Option B.9 – Respiration
B.9.1 – Compare aerobic and anaerobic respiration of glucose in terms of oxidation/reduction and energy released
The human body needs a constant supply of energy to allow the necessary chemical reactions of life to take place. Through respiration, the body breaks down energy-rich molecules like glucose to release energy.
When energy is released in cells, it is stored in the molecule ATP (adenosine triphosphate), which is then used up as it is needed in reactions.
Both aerobic and anaerobic respiration begin with the process of glycolysis, which does not require oxygen. In this stage, the glucose is partially oxidised and broken into a three carbon molecule, pyruvate. To allow this to happen, the coenzyme NAD+ is reduced.
Aerobic Respiration
The later stages of respiration require oxygen to completely oxidise the respiratory substrate. It is in these stages that most of the energy is released – up to 36 ATP molecules are produced.
Once the pyruvate has been formed in glycolysis, it will be oxidised to form CO2 and H2O. There is a series of complex reactions in between, but in the end, oxygen acts as a terminal electron acceptor.
It is important to realise that this is not a simple process – there are a number of intermediate products that are formed through oxidation and reduction before the final products are formed.
Anaerobic Respiration
When there is no supply of oxygen available, the process is different, and releases less energy. Once glycolysis is complete, releasing 2 ATP molecules, the pyruvate cannot be oxidised further, so no more energy can be released. Instead, the NAD+ molecules is oxidised so that it can continue to participate in the reactions of glycolysis.
There are many organisms that do not require much energy to survive, and will only use anaerobic respiration.
B.9.2 – Outline the role of copper ions in electron transport and iron ions in oxygen transport
Electron Transport
Since respiration involves oxidation and reduction, there must be transfer of electron pairs. The mitochondria of cells, where respiration takes place, contain oxidising enzymes called cytochromes, which are involved in final stages of aerobic respiration as electron transport carriers. These cytochromes contain iron ions that are surrounded by a porphyrin ligand, creating a ring. The metal ion is called the prosthetic group. The ligand donates electron pairs to allow the oxidation of glucose.
This is repeated throughout all the steps involved in the oxidation of glucose. Using the electron transport carriers, the electrons flow down an electrochemical gradient. All the iron is converted back to Fe3+ during the reduction of oxygen to water.
The terminal electron carrier also contains Cu2+. It is this carrier that is involved in the reduction of oxygen.
Oxygen Transport
Iron is located in the heme group of the haemoglobin molecule, found in the red blood cells in the body. The iron binds to the oxygen molecule, allowing it to be transported in the blood to carry oxygen to cells for use in respiration. However, no oxidation takes place here – instead, the protein simply changes shape and becomes oxygenated.
Each molecule of haemoglobin contains four heme groups, allowing it to transport four oxygen molecules. The heme is the prosthetic group.
Carbon monoxide (CO) and cyanide ions (CN–) are both poisons because they will also bind irreversibly with the iron so that it can no longer transport oxygen.