Fuels for the body
Living things require energy for warmth, movement and the synthesis of necessary biomolecules such as enzymes and other proteins, carbohydrates and triglycerides. Plants use photosynthesis to take in energy from the sun and to store it as chemical energy. Cellular respiration in plants and animals releases this energy.
One of the key molecules to the energy systems of both plants and animals is glucose. In plants, it is the building block of the structural material cellulose and the energy storage molecule starch. In animals, glucose forms an equivalent energy storage molecule: glycogen.
Plants use photosynthesis in their cells to produce glucose and oxygen gas from water and carbon dioxide. The thermochemical equation for the reaction is:
6CO2(g) + 6H2O(l) → C6H12O6(aq) + 6O2(g) ΔH = +2840 kJ
In plants, glucose is used to provide energy, or it can undergo polymerisation to starch for stored energy or polymerisation to cellulose for structure.
Use this page to revise the following concepts within fuels for the body:
Cellular respiration
Cellular respiration is the process where glucose is oxidised in plant and animal cells as an energy source. The energy collected from the sun when glucose was formed is released into the organism.
Glucose is released from the breakdown of carbohydrates in the digestive system of animals. In animals, the glucose is transported in blood to body cells where cellular respiration can occur.
The equation for the reaction is:
C6H12O6(aq) + 6O2(g) → 6CO2(g) + 6H2O(l) ∆H = -2840 kJ
Respiration can occur in either plant or animal cells but photosynthesis only occurs in plants.
Energy from food
Glucose is not the only molecule that provides energy to living things. Energy can be derived from each of the three major food groups: carbohydrates, proteins and triglycerides (fats and oils).
The energy value is the amount of energy available to the body. For example, the energy value of carbohydrates is 16 kJ g-1. This is an average value, as the different structures of each carbohydrate will lead to a slightly different value.
The following table shows that fats and oils have a much higher energy density than carbohydrates.
Energy values of the major food groups
| Food Group | Energy value kJ g-1 |
|---|---|
| Carbohydrates | 16 |
| Fats and oils | 37 |
| Protein | 17 |
Carbohydrates contain the elements carbon, hydrogen and oxygen and usually have a molecular formula of Cx(H2O)y. The structures of three important carbohydrates are shown below. The diagram illustrates that:
- Glucose is an example of a monosaccharide, containing one hexagonal cyclic structure
- Sucrose, like maltose and galactose, is a disaccharide
- Starch is a polysaccharide formed from many glucose molecules.
Fats and oils are examples of triglycerides, large non-polar molecules with three hydrocarbon chains attached to a glycerol molecule. The first stage in digestion of fats is the breaking of the ester bonds shown in the diagram below to form three fatty acids and a glycerol molecule. The fatty acids are long hydrocarbon chains, represented by the R-group.
The fatty acids are transported in the blood to muscle cells where they can be oxidised to form carbon dioxide and water, releasing large quantities of energy. The balanced equation for the oxidation of the fatty acid above is:
2C22H42O2(l) + 63O2(g) → 44CO2(g) + 42H2O(l)
The diagram below shows the structure of a typical fatty acid in more detail (tetradecanoic acid, more commonly called myristic acid). It contains a long hydrocarbon chain and a carboxyl group.
Carbohydrates have lower energy values than fats because carbohydrates are partially oxidised. Their molecules contain a significantly higher proportion of oxygen atoms.
Proteins can be used as an energy supply during intense exercise. A segment of a protein chain is shown below. They are large polymer molecules that can be broken down into amino acids. Amino acids can be oxidised to release energy. The presence of nitrogen in amino acids requires more complex reactions for this oxidation. Like in the oxidation of carbohydrates and fatty acids, CO2 and water are eventual products of this oxidation.
