Aerobic respiration is the release of energy from glucose or another organic substrate in the presence of Oxygen. Strictly speaking aerobic means in air, but it is the Oxygen in the air which is necessary for aerobic respiration. Anaerobic respiration is in the absence of air. Here is a molecular model of a glucose molecule. You do not need to memorise the diagram for you GCSE exam, but it should help you to understand that a molecule of glucose contains six atoms of Carbon (shown in blue), twelve atoms of Hydrogen (shown in green), and six atoms of Oxygen (shown in red).
Anaerobic respiration is a form of respiration using electron acceptors other than oxygen. Although oxygen is not used as the final electron acceptor, the process still uses a respiratory electron transport chain; it is respiration without oxygen. In order for the electron transport chain to function, an exogenous final electron acceptor must be present to allow electrons to pass through the system. In aerobic organisms, this final electron acceptor is oxygen. Molecular oxygen is a highly oxidizing agent and, therefore, is an excellent acceptor. In anaerobes, other less-oxidizing substances such as sulfate (SO42-), nitrate (NO3-), or sulfur (S) are used. These terminal electron acceptors have smaller reduction potentials than O2, meaning that less energy is released per oxidized molecule. Anaerobic respiration is, therefore, in general energetically less efficient than aerobic respiration. Anaerobic respiration is used mainly by prokaryotes that live in environments devoid of oxygen. Many anaerobic organisms are obligate anaerobes, meaning that they can respire only using anaerobic compounds and will die in the presence of oxygen.
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