Carbon Dioxide and Organisms

Carbon dioxide is both a vital requirement for producers – the first organisms in any food chain, and at times a dangerous waste product. Where plants use carbon dioxide during the Calvin cycle of photosynthesis, at night they also produce small quantities due to respiration. Fundamentally, carbon dioxide reacts with ribulose bisphosphate to produce glycerate 3-phosphate in the light independent reaction. In turn glycerate3-phosphate is reduced to triose phosphate using reduced NADP and ATP from the light dependent reaction. This follows through to produce products such as starch, and regenerate RuBP. Without this process, plants cannot produce the organic molecules which are a critical starting point for all food chains and webs. One way that carbon dioxide is released is through respiration. During the Krebs cycle carbon dioxide is removed when turning citrate into oxaloacetate, removing 2
CO2 in each cycle. Directly, organisms release 6 CO2 molecules for each molecule of glucose used in the respiratory pathway. It is important that when breathing, excess carbon dioxide is removed from the body. In solution, carbon dioxide forms a weak acid called carbonic acid. This lowers the pH of the blood (in humans) and in return, this change in pH can affect many enzymes as well as the haemoglobin molecule. The lowered pH causes the shape of the haemoglobin molecule to change so it can offload more oxygen. This change in properties of the haemoglobin molecule displaces the oxygen dissociation curve to the right (Bohr shift) and lowers its affinity for oxygen. To remove excess carbon dioxide, the sympathetic nerve from the medulla oblongata will send a series of impulses to the SAN in the heart as a result of chemoreceptors in the carotid artery detecting an increase in CO2 concentration. The sympathetic nerve causes the SAN to increase the amount of heart beats per minute, so removing the carbon dioxide faster by transporting more blood to the lungs. Another way of carbon dioxide accumulating in the atmosphere is via the carbon cycle when dead or decaying organisms are broken down by saprobionts and carbon dioxide is produced by microbial respiration. Unfortunately, carbon dioxide may indirectly cause adverse effects to the climate in which organisms live. When radiation from the sun reaches the earth’s crust, it is 5 reflected back. Carbon dioxide along with other greenhouse gases reflects his radiation and prevents it escaping. As more carbon dioxide accumulates, more heat is retained leading to rising temperatures and a warmer climate. An increasing temperature can have a number of effects on living organisms. At first thought, it might be considered that a combination of a greater concentration of carbon dioxide and a higher temperature will increase the rate of photosynthesis but this may not be the case. The enzyme Rubisco is involved in catalysing the reaction in which ribulose bisphosphate combines with carbon dioxide. Rubisco also combines with oxygen however in a process called photorespiration. Photorespiration increases rapidly at higher temperatures so a rise in temperature could result in photosynthesis become less efficient, not more.
Crop yield is also related to pest damage. A higher temperature is likely to affect pest numbers. It might do this in a number of ways. The European corn borer is a pest of maize. Scientists have found that the number of generations of this pest decrease with distance north. A higher mean temperature will therefore lead to more generations and greater crop loss. The UK is at the northern limit of distribution for some pest insects. One of these is the southern green shield bug. This is a serious pest of some crops in continental Europe. Over the past few years, biologists have found more of these plant-sucking pests in southern England.
To conclude, carbon dioxide is both a harmful waste product and a necessity for the vital process of photosynthesis.