Biological Oxygen Transport - Essay Example

Biological Oxygen Transport Oxygen is one of the most important elements to sustain life on this planet. Almost every life form, be it plants, invertebrates and vertebrate animals, require oxygen to carry out essential biological processes. It is crucial for the generation of energy through the oxidation of carbohydrates and other fuel forms. The generated energy is used to carry out important functions including growth and movement in animals. Life of every structural and functional unit of any life form depends upon the availability and utilization of oxygen. Keeping in view the importance of oxygen for life, its transfer in animals and plants has become a wide area of research and study. The term biological oxygen transport refers to the incorporation of oxygen from the environment into the cell. It covers all the processes right from breathing to the respiration. Such transport process encompasses different levels of organization from organism to the level of sub cellular organelles. The transport of oxygen in various biological systems majorly depends upon the process of diffusion and convection. Generally there are two biological systems: plant and animals. Plants do not have any specialized organ for the transport of oxygen. Roots stem and leaves are major sites involved in uptake and release of oxygen. Oxygen is absorbed for respiration and is released into the atmosphere as a result of photosynthesis. Leaves being exposed to air are the major sites of oxygen uptake. Air mixed oxygen enters the empty spaces in the leaves through the openings called stomata. The entry of oxygen to the intracellular air spaces is followed by the process of rapid diffusion. The interior of the cells have less oxygen concentration as compare to the exterior. So, following the principle of diffusion, oxygen moves from the higher exterior concentration to the lower concentration inside the cell. This transport is accomplished by passing through various cellular-barriers such as cell wall and cell membrane. Oxygen being non polar easily diffuses through these barriers. Oxygen can also be transported through the stem and the roots. The cork of roots and stem contain openings called lenticels, for the transport of oxygen into and out of the plant body. Moreover plants with soft green stems bear stomata for the transport of oxygen. As far as the transport of oxygen within the plants is concerned, experiments have shown that the plant contains non –tortuous gas filled channels between the stem and the root. Apart from the lenticels; the oxygen requirement of roots is met by the transport of oxygen through these channels. Roots also possess numerous gas spaces that aid in the transport of oxygen. This oxygen is ultimately transported to the power house of the cell (Greenwood). Like plants, animals also require oxygen to sustain life. They have different oxygen transport mechanisms, depending upon the habitat and level of complexity. When we talk about the lower invertebrates, there is no specialized organ or system for uptake of oxygen. The transport is generally carried out by simple diffusion through the skin. The animals inhabiting sea environment use to respire through the general body surface. Such invertebrates include cnidarians, sponges and acoelomate worms (Overhill). When we move towards slightly complex invertebrates, we find them to be having some transport mechanisms. Although the basic mechanism of diffusion is same, it is now aided with specified organs and carrier proteins between the sites of gas exchange. Researchers have shown that various invertebrates and some lower vertebrates possess carrier proteins such as hemerythrins and hemocyanins that have actively participated in transport of oxygen, over the whole evolutionary process. As we move towards the complex organisms, we see origination of specific systems involved in oxygen transport. The aquatic insects obtain oxygen through convection. The bulk of water entering the body, uniformly distributes oxygen to various parts of body. The terrestrial insects have developed tracheal system for the transport of oxygen from the surrounding to the body cells. These insects trap a layer of oxygen around their body. This trapped oxygen is transferred to the interior though the tracheal openings on the insect’s surface. This kind of respiration is called plastron respiration (Chapman and Chapman). Birds or avian have developed a unique system for the transport of oxygen. In addition to small lungs, birds also possess nine air sacs for the efficient transfer of oxygen to every part of the body. As they are involved in more energy consuming activities therefore the high oxygen demand is met through the air sac system. They act as bellows to transport air through the lungs. The air sacs are provided with small number of capillaries. Although they are not significantly involved in gas exchange but still possess key importance in oxygen transport system in birds. In fishes, the transport of oxygen is quite difficult as compare to the terrestrial organisms. The aquatic environment has less than 1 percent dissolved or available oxygen. Fishes and other aquatic vertebrates cope with the oxygen demand through gills. The gills possess filaments that are in turn provided with a rich network of capillaries. Keeping in view the Flicks law, they offer a greater surface area for the uptake of oxygen. This absorbed oxygen is transported to various parts through the openly circulated blood. Mammals’ posses the most advanced process of oxygen transport. Oxygen is efficiently transported from the atmosphere to each cell in the tissue. Body gains oxygen by inhaling air through the specialized nasal and oral passages. This inhaled air contains oxygen, which is transported to the tubular trachea, followed by larynx and pharynx. The trachea bifurcates into two tubes that lead air into the lungs. These tubes are called bronchi that divide and re-divide to form bronchioles. The whole system is like a tree, as a network is obtained after successive branching. These finely divided passageways ultimately end up into the sac like structures called alveoli. Once oxygen has entered into the lungs, it is transported to each cell through the circulatory system. Blood being the fluid of the vascular system contains oxygen carrying protein hemoglobin. Hemoglobin consists of four polypeptide chains; two alpha and two beta chains. It contains a non polypeptide unit called heme group. This group is responsible for carrying oxygen from lungs to the tissues. Each polypeptide chain has its own heme group. Therefore each hemoglobin molecule carries four molecules of oxygen. The binding of one molecule of oxygen enhances the binding of additional oxygen molecules. Thus the function of hemoglobin is significantly cooperative. In addition to hemoglobin, human blood also contains myoglobin for the transport of oxygen in muscles. The oxygen carrying proteins present in the oxygen lean blood pick up oxygen on their passage through the lungs. This oxygen lean blood is transported through the pulmonary arteries. The pulmonary arteries divide into capillaries and surround the alveoli. Alveoli are rich in oxygen. Oxygen is easily diffused into the blood through the alveolar wall. The diffused oxygen binds to the hemoglobin present in the erythrocytes. As mammals possess double circuit circulation, the oxygenated blood is transported to the left atrium through pulmonary veins. Heart pumps the oxygen carrying blood to every organ of body through the respective arteries. On reaching the organ or tissue, the arteries divide into daughter vessels or arterioles having smaller diameter as compare to the parent vessel. The arterioles in turn divide into capillaries. The capillaries being one cell thick offers the most suitable and efficient site for gaseous exchange. On reaching the tissue level, the oxygen is detached from the hemoglobin. Here the venular blood act as oxygen sink, thus facilitate the release of oxygen. Moreover the diffusive shunts created between arterioles and venule and between the capillaries allow diffusion to the tissues. In fact, carbon dioxide has higher partial pressure as compare to oxygen. Carbon dioxide diffuses into the plasma and become bi-carbonate ions. The hydrogen ions released as result of this diffusion cause a drop in the pH of plasma. The lower pH or hydrogen ions alter the quaternary structure of hemoglobin and transform it into acid hemoglobin or hydrogen bound hemoglobin. This results in loss of oxygen affinity. This change is called cooperativity. This change facilitates the release of second oxygen molecule after the release of first molecule. This process is followed by successive release of oxygen molecules till four molecules detach from hemoglobin. Thus, cooperativity helps in loading and unloading of large amount of oxygen (Dart Mouth). Similarly, partial pressure gradient facilitates the transfer of oxygen in the muscles. This is achieved through myoglobin. It is different from hemoglobin as it carries only one molecule of oxygen. It absorbs and releases oxygen at much lower partial pressure as compare to hemoglobin. The released oxygen nourishes every cell. The released oxygen in the interstitum, crosses the plasma membrane into the cell. This is facilitated by the low partial pressure in each compartment. Oxygen diffuses through the cytosol and ultimately reaches mitochondria. Partial pressure of oxygen at mitochondria is low as 1 to 2 torr. Mitochondria utilizes oxygen molecule for metabolism and ATP synthesis (Friedman). The synthesized ATP is used to carry out various functions of cell. Thus, we can conclude that from simpler to the complex biological systems, transport of oxygen is achieved through mechanism specialized for a particular complexity level. Word Count: 1544 References Chapman, R. F. and Reginald Frederick Chapman. The insects: structure and function. Vol. 4. Cambridge University Press, 1998. Dart Mouth. Oxygen Transport . 3 March 2010 . Friedman, Morton H. Principles and models of biological transport. Springer , 2008. Greenwood, D. J. "Studies on the Transport of Oxygen Through the Stems and Roots of Vegetable Seedlings." Jstor (1967): 337-347. Overhill, Raith. An introduction to the invertebrates. Cambridge University Press, 2006. Read More