Carbohydrates: Classification, Function, Benefits & Working

One of the essential macronutrients that are required by the human body is carbohydrates. In simple terms, it can be described as a form of organic macronutrient which provides the energy for the regular proper functioning of the human body and is obtained through diet mostly. This compound is made of essential elements like Carbon, Oxygen, and Hydrogen.

Hence chemically or in the perspective of biochemical science carbohydrate can be defined as, a molecule consisting of Carbon, Hydrogen, and Oxygen atoms, usually with a hydrogen-oxygen atom ratio of 2:1 and an empirical formula of Cm(H2O), wherein which few carbohydrates like uronic acids, deoxy sugars such as fructose are excluded. The term is most common in biochemistry, where it is a synonym of saccharide, a group that includes sugars, starch, and cellulose.

Carbohydrates are an essential fuel for the central nervous system and energy for working muscles. They also prevent protein from being used as an energy source and enable fat metabolism. It supports brain function and influence, mood, memory, etc.

Classification of Carbohydrates

Carbohydrates are classified based on their complexity and structure. Based on this they could be Simple and Complex carbohydrates.

Simple carbohydrates contain just one or two sugars such as fructose and galactose. These single sugars are also called monosaccharides and carbs with two sugars such as lactose and maltose are referred to as disaccharides. 

Monosaccharide carbohydrates cannot be hydrolyzed further to give simpler units of polyhydroxy aldehyde or ketone. If a monosaccharide contains an aldehyde group, then it is called aldose and on the other hand, if it contains a keto group then it is called a ketose.

On hydrolysis, disaccharides yield two molecules of either the same or different monosaccharides. The two monosaccharide units are joined by oxide linkage which is formed by the loss of water molecules and this linkage is called glycosidic linkage. Sucrose is one of the most common disaccharides which on hydrolysis gives glucose and fructose. Maltose and Lactose are the other two important disaccharides.

Complex carbohydrates are referred to as polysaccharides and they contain more than two sugar molecules. It is a polymer of α glucose and consists of two components i.e., Amylose and Amylopectin. Cellulose is also one of the polysaccharides that are mostly found in plants. They are usually found in high starchy foods. In the perspective of diet and bodily requirement these simple and complex carbs can briefly be classified into three types:

Sugars: They come under simple carbohydrates as they appear in the most basic form of carbs. They can be added to foods such as sugar in candy, processed foods, and desserts. Naturally, they are found in fruits, vegetables, and milk.

Starches: They come under complex carbohydrates and are made up of huge amounts of sugars strung together in a chain making a complex carb molecule. Starch is broken down by the human body once consumed during metabolism to use as energy. They are found in abundance in bread, cereal, pasta, and a few vegetables like potatoes.

Fiber: Being another complex carb, fibers are super healthy as it takes a lot of time for the human body to break it down, and hence once a person consumes it, they feel full and prevent the person from overeating. They also have additional benefits such as it prevents stomach and intestinal issues, preventing cholesterol and blood sugar, and also helping in the absorption of enough water into the body.

Working of Carbohydrates in Human Body

In the human body, carbohydrates break down into smaller units of sugar, such as glucose and fructose. The small intestine absorbs these smaller units which are then directed towards the bloodstream and reaches the liver via blood.

The liver converts all of these sugars into glucose which is again carried through the bloodstream from the liver along with insulin and gets converted into energy for basic body functioning and physical activity. If the glucose is not immediately needed for energy, the body can store up to 2,000 calories of it in the liver and skeletal muscles in the form of glycogen.

Once glycogen stores are full, carbs are stored as fat. If there is insufficient carbohydrate intake, the body will consume protein as fuel instead of carbs and this causes issues in the health as protein is required to synthesize muscles. It also puts stress on the kidneys and can lead to the passage of painful by-products in the urine.

Metabolism of Cabohydrates

Metabolism refers to the process of breaking down of carbohydrates into simple energy absorbing units. It is a series of biochemical processes. In plants, carbohydrate is synthesized from carbon dioxide and water through photosynthesis, storing the absorbed energy internally, often in the form of starch or lipids.

These plant components are consumed by higher animals and fungi and are used as fuel for cellular respiration. Oxidation of one gram of carbohydrate can provide approximately 16 KJ of energy. The human body stores between (300-500)g of carbohydrates depending on the body weight, with the skeletal muscle contributing to a large portion of the storage.

Energy obtained from metabolism, that is the oxidation of glucose, is usually stored temporarily within cells as Adenosine Triphosphate (ATP) is the energy within the cells. Organisms capable of anaerobic and aerobic respiration metabolize glucose and oxygen (aerobic) to release energy with carbon dioxide and water as by-products.


It is the metabolic reaction that cells undergo to break down large molecules extracting energy. It is performed in majorly two metabolic pathways, glycolysis, and citric acid cycle. In glycolysis, oligosaccharides and polysaccharides are cleaved first to smaller monosaccharides by enzymes called glycoside hydrolases.

The monosaccharide units can then enter into monosaccharide catabolism. ATP investment is required in the early steps of glycolysis to phosphorylate Glucose to Glucose 6-Phosphate (G6P) and Fructose 6-Phosphate (F6P) to Fructose 1,6-bisphosphate (FBP), thereby pushing the reaction forward irreversibly.

In some cases, as with humans, not all carbohydrate types are usable as the digestive and metabolic enzymes necessary are not present.

Structure of Carbohydrates

Glucose: One of the most important monosaccharides is glucose. The two commonly used methods for the preparation of glucose are, starch and sucrose. If sucrose is boiled with dilute acid in an alcoholic solution then glucose and fructose are obtained.

From Starch, glucose can be obtained by hydrolysis of starch and by boiling it with dilute H2SO4 at 393K under elevated pressure. Glucose is named as D(+) glucose, D represents the configuration whereas (+) represents the dextrorotatory nature of the molecule.

The two cyclic structures differ in the configuration of the hydroxyl group at C1 called anomeric carbon. Such isomers i.e. α and β form are known as anomers. The cyclic structure is also called pyranose structure due to its analogy with pyran.

Fructose: It is an important ketohexose. The molecular formula of fructose is C6H12O6 and contains a ketone functional group at carbon number 2 and has six carbon atoms in a straight chain. The ring member of fructose is in analogy to the compound Furan and is named furanose.

Health Benefits, Sources and Risks

Carbohydrate is usually said to be the main source of weight gain, however, it is not true. What matters is the type of carbohydrate. Good carbohydrates are found in healthy whole grains such as whole-wheat bread, rye, barley, and quinoa.

Fruits, beans, whole grains, vegetables, and leafy vegetables are rich in good and healthy carbohydrates, which are an essential source of proteins, minerals, and phytonutrients as well. Unhealthy sources of carbohydrates include white bread, pastries, desserts like pies, sodas, and other highly processed refined foods.

These contain easily digested carbohydrates that may contribute to weight gain, diabetes, and heart diseases. The wrong form of carbohydrates is usually obtained via these unprocessed foods and items like creams and sodas.

Carbohydrates account for half of the healthy human body.

Mental health: Studies have shown that carbohydrates contribute to the improvement of mental health. People on high fat and low carb diet were usually found to possess higher anxiety, depression, and anger than those who were on low fat and high carb diet. This might be due to the influence of carbohydrates in producing serotonin in the brain. Carbohydrates influence memory too. A 2008 study showed that overweight women possessed low cognitive, visual attention, and spatial memory skills than those who had a healthy body with a good amount of carbohydrate diet.

Weight loss: The right kind of carbohydrates can help in losing weight and maintaining a healthy body. This is because good carbohydrates including whole grains, vegetables(unpeeled) contain a sufficient amount of carbohydrates in the form of fiber.

Not consuming enough carbohydrates can lead to huge risk factors such as physical weakness, dizziness. It can also result in hypoglycemia which is a deficiency of glucose. People who don’t consume enough carbohydrates may also suffer from insufficient fiber, which can cause digestive problems and constipation.

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