Biology  5/e   Raven/Johnson
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Chemistry of carbon dioxide: Carbon dioxide (CO₂) is one of the inorganic compounds thought to have been common on the early earth. All of the carbon atoms found in organic molecules today have come from CO₂ molecules. Carbon dioxide is produced as a by-product of aerobic cellular respiration and some forms of anaerobic respiration. Carbon dioxide is not polar, but reacts with water to form carbonic acid (H₂CO₃).

Diffusion: Unless at a temperature of absolute zero, molecules do not sit still, but move about randomly because of thermal agitation. Over time, there is a net movement of molecules from areas where they are more concentrated to areas of lower concentration. If no other factors interfere, the end result is a uniform distribution or concentration of the molecules throughout the whole system or area in question.

Oxidative respiration: Oxidative respiration is the process by which all vertebrates (and almost all other organisms) obtain the energy they need to survive. Energy-rich organic compounds such as glucose are oxidized, and the energy that is released is used to drive proton pumps and thus synthesize ATP. This is an aerobic process – the energy that drives the proton pump is supplied by electron transport, and oxygen is the ultimate electron acceptor. Cells must obtain oxygen if they are to carry out aerobic oxidative respiration. If oxygen is not available, anaerobic respiration occurs (i.e., fermentation) and there is a much lower yield of ATP per initial molecule of glucose. Carbon dioxide is produced as a by-product of oxidative respiration.

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