Lesson Explainer: Carbohydrates Biology • First Year of Secondary School

In this explainer, we will learn how to describe the structure of different carbohydrates and outline the roles of carbohydrates in organisms.

When you think of carbohydrates, you may think of starchy foods like bread, rice, pasta, and potatoes. And you would be correct! Starch is a type of carbohydrate and is a source of readily available energy in the human diet because it is easily digestible and broken down to release energy. Starch is common in these foods that come from plants because starch is one way that plant cells store the molecules they use to generate cellular energy.

Carbohydrates are biological macromolecules with several different functions. They play an important role as a source of energy and a form of energy storage in cells. They are also important structural molecules in the cell walls of plant cells and the cells of other organisms. Carbohydrates include simple sugars as well as complex carbohydrates.

A carbohydrate is a molecule that contains carbon, hydrogen, and oxygen atoms in a particular ratio. Carbo- means “carbon” and -hydrate means “water,” which we know is . The ratio that most carbohydrates follow is . This means that there is an “” number of carbon atoms and an “” number of water molecules arranged in a particular carbohydrate.

Certain carbohydrates, called complex carbohydrates, are macromolecules that are also referred to as polysaccharides. A polysaccharide is a type of polymer. You may know that a polymer is a large molecule made of several smaller repeating units, or parts, called monomers. The monomers that make up complex carbohydrates are sugars, which are also called monosaccharides.

The simplest carbohydrates are monosaccharides: Mono- means “one“, that is because one molecule of monosacchride represents a single sugar unit. Monosaccharides usually follow the ratio , where is a number between 3 and 7. For example, most hexose sugars have the chemical formula , which is equivalent to . Monosaccharides are generally soluble in water. Monosaccharides can be categorized by the number of carbon atoms they possess. Hexose sugars have 6 carbons each (hex- means “six”). Examples of hexose monosaccharides include glucose, fructose, and galactose. Pentose sugars have 5 carbon atoms each (pent- means “five”). The ribose and deoxyribose you find in nucleic acids are pentose monosaccharides. You may have noticed that many of these sugar-related terms rhyme with each other. That is because the suffix -ose is a word part that refers to sugar. The chemical structures of some monosaccharides are shown in Figure 1.

Glucose is particularly important to cells since it is one of the reactants in cellular respiration. A diagram illustrating cellular respiration is shown in Figure 2. The energy found within the bonds between the atoms in glucose molecules is released and transferred to ATP to power our other life processes:

Monosaccharides can be bonded together during a chemical reaction called a condensation reaction, also called dehydration synthesis. This occurs when the hydroxyl groups of two monosaccharides rearrange and join in what is called a “glycosidic bond” as shown in Figure 3. The reaction releases a water molecule, which is how the condensation reaction gets its name. To help you remember this, it may help to think of how water is released from the air when it “condenses” into droplets.

Two monosaccharides bonded together is called a “disaccharide.” Di- means “two.” Maltose, sucrose, and lactose are examples of disaccharides. Disaccharides follow the chemical formula , where . Sucrose is what we commonly use as table sugar. It consists of a molecule of glucose bonded with a molecule of fructose. Lactose is found in milk, and it consists of a molecule of glucose bonded with a molecule of galactose. Maltose is found in many grains, such as wheat and barley. It consists of two molecules of glucose bonded with each other. Table 1 summarizes the compositions of common disaccharides.

Example 1: Identifying the Type of Chemical Reaction That Occurs When Monosaccharides Bond

What type of reaction occurs when two or more monosaccharides join together?

Answer

Monosaccharides are the simplest carbohydrates. Three common monosaccharides are glucose, fructose, and galactose. Mono- means “one.” Two monosaccharides can join together to form a disaccharide. Di- means “two.” An example of a disaccharide is sucrose, or common table sugar. Sucrose is composed of a molecule of glucose bonded to a molecule of fructose. Many monosaccharides can join together to form a polysaccharide. Poly- means “many.” Some examples of common polysaccharides are starch, cellulose, and glycogen. All three of these polysaccharides are polymers of glucose monomers. In order to make a disaccharide or a polysaccharide, monosaccharides have to join together. The hydroxyl groups of two molecules () rearrange so that both molecules share a bond with one oxygen atom (). This means that two hydrogen atoms () and one oxygen atom () are removed. These atoms bond together into a molecule of water (), which is released. Because a molecule of water is released, this chemical reaction is called a “condensation reaction.”

This means that the type of reaction that occurs when two or more monosaccharides join together is a condensation reaction.

Example 2: Identifying Monosaccharides from a List of Carbohydrates

Which of the following is an example of a monosaccharide?

1. Lactose
2. Sucrose
3. Glucose
4. Maltose

Answer

A monosaccharide is a type of simple sugar. Disaccharides are also considered to be simple sugars, but disaccharides are made of two monosaccharides bonded together. Mono- means “one,” and di- means “two.” Monosaccharides can also be distinguished because they typically follow the chemical formula , while disaccharides follow the chemical formula , where .

Lactose is composed of two monosaccharides, glucose and galactose (each ), bonded together.

Sucrose is composed of two monosaccharides, glucose and fructose (each ), bonded together.

Maltose is composed of two monosaccharides, glucose and glucose (each ), bonded together.

This can be seen in the following:

So, the answer that is an example of a monosaccharide is glucose.

Monosaccharide monomers can also polymerize into larger polymer carbohydrates called polysaccharides. Polysaccharides are insoluble in water, so they are ideal for energy storage and also for use as structural components within the cell. Examples of polysaccharides include starch, glycogen, and cellulose. All three of these polysaccharides are made of several glucose molecules bonded together in specific arrangements.

The monomers in starch and glycogen are a different isomer of glucose than the monomers in cellulose. An isomer is a molecule with the same chemical formula but with a different arrangement of atoms. The monosaccharides glucose, galactose, and fructose are actually all isomers with the chemical formula . The isomers of glucose found in starch, glycogen, and cellulose are called “alpha-glucose” and “beta-glucose,” and they can be seen in Figure 4. Alpha-glucose and beta-glucose are almost exactly the same, except that the hydrogen and the hydroxyl attached to the first carbon (also called -1) are flipped.

Cellulose is a polysaccharide made of beta-glucose monomers, illustrated in Figure 5. Because of their structure, the beta-glucose molecules bond together in an alternating orientation. Glycosidic bonds form between the first (-1) and the fourth (-4) carbon atoms in the beta-glucose monomers, so they are called “1,4 beta glycosidic bonds.” This orientation creates long, rigid, unbranched chains. These chains easily form hydrogen bonds between them, allowing them to stack together into larger structures. Cellulose is an extremely tough material and is hard to break apart. For this reason, cellulose is well adapted for its function as a structural polysaccharide found in the walls of plant cells.

Starch and glycogen are made of alpha-glucose monomers, illustrated in Figure 6. The alpha-glucose monomers also form 1,4 glycosidic bonds, but all the molecules point in the same direction. Starch is made by plant cells, and it can occur in long branched or unbranched chains. Glycogen is made by animal cells, and it occurs in highly branched complexes. Where branches occur in these molecules, one monomer will form a 1,4 glycosidic bond and a 1,6 glycosidic bond with two other monomers, which allows two chains to join together.

Both starch and glycogen function as energy storage molecules. In order to release this energy from storage, the polysaccharides must first be broken apart into monosaccharides in a process called hydrolysis. Hydrolysis reactions, shown in Figure 7, use a molecule of water to break a bond apart. It is basically the exact opposite of a condensation reaction. Once they are broken down again, the monosaccharides can be used as reactants in cellular respiration.

Carbohydrates are molecules that function in energy storage and transfer as well as provide structure to certain types of cells. The different compositions of different carbohydrates make them adapted for their different functions. These structures and functions are summarized in Table 2.

Let’s review what we have learned in this explainer.