The carbohydrate is an organic compound made up of carbon, hydrogen, and oxygen atoms. It is viewed as hydrates of carbon, hence its name. Its other name is saccharide, which is derived from the Greek word “sakkharon”, meaning “sugar”. The basic unit, or building block of a carbohydrate, is the monosaccharide. Monosaccharides can join through chemical bonds to produce disaccharides, oligosaccharides, and polysaccharides. The prefixes indicate the number of monosaccharides joined together.
Carbohydrates play important roles in the human body. Polysaccharides, like glycogen, serve as storage of energy for future use. Glucose is the monosaccharide used in cellular reactions to manufacture energy (ATP). The monosaccharides ribose and deoxyribose are backbones for RNA and DNA respectively. Moreover, carbohydrates are also important structural components of cells and essential biomolecules for immunity, fertilization, blood clotting, and development.
Unlike plants, humans are not capable of performing photosynthesis to produce carbohydrates. They need to eat food with carbohydrates to supply their body with the compound. Foods with a high amount of carbohydrates include cereals, grains, root tubers, fruits, and vegetables. The carbohydrates contained in these foods are made up of trillions (or more) of polysaccharides that are joined together by chemical bonds. Since our body can only use glucose (simple sugar), food polysaccharides must be broken down into smaller units before they can be used in cell metabolism (e.g. glycolysis and glycogenesis). Let us look at the various processes of carbohydrate metabolism in humans.
Digestion of Carbohydrates
The first stage of carbohydrate metabolism in humans is the physical and chemical breakdown of large carbohydrates into the simplest form (glucose). This occurs in the mouth down to the intestines.
In the mouth, carbohydrates are broken down into tiny pieces by chewing or mastication. The enzyme salivary amylase will break down more complex carbohydrates into simpler ones. For example, starch is broken down by amylase (ptyalin) to maltose.
From the mouth, carbohydrates will pass through the esophagus until they reach the stomach, where they would be further broken into smaller forms (e.g. maltose, fructose and sucrose) and mixed with gastric acids.
After few hours in the stomach, carbohydrates are ready to move into the duodenum, the first portion of the small intestine. The pancreas will release digestive enzymes such as maltase, lactase, and sucrase to splice maltose, lactose, and sucrose into glucose, the only form of carbohydrate absorbed into the bloodstream.
The blood will carry all glucose molecules into all cells of the body. Depending on the conditions of the body, cells can process glucose to generate energy, store glucose for future use or convert glucose into fats.
Carbohydrates to Energy
To generate energy, glucose should undergo a series of chemical reactions in the cell cytoplasm and mitochondria.
Glycolysis is a chemical reaction in the cytoplasm involving glucose and several enzymes. In the reaction, glucose is converted into two molecules of pyruvates through enzymatic reactions. Two molecules of ATP are generated throughout glycolysis. ATP or adenosine triphosphate is the energy-rich molecule universally used by organisms to do work.
After glycolysis, pyruvate will be transferred to the mitochondrion to join two more series of chemical reactions namely Kreb Cycle and Electron Transport Chain. The end products of these two complex chemical reactions are more ATP molecules that would power all cellular activities such as cell division, gene expression, gene transcription, and protein synthesis.
Storage of Carbohydrates
If the cell senses an abundance of glucose and ATP, it would break down glucose molecules to chains of glycogen for storage in the liver and muscles.
The opposite of glycogenesis is glycogenolysis or the breakdown of glycogen into glucose molecules. Glycogenolysis occurs when the levels of glucose and ATP are below normal levels.
Glycogenesis and glycogenolysis are tightly regulated by hormones such as insulin, adrenaline, and glucagon.
Carbohydrates to Fats
Carbohydrates consumed in excess of caloric expenditure are stored in adipose tissues as fats. Like glycogen, fats are stored energy; they are broken down and converted into glucose when the body is depleted with both ATP and glucose. However, fats are broken down only when almost all glycogen molecules stored in the body are used up.
Carbohydrate Metabolism. Online Educ. Support. University of South Australia