Do chemical reactions involve energy?
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Molecules house energy in the bonds between atoms. So, when a chemical reaction takes place and the molecules are broken at their bonds and bonds are formed for the new (product) molecules, energy has to be involved. Generally speaking there are two types of chemical reactions-those that release energy (energy releasing) and those that require the input of energy (energy demanding). If a chemical reaction is said to be energy releasing that means that more energy will be released in the disruption of the bonds of the reacting molecule than is needed to form the new bonds in the product molecule(s) (see Energy Transfer Figure). |
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Said differently, if the energy within the bonds of the products is less than the energy associated with the initial energy in the bonds of the reactants, then the reaction can proceed without a need for an input of outside energy. In this situation, there is leftover energy. On the other hand, if the energy that is required to form the bonds of a new molecule(s) is greater than the energy that will be released by disrupting the reacting molecule(s), then an outside energy source will be needed. This is often the case when complex molecules are being built in our body. To do so, the energy released from energy releasing reactions is used to “drive” the energy demanding reactions.
Beyond those chemical reactions that either release or require appreciable amounts of energy there are many chemical reactions that take place without a release or demand for energy. Here the energy associated with the bonds of the reactants and products of chemical reactions is the same. These would be the reversible reactions we discussed earlier, where one enzyme catalyzes the reaction in both directions.
How does food energy become our body’s energy?
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On a daily basis we acquire energy from foods in the form of carbohydrates, protein, fat, and alcohol. However, we cannot use the energy from these molecules directly; they must first engage in chemical reaction pathways that break them down to capture some energy in so-called “high-energy molecules.” By far, the most important high-energy molecule is adenosine triphosphate or, more commonly, ATP (see ATP Figure). |
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When energy is needed to power an event in our body it is ATP that is used directly. So, the energy in carbohydrate is used to generate ATP, which in turn can directly power an energy requiring event or operation in our body. As you might expect, the release of the energy from these little molecular powerhouses is controlled. Specific enzymes are employed to couple ATP with an energy requiring chemical reaction or event and the transfer of energy.
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Interestingly, not all of the energy released in the breakdown of carbohydrates, protein, fat, and alcohol is incorporated in ATP. It seems that we are able to capture only about 40 to 45 percent of the energy available in those molecules in the formation of ATP. The remaining 55 to 60 percent of the energy is converted to heat, which helps us maintain our body temperature (see Heat Release Figure). The final product of the chemical reaction pathways that breakdown carbohydrates, proteins, fat, and alcohol is primarily carbon dioxide, which we then must exhale, and water, which helps keep our body hydrated. |
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Looking at the ATP molecule above, we notice what looks like a phosphate tail (see Figure 6). Phosphate is made up of phosphorus (P) bonded to oxygen (O) and, as indicated in its name, ATP contains three phosphates. The energy liberated during the breakdown of energy nutrients is used to link phosphates together to make ATP. These phosphate links are thus little storehouses of energy. When energy is needed, special enzymes in our cells are able to break the links between adjacent phosphate groups. This releases the energy stored within that link, which can be harnessed to drive a nearby energy-requiring reaction or process.
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