Re: what elements does glucose contain?

Hi Kate,

Glucose is probably the best known sugar or carbohydrate molecule. As the name suggests, “carbohydrates” are molecules that contain carbon, hydrogen, and oxygen atoms. Glucose contains only these 3 elements.

I’ve attached a figure showing a model of the structure of a glucose molecule. In the diagram, as is a convention for structural diagrams in organic chemistry, the corners represent carbon atoms. Oxygen atoms are depicted by “O” symbols, and the hydrogen atoms attached to oxygen are also explicitly shown by “H” symbols. The lines connecting corners and letter symbols represent covalent chemical bonds. There are many more hydrogen atoms in a glucose molecule that are not shown in this diagram. A chemist looking at this diagram would recognize they are implicitly present. This is because carbon atoms make 4 bonds. So, for example, in the attached diagram, we see many carbons (corners) with 3 explicitly shown bonds and one with only 2 bonds shown. The molecular formula for glucose is C₆H₁₂O₆. This means that there are 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms in each molecule of glucose. Can you fill in all the hydrogens not explicitly shown in the diagram to get the correct number?

The main biological function of glucose is in energy metabolism. Glucose is a source of biological fuel because it can be burned (oxidized). You are familiar with one form of oxidation reaction, which is burning by fire. Cars oxidize fuel by fire in order to obtain kinetic energy for motion. This type of burning involves reaction of oxygen with the fuel, catalyzed by a spark. Organisms also oxidize fuel to obtain energy, but they do it a different way. Glucose is oxidized (reacted with oxygen) in reactions catalyzed by enzymes. In the end, the oxidation reactions in engines and organisms share many similarities. Both liberate energy and both result in product molecules that are smaller (have fewer atoms) than the starting fuel. The most common products of both types of oxidation reactions are carbon dioxide and water.

Many people believe that glucose “stores” energy in its chemical bonds. This is not in fact true. Rather, molecules like glucose that are said to be “high energy” or “energy storage” molecules are described as such because they have the potential to participate in reactions like oxidation, which liberate energy. It takes energy to break chemical bonds and the formation of bonds releases energy. So just breaking up glucose into its constituent atoms would cost energy. Energy is released in the new bonds that are formed in the products. For oxidation of glucose, and many other reactions, it turns out that the energy released due to formation of the new bonds is greater than the initial energy cost to break the bonds that are broken in glucose. This means that in total, energy is released by the entire reaction; these reactions are said to be “exothermic”.

As for how the particular structure of glucose leads to its function…that is a difficult question with some rather subtle and speculative answers. In general, all carbohydrates can be oxidized to liberate energy. So why did nature choose glucose as one of its primary energy storage molecules? The many -OH (hydroxyl) groups on glucose act as chemical “handles” for recognition by enzymes, and also serve as reactive centers for the formation of polymers of glucose, known as starches. Still, many other sugar molecules have similar structures to glucose. Chemists and biologists have speculated that glucose may be “special” due to its relatively weak tendency to react with amino groups in proteins and enzymes. This type of reaction can be very detrimental to the function of these proteins. However, many other reasons for nature’s preference for glucose are plausible.

Biological systems have become the way they are by evolution, which is usually not a straightforward or predictable process. In evolution, once good solutions are found, new systems become built upon them. It then becomes very difficult to change the original solution to something else, even to something better, since so many dependent processes would be disrupted. Judging by its almost universal role in life as a primary energy storage molecule, glucose must have been chosen or “fixed” very early in the evolution of life on earth.

See http://en.wikipedia.org/wiki/Glucose for some more information on glucose.