Carbohydrates Metabolism
Carbohydrates, also called saccharides or carbs are the most abundant organic compounds in nature that provide energy for our bodies through metabolism. A carbohydrate primarily contains carbon, hydrogen, and oxygen in its structure. Carbohydrates produces during the hydrolysis of polyhydroxy aldehyde or ketone compounds. Carbohydrates are mainly found in plant foods and also occur in dairy products in the form of sugar called lactose. The term sugar is applied to carbohydrates soluble in water and sweet in taste. Like proteins and fats, carbohydrates or polysaccharides are also important components of our bodies.
Carbohydrates in our body are generally involved in the development of the immune system, health diseases, blood clotting, and reproduction. It also provides energy during metabolism to control several biological functions.
The food products bread, pasta, beans, potatoes, rice, and cereals contain a high level of carbohydrates that participate in a wide range of biological functions in our bodies.
Benefits of Carbohydrates
Like proteins and fats, carbohydrates are one of the three main nutrients found in our daily foods and drinks. They are beneficial for our bodies and also participate in a wide range of biological functions.
The most common biological functions of carbohydrates may include:
- Carbohydrates provide the most dietary energy for all organisms present on our earth. Thus, our body metabolizes carbohydrates into glucose and it is the main source of energy for our body’s cells, tissues, and organs.
- The monosaccharide ribose is an essential component of adenosine triphosphate (ATP), flavin adenine dinucleotide (FAD), nicotinamide adenine dinucleotide (NAD), nicotinamide adenine dinucleotide phosphate (NADP), and the genetic molecule ribonucleic acid (RNA).
- Deoxyribose is also an essential component of deoxyribonucleic acid (DNA).
- Carbohydrates are the precursors for many organic substances such as amino acids. They also perform numerous roles in living organisms.
- Carbohydrates such as glycoproteins and glycolipids help to construct the cell membrane. Therefore, they participate in various cellular functions such as cell growth, adhesion, and fertilization.
- Carbohydrates are the structural components of many organisms. For example, cellulose (fiber) of plants, the exoskeleton of some insects, and the cell wall of microorganisms.
- The storage form of carbohydrates or glycogen generally fulfills the immediate demands of energy for our body through metabolism.
Classification of Carbohydrates
The name carbohydrates or saccharides can be obtained from the Ancient Greek word sákkharon means ‘sugar’. This group of organic compounds includes sugars, starch, and cellulose.
Many saccharide forms of carbohydrates and their derivatives play important biological roles in the development of immune health, fertilization processes, pathogenesis, blood clotting, and development.
Carbohydrates or saccharides are broadly classified into four major groups based on the number of sugar units.
- Monosaccharides
- Disaccharides
- Oligosaccharides
- Polysaccharides
Monosaccharides and oligosaccharides are sweet in taste, crystalline in nature, and soluble in water. Monosaccharides and disaccharides are the smallest carbohydrates that generally contain sugar units in their structure.
Polysaccharides are polymers of monosaccharide units with high molecular weight, and they also form colloids with water. They are usually tasteless in nature.
Monosaccharides
Monosaccharides or simple sugars are the simplest groups of carbohydrates with the general molecular formula Cn(H2O)n. The monosaccharides further divide into various categories based on the functional group and the number of carbon atoms.
Based on the functional group, monosaccharides are mainly of two types,
- Aldoses: When the functional group in monosaccharides is an aldehyde, they are called aldoses. For example, glyceraldehyde and glucose.
- Ketoes: When the functional group in monosaccharides is a ketone, they are called ketoes. For example, dihydroxyacetone and fructose.
Based on the number of carbon atoms, the monosaccharides are in the following types,
| Monosaccharides | Empirical formula | Aldose | Ketose |
| Trioses | C3H6O3 | Glyceraldehyde | dihydroxyacetone |
| Tetroses | C4H8O4 | Erythrose | Erythrulose |
| Pentoses | C5H10O5 | Ribose | Ribulose |
| Hexoses | C6H12O6 | Glucose | Fructose |
| Heptoses | C7H14O7 | Glucoheptose | Sedoheptulose |
These terms with functional groups uses when naming carbohydrates. For example, glucose is an aldohexose while fructose is an aldoketose.
Disaccharides
A disaccharide contains two similar or dissimilar monosaccharide units held together by a glycosidic bond. They are crystalline and generally sweet in nature and soluble in water.
The disaccharides are of two types:
- Reducing disaccharides with a free aldehyde or keto group such as maltose and lactose.
- Non-reducing disaccharides without a free aldehyde or keto group such as sucrose and trehalose.
Oligosaccharides
The name oligosaccharides is obtained from the Greek word oligo, which means few. An oligosaccharide is a saccharide polymer containing three to ten monosaccharides or simple sugar molecules.
Oligosaccharides are a type of carbohydrate that is found naturally in various plant foods such as fruits and vegetables. Therefore, it is possible to eat them regularly but they are relatively less abundant in the diet when compared to other more common carbohydrates.
Oligosaccharides with more than 3 monosaccharide units are not usually digested by human enzymes. These types of carbohydrate metabolism cannot be performed in the human digestive tract. Therefore, they travel through your gut to the colon and feed or support the growth of beneficial bacteria.
Raffinose is an example of a trisaccharide found generally in vegetables and whole grains. This type of carbohydrate contains galactose, glucose, and fructose. This type of carbohydrate is generally considered a good source of prebiotic fiber (bacterial food).
Polysaccharides
The name polysaccharides originates from the Greek word poly, which means many. An oligosaccharide is a saccharide polymer containing three to ten monosaccharides or simple sugar molecules. Thus, polysaccharides consist of long chains and contain several smaller monosaccharide molecules.
These complex types of bio-macromolecules provide energy in animal cells and form a structural component of a plant cell. Therefore, the metabolism of carbohydrates plays important biological roles in living organisms. Polysaccharides such as starch and glycogen can store energy for animals while cellulose is the main structural component of plants.
Examples of Carbohydrates
Foods that contain high levels of carbohydrates are an important part of our healthy diet because they are the most abundant dietary source of energy for all organisms.
They have come in a variety of forms. However, the most common and abundant forms of carbohydrates are sugars, fibers, and starches.
Examples of Monosaccharides
Some important examples of carbohydrates or saccharides with their occurrence and metabolic importance are listed below in the table,
| Carbohydrate | Occurrence | Biochemical importance | |
| Trioses | Glyceraldehyde | The phosphate form occurs mostly in cells | Glyceraldehyde 3-phosphate is an intermediate in glycolysis. |
| Dihydroxyacetone | Functioning as an intermediate when glycolysis occurs. | ||
| Tetroses | D-Erythrose | Widespread | An intermediate in the HMP pathway during carbohydrate metabolism. |
| Pentoses | D-Ribose | Found widely as a constituent of RNA and nucleotides | For the structure of RNA, coenzymes (ATP, NAD, and NADP). |
| D-deoxyribose | Found in DNA | For the structure of DNA. | |
| D-Ribulose | Produced during metabolism | An important metabolite in the hexose monophosphate shunt. | |
| D-Xylose | Found in glycoproteins and gums | Involved in the formation of glycoproteins | |
| L-Xylulose | An intermediate in the uronic acid pathway | Excreted in urine in essential pentosuria | |
| D-Lyxose | Heart muscle | A constituent of lyxoflavin in the heart muscle. | |
| Hexoses | D-Glucose | Constituents of starch, glycogen, cellulose, maltose, lactose, and sucrose. | Sugar fuel of life and also the structural unit of cellulose in plants. |
| D-Galactose | A constituent of lactose (milk sugar) | Converted to glucose, failure leads to galactosemia | |
| D-Mannose | Found in plant polysaccharides and animal glycoproteins | For the structure of polysaccharides | |
| D-Fructose | A constituent of sucrose and inulin | Its phosphate forms are intermediates of glycolysis | |
| Heptoses | D-Sedoheptulose | Found in plants | Its 7-phosphate is an intermediate in the hexose monophosphate shunt and photosynthesis |
Examples of Disaccharides
| Disaccharide | Occurrence | Biochemical importance | |
| Sucrose | Occurs in cane sugar and beet sugar, pineapple | The most commonly used table sugar supplies calories. | |
| Lactose | Milk sugar | Exclusive carbohydrate source for breastfed infants. Lactase deficiency leads to flatulence. | |
| Maltose | The product of starch hydrolysis occurs during the germination of seeds | An important intermediate in the digestion of starch. | |
Nutrition Sources of Carbohydrates
Carbohydrates are important types of food nutrients found in a wide variety of natural and processed foods. Like vitamins and minerals, carbohydrates are also nutrients that cannot be produced by the human body. Therefore, we should take them through our daily diet.
There are both healthy and unhealthy sources of carbohydrates or polysaccharides found in our daily food items.
- Healthy sources of carbohydrates: Animal and plant-based products such as fresh fruits, vegetables, corn, potatoes, milk, milk products, etc.
- Unhealthy sources of carbohydrates: soda, white bread, artificial sugar, pastries, etc.
They may also be found in different forms such as sugars, starches, and fibers.
Sources of Sugars
Sugars appear in the human diet generally in the form of table sugar (sugarcane or sugar beets), lactose (milk), glucose, and fructose. These types of carbohydrates occur naturally in honey, many fruits, and some vegetables.
Carbohydrates containing items such as table sugar, milk, or honey are also added to drinks and many prepared foods such as jam, biscuits, and cakes.
Sources of Starch
Starch is a carbohydrate or polysaccharide that is found abundantly in plant-based foods such as cereals (wheat, maize, rice), potatoes, etc. Cereal-based flour also contains a huge amount of starch and uses for the production of bread, pizza, or pasta.
Sources of Fibers
Cellulose is a carbohydrate that is one of the main components of insoluble dietary fiber. Therefore, these types of carbohydrates are not metabolized by humans but help to maintain a healthy digestive system by facilitating bowel movements.
