Apr 7, 2012

Fueling the Body: The Power of Carbohydrates

Carbohydrates, which mean hydrated carbon, are composed of C, H, and O in the ratio of 1:2:1. Their general formula is C(H2O)x, where x is a whole number ranging from three to thousands. Chemically, carbohydrates are defined as polyhydroxy aldehydes or ketones, or complex substances that yield polyhydroxy aldehyde or ketone subunits upon hydrolysis. They are primarily sourced from green plants, as they are the products of photosynthesis and serve as precursors for other compounds in plants through various chemical changes.

Carbohydrates can combine with other molecules to form complex conjugated molecules, such as glycolipids (carbohydrates + lipids) and glycoproteins (carbohydrates + proteins).

Carbohydrates are classified into three major classes: monosaccharides, oligosaccharides, and polysaccharides.


Monosaccharides, which are simple sugars, cannot be further hydrolyzed into simpler units. They are sweet in taste and soluble in water. Chemically, they are either polyhydroxy aldehydes or ketones. Except for one carbon atom, all other carbon atoms in a monosaccharide have a hydroxyl group. The remaining carbon atom is either a part of an aldehyde group or a keto group. Monosaccharides with an aldehyde group are called aldoses, while those with a keto group are called ketoses. Examples of aldoses include glyceraldehyde, and examples of ketoses include dihydroxyacetone. Monosaccharides with 3-7 carbon atoms are found in nature, and they have a general formula of Cn(H2O)n.

Note: Glucose and fructose have the same molecular formula of C6H12O6, but they have different molecular structures or arrangements of atoms.




 In solution, most monosaccharides form ring structures, such as the 5-carbon ring of ribose and the 6-carbon ring of glucose.

Oligosaccharides are sugars that yield 2 to 10 monosaccharides upon hydrolysis. They are less sweet in taste and less soluble in water.

Disaccharides are formed when two monosaccharides combine. Examples include maltose (glucose + glucose), lactose (glucose + galactose), and sucrose (glucose + fructose).

Trisaccharides are formed when three monosaccharides combine.

The bond formed between two monosaccharides is called a glycosidic linkage. Water is removed during the formation of this linkage. Examples of glycosidic linkages include 1,4-glycosidic linkage in maltose and 1,2-glycosidic linkage in sucrose.


Polysaccharides are composed of many glucose monomers linked by glycosidic bonds, and they are usually branched. They are tasteless and sparingly soluble in water. Examples of polysaccharides include starch and glycogen.

Starch is a polymer of glucose and consists of linear and branched chains. It is insoluble in water, which allows it to be stored in plant cells. There are two types of starch: amylose and amylopectin. Amylose has unbranched chains of glucose and is soluble in hot water, while amylopectin is branched and insoluble in hot or cold water. Starch gives a blue color with iodine.



Glycogen is a polysaccharide that animals use to store glucose. It consists of long branched chains of amylose and is insoluble in water. Glycogen gives a red color with iodine.

Cellulose: It is a polymer of glucose. The rings of glucose are arranged in a flip-flop manner. Cellulose is the building and supporting material of the cell wall. It is highly insoluble in water and gives no color with iodine. Examples of cellulose-containing materials are wood, paper, and cotton fibers.



Functions of Carbohydrates

Carbohydrates exhibit a variety of functions in living organisms.

Pentose: Ribose, a type of pentose, is a constituent of RNA (ribonucleic acid) and deoxyribose is a constituent of DNA (deoxyribonucleic acid). These nucleic acids are essential for genetic information storage and transfer in cells. Additionally, ribose is used in the synthesis of some coenzymes, such as NAD (nicotinamide adenine dinucleotide), which play important roles in cellular metabolism.

Disaccharide: Glucose, which is a monosaccharide, is the main source of energy for living organisms. It is utilized through cellular respiration to produce ATP (adenosine triphosphate), the energy currency of cells. Maltose, a disaccharide composed of two glucose units, is used in the production of alcohol, such as in brewing and fermentation processes. Lactose, another disaccharide composed of glucose and galactose, is an important source of food for young mammals, including human infants. Sucrose, commonly known as table sugar, is a disaccharide composed of glucose and fructose, and it serves as a source of energy for plants and is also widely used as a sweetener in food and beverages.

Polysaccharides: Polysaccharides have various functions in living organisms. They can act as food and energy stores, as well as structural materials.

Starch: Starch is a polysaccharide composed of glucose monomers and serves as a storage form of glucose in plants. It consists of both linear and branched chains, and it is insoluble in water. This insolubility allows starch to be stored in plant cells without affecting the osmotic balance. There are two types of starch: amylose, which has unbranched chains of glucose and is soluble in hot water, and amylopectin, which has branched chains and is insoluble in both hot and cold water. Starch can be broken down into glucose molecules for energy during plant metabolism.

Glycogen: Glycogen is the animal equivalent of starch and serves as a storage form of glucose in animals, including humans. It is stored in animal cells, particularly in the liver and muscles, and can be broken down into glucose for energy during periods of high energy demand, such as during exercise or fasting. Glycogen has a highly branched structure, which allows for efficient storage and rapid release of glucose when needed.

Cellulose: Cellulose is a polysaccharide that forms the structural material of the cell walls in plants. It provides rigidity and support to plant cells, helping them maintain their shape and resist external pressures. Cellulose is composed of long chains of glucose molecules arranged in a flip-flop manner, forming a complex network of fibers. Unlike starch and glycogen, cellulose is highly insoluble in water and is not used as an energy source in most animals, including humans. However, some animals, such as ruminant mammals, can digest cellulose with the help of specialized microorganisms in their digestive system.

In addition to their roles as energy sources and structural materials, carbohydrates also play important roles in cell signaling, cell recognition, and immune response through the formation of glycolipids and glycoproteins, which are complex carbohydrate-containing molecules on the cell surface. These glycolipids and glycoproteins are involved in cell-cell communication, cell adhesion, and recognition processes, playing critical roles in various physiological and pathological processes in living organisms.

Furthermore, carbohydrates also serve as important components of many biological molecules, such as nucleotides, which are the building blocks of nucleic acids (DNA and RNA), and ATP (adenosine triphosphate), which is the primary energy currency in cells. Carbohydrates also play a role in the modification and stabilization of proteins through processes like glycosylation, where carbohydrates are attached to proteins to form glycoproteins, regulating their functions and stability.

Carbohydrates also have important functional roles in the immune system. For example, certain types of carbohydrates, such as antigens on the surface of pathogens, are recognized by the immune system as foreign, triggering immune responses to fight against infections. Carbohydrates also play a role in blood typing, where specific carbohydrate antigens on red blood cells determine an individual's blood type.

In summary, carbohydrates are essential biomolecules with diverse functions in living organisms. They serve as a major source of energy, provide structural support, play roles in cell signaling and recognition, and are involved in various physiological and pathological processes. The complex and diverse roles of carbohydrates make them crucial for the proper functioning of cells and organisms as a whole.


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