2.4: Carbohydrates

Biotech Focus

Carbohydrates are not only important components of foods, but they are essential macromolecules that perform a range of critical biological functions in cells. In addition to providing structure to cells (in plants) and being a source of cellular energy (in all cells), some carbohydrates have been approved for the treatment of various diseases. However, their use is very limited. To learn why and read about the future potential of carbohydrates as medicine, check out the article Exploring Carbohydrates for Therapeutics: A Review on Future Directions.

Introduction

Carbohydrates play essential roles in the human body as well as in other organisms. They are an important source of energy to cells and also provide structural support.

Learning Objectives

By the end of this section, you will be able to:

• Discuss the role of carbohydrates in cells and in the extracellular materials of animals and plants
• Explain the basic composition of a carbohydrate
• List the major types of carbohydrates
• Explain how monosaccharides are classified
• Explain the major forms of monosaccharides
• Explain the different types of common monosaccharides, disaccharides, and polysaccharides
• Explain the difference between a dehydration synthesis reaction and a hydrolysis reaction

Molecular Structures

Carbohydrates are biological molecules made of carbon, hydrogen, and oxygen. This composition gives carbohydrates their name: they are made up of carbon (carbo-) plus water (-hydrate). Carbohydrates can be represented by the stoichiometric formula (CH₂O)_n, where n is the number of carbons in the molecule. In other words, there is as number of carbon atoms in a carbohydrate molecule equals that of oxygen atoms and there are twice as many hydrogen atoms as carbon.

Carbohydrates can be classified into three categories:

1. monosaccharides
2. disaccharides
3. polysaccharides

Monosaccharides

Monosaccharides (mono- = “one”; sacchar- = “sugar”) are the simplest form of a carbohydrate and are often referred to as simple sugars. They typically range in size from three to seven carbons (Figure \(\PageIndex{1}\)). Most monosaccharide names end with the suffix "-ose" (e.g. glucose). They can be classified in several ways, one of which is based on the number of carbons.

Examples of carbohydrates classified using this method are:

• triose sugars (three carbons)
• pentose sugars (five carbons)
• hexose sugars (six carbons)

Figure \(\PageIndex{1}\): Monosaccharides can be classified based on the number of carbons in the backbone. The triose sugar, glyceraldehyde, (C₃H₆O₃) has three carbons (carbons are highlighted in red). The pentose sugar, ribose, (C₅H₁₀O₅) has five sugars. The hexose sugar, glucose, (C₆H₁₂O₆) is made of 6 carbons. (Monosaccharides by Kareen Martin; CC BY 4.0)

A monosaccharide is composed of carbon, oxygen, and hydrogen atoms. Most of the oxygen atoms in the monosaccharide can be found in hydroxyl (OH) groups. However, one oxygen atom is found as part of a functional group called a carbonyl (C=O) (Figure \(\PageIndex{2}\)). The position of the carbonyl group is another way to classify a monosaccharide.

Using this method, there are two types of monosaccharides:

1. Aldose: the carbonyl (C=O) is located at the end of the sugar, forming an aldehyde group
2. Ketose: The carbonyl (C=O) is located within the molecule, forming a ketone group

Figure \(\PageIndex{2}\): Monosaccharides can be classified based on the position of their carbonyl group (C=O) in the carbon chain (red boxes). Aldose sugars (e.g. glyceraldehyde) have a carbonyl group at the end of the carbon chain, and ketose sugars (e.g. dihydroxyacetone) have a carbonyl group in the middle of the carbon chain. (Aldose vs Ketose by Kareen Martin; CC BY 4.0)

The most common monosaccharides found in cells are the hexose sugars glucose, fructose, and galactose and the pentose sugars ribose and deoxyribose. Both ribose and deoxyribose are found in nucleic acids. Galactose can be found as part of lactose (i.e. milk sugar) and fructose is a component of the disaccharide in sucrose, which is found in fruit. Glucose is an important source of energy to eukaryotic cells. During cellular respiration, energy is released from glucose, and that energy is used to help make adenosine triphosphate (ATP). Plants synthesize glucose using carbon dioxide and water, and glucose in turn is used for energy requirements for the plant. In plants and certain algae excess glucose is stored as starch that is broken down by humans and other animals that feed on these materials. In animals and certain fungi, excess glucose is stored as glycogen.

The chemical formula for glucose, galactose, and fructose is C₆H₁₂O₆. While their chemical formulae are identical, their structures are different (Figure \(\PageIndex{3}\)). Compounds that have the same molecular formulae, but have different structures are known as structural isomers. Therefore, glucose, galactose and fructose are structural isomers of one another.

Figure \(\PageIndex{3}\): Hexose sugars. Glucose, galactose, and fructose are all hexose sugars with the same chemical formula (C₆H₁₂O₆). They are known as structural isomers, meaning they have the same chemical formula but a different arrangement of atoms. (Structural Isomers of Glucose by Kareen Martin; CC BY 4.0)

When placed into aqueous solutions, monosaccharides form ring-shaped molecules (Figure \(\PageIndex{4}\)). The ring form of a monosaccharide is more stable than its linear counterpart. This is because the reactive carbonyl function group found in the linear form is no longer exposed and becomes "trapped" in the ring.

Figure \(\PageIndex{4}\): Monosaccharides can be found in ring forms. The ring forms of the hexose sugars glucose, fructose, and galactose are shown at the top of the figure and the pentose sugars deoxyribose and ribose are shown below. (Carbohydrate Ring Forms by Kareen Martin; CC BY 4.0)

The linear molecule is converted into a ring form when the hydroxyl group on the fifth carbon reacts with the carbonyl group on the first carbon. Numbering of the carbons in the resulting ring-shaped monosaccharide begins with the carbon to the right of the oxygen and proceeds in a clock-wise fashion (Figure \(\PageIndex{5}\)).

Figure \(\PageIndex{5}\): Transformation of linear glucose to the ring form. Conversion of a linear glucose molecule to its ring form occurs when the hydroxyl group at the 5th carbon reacts with the carbonyl group on the 1st carbon (left image, dotted red arrow). The linear form begins to convert to its ring conformation, with a bond forming between the 5th and 1st carbon (middle image, arrow). The hydrogen from the 5th carbon (shown in red) bonds to the oxygen at the 1st carbon to form a hydroxyl group. The ring formation of glucose has a specific numbering scheme with the 1st carbon located to the right of the oxygen (right image). (Glucose Linear to Ring by Patricia Zuk, CC BY 4.0)

Glucose and galactose in their ring structure, differ in the orientation of the hydroxyl group (OH) at the fourth carbon, with glucose having this functional group located below the plane of the ring. Furthermore, glucose can have two different arrangements of the hydroxyl group around the first carbon. If the hydroxyl group at carbon 1 found is below the plane of the ring, it is said to be in the alpha (α) position, and if it is above the plane, it is said to be in the beta (ß) position (Figure \(\PageIndex{6}\)). The beta-form is more stable than the alpha form and is typically found in large polymers like cellulose, whereas the alpha form can be found in plant starches like amylose. Fructose, with its unique shape does not have alpha and beta conformations.

Figure \(\PageIndex{6}\): Monosaccharide conformations. Glucose and galactose can be distinguished using the hydroxyl groups located at carbon four (blue circles). The alpha and beta forms of glucose can be distinguished using the hydroxyl groups located at carbon one (red squares). (Glucose and Galactose Forms by Kareen Martin, CC BY 4.0)

Disaccharides

The joining of two monosaccharides forms a disaccharide (di- = “two”; sacchar- = “sugar” ). Disaccharides are formed through dehydration synthesis and result in the formation of a glycosidic bond between the two monosaccharide subunits. During this process, the hydroxyl group (–OH) of one monosaccharide combines with a hydrogen atom of another monosaccharide, releasing a molecule of water (H₂O) and forming a covalent bond between atoms in the two sugar molecules (Figure \(\PageIndex{7}\)).

Common disaccharides are:

• sucrose - formed by joining glucose to fructose
• lactose - formed by joining glucose to galactose
• maltose - formed by joining glucose to glucose

Figure \(\PageIndex{7}\): Sucrose is a disaccharide made of a monomer of glucose and a monomer of fructose. Sucrose is formed through a dehydration reaction that forms a glycosidic linkage between carbon 1 in glucose and carbon 2 in fructose. The components of the water molecule removed to make the bond is shown with a dotted circle. (Sucrose Formation by Kareen Martin; CC BY 4.0)

The breakdown of these disaccharides is done through a hydrolysis reaction and will yield glucose plus either galactose or fructose. In cells, this hydrolytic breakdown is catalyzed by enzymes. For example, the enzyme sucrase catalyzes the breakdown of sucrose into glucose and fructose Figure \(\PageIndex{8}\)).

Figure \(\PageIndex{8}\): Sucrose breakdown by sucrase. The enzyme sucrase catalyzes the hydrolytic breakdown of sucrose into the monomers glucose and fructose. The location of the incorporated water molecule is shown with blue boxes. (Sucrose Breakdown by Patricia Zuk, CC BY 4.0)

Polysaccharides

The joining of three of more monosaccharides creates a polysaccharide (poly- = “many”). Each polysaccharide is made of either a few or hundreds of monosaccharides with each joined through a dehydration synthesis reaction. Unlike simple sugars, polysaccharides are not soluble in water and do not taste very sweet. This is because the majority of polysaccharides found in living organisms are made of glucose monomers. Starch, glycogen, cellulose, and chitin are examples of polysaccharides. Starch and glycogen are considered storage polysaccharides owing to their roles in energy utilization. Cellulose and chitin are examples of structural polysaccharides that are incorporated into the structure of the organism.

Starch is the stored form of glucose made by plants and can be found in two forms: amylose and amylopectin (Figure \(\PageIndex{9}\)). Amylose is the simplest of these two forms with the alpha form of glucose linked by glycosidic bonds found between the first carbon of one glucose monomer and the fourth carbon of the next monomer (i.e. 1-4 glycosidic linkages). This creates a starch with a helical (i.e. coiled) conformation. Amylopectin has regions of coils, in addition to branch points that are formed by glycosidic bonds that form between the first carbon of one glucose monomer and the sixth carbon of another (i.e. 1-6 glycosidic linkages). Excess glucose in plants is stored as starch in different plant parts, including the roots and seeds. The starch that is consumed by animals is broken down into smaller molecules, such as glucose. The cells can then utilize the glucose.

Figure \(\PageIndex{9}\): Amylose and amylopectin. The starch molecules amylose and amylopectin are composed of α-glucose subunits. Amylose (left image) is a simple, coiled starch made of α-glucose monomers joined through 1-4 glycosidic linkages (red box). Amylopectin (right image) is made of α-glucose monomers joined through 1-4 glycosidic linkages, in addition to 1-6 glycosidic linkages (blue box). The 1-6 glycosidic linkages produce a branch point in this starch. (Amylose and Amylopectin by Patricia Zuk, CC BY 4.0; modified from Carbohydrates, CC BY-SA 4.0)

The main polysaccharide of vertebrates is glycogen. Glycogen is a complex, branched polysaccharide whose monomers are the beta form of glucose (Figure \(\PageIndex{10}\)) Glycogen is the animal equivalent of starch. Branch points are the result of 1-6 glycosidic bonds, with the coiled sections being the result of 1-4 glycosidic bonds. Excess glucose is converted into glycogen by the cells of the liver and skeletal muscle. When glucose levels in an organism drop, glycogen is broken down to release glucose.

Figure \(\PageIndex{10}\): Glycogen structure. Glycogen is a highly branched molecule made of ß-glucose monomers connected by 1-4 glycosidic linkages (solid ed box) and 1-6 glycosidic linkages (dotted red box). The 1-6 glycosidic linkage will produce a branch point in the molecule. Glycogen is the animal equivalent of starch and usually stored in liver and muscle cells. (Glycogen Structure by Kareen Martin; CC BY 4.0)

Cellulose is one of the most abundant natural biopolymers. The cell walls of plants are mostly made of cellulose, which provides structural support to the cell. Wood and paper are mostly cellulosic in nature. The chemical formula for cellulose is (C₆H₁₀O₅)_n, where n is the number of glucose monomers. Cellulose is made up of glucose monomers that are linked by 1-4 glycosidic bonds. (Figure \(\PageIndex{11}\)). While this is the same glycosidic linkage found in starch, in cellulose the glucose monomer is the beta form and every other beta-glucose monomer in cellulose is flipped in its orientation. This allows for the creation of extremely long chains of glucose that are rigid and have a high tensile strength—two parameters that are important to plant cells.

Figure \(\PageIndex{11}\): Cellulose structure. In cellulose, glucose monomers (beta form) are linked in unbranched chains (left image). Because of the way the beta-glucose subunits are joined, every glucose monomer is flipped relative to the next one (OH groups at carbon 3 shown circled in red). Cellulose is found in the cell walls of plants and gives them rigidity as seen in celery (right image). (Cellulose Structure by Kareen Martin; CC BY 4.0)

Cellulose passing through the digestive system is called dietary fiber. While the glucose-glucose bonds in cellulose cannot be broken down by human digestive enzymes, herbivores such as cows, buffalos, and horses are able to digest grass that is rich in cellulose and use it as a food source. In these animals, certain species of bacteria reside in the digestive system and secrete the enzyme cellulase. The appendix in some ruminants also contains bacteria that break down cellulose. Cellulases can break down cellulose into glucose monomers that can be used as an energy source by the animal.

Carbohydrates serve other functions in different animals. Arthropods, such as insects, spiders, and crabs, have an outer skeleton, called the exoskeleton, which protects their internal body parts. This exoskeleton is made of the biological macromolecule chitin, which is a nitrogenous carbohydrate. Chitin is made of repeating units of N-acetylglucosamine (GlcNAc), a modified sugar derived from glucose and containing nitrogen. (Figure \(\PageIndex{12}\)).

Figure \(\PageIndex{12}\): Chitin structure. Chitin is a polysaccharide made of repeating units of N-acetylglucosamine (GlcNAc). N-acetylglucosamine is a modified glucose monomer that has an amine and acetyl group (shown in the green box) bound to the 2nd carbon. Insects like the ladybug shown on the right, have a hard outer exoskeleton made of chitin. (Chitin Structure by Kareen Martin; CC BY 4.0)

While the polysaccharides discussed in this page are all built of glucose monomers, differences in their molecular structure allow for the different functions of energy storage (starch and glycogen) and structural support and protection (cellulose and chitin).

Key Concepts

Carbohydrates are a group of macromolecules that are a vital energy source for the cell and provide structural support to plant cells, fungi, and all of the arthropods that include lobsters, crabs, shrimp, insects, and spiders.

Some important concepts to remember are:

• Carbohydrates are classified as monosaccharides, disaccharides, and polysaccharides depending on the number of monomers in the molecule
• The monomer of a carbohydrate is the monosaccharide
• Monosaccharides are linked together by glycosidic bonds that are formed as a result of dehydration synthesis reactions
• Disaccharides are formed by joining two monosaccharides by a glycosidic bond
• Polysaccharides are formed by joining numerous monosaccharides by glycosidic bonds
• Glucose, galactose, and fructose are common monosaccharides
• Common disaccharides include lactose, maltose, and sucrose
• Starch and glycogen are storage polysaccharides, made of glucose and found in plants and animals, respectively.
• Cellulose and chitin are structural polysaccharides

Glossary

Carbohydrate - a macromolecule composed of carbon, hydrogen, and oxygen; primarily used for energy storage and structural functions.

Cellulose - a structural polysaccharide found in the cell walls of plants; composed of glucose molecules linked by glycosidic bonds; indigestible by humans but important as dietary fiber.

Disaccharide - a carbohydrate composed of two monosaccharides linked by a glycosidic bond, such as sucrose, lactose, and maltose.

Fiber - a type of carbohydrate that is not digested by human enzymes but aids in digestion and overall health; found in plant-based foods.

Fructose - a monosaccharide found in fruits and honey; often referred to as fruit sugar.

Glucose - a monosaccharide that serves as the primary source of energy for cells; often referred to in humans as as blood sugar.

Glycogen - a polysaccharide that serves as the primary storage form of glucose in animals and humans; stored mainly in the liver and muscles.

Glycosidic bond - a covalent bond that links carbohydrate molecules together; formed between the hydroxyl groups of two sugars.

Lactose - a disaccharide composed of glucose and galactose; commonly found in milk and dairy products.

Monosaccharide - the simplest form of carbohydrate, consisting of a single sugar unit, such as glucose, fructose, or galactose.

Polymer - a large molecule made up of repeating units called monomers

Polysaccharide - a complex carbohydrate composed of multiple monosaccharide units, such as starch, cellulose, and glycogen.

Starch - a polysaccharide found in plants that serves as their main form of energy storage; examples include amylose and amylopectin.

Sucrose - a disaccharide made up of glucose and fructose; commonly known as table sugar.

Attribution

text modified from OpenStax Biology 2e carbohydrates and OpenStax Biology 2e Biological Molecules