- Discuss the role of carbohydrates in cells and in the extracellular materials of animals and plants
- Explain the classifications of carbohydrates
- List common monosaccharides, disaccharides, and polysaccharides
Carbohydrates
Most people are familiar with carbohydrates, one type of macromolecule, especially when it comes to what we eat. To lose weight, some individuals adhere to “low-carb” diets. Athletes, in contrast, often “carb-load” before important competitions to ensure that they have enough energy to compete at a high level. Carbohydrates are, in fact, an essential part of our diet; grains, fruits, and vegetables are all natural sources of carbohydrates. Carbohydrates provide energy to the body, particularly through glucose, a simple sugar that is a component of starch and an ingredient in many staple foods. Carbohydrates also have other important functions in humans, animals, and plants.Molecular structures
Carbohydrates can be represented by the stoichiometric formula (CH 2 O) n , where n is the number of carbons in the molecule. In other words, the ratio of carbon to hydrogen to oxygen is 1:2:1 in carbohydrate molecules. This formula also explains the origin of the term “carbohydrate”: the components are carbon (“carbo”) and the components of water (hence, “hydrate”). Carbohydrates are classified into three subtypes: monosaccharides, disaccharides, and polysaccharides.
Nomenclature
One issue with carbohydrate chemistry is the nomenclature. Here are a few quick and simple rules:
- Simple carbohydrates end with an "...ose"; such as glucose, lactose, or dexrose
- Simple carbohydrates can be classified based on the number of carbon atoms in the sugar, such as a triose (3-carbons), pentose (5-carbons)or hexose (6-carbons).
- Simple Carbohydrates can be classified based on the functional group found in the molecule, such as either a ketoses or aldoses
- Polysaccharides are often organized by the number of sugar molecules in the chain, such as a monosaccharide, disaccharide or trisaccharide.
These will be explained in detail below. For a short video on carbohydrate classification see the Khan academy video (10 minutes in length) by clicking here .
Monosaccharides
Monosaccharides (mono- = “one”; sacchar- = “sweet”) are simple sugars, the most common of which is glucose. In monosaccharides, the number of carbons usually ranges from three to seven. Most monosaccharide names end with the suffix -ose. If the sugar has an aldehyde group (the functional group with the structure R-CHO), it is known as an aldose, and if it has a ketone group (the functional group with the structure RC(=O)R'), it is known as a ketose. Depending on the number of carbons in the sugar, they also may be known as trioses (three carbons), pentoses (five carbons), and or hexoses (six carbons). See [link] for an illustration of the monosaccharides.