Carbohydrates are one of the three macronutrients (along with fat and protein) and are the body’s main source of energy. There are two types of carbohydrates – simple and complex. Simple carbohydrates are made up of single sugars (monosaccharides) or two sugars (disaccharides), while complex carbohydrates are made up of long chains of sugar molecules (polysaccharides).
The five types of carbohydrates are:
- Monosaccharides
- Disaccharides
- Oligosaccharides
- Polysaccharides
- Fiber
Monosaccharides
Dextrose, or glucose, is the most common monosaccharide found in nature. It is a component of sucrose (table sugar), maltose (beer sugar), and cellulose (wood fiber). Glucose is also present in fruit juices, honey, and some wines. Fructose, or “fruit sugar”, is less sweet than dextrose but has a more persistent sweetness due to its slower rate of metabolism by the body.
Fructose is present in honey, tree fruits (such as apples and pears), berries, melons, and some root vegetables (such as onions). It is also used as an artificial sweetener because it does not promote tooth decay like other sugars do. Galactose is found with lactose (milk sugar) in milk products but can also be obtained from certain fruits such as grapes and plums.
Disaccharides
Sucrose is a disaccharide of glucose and fructose and is often referred to as table sugar. It is found naturally in fruits and vegetables and is used as a sweetener in many processed foods. Lactose is a disaccharide of glucose and galactose and is the main sugar found in milk. Maltose is a disaccharide of two glucose molecules and is found in malted grains such as barley.
Disaccharides are generally not considered to be healthy because they can cause tooth decay, weight gain, and blood sugar spikes when consumed in large amounts. However, they can be part of a healthy diet when consumed in moderation.
Oligosaccharides
The most common type of oligosaccharide is the disaccharide. Disaccharides are composed of two monosaccharide units joined by a covalent bond. The most common disaccharide is sucrose, which is composed of glucose and fructose. Other important disaccharides include lactose (glucose + galactose) and maltose (glucose + glucose). Oligsaccharides with more than two saccharide units are relatively rare in nature but can be found in some plant extracts and honey.
Oligsaccharides play important roles in many biological processes. For example, many oligsaccharides act as Lewis acids or bases, which means they can participate in chemical reactions by accepting or donating electrons. This property allows them to serve as cofactors in enzymatic reactions or to modulate the activity of enzymes. Additionally, some oligsaccharides function as ligands that bind to specific proteins or cell-surface receptors; this interaction can trigger a change in the protein’s conformation or activity. Finally, many oligsaccharides serve as essential structural components of cell membranes and other macromolecular assemblies.
Nucleotides
Nucleotides consist of a nitrogen-containing base attached to a sugar molecule (ribose or deoxyribose) and a phosphate group. The sugar molecules bond to each other through phosphate groups to form the backbone of the nucleic acid strand. The bases project inward from the backbone and pair up with each other according to specific rules: A always pairs with T (or U in RNA), and C always pairs with G. This base-pairing rule is what makes DNA and RNA unique among all other biological molecules.
The function of nucleic acids is to store and transmit genetic information. DNA contains the instructions needed for an organism to develop, grow, reproduce, and function. This information is encoded in sequences of bases-the “letters” of the genetic alphabet-that specify the order of amino acids in proteins. Proteins make up most of the structure and perform most of the functions in cells. Some RNAs also play important roles in cellular function, including carrying out chemical reactions or serving as structural components of ribosomes-the cell’s protein-synthesizing machinery.