Carbohydrates-Definition, Classification, and Functions

Carbohydrates are the major source of energy in any species. These simple molecules are made up of carbon, hydrogen, and oxygen atoms. There are two types of carbohydrates: simple carbohydrates, and complex carbohydrates. Simple carbohydrates are categorized as monosaccharides, disaccharides, and polysaccharides. Monosaccharides consist of only a single sugar molecule (e.g. fructose), while disaccharide contains 2 simple carbohydrates bound covalently together (e.g. sucrose). Polysaccharides are composed of many simple carbohydrate chains linked by glycosidic bonds; they may have various molecular weights.

What is Carbohydrate?

Carbohydrate is an organic substance that contains carbon, hydrogen, and oxygen. These three elements act together to create the structure of the plant. Carbohydrates are the primary building blocks of the plant and allow the plant to store its own food in a usable form. Most plants have two different types of carbohydrates; cellulose and starch. Cellulose is a type of polysaccharide that is responsible for the structural integrity of the plant. Starch is a complex carbohydrate that is present in seeds and roots and is broken down into smaller molecules called glucose. Glucose is a monosaccharide (single sugar) that gives the body fuel and energy. In plants, glucose is stored as a reserve to help with fruit set and seed production. It is produced from sucrose, which is broken down from starches.

Carbohydrates Classifications

Carbohydrates are compounds containing carbon and hydrogen. They are composed of carbon atoms bonded to each other (CO) and two hydrogen atoms.

There are five major types of carbohydrates: monosaccharides, disaccharides, trisaccharides, oligosaccharides, and polysaccharides.

Each type of carbohydrate consists of different combinations of these components and may have different physical properties.

1. Monosaccharide (one sugar)

Number ofCarbonsCategory NameExamples
4TetroseErythrose, Threose
5PentoseArabinose, Ribose, Ribulose, Xylose, Xylulose, Lyxose
6HexoseAllose, Altrose, Fructose, Galactose, Glucose, Gulose, Idose, Mannose, Sorbose, Talose, Tagatose
7HeptoseSedoheptulose, Mannoheptulose

2. Disaccharides (two sugars)

Saccharose, also known as sucrose, is regular table sugar that has been refined from sugar cane or sugar beets. Brown sugar, confectioner’s sugar, evaporated or dried cane juice, and turbinado sugar all contain it as their primary ingredient. The molecules of lactose are made up of glucose and galactose. It is noteworthy because it is connected to lactose intolerance, a condition characterized by intestinal discomfort brought on by a lack of lactase, an intestinal enzyme essential for absorbing and digesting lactose in milk. Abdominal pain, bloating, gas, and diarrhea are all symptoms of the fermentation of undigested lactose in the colon. 

Since lactose is consumed by the bacteria that turn milk into yogurt, yogurt does not result in these issues.

DisaccharideDescriptionComponent monosaccharides
sucrosecommon table sugarglucose 1α→2 fructose
maltoseproduct of starch hydrolysisglucose 1α→4 glucose
Trehalosefound in fungiglucose 1α→1 glucose
lactosemain sugar in milkgalactose 1β→4 glucose
melibiosefound in legumesgalactose 1α→6 glucose

Trisaccharide (three sugars): Maltotriose, maltoheptaose, melibiose, panose, stachyose, nigerose, kestose, fructohexaose, lactulose, gentiooxaose.

Oligosaccharide (many sugars): Laminaribiose, laminarasitol, laminarin, laminaran, fucoidan, chondroitin sulfate, chondroitin.

Polysaccharides (many sugars) (polymers of saccharides):

Unlike sugars, many polysaccharides are insoluble in water. In the human small intestine, polysaccharides and oligosaccharides are resistant to digestion and absorption; however, in the large intestine, microorganisms completely or partially ferment these substances. The following polysaccharides are significant in biology, nutrition, or food preparation.


In plants, starch is the main type of carbohydrate that is stored. Amylopectin, a highly branched polysaccharide, and amylose, an essentially linear polysaccharide, are the two main components of starch. Both types of starch are -D-glucose polymers. Natural starches contain 10-20% amylose and 80-90% amylopectin. Amylopectin is completely insoluble, whereas amylose forms a colloidal dispersion in hot water (which helps to thicken gravies).

  • Amylose molecules typically contain 200 to 20,000 glucose units, which are bonded at angles that result in a helix shape.
  • The fact that amylopectin is highly branched sets it apart from amylose.
  • Every twenty to thirty glucose units along the chain, short side chains of about 30 glucose units are joined with 1-> 6 linkages. Amylopectin molecules may contain up to two million glucose units.

Functions of Carbohydrates

  • Carbohydrates are simple sugars that are used for many different processes in plants.
  • Carbohydrate metabolism is the conversion of food into energy within the body.
  • In plants, they are converted into starches, cellulose, and fibers.
  • When carbohydrates accumulate in excess amounts in the tissues of a plant, it creates high sugar levels. These excess carbohydrates become stored as starch, which is the most abundant carbohydrate storage material in plants.
  • If sugar accumulates in the cells of a plant, then this causes problems in cellular functioning.
  • Plants may not be able to produce enough photosynthetic products (such as oxygen and glucose) due to the increased level of carbohydrates.
  • Glucose is the simplest type of carbohydrate molecule.
  • Sucrose consists of two molecules of glucose bound together by a disaccharide linker.
  • Fructose is a monosaccharide derived from sucrose.
  • Other types of carbohydrates include maltodextrins, lactose, gum arabic, gums, and pectin.

Frequently Asked Questions on Carbohydrates

What are carbohydrates?

Simple carbohydrates include glucose, fructose, and galactose. Glucose, the simplest of the three, is derived from starch. Fructose and galactose are both present in fruits. The body uses these sugars for fuel. Complex carbohydrates are composed mainly of cellulose, hemicellulose, and lignin. Carbohydrates have not created inside cells but rather absorbed from outside. Plants take up carbohydrates from their substrate and convert them into energy.

What do we need carbohydrates for?

For a plant’s survival, carbohydrates are necessary. Without enough carbohydrates, our plants cannot survive. A plant’s primary function is to store carbohydrates. A plant absorbs water and nutrients through its roots from the surrounding soil. Then using the energy stored in the carbohydrate molecule, the plant converts the nutrients into the food. Once all the necessary nutrients are assimilated, the plant then expends the remaining energy as an electrical activity to create new cellular material. When the plant begins to wilt, the plant stops absorbing water and nutrients and therefore stops converting it into food. Since the plant no longer has enough energy stores, it dies.

Where do carbohydrates come from?

The majority of carbohydrates are obtained from the atmosphere. Air contains about 20 percent oxygen and 80 percent nitrogen. Oxygen is essential for respiration, and nitrogen helps catalyze chemical reactions. In addition, some carbohydrates are generated naturally by the decomposition of dead organisms. Humus, decaying matter, and compost are good sources of carbohydrates.

How many carbohydrates should I use?

In general, a mature plant requires around 1 pound per square foot of area each week. In general, the number of carbohydrates will be equivalent to the number of times the plant was watered. If the plant receives more than 30 hours of sunlight daily, the plant needs fewer carbohydrates. If the plant does not receive sufficient sunlight, carbohydrates are needed at a higher rate.

How do we know how much carbohydrates to use?

A quick way to determine the number of carbohydrates is to simply count the number of days since the last watering. As long as the plant needs water, it also needs carbohydrates.

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