Why is a transport system necessary?
All living organisms need to exchange materials with their external environment. Materials are taken in, distributed around the body and wastes which are produced eliminated. In addition, the products of some structures are used at sites away from where they are produced. Such products must also be circulated.
Cells exchange materials with their immediate surroundings by the processes of diffusion and osmosis. In small organisms, the distances between their body surfaces and the cells are small enough to enable efficient exchange by simple diffusion. Thus, protozoa, algae and flatworm carry out exchanges by diffusion. However, as organisms increase in size, their surface area to volume ratios decrease while their distances of cells from external surfaces increase. For larger organisms diffusion through external surfaces is inefficient as a means of transport.
As organisms evolved and increased in size, special absorptive surfaces in localized regions of the body were developed. Examples of such organs are gills, lungs, alimentary canal, leaves and roots. Transport systems became essential in order to distribute materials rapidly between the cells and these specialized structures, as well as between different parts of the body. Most transport systems consist of a transporting fluid, conducting structures and mechanisms for maintaining the flow of the fluid through the structures.
The necessity for transport systems becomes more obvious when the types of materials transported are examined.
What types of materials do transport system carry
Variety of materials transported in the bodies of plants and animals may be divided into 3 groups: useful materials, harmful materials and wastes.
Useful materials-nutrients and oxygen that are diffused from the ileum to the cells when needed.
Harmful materials-some substances are toxic to the body eg alcohol and drugs that are broken down in the liver. Carbon dioxide in the body is harmful and most be released. It travels in the blood to the lungs where it is expired.
Waste substances-these are produced from metabolic reactions in the body. Carbon dioxide in respiration, oxygen produced in photosynthesis and other substances that result from these reactions.
Heat-is transported in the blood to all the body cells and keeps the body’s temperature constant.
Substances transported in plants
Water and minerals-these materials are transported from the roots to the living cells through the plant. Water and minerals are especially needed by cells which are metabolically active eg to growing points, developing flowers and fruits and cells carry out photosynthesis. However, most of the water leaves the plant in transpiration as part of its transport mechanism.
Manufactured materials eg sucrose and amino acids which are produced after photosynthesis. They are in soluble form and are moved from points of production or of storage to other areas of the plant which needs them.
Waste products-these materials are transported from wherever they are made to old leaves, stems and bank which will eventually fall off. Some waste is transported to storage areas.
Plant growth substances-plant hormones called auxins are transported by diffusion from growing points to the areas behind the tips where they controlled the elongation of cells.