A monomer is a single unit of a substance (eg. Amino acid). A polymer is a chain of monomers (eg protein). An isomer is a molecule of an element with the same chemical formula but a different chemical structure (e.g sucrose and maltose)
Aerobic respiration takes place in the presence of oxygen. Fermentation occurs in the absence of oxygen. You will get 2 ATP from glycolysis from both, however with oxygen present as in aerobic respiration you will get more energy.
