What is the relationship between metabolic rate and body temperature in animals
ID: 28686 • Letter: W
Question
What is the relationship between metabolic rate and body temperature in animals in general? Given that relationship, why does lowering brain temperature (by 3o C) conserve oxygen, and why is decreasing oxygen demand so critical for neural function (hint: as opposed to muscle function)?Explanation / Answer
Themes > Science > Zoological Sciences > Animal Physiology > Respiration (J.L. Bailey, J.Y. Grivetti, R.C. Adams, R. Bailey, D.E. Facey, R. Bailey) (presented by Saint Michael's College, Colchester, VT, USA) Do you like to breathe? Well, we do, and we learned a whole lot about how it works in mammals and in other groups of animals. If you’d like to know more, take a breather and enter our website. Just sit back, relax, take deep breaths, and inhale all of our information. Go for it! What is respiration? Why do animals respire and why is it important? Respiration is the process by which animals take in oxygen necessary for cellular metabolism and release the carbon dioxide that accumulates in their bodies as a result of the expenditure of energy. When an animal breathes, air or water is moved across such respiratory surfaces as the lung or gill in order to help with the process of respiration. Oxygen must be continuously supplied to the animal and carbon dioxide, the waste product, must be continuously removed for cellular metabolism to function properly. For example, if this does not happen and carbon dioxide levels increase in the body, pH levels decrease and the animals may eventually die (see Question: Why is the regulation of body pH important?). Oxygen is valuable because it is important in many ATP-producing cycles occurring throughout the body such as, the Krebs cycle, and the electron transport chain. Glycolysis breaks down glucose, a six-carbon sugar, into the three-carbon molecule of pyruvic acid. The series of reactions associated with glycolysis are necessary for anaerobic and aerobic pathways to work, and are also the most fundamental in cellular metabolism. In the presence of 02, the pyruvic acid, which came about from the breakdown of glucose, is further oxidized. However, under anaerobic conditions the pyruvic acid is reduced to lactic acid. Glycolysis follows a specific pathway and ultimately, the oxidation of 1 mol of glucose to pyruvic acid ends in a net gain of only 2 mol of ATP and 2 NADH molecules. The Krebs cycle is a series of eight major reactions following glycolysis. In these reactions, acetate residues are degraded to CO2 and H2O. With each turn of the Krebs cycle, 2 CO2 molecules and 8 H+ atoms are removed. These hydrogen atoms, which are removed two at a time, are transported by NADH and FADH2 and further go into the electron transport chain. The electron transport chain, also known as the respiratory chain, oxidizes the NADH and FADH2 from the Krebs cycle to H2O by oxygen. This cycle involves electrons that move through about seven steps in order of their decreasing electron pressures, more specifically, from the high reducing potential of NADH to FADH2 to oxygen, the final electron acceptor. The electron transfer is the final pathway for all electrons during aerobic metabolism, and it uses the energy from the transfer for the phosphorylation of ADP to ATP. A total of 38 ATP molecules are collectively released from the three cycles of glycolysis, the Krebs cycle, and the electron transport chain working together. Without oxygen, the Kreb's cycle and electron transport chain would be disabled and only 2 ATPs would be produced by glycolysis. To maintain an adequate supply of oxygen to cells, animals must have an efficient means of gas transfer and respiration. Oxygen debt In some animals, such as mammals, if the supply of oxygen to active muscle cells is not sufficient to produce enough ATP to maintain intense activity, the only source of additional ATP will be from glycolysis. Without sufficient oxygen, some of the pyruvic acid produced is reduced to lactic acid, which accumulates in the tissues, resulting in fatigue. Excess lactic acid may also enter the blood, decreasing blood pH and affecting other tissues in the body. When muscle activity decreases, extra oxygen is needed to convert the lactic acid back to pyruvic acid, which is then utilized by the Kreb's cycle. This extra oxygen represents the animal's oxygen debt. Some animals, such as the goldfish and some intertidal invertebrates, can avoid oxygen debt through the use of biochemical pathways that convert lactic acid to alcohol, which can then be excreted. What is the difference between air and water as respiratory environments? How does this affect the amount of energy spent obtaining oxygen in water and air and therefore the structures used in ventilation? Water and air are radically different as respiratory environments in a number of ways. The most significant difference is that water contains only 1/13 as much O2 as air does, or 1% to 21% (water to air) by volume. Water also is over 800 times denser than air and 50 times more viscous, so aquatic breathers must use more energy to simply move water across their respiratory surfaces. Fish, for example, use as much as 10% of the oxygen they take in to provide breathing muscles with enough oxygen to burn the energy needed to keep water passing over the gills in the right direction. Humans use only 1-2 % of their oxygen intake to keep breathing. Temperature also has an effect on the amount of oxygen each environment can hold. As water temperature increases the amount of dissolved oxygen decreases. Air also shows a slight reduction in oxygen content with increasing temperature, but it isn't physiologically significant because there is so much oxygen in air to begin with. Gas diffusion rates are also lower in water than in air. Salt water contains less oxygen than fresh water because the higher salt concentration decreases gas solubility. All of this produces a vast difference between aquatic and terrestrial organisms in the amount of energy expended to obtain oxygen. How is oxygen carried through the blood and passed onto other cells? What role does hemoglobin play in oxygen transfer? What conditions affect hemoglobin/oxygen affinity? Hemoglobin (Hb) is found in red blood cells, being the principle part of a red blood cell. Hemoglobin is a large protein with four polypeptide chains and four heme groups. Each heme group has an iron atom attached to it, which is where oxygen attaches to be carried to cells and tissues. It is important to note that the O2/Fe bond, that is initially made so the oxygen can be transported, can be readily broken in the right conditions. These conditions are altered depending on if oxygen needs to be picked up or released to tissue cells. The reason hemoglobin is found in red blood cells only is that the conditions needed for efficient oxygen transport by the Hb molecules can be quickly changed, and all of this can be done without changing the conditions throughout the body. Some of the conditions necessary for oxygen and carbon dioxide transport may be unsuitable for other reactions that need to take place throughout the body, so keeping Hb within the red blood cells allows oxygen transport to occur without interfering with other bodily functions. Conditions that control the ability of hemoglobin (Hb) to bind to oxygen include the partial pressure of O2 in the surrounding respiratory medium (air or water), temperature, pH, CO2 levels. A high partial pressure of O2 in the surrounding respiratory medium will increase the rate at which the O2 diffuses into the blood. Hemoglobin's affinity for oxygen typically decreases if temperature increases, pH decreases, or CO2 levels increase. There are a few different kinds of hemoglobin, all doing the same job, but each having its own affinity to O2. Normally hemoglobin will pick up an O2 when the partial pressure of the O2 in the blood (O2 dissolved in solution) is high, and there are fewer than 4 O2 molecules on the hemoglobin, 4 being the maximum number able to be carried. When an O2 molecule is attached to a hemoglobin molecule it is not affecting the partial pressure of the O2 in the blood, as there is a low concentration of O2 in the blood plasma, just not enough to supply the cells of the body. The best scenario for oxygen transfer from the lungs to body cells and tissues is hemoglobin to have high affinity at the respiratory surface (high amount of O2 diffusing across the lung surface) and low oxygen affinity (give the oxygen away) near body cells that need it (low O2 content). Other factors that affect hemoglobin/oxygen affinity include a decrease in pH, which reduces hemoglobin/oxygen affinity (the Bohr effect). A decrease in pH reduces Hb/ O2 affinity because the shape of the oxygen-binding sites of the hemoglobin molecule changes, making it more difficult for them to bind to oxygen. (See "Why are red blood cells important to carbon dioxide transport?" for a complete explanation of the mechanisms involved). A rise in body temperature reduces Hb/O2 affinity as the increased energy (heat) will prevent bonds from forming or break bonds currently in place. Increased CO2 content can affect the affinity because CO2 can bind to sites where O2 would normally bind. Hemoglobin normally picks up CO2 at the tissues and releases it at the respiratory surface in exchange for oxygen to complete the chain. When the concentration of CO2 is too high it takes the place of oxygen on Hb at higher than normal rates. Oxygen dissociation curves graphically represent the percent of hemoglobin's oxygen binding sites that are holding oxygen at different partial pressures of oxygen. The sigmoid (S-shaped) curve is due to subunit cooperativity between the four oxygen binding sites on a hemoglobin molecule. When no binding sites are occupied by oxygen, it is relatively difficult to get the first oxygen to bind. After it does, however, the structure of the hemoglobin molecule is altered a bit, and the second binding site becomes more accessible. This makes it a bit easier for the second molecule of oxygen to bind. After this, additional oxygen molecules bind rather easily to the third and fourth binding sites. Therefore, oxygen binds slowly at first, and then more quickly, giving the dissociation curve a sigmoid shape. Carbon dioxide transported in the blood The transportation of carbon dioxide is a very significant process of the gas-transfer systems within many animals. There are three main ways in which CO2 is transported in the blood. A small percentage of the CO2 that is in the blood is dissolved molecular CO2. A larger amount of CO2 reacts with –NH2 groups of hemoglobin and other proteins to form carbamino compounds. However, most of the CO2 that is transported in the blood is in the form of bicarbonate (HCO3-). In general, CO2 is diffused into the blood from the tissues. The blood transports CO2 to the respiratory surfaces of the lungs or gills, where it is released into the environment. The blood mainly consists of plasma and erythrocytes (red blood cells). Most of the CO2 entering and leaving the blood does so through erythrocytes. Red blood cells important to carbon dioxide transport Most of the CO2 entering or leaving the blood go through red blood cells for two reasons. One reason is due to the enzyme carbonic anhydrase. This enzyme is present in red blood cells and not in the plasma. The enzyme is important in the transportation of CO2 because, within the red blood cells, it catalyzes the reaction of CO2 with OH- resulting in the formation of HCO3- ions. As the level of HCO3- ions increases within the erythrocytes, the HCO3- ions diffuse through the erythrocyte membranes into the plasma of the blood. In order to maintain electrical balance within the erythrocytes, an anion exchange occurs in a process called a chloride shift. In this process, HCO3- ions leave the red blood cells while a net influx of Cl- ions from the plasma enters the red blood cells. The membrane of red blood cells is very permeable to both ions because the membrane has a high concentration of a special anion carrier protein, the band III protein. This protein allows for a passive diffusion of the Cl- and HCO3- ions to and from the red blood cells and plasma. This keeps the bicarbonate from building up in the red blood cells, which would slow down or stop the reversible conversion of CO2 to HCO3-. Facilitated diffusion occurs in the movement of CO2 across the respiratory surfaces as bicarbonate (HCO3-) diffuses out of the red blood cells and into the epithelium where it is converted back to CO2. Excretion of CO2 is limited by the rate of bicarbonate-chloride exchange across the erythrocyte membrane. The second reason why most of the CO2 is transported to and from the blood by passing through the erythrocytes is that O2 binds to Hemoglobin (Hb) at the respiratory surface, causing hydrogen ions (H+) to be released. The increase in H+ ions combines with HCO3- to form CO2 and OH-. Thus, more CO2 is formed and can leave the blood across the respiratory surface. Excess H+ binds to OH-, forming water and allowing the pH to increase enough to promote the binding of oxygen to Hb. The release of O2 from Hb in the tissues makes the Hb available to bind to H+, promoting the conversion of CO2 to HCO3-, which helps draw CO2 from the tissues. Therefore, CO2 that is being transported into and out of the red blood cells minimizes changes in pH in other parts of the body because of proton binding to and proton release from hemoglobin, as it is deoxygenated and oxygenated, respectively (Figure 1.).
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