11.6: The Energy of Biochemical Reactions
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Learning Objectives
- To relate the concept of energy change to chemical reactions that occur in the body.
The chemistry of the human body, or any living organism, is very complex. Even so, the chemical reactions found in the human body follow the same principles of energy that other chemical reactions follow.
Where does the energy that powers our bodies come from? The details are complex, but we can look at some simple processes at the heart of cellular activity.
An important reaction that provides energy for our bodies is the oxidation of glucose (\(\ce{C6H12O6}\)):
\[\ce{ C6H12O6(s) + 6O2(g) \rightarrow 6CO2(g) + 6H2O(ℓ)} + \text{670 kcal} \label{Eq1} \]
Considering that 1 mol of \(\ce{C6H12O6(s)}\) has a volume of about 115 mL, we can see that glucose is a compact source of energy.
Glucose and other sugars are examples of carbohydrates, which are one of the three main dietary components of a human diet. All carbohydrates supply approximately 4 kcal/g. You can verify that by taking the heat of reaction for glucose oxidation and dividing it by its molar mass. Proteins, the building blocks of structural tissues like muscle and skin, also supply about 4 kcal/g. Other important energy sources for the body are fats, which are largely hydrocarbon chains. Fats provide even more energy per gram, about 9 kcal/g.
Another important reaction is the conversion of adenosine triphosphate (ATP) to adenosine diphosphate (ADP), which is shown in Figure \(\PageIndex{1}\). Under physiological conditions, the breaking of an O–P bond and the formation of an O–P and two O–H bonds gives off about 7.5 kcal/mol of ATP . This may not seem like much energy, especially compared to the amount of energy given off when glucose reacts. It is enough energy, however, to fuel other biochemically important chemical reactions in our cells.
Even complex biological reactions must obey the basic rules of chemistry.
Career Focus: Dietitian
A dietitian is a nutrition expert who communicates food-related information to the general public. In doing so, dietitians promote the general well-being among the population and help individuals recover from nutritionally related illnesses.
Our diet does not just supply us with energy. We also get vitamins, minerals, and even water from what we eat. Eating too much, too little, or not enough of the right foods can lead to a variety of problems. Dietitians are trained to make specific dietary recommendations to address particular issues relating to health. For example, a dietitian might work with a person to develop an overall diet that would help that person lose weight or control diabetes. Hospitals employ dietitians in planning menus for patients, and many dietitians work with community organizations to improve the eating habits of large groups of people.
Key Takeaway
- Energy to power the human body comes from chemical reactions.