Unit 3 - Organisms exchange substances with their environment
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Carbon Dioxide may Affect Organisms Directly or Indirectly. Describe and Explain these Effects.
One way that Carbon Dioxide affects organisms is through its effect on haemoglobin. This is a
protein molecule with a quaternary structure that carries Oxygen around the body in the blood.
When the concentration of Carbon Dioxide is increased, haemoglobin has a lower affinity for
Oxygen (Bohr Shift). This means that the Oxygen bound to the haemoglobin is more readily
released to respiring tissues, such as muscles, which have a high Carbon Dioxide
concentration. Carbon Dioxide has this effect on haemoglobin because it causes it to change
shape due to the acidic pH of Carbon Dioxide. This results in more Oxygen being available for
respiration, which is needed in tissues such as muscles. It therefore has a positive effect on
organisms, as it allows respiration to continue at the required rate so tha tissues do not die from
Oxygen deprivation.
Another way Carbon Dioxide affects organisms is through breathing rate. When there is an
increased internal concentration of Carbon Dioxide caused by increased activity, the breathing
rate of the organism increases. This is because the production of Carbon Dioxide stimulates the
breathing centre to cause inhalation to increase the concentration of Oxygen. The exhalation
that then occurs lowers the levels of Carbon Dioxide in the body. When more Carbon Dioxide is
produced, the breathing rate therefore increases because the high Carbon Dioxide
concentration means that there is less Oxygen that can be provided to respiring tissues. Carbon
Dioxide also has an effect on the rate of respiration in organisms. When there is an increased
concentration of Carbon Dioxide, the rate of respiration increases, so that the tissues have a
constant supply of Oxygen. This Oxygen is produced by plants in photosynthesis. The
increased rate of photosynthesis also causes more Carbon Dioxide to be produced, which is
then released and used by plants for photosynthesis.
A third way Carbon Dioxide affects organisms is through heart rate. When Carbon Dioxide
concentration increases due to increased respiration caused by increased activity, the pH of the
blood is lowered. This pH change is picked up on by chemical receptors in the carotid arteries,
which increases the frequency of impulses to the medulla oblongata. This then increases the
frequency of impulses to the sinoatrial node, which increases the heart rate. This is important for
the organism, because it lowers the levels of Carbon Dioxide in the body, as the increased
blood flow removes it faster, and the haemoglobin has a lower affinity for Oxygen so the
respiring tissues are supplied with Oxygen. This has a direct effect on the organism, as it
causes heart rate to increase, and also the breathing rate to increase so that there is a good
supply of Oxygen for respiration to take place and to keep up with the heart rate.
Another way that Carbon Dioxide affects organisms is through photosynthesis. Plants use
Carbon Dioxide in photosynthesis, and so when the concentration of Carbon Dioxide in the
environment increases, rate of photosynthesis is also able to increase. This produces more
Oxygen and glucose, which are needed for respiration in plants. Rate of photosynthesis is able
to increase, as there is more Carbon available to be used in Glycolysis and the Link Reaction,
and more Carbon Dioxide for use in the Krebs Cycle. This has a direct effect on plants, as they
are able to photosynthesise more, and produce increased amounts of Oxygen and glucose
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