Our Body is a Wonder Machine

Photosynthesis is the process by which plants make two essential things on which the very survival of animals is hinged. These are namely a sugar called Glucose and oxygen. In the beginning oxygen was a poison to many microbes. But since it was a question of survive or perish, slowly they adapted to respiration using oxygen. Those which could not change receded to great depth where oxygen cannot reach.

Glucose is a like a charged battery which stores energy. Where does this energy come from? Obviously it is the Sun on which the life on the Earth is based upon. Plants use carbon dioxide and water and a mediator called chlorophyll to make glucose and oxygen. Anyone familiar with thermodynamics knows that this reaction is not favorable as its overall Gibbs free energy is positive. Second law of thermodynamics requires that only those reactions are spontaneous for which this energy change is negative. In simple words the energy of products should be lower than that of reactants. Here the energy content of the products is higher by 2880 Kilo Joules per mole. It is the Sun who provides this energy and plants store it in the glucose.

Many of the reactions that take place in living organisms require a source of free energy to drive them. The immediate source of this energy in heterotrophic organisms, which include animals, fungi, and most bacteria, is the sugar glucose. Now reverse reaction that is the oxidation of glucose to carbon dioxide and water takes place when animals consume glucose and oxygen. Thus 2880 KJ/mole energy is liberated.

Of course it would not do to simply “burn” the glucose in the normal way; the energy change would be wasted as heat, and rather too quickly for the well-being of the organism! Effective utilization of this free energy requires a means of capturing it from the glucose and then releasing it in small amounts when and where it is needed. This is accomplished by breaking down the glucose in a series of a dozen or more steps in which the energy liberated in each stage is captured by an “energy carrier” molecule, of which the most important is adenosine diphosphate, known shortly as ADP. At each step in the breakdown of glucose, an ADP molecule reacts with inorganic phosphate and changes into adenosine triphosphate ATP

The 30 kJ mol–1 of free energy stored in each ATP molecule is released when the molecule travels to a site where it is needed and loses one of its phosphate groups, yielding inorganic phosphate and ADP, which eventually finds its way back the site of glucose metabolism for recycling back into ATP. The complete breakdown of one molecule of glucose is coupled with the production of 38 molecules of ATP according to the overall reaction

For each mole of glucose metabolized, 38 × (30 kJ) = 1140 kJ of free energy is captured as ATP, representing an energy efficiency of 1140/2880 = 0.4. That is, 40% of the free energy obtainable from the oxidation of glucose is made available to drive other metabolic processes. The rest is liberated as heat.


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