body, the amino acids are reassembled into the proteins the individual requires – the process of protein synthesis.
Excess amino acids are broken down in the liver.
Lipid molecules are too large to pass through the gut wall and must be digested first.
In the body's cells, they are reassembled into the lipids the cell needs, for instance, for the cell membranes.