Diffusion is insufficient to meet the oxygen requirement of the multicellular organisms.
Diffusion is the process of transport of molecules from the high concentration areas to the low concentration areas. Diffusion is the random movement of the molecules to make the difference in concentration to become equal . Diffusion takes place when there is a difference in the concentration of molecules between two areas, this is called the concentration gradient.
Multicellular organisms like humans and other animals have complex body structures. All the cells combine and form specialized tissue structures that work for specific functions. In multicellular organisms we have different systems like circulatory systems for blood transport, digestion systems for food transport etc.
Diffusion is a slow process, it takes a lot of time for the oxygen to circulate to all the cells of the body. Therefore it is difficult to transport the oxygen to all the cells of the body through diffusion in multicellular organisms.
multicellular organisms need a speed transport of oxygen throughout the body. So, they have a specialized circulatory system to transport oxygen. However, in multicellular organisms exchange of oxygen and carbon dioxide in alveoli of the respiratory system through the process of diffusion.
Unlike unicellular organisms, multicellular organisms cannot interact directly with the outer side environment. So, it is difficult for multicellular organisms to meet the oxygen requirement of the body through the process of diffusion.