Carbon dioxide is a greenhouse gas that directly affects the growth of plants and the distribution of insects, but also indirectly influences the activity of organisms and could even lead to speciation of new organisms.
Carbon dioxide levels affect the rate of plant growth, as CO2 is an essential part of the light independent stages of photosynthesis. The CO2 is fixated with a molecule of ribulose bisphosphate by the rubisco enzyme to create an unstable 6-carbon compound. This dissipates to form 2 more stable molecules of glycerate-3-phosphate. This then undergoes reduction by NADPH and phosphorylation by ATP to form 2 molecules of triose phosphates. Some of the triose phosphates are assimilated to make glucose and the rest is regenerated by another molecule of ATP to reform ribulose bisphosphate. As CO2 is usually a limiting factor, increasing the CO2 concentration, usually increases productivity in terms of percentage yield of plants/crops.
Fluctuating CO2 concentrations have an indirect impact on the speciation of fish. This is because CO2 concentrations of the atmosphere increase, thus causing an equilibrium shift of CO2 (g) + H2O (l) <----> H2CO3 and thereby increasing the pH of water. This change in selection pressures would provide a selective advantage to fish with alleles that favour the acidic conditions, as they are more likely to survive and thus more probable to pass on their advantageous allele to the next generation, thus causing a change in allele frequency. This, over time, would lead to variation and eventually speciation.
Carbon dioxide also plays a huge role in the internal functioning of the human body with particular reference to the transport of oxygen. Oxygen is transported around the body in the blood, where it is bound to haemoglobin forming oxyhaemoglobin. As haemoglobin is a protein molecule and proteins are affected by pH changes; as the partial pressures of CO2 change, so does the shape of the haemoglobin. This affects the affinity of the haemoglobin to the oxygen. This is advantageous as it means that muscle, where there is a high rate of aerobic respiration, has a greater demand for oxygen and a high production of CO2. These high partial pressures of CO2 lower the affinity of the haemoglobin to oxygen, thus satisfying the oxygen demand by muscle cells, where it can be used as a terminal electron acceptor. This is known as the Bohr Effect.
Carbon dioxide levels in the blood also have a direct impact on the heart rate in mammals. If the CO2 levels in the blood increases, it causes the pH of the blood to decrease. This change in pH is detected by specialised chemoreceptors found in the carotid artery. They convert this chemical stimulus into electrical impulses that are sent to the medulla oblongata via nerve fibres. The medulla oblongata communicates this information and sends impulses along sympathetic nervous fibres to the S.A.N, causing an increase in heart rate and thus the increased removal of CO2. This, therefore, returns the pH levels back to a natural homeostatic level, thus completing the negative feedback loop.
Carbon dioxide levels also indirectly affect the distributions of ectotherms, such as lizards. As atmospheric CO2 levels rise, so do the global temperatures, because CO2 is a greenhouse gas that traps IR radiation reflected from the earth and re-radiates it back towards the ground, thereby increasing the temperature. And as ectotherms cannot maintain their body temperature internally, external changes to temperature affect the behavioural pattern and habitat locations. They will tend to migrate to temperatures that are more favourable.
The activity of detritivores and saprobionts are also indirectly affected by CO2 concentrations. As described previously, the increasing CO2 levels result in increased plant growth. This eventually means increased plant death, which means that organic matter is not a limiting factor for detritivores and saprobionts. The detritivores break down the dead organic matter to increase the surface area via holozoic nutrition. The saprobionts then digest the organic matter through the release of extra-cellular enzymes. They then absorb the products of digestion and respire aerobically.
So, as can be seen above, changing carbon dioxide concentrations can have an affect both inside an organism and also in greater ecosystems. As a result, it is an essential molecule in every way, from photosynthesis and the production of glucose to its role of metabolism and blood.