TRANSPORT IN HIGHER AND LOWER ORGANISMS
As organisms become larger, their body surface areas increase by the square of dimensions, whereas the volume increases by the cube. Hence, an organisms increase in size, their surface area to volume ratio (SA/V) decreases.
The implication of this is that small organisms have larger surface area in relation to the volume (i.e. per unit of volume) than larger organisms. This could be demonstrated using three cubes of 1cm, 2cm, and 3cm. The relationship between the surface area and volumes of bodies can be interpreted or calculated using the table below.
Note: a cube has six sizes.
From the table above it can be seen or observed that the smaller the size of the body of an organism (e.g. cube X units 1cm) the larger the surface area (i.e. b) while the larger the size of the body of an organism (e.g. cube Z with 3cm) the smaller the surface area (i.e. 2)
In lower or unicellular organisms such as Amoeba, Paramecium, Euglena, and Chlamydomonas, the surface area of volume ratio (SA/V) of the body is large. As a result, essential nutrients like food, oxygen, and water as well as excretory products, e.g. water and carbon dioxide move in and out of the body by diffusion. Diffusion, therefore, is most effective when the surface area is large.
In multicellular organisms with a small body surface relative to their large volume, diffusion is inadequate for the exchange of metabolic materials within their body and between them and their external environment. This is because large quantities of nutrients and waste products have to be transported, over long distances, to and from their numerous body cells.
In other words, as SA/V ratio in multicellular organisms decrease, with increasing sizes, the rate at which substances diffuse into and out of their cells decreases. Hence, most multicellular organisms have developed complex transport system.
Importance of transport systems in mammals are as follows:
1. The primary purpose of transport systems is to move materials throughout the body of the organisms.
2. Mammals have various substances/materials for specific functions located in different parts/organs of the body. For example, oxygen is present in the lung while digested food is located in the alimentary canal.
3. Through transport metabolic products/excretory products are eliminated/removed via the kidney/lung/skin.
4. Hormones which are produced by the endocrine glands are made available to far away cells which they serve.
5. Blood is the main medium of transport, assisted by the lymphatic system/lymph.
6. Mammals being large and complex organisms have a surface area comparatively smaller than the volume ratio is low. Diffusion alone is inefficient.
7. Therefore there is the need to develop an efficient transport system to cope with this problem.
Materials for Transportation
Materials that are transported in plants and animals are many. In this respect, discussion will be based
on materials transported, sources of materials and where they are transported to.
materials for transport in animals
Materials that are transported in animals include:
1. Oxygen: oxygen is transported from the lungs to all the living cells of the body for tissue respiration.
2. Carbon dioxide: this is an excretory waste product transported from the cells where they are produced to the lungs where they are excreted.
3. Urea: urea is also an excretory product transported from the cells to where they are removed, e.g. liver.
4. Excess salts: these are also produced from cells and excreted by the skin and kidney.
5. Water: water is also an excretory product produced by the cells and transported to various organs such as skin, lungs, kidney and liver where they are excreted.
6. Amino acids: these are products of protein digestion transported from small intestine to various cells.
7. Vitamins: vitamins are also products of digestion transported from small intestine to various cells for use.
8. Sugars: sugars are products of starch and carbohydrate digestion. They are transported from the ileum to various cells of the body for tissue respiration.
9. Fatty acids and glycerol: these are products of fats and oil digestion. They are transported from the small intestine to the cells where they are required for body metabolism.
10. Mineral salts: mineral salts are transported from the small intestine to the various cells where they are needed for metabolism.
11. Hormones: hormones are transported from the endocrine glands that secret them for the various organs or tissues on which they act.
12. Antibodies: antibodies are produced by white blood corpuscles and transported by the blood to all parts of the body where they defend the body against infection.
Materials for transport in plants
Materials that are transported in plants include:
1. Manufactured food: it is transported from the leaves mainly to all living cells of the body for tissue respiration or for storage in storage organs.
2. Excretory products: e.g. carbon dioxide and water, are transported from all the living cells to where they are excreted.
3. Water: water absorbed from the soil is transported to the leaves and other parts of the plant for photosynthesis and other functions.
Other materials transported in plants are:
5. Nitrogenous waste products/latex
6. Amino acids
9. Auxins or hormones
10. Mineral salts such as nitrates and phosphate.
Media of Transportation
In all organisms, a liquid or fluid is the medium of transportation of materials. Generally, there are four major media of transportation which are:
1. Cytoplasm: cytoplasm is used as the medium of transportation of materials in lower unicellular organisms such as Amoeba and Paramecium. Materials such as glucose, amino acids, oxygen, water and carbon dioxide are transported from one part of the cell to another through cytoplasm.
2. Cell sap or latex: cell sap or latex is used as the medium of transportation of materials in plants. Cell sap is a concentrated solution found in the vacuole of cells which serves as a stronger solution. As a result of this the cell sap is able to transport water and dissolved mineral salts from the soil through the root hairs to the upper parts of the plants.
3. Blood: the blood is a powerful medium of transportation of materials in most animals especially vertebrates. The blood in its fluid state is able to move large materials over the entire body through blood vessels like arteries, veins and capillaries from where they are produced or obtained to their point of destination.
4. Lymph: lymph is one of the media of transportation in higher animals. It is a fluid similar in composition to tissue fluid, although it contains extra lymphocytes, there is no red cell present. It returns fluid to the main veins through opening in the subclavian (left jugular) vein below the neck. Examples of lymph vessel is the lateal which transports fatty acids and glycerol.
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