This four-part series of articles will provide an overview of the molecules of life, which are found in all living organisms: Carbohydrates, Proteins, Lipids, and Nucleic Acids. This is the first of the series.
What Are They?
You’ve probably heard of the term “carbs.” Since the Atkins Diet, this has almost become a “dirty” word. But what are carbs? Carbohydrates, as the name implies, are large molecules (macromolecules), which consist mainly of Carbon (carbo-), Hydrogen, and Oxygen (-hydrate, the atoms in water). The carbohydrate family of biological molecules can be divided into three main groups: simple sugars, storage carbohydrates or structural carbohydrates. We will briefly discuss each of these carbohydrates.
Examples of simple sugars include glucose (blood sugar) and fructose (fruit sugar). These sugars are referred to as monosaccharides, meaning “single” (mono-) “sugar” (saccharide is Greek for “sweet” or “sugar”). If two monosaccharides are connected together, then it’s called a disaccharide. Examples of disaccharides include sucrose (table sugar; glucose-fructose). The chemical bond, or connection, between sugars is covalent (shares electrons) and is called a glycosidic bond.
If many sugars are chained together, called a polysaccharide, then it can either be a storage or a structural carbohydrate. Storage carbohydrates are simply that, they store the sugar. These are generally long chains (upwards of 1,000 or more) of glucose. Then when the body needs sugar, it simply “plucks” a sugar off. Kind of like plucking of single grapes from a cluster. You eat one at a time, not the whole cluster at once. In plants it’s called starch, while in animals it’s called glycogen. However, they are not the same molecule. Starch is a long chain of glucose, but glycogen is more of a “cluster” as it has many branched chains of glucose. Yet both are polysaccharides of glucose.
If glucose were not stored this way, then all the glucose in the cell would be consumed very quickly. But when stored in chains, the glucose is “safe” and is only used when needed.
Structural carbohydrate on the other hand are simply used to provide structure for the organisms in which they’re found. Cellulose is a classic example. This carbohydrate is found in plants and it’s what makes up their cell walls, resulting in the rigidity of plant leaves, grass, and stems. What’s really neat is that cellulose and starch are almost identical in structure, but it’s the angle of the glycosidic bonds between the glucose monomers that differs. That “little” difference is all it takes for one, the starch, to be a food source for humans, while cellulose cannot. This is because we don’t have the correct “scissors” to pluck off the “grapes” (glucose) from the cellulose “cluster.” However, herbivores (plant eaters) do. This is why they can munch on leaves and grass and to them, it’s food! Much like French fries are to humans!
Another example of a structural carbohydrate is chitin, which is a major component of insect’s exoskeletons as well as the shells of crustaceans.
Finally, here’s a hint in identifying which molecules are carbohydrates: if the name of a molecule ends in “-ose,” then it is most likely (roughly 90% of the time) a carbohydrate. Examples include: glucose, fructose, galactose, sucrose, lactose, maltose, cellulose, ribose, etc.
Other Articles in this Series: