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Structure and Function of Carbohydrates

Author: Sophia

what's covered
In this lesson, you will learn about the structure and function of carbohydrates. Specifically, this lesson will cover:

Table of Contents

1. The Importance of Carbohydrates

Most people are familiar with carbohydrates, one type of macromolecule, especially when it comes to what we eat. To lose weight, some individuals adhere to “low-carb” diets. Athletes, in contrast, often “carb-load” before important competitions to ensure that they have enough energy to compete at a high level. Carbohydrates are, in fact, an essential part of our diet; grains, fruits, and vegetables are all natural sources of carbohydrates. Carbohydrates provide energy to the body, particularly through glucose, a simple sugar that is a component of starch and an ingredient in many staple foods. Carbohydrates also have other important functions in humans, animals, and plants.

IN CONTEXT

Carbohydrates can be represented by the stoichiometric formula left parenthesis CH subscript 2 straight O right parenthesis subscript straight n, where n is the number of carbons in the molecule. In other words, the ratio of carbon to hydrogen to oxygen is 1:2:1 in carbohydrate molecules. This formula also explains the origin of the term “carbohydrate”: the components are carbon (“carbo”) and the components of water (hence, “hydrate”). Carbohydrates are classified into three subtypes: monosaccharides, disaccharides, and polysaccharides.


2. Monosaccharides

Monosaccharides (mono– = “one”; sacchar– = “sweet”) are simple sugars, the most common of which is glucose. In monosaccharides, the number of carbons usually ranges from three to seven. Most monosaccharide names end with the suffix –ose. If the sugar has an aldehyde group (the functional group with the structure R-CHO), it is known as an aldose, and if it has a ketone group (the functional group with the structure RC(=O)R′), it is known as a ketose.

Depending on the number of carbons in the sugar, they also may be known as trioses (three carbons), pentoses (five carbons), and or hexoses (six carbons). See Figure 1 for an illustration of the monosaccharides.

Chemical structures of Monosaccharides. On the top left, it is the chemical structure of Glyceraldehyde with aldose. On the top right, it is the chemical structure of Dihydroxyacetone with ketose. On the bottom left is the chemical structure of Glyceraldehyde with triose on the bottom. In the bottom center is the chemical structure with Pentose. On the bottom right is the chemical structure of Glucose with Hexose. Each chemical structure is labeled with their extra component labeled beneath it.
Figure 1

key concept
Monosaccharides are classified based on the position of their carbonyl group and the number of carbons in the backbone. Aldoses have a carbonyl group (indicated in orange in Figure 1) at the end of the carbon chain, and ketoses have a carbonyl group in the middle of the carbon chain. Trioses, pentoses, and hexoses have three, five, and six carbon backbones, respectively.

The chemical formula for glucose is straight C subscript 6 straight H subscript 12 straight O subscript 6. In humans, glucose is an important source of energy. 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. Excess glucose is often stored as starch that is catabolized (the breakdown of larger molecules by cells) by humans and other animals that feed on plants.

Galactose and fructose are other common monosaccharides—galactose is found in milk sugars and fructose is found in fruit sugars. Although glucose, galactose, and fructose all have the same chemical formula (straight C subscript 6 straight H subscript 12 straight O subscript 6), they differ structurally and chemically (and are known as isomers) because of the different arrangement of functional groups around the asymmetric carbon; all of these monosaccharides have more than one asymmetric carbon. See Figure 2 for an illustration of glucose, galactose, and fructose, which are all hexoses. They are structural isomers, meaning they have the same chemical formula (straight C subscript 6 straight H subscript 12 straight O subscript 6) but a different arrangement of atoms.

Chemical structures of glucose, galactose, and fructose. The three structures are aligned side by side.
Figure 2

Monosaccharides can exist as a linear chain or as ring-shaped molecules; in aqueous solutions, they are usually found in ring forms (Figure 3). Glucose in a ring form can have two different arrangements of the hydroxyl group (−OH) around the anomeric carbon (carbon 1 that becomes asymmetric in the process of ring formation). If the hydroxyl group is below carbon number 1 in the sugar, 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 (β) position.

In Figure 3, five and six carbon monosaccharides exist in equilibrium between linear and ring forms. When the ring forms, the side chain it closes on is locked into an α or β position. Fructose and ribose also form rings, although they form five-membered rings as opposed to the six-membered ring of glucose.

Chemical structures of five and six carbon monosaccharides exist in equilibrium between linear and ring forms. The top part of the image shows the conversion between linear and ring forms of glucose. The bottom part of the image shows the ring forms of ribose and fructose.
Figure 3

terms to know
Monosaccharides
Simple sugars, the most common of which is glucose.
Aldehyde Group
The functional group with the structure R-CHO.
Ketone Group
The functional group with the structure RC(=O)R′.


3. Disaccharides

Disaccharides (di– = “two”) form when two monosaccharides undergo a dehydration reaction (also known as a condensation reaction or dehydration synthesis). During this process, the hydroxyl group of one monosaccharide combines with the hydrogen of another monosaccharide, releasing a molecule of water and forming a covalent bond. A covalent bond formed between a carbohydrate molecule and another molecule (in this case, between two monosaccharides) is known as a glycosidic bond (Figure 4). Glycosidic bonds (also called glycosidic linkages) can be of the alpha or the beta type. An alpha bond is formed when the OH group on the carbon-1 of the first glucose is below the ring plane, and a beta bond is formed when the OH group on the carbon-1 is above the ring plane.

In Figure 4, sucrose is formed when a monomer of glucose and a monomer of fructose are joined in a dehydration reaction to form a glycosidic bond. In the process, a water molecule is lost. By convention, the carbon atoms in a monosaccharide are numbered from the terminal carbon closest to the carbonyl group. In sucrose, a glycosidic linkage is formed between carbon 1 in glucose and carbon 2 in fructose.

The chemical structure of sucrose being formed when a monomer of glucose and a monomer of fructose are joined in a dehydration reaction to form a glycosidic bond. On the top of the image, it shows the chemical structure of Glucose and fructose, and on the bottom of the image, it shows sucrose and the glycosidic bond.
Figure 4

IN CONTEXT

Common disaccharides include lactose, maltose, and sucrose (Figure 5). Lactose is a disaccharide consisting of the monomers glucose and galactose. It is found naturally in milk. Maltose, or malt sugar, is a disaccharide formed by a dehydration reaction between two glucose molecules. The most common disaccharide is sucrose, or table sugar, which is composed of the monomers glucose and fructose.

Chemical structure of common disaccharides, maltose, lactose, and sucrose. The chemical structures are organized vertically. On the top is maltose, in the center is lactose, and on the bottom is sucrose.
Figure 5. Common disaccharides include maltose (grain sugar), lactose (milk sugar), and sucrose (table sugar).

term to know
Disaccharides
When two monosaccharides undergo a dehydration reaction. This is also known as a condensation reaction or dehydration synthesis.


4. Polysaccharides

A long chain of monosaccharides linked by glycosidic bonds is known as a polysaccharide (poly– = “many”). The chain may be branched or unbranched, and it may contain different types of monosaccharides. The molecular weight may be 100,000 daltons or more depending on the number of monomers joined. Starch, glycogen, cellulose, and chitin are primary examples of polysaccharides.

term to know
Polysaccharide
A long chain of monosaccharides linked by glycosidic bonds.

4a. Starch

Starch is the stored form of sugars in plants and is made up of a mixture of amylose and amylopectin (both polymers of glucose). Plants are able to synthesize glucose, and the excess glucose, beyond the plant’s immediate energy needs, is stored as starch in different plant parts, including roots and seeds. The starch in the seeds provides food for the embryo as it germinates and can also act as a source of food for humans and animals. The starch that is consumed by humans is broken down by enzymes, such as salivary amylases, into smaller molecules, such as maltose and glucose. The cells can then absorb the glucose.

IN CONTEXT

Starch is made up of glucose monomers that are joined by α 1-4 or α 1-6 glycosidic bonds. The numbers 1-4 and 1-6 refer to the carbon number of the two residues that have joined to form the bond. As illustrated in Figure 6, amylose is starch formed by unbranched chains of glucose monomers (only α 1-4 linkages), whereas amylopectin is a branched polysaccharide (α 1-6 linkages at the branch points).

Chemical structure of amylose and amylopectin. The image shows the unbranched chain of glucose monomers.
Figure 6

Amylose and amylopectin are two different forms of starch. Amylose is composed of unbranched chains of glucose monomers connected by α 1,4 glycosidic linkages. Amylopectin is composed of branched chains of glucose monomers connected by α 1,4 and α 1,6 glycosidic linkages. Because of the way the subunits are joined, the glucose chains have a helical structure. Glycogen (not shown) is similar in structure to amylopectin but more highly branched.

term to know
Amylose
Unbranched chains of glucose monomers connected by α 1,4 glycosidic linkages.

4b. Glycogen

Glycogen is the storage form of glucose in humans and other vertebrates and is made up of monomers of glucose. Glycogen is the animal equivalent of starch and is a highly branched molecule usually stored in liver and muscle cells. Whenever blood glucose levels decrease, glycogen is broken down to release glucose in a process known as glycogenolysis.

Cellulose is the most abundant natural biopolymer. The cell wall of plants is mostly made of cellulose; this provides structural support to the cell. Wood and paper are mostly cellulosic in nature. Cellulose is made up of glucose monomers that are linked by β 1-4 glycosidic bonds (Figure 7).

Image that shows the chemical structure of cellulose fibers and cellulose structure. On top are the cellulose fibers, and on the bottom is the cellulose chemical structure.
Figure 7

Figure 7 illustrates that in cellulose, glucose monomers are linked in unbranched chains by β 1-4 glycosidic linkages. Because of the way the glucose subunits are joined, every glucose monomer is flipped relative to the next one, resulting in a linear, fibrous structure. Also shown in Figure 7, every other glucose monomer in cellulose is flipped over, and the monomers are packed tightly as extended long chains. This gives cellulose its rigidity and high tensile strength—which is so important to plant cells. While the β 1-4 linkage cannot be broken down by human digestive enzymes, herbivores such as cows, koalas, buffalos, and horses are able, with the help of the specialized flora in their stomach, to digest plant material that is rich in cellulose and use it as a food source. In these animals, certain species of bacteria and protists reside in the rumen (part of the digestive system of herbivores) and secrete the enzyme cellulase. The appendix of grazing animals also contains bacteria that digest cellulose, giving it an important role in the digestive systems of ruminants.

try it
Check out this article on the importance of your gut microbiota. https://pmc.ncbi.nlm.nih.gov/articles/PMC4425030/ The bacteria, or flora, in your gut help break down the cellulose that would otherwise be indigestible. There are MANY other roles that the bacteria in your gut play so eating probiotics and keeping a healthy microbiota is important. Also, as a part of cellular respiration, the bacteria produce gas. That gas in your gut has to escape somehow! Where do you think it goes?

key concept
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. Carbohydrates are classified as monosaccharides, disaccharides, and polysaccharides depending on the number of monomers in the molecule. Monosaccharides are linked by glycosidic bonds that are formed as a result of dehydration reactions, forming disaccharides and polysaccharides with the elimination of a water molecule for each bond formed. Glucose, galactose, and fructose are common monosaccharides, whereas common disaccharides include lactose, maltose, and sucrose. Starch and glycogen, examples of polysaccharides, are the storage forms of glucose in plants and animals, respectively. The long polysaccharide chains may be branched or unbranched. Cellulose is an example of an unbranched polysaccharide, whereas amylopectin, a constituent of starch, is a highly branched molecule. Storage of glucose, in the form of polymers like starch or glycogen, makes it slightly less accessible for metabolism; however, this prevents it from leaking out of the cell or creating a high osmotic pressure that could cause excessive water uptake by the cell.

terms to know
Glycogen
The storage form of glucose in humans and other vertebrates. It is made up of monomers of glucose.
Cellulose
The most abundant natural biopolymer.

summary
In this lesson, you learned about the structure of carbohydrates and how they are important for our bodies. Carbohydrates are a vital energy source for our bodies. There are three different types of carbohydrates: monosaccharides, disaccharides, and polysaccharides. You also learned about starch and glycogen, two examples of polysaccharides.

Source: THIS TUTORIAL HAS BEEN ADAPTED FROM LUMEN LEARNING’S “NUTRITION FLEXBOOK”. ACCESS FOR FREE AT https://courses.lumenlearning.com/suny-nutrition/. LICENSE: creative commons attribution 4.0 international.

Terms to Know
Aldehyde Group

The functional group with the structure R-CHO.

Amylose

Unbranched chains of glucose monomers connected by α 1,4 glycosidic linkages.

Cellulose

The most abundant natural biopolymer.

Disaccharides

When two monosaccharides undergo a dehydration reaction. This is also known as a condensation reaction or dehydration synthesis.

Glycogen

The storage form of glucose in humans and other vertebrates. It is made up of monomers of glucose.

Ketone Group

The functional group with the structure RC(=O)R'.

Monosaccharides

Simple sugars, the most common of which is glucose.

Polysaccharide

A long chain of monosaccharides linked by glycosidic bonds.