Carbohydrates are the plant’s main energy source, they participate in many biological processes such as cellular communication, intervene in bacterial and viral infections and in immunity. To this functional role, a structural role is added as it is part of the cell walls of bacteria and plants.

Chemically they are defined as polyhydroxyaldehydes or polyhydroxy ketones and there are three main classes according to their size: monosaccharides, oligosaccharides and polysaccharides. The word “saccharide” comes from the Greek sakcharon, meaning “sugar.”

Monosaccharides are the simplest sugars and consist of a single unit of polyhydroxyaldehyde or ketone. The most abundant monosaccharide in nature is the D-glucose of six carbon atoms.

Oligosaccharides consist of short chains of monosaccharide units linked by glycosidic bonds. The most abundant and simple ones are the disaccharides, formed by the union of two monosaccharides and the most well-known disaccharide is sucrose, formed by D-glucose and D-fructose. Most oligosaccharides with three or more units of monosaccharides are not found in free form in the cell but linked to other types of molecules (lipids or proteins) forming glucoconjugates (glycolipids or glycoproteins).

Polysaccharides are polymers that contain more than 20 monosaccharide units, which can reach hundreds or thousands of monosaccharide units. Polysaccharides can be linear chains, such as cellulose (the most abundant biomolecule on Earth), or branched, such as glycogen (the main reserve sugar in animals). Both glycogen and cellulose consist of repetitive D-glucose units but differ in the type of glycosidic bonds they possess and therefore have very different biological properties and functions. The polysaccharides of microorganisms, algae, plants and animals, are all physically and/or chemically related to other molecules such as proteins, polynucleotides, lipids, lignin and some mineral inorganic substances.

Polysaccharides can be classified based on their chemical composition into two groups: homopolysaccharides, constituted by the repetition of units of a single type of monosaccharide (for example, cellulose or glycogen), and heteropolysaccharides, which consist of the repetition of units of more than one type of monosaccharide (for example, pectins or hemicelluloses).

If we focus on the physiology of plants, sugars and mainly sucrose, are the major component of the phloem fluid, since they normally represent between 80 and 90% of the total dry matter.

In plants, carbohydrates can come directly from photosynthesis or the mobilization of accumulated reserves. Therefore, in plants, there are source organs (carbohydrate producers) and sink organs (consumers or accumulators of carbohydrates). In the reserve organs carbohydrates can be stored in the same way they are transported, as sucrose (examples: beet, sugar cane and fruits), or they can accumulate in the form of glucose, (as is the case with grapes ) or as starch (in seeds and tubers such as potatoes).

There are many carbohydrates of agro-industrial and even pharmacological interest since they act as immunostimulants, antivirals, antioxidants, antitumor agents, radioprotectors, hepatoprotectors or anti-fatigue. Some examples:

• Vegetable fibers, which include cellulose, which as mentioned above is the most abundant biomolecule found in nature, constitutes the majority of terrestrial biomass, and hemicellulose, which act as stimulants of gastrointestinal motility.
• Pectins: present in the primary cell wall of plants, act as modulators of the intestinal immune system, cholesterol-lowering effect, and decrease the speed of intestinal absorption of other sugars.
• Β-glucans: present in cereal grains, act as a cholesterol levels reducers, have hypotensive properties and control blood glucose levels.
• Starch: main storage-sugar for seeds and tubers. It is widely used in the food industry as a thickener.
• Gums: obtained from exudates or vegetable seeds. It is used in the formulation of foods and have a satiating effect and decrease the speed gastric emptying.
• Inulin: prebiotic obtained from vegetable sources.