Human Biochemistry

All living things require the input of energy to exist – this energy is used to drive the thousands of biochemical reactions that occur to allow the organism to grow, reproduce and sustain life. This energy comes almost always from the Sun, in the rst instance – energy from sunlight is captured by photosynthetic organisms (e.g. plants, algae, certain bacteria) and converted into carbohydrates. These are then broken down by a process called cellular respiration, to produce energy-rich molecules (e.g. adenosine triphosphate, or ATP) that release energy to drive biochemical reactions. Photosynthetic organisms can by ingested by nonphotosynthetic animals, and the carbohydrates (and other biomolecules) can be broken down and used for cellular respiration. As we ourselves are non-photosynthetic organisms, we must obtain our energy through what we ingest, i.e. via our diet, so that our cells are able to carry out all the necessary biochemical reactions. The amount of energy required by an individual will depend on the amount of physical activity they perform, but in general an average man requires about 10 500 kilojoules (kJ), equating to 2500 kilocalories (kcal) per day, while an average woman needs approximately 8400 kJ (2000 kcal) per day. The amount of energy found within di erent foods we buy is often displayed on the food packaging. This energy value is worked out through a process known as food calorimetry. A food (or bomb) calorimeter can be used, which measures the heat of combustion. Here, a known mass of a particular food is ignited and completely burnt in the presence of oxygen. The energy released is transferred to water and the rise in temperature of the water is measured. The energy contained in the food can then be calculated using the following equation: q = mc∆T where: q = heat evolved (J) m = mass of water (g) c = speci c heat capacity of water (4.18 J g−1 K−1 or 4.18 J g−1 °C) (This is included in the IBO Chemistry Data booklet.) ∆T = temperature