Previous kitchen chemistry posts have discussed solids, liquids, and gases, as well as the combustion of hydrocarbons. In this post we will look at carbohydrates, such as starch and sugars. Carbohydrate are made up out of carbon, hydrogen, and oxygen, with twice as much hydrogen as oxygen. Glucose (C6H12O6, illustrated below) is an example:
Since each carbohydrate molecule has twice as many hydrogen atoms as oxygen atoms, carbohydrates can be viewed as being made up from carbon and water (H2O) – hence the name. Indeed, if sugar is heated in a metal pan on high heat, it eventually breaks down into black carbon and water vapour.
The body uses starch by breaking it down into sugars. Indeed, if an unsweetened, unsalted cracker is placed on the tongue and allowed to become soaked in saliva, after a while a faint sweet taste will generally be noticed, as amylase in the saliva breaks down the starch into the sugar maltose (C12H22O11). Later on in the digestive process, maltose is broken down into glucose. It is easy to use iodine to show how saliva digests starch (experiment 1, experiment 2, experiment 3). Plants have a similar process, by which starch in unripe fruit is broken down into sugars as the fruit ripens.
Glucose is the body’s main energy source (although it is unhealthy to overload the body’s systems by eating straight sugar – it is best to allow the body to slowly break starch down into glucose). Initially, glucose is broken down into pyruvic acid (actually, the pyruvic acid is in the form of pyruvate ions, but I’m keeping things simple). This process produces a small amount of energy, and uses no oxygen (an important fact which we will discuss in the next post). It also produces some hydrogen atoms (which aren’t actually loose, but are temporarily attached to the coenzyme NAD, and must be recycled in a later stage of metabolism):
In a complicated process, the pyruvic acid (and the hydrogen atoms) combine with oxygen to give carbon dioxide and water. This process releases much more energy:
The overall reaction is C6H12O6 + 6 O2 → 6 CO2 + 6 H2O, the same as if the glucose was burned. In total, eighteen times as much energy is produced as in the first stage. However, in the absence of oxygen, that first stage (called glycolysis) is all that is possible.
Because the reaction produces water, some desert animals do not need to drink – their water supply comes from this metabolic process, together with other food sources. Merriam’s kangaroo rat is one example: