Carbon dioxide is an unreactive and colourless gas that naturally makes up small quantities of the earth’s atmosphere. The molecule is made up of one carbon atom and 2 oxygen atoms that form 2 carbon-oxygen double bonds with the molecular formula CO2. Carbon dioxide has several important roles, which contribute to the continuation of life on Earth. It plays a role in maintaining the Earth’s atmosphere, cellular reactions such as photosynthesis in plant’s and respiration in human’s as well as regulating conditions in the body and anthropogenic activity (NESTA, 2006). This essay aims to describe how carbon dioxide contributes to all these roles related to plants and humans and furthermore, explain why the presence of carbon dioxide is important for all these roles to take place.
The earth’s atmosphere is only made up of 0.04% carbon dioxide naturally (BBC, 2006), where respiration, photosynthesis and decomposition of dead matter through the carbon cycle has provided a natural balance. Increased anthropogenic activity has raised the carbon dioxide percentage in the atmosphere considerably more due to the burning of fossil fuels. Greenhouse gases such as carbon dioxide, are gases that absorb infra-red radiation from the Sun, and release it into the Earth’s atmosphere rather than back into space. This released heat allows the earth to heat up and this is how the Earth is maintained at a temperature suitable enough for organisms to survive. Carbon dioxide, however, is not the most abundant greenhouse gas. Water vapour has the highest percentage in the Earth’s atmosphere but the reason it is not as effective as carbon dioxide in trapping the heat is due to its short life span in the atmosphere until it becomes incorporated into the weather cycle. Carbon dioxide has a longer life span and so can build up in the atmosphere, so it contributes largely to the global warming that is troubling the planet Earth currently (NASA, 2016). This shows that carbon dioxide is important since without it’s presence in the Earth’s atmosphere, the temperature of Earth would drop to levels in which plants and humans would not be able to survive.
Plants use the naturally occurring carbon dioxide in the atmosphere as a key reactant in photosynthesis. The BBC’s simple general reaction involves the reduction of carbon dioxide into glucose (BBC, 2006);
6CO2 + 6H2O —-> C6H12O6 + 6O2
Sunlight energy
Photosynthesis has 2 stages; the light dependent reaction, which takes place in the thylakoid membranes of chloroplasts and the light independent reaction (also called the Calvin Cycle), which takes place in the stroma of the chloroplasts. Carbon dioxide is used as a reactant mainly in the light independent stage since this is when it enters the plant. The light dependent reaction requires the use of light energy from the sun that drives the photophosphorylation reactions in the photosystems. These reactions form oxygen, ATP and reduced NADP. The ATP and reduced NADP are required for the Calvin cycle. For the Calvin cycle to begin, the carbon dioxide diffuses through pores on the underside of the leaf called the stomata (Lodish et al., 1999). Once inside the leaf, as shown in figure 3, the carbon dioxide binds with the 5-carbon compound ribulose bisphosphate to form two molecules of glycerate-3-phosphate which is a 3-carbon compound. This reaction is catalysed by the enzyme ribulose bisphosphate carboxylase(rubisco). Two triose phosphate molecules are formed with the aid of energy from ATP and H+ ions from reduced NADP and these triose phosphate molecules are then converted into many useful organic substances such as glucose. This is called carbon fixation whereby inorganic carbon (CO2) has been converted into organic carbon molecules (Lambers, 2015). Carbon dioxide is essential for photosynthesis to occur since it’s the starting point for the Calvin cycle, without it, the RuBP wouldn’t have anything to combine with, therefore no sugars would be produced. The sugars produced by plants through photosynthesis are the sugars that humans use for respiration (glucose) to produce energy. So carbon dioxide is very important for photosynthesis in plants.
Aerobic respiration is a vital cellular process that occurs in all living organisms. The general simple reaction involves the oxidation of glucose;
C6H12O6 + 6O2 —-> 6CO2 + 6H2O
As can be seen from the general equation, carbon dioxide is a major product of respiration and this is mainly due to it’s role in the link reaction and the Kreb’s cycle (Royal Society of Chemistry (2004). There are four stages in aerobic respiration; glycolysis, the link reaction, the Kreb’s cycle and oxidative phosphorylation. Glycolysis involves the splitting of glucose into 2 pyruvate molecules through substrate level phosphorylation and oxidation. The link reaction follows, where each pyruvate molecule is decarboxylated while NAD is reduced, allowing for the coenzyme A to combine with the acetate to form acetyl CoA. The loss of carbon dioxide in the link reaction is important because this results in the 3-carbon pyruvate molecule losing a carbon atom in the form of CO2 to form the 2 carbon compound acetate. Without this loss of carbon dioxide, acetate would not have been produced and so the Kreb’s cycle, which relies heavily on the acetyl CoA being a 2 carbon compound, could not occur, therefore not enough energy would have been produced through respiration for the living organism to function (Parsons, 2004). The Kreb’s cycle also involves decarboxylation twice to form the 4-carbon compound oxaloacetate from the 6-carbon compound citrate and this happens alongside substrate level phosphorylation and numerous redox reactions. The importance of the carbon dioxide in the Kreb’s cycle is similar to it’s use in the Link reaction, since the carbon-numbered compounds are necessary for the Kreb’s cycle to be maintained and for some of it’s products to be reused in previous stages or used in the next stage, oxidative phosphorylation (Parsons, 2004). Carbon dioxide is released as a waste product to the blood where it travels to the lungs and is exhaled into the atmosphere.
The carbon dioxide that is released from respiring tissues is removed from the tissues and transported to the lungs. Most of the carbon dioxide is transported in the form of hydrogencarbonate ions (HCO3-) (Kennedy and Sochacki, 2008). When CO2 diffuses into red blood cells, it reacts with water in the following reaction:
CO2 + H2O ———> H2CO3
The reaction is catalysed by the enzyme carbonic anhydrase and the product is carbonic acid. The carbonic acid readily dissociates, since it’s a weak acid, into hydrogen ions and hydrogencarbonate ions (HCO3-).
CO2 + H2O <———> H+ + HCO3-
The produced hydrogencarbonate ions then diffuse out of the red blood cells and into the plasma to be transported to the lungs. Remaining is a high concentration of hydrogen ions which means there is high acidity and therefore, a low pH. The blood that enters the respiring tissues is oxyhaemoglobin (haemoglobin carrying oxygen). The low pH levels make the oxyhaemoglobin release more oxygen. This is called the bohr shift since oxygen is readily released by the blood when there are higher carbon dioxide levels. (Kennedy and Sochacki, 2008)
This shows that carbon dioxide is important in humans because when muscles need to respire more, if there wasn’t an increase in carbon dioxide level, then there would be a lack of oxygen in those cells, which could result in anaerobic respiration used unnecessarily.
Carbon Dioxide is very important to both plants and human’s. The Earth’s atmospheric temperature is maintained due to the trapping of heat by carbon dioxide and this allows for plants and humans to live on Earth. Photosynthesis is a reaction that can’t occur without carbon dioxide since it involves carbon fixation and this allows for sugars to be made necessary for the plants survival and also human’s survival since they require plant’s sugars such as glucose for respiration. Respiration also needs carbon dioxide since it’s produced in the link reaction and the Kreb’s cycle. The bhor shift relies on carbon dioxide to be an indicator of where there is less oxygen in respiring cells so that more oxygen can be added for further respiration of cells.