Small multicellular organisms like the Hydra and flatworm increase their S.A/V ratio by developing structures such as the gut which provide extra surface for material exchange. However, these multicellular organisms still could not grow beyond a certain size. This is because as a size increases the process of diffusion becomes a limiting factor, preventing the development of larger and more complex bodies.
In the hydra and flatworm, all the body cells are very nest the material exchange surfaces. Diffusions is sufficient to transport materials between the exchange surfaces and the cells. Diffusion is only efficient over distances between the body cells and the exchange surface increase. an k crease in body size means an increase in the distances between the body cells and the exchange surfaces. Since diffusion could not cope with the transport of materials beyond a short distance, a need arose for a transport system to carry materials rapidly between the material exchange surfaces and the body cells.
In large multicellular organisms, the limiting factors due to increase in size and complexity, were overcome by the development of
• Structures, such as gills, lungs, intestines and leaves (in plants) , with large external and internal surfaces for material exchange; and
• A transport system where a fluid carries materials through out the organism. Certain mechanisms provide the necessary driving force to keep the fluid moving continuously. The suspensions and solutes in the fluid flow at the same speed. This type of transport System is called a mass flow system. It is present in all higher plants and animals.
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