- Describe the general structures of the simple sugars glucose and fructose.
- Describe how two simple sugars can be linked to form a disaccharide.
- Compare the simple sugar components of sucrose, lactose, and maltose.
- Identify the structure and function of the polysaccharides starch, glycogen, and cellulose.
Check Your Understanding
Recalling Prior Knowledge
- What are aldehydes, ketones, and alcohols?
- What is the relationship between monomers and polymers?
Carbohydrates constitute a class of compounds that serves many functions in living organisms. Among the most important are the storage of energy in the form of starch and glycogen. Carbohydrates can also act as structural components, such as cellulose in plants, and are a part of the important genetic molecules DNA and RNA. In this chapter, you will learn about the basic structures and functions of carbohydrates.
Some foods that are high in carbohydrates include bread, pasta, and potatoes. Endurance athletes often prepare for competition by consuming large amounts of carbohydrate-heavy foods to allow them to perform at a high level for long periods of time (Figure below).
Bread, potatoes, pasta, and vegetables are high-carbohydrate foods that endurance athletes often consume in large quantities in a procedure called carbo-loading.
The term carbohydrate comes from the fact that the majority of carbohydrates contain carbon, hydrogen, and oxygen in a ratio of 1:2:1, making for an empirical formula of CH2O. This is somewhat misleading, because the molecules are not actually hydrates of carbon at all. Carbohydrates are monomers and polymers of aldehydes and ketones that have multiple hydroxyl groups attached.
Carbohydrates are the most abundant source of energy found in most foods. The simplest carbohydrates are also called simple sugars and are plentiful in fruits. A monosaccharide is a carbohydrate consisting of one sugar unit. Common examples of simple sugars or monosaccharides are glucose and fructose. Glucose is also referred to as corn sugar or grape sugar and is abundant in plants and animals. Fructose occurs in many fruits and is also found in honey. They are structural isomers of one another. Glucose has a carbonyl on the end of its chain, so it is an aldehyde, while the carbonyl of fructose is on an interior carbon, making it a ketone (Figure below).
Glucose and fructose are monosaccharides, or simple sugars.
Glucose and fructose are both very soluble in water. In aqueous solution, the predominant forms are not the straight-chain structures shown in Figure above. Rather, they adopt a cyclic structure (Figure below). Glucose forms a six-membered ring, while fructose forms a five-membered ring. Both rings contain an oxygen atom.
The cyclic form of sugars is the favored form in aqueous solution.
Simple sugars form the foundation for more complex carbohydrates. The cyclic forms of two sugars can be linked together by means of a condensation reaction. Figure below shows how a glucose molecule and a fructose molecule combine to form a sucrose molecule. A hydrogen atom from one molecule and a hydroxyl group from the other molecule are eliminated as water, resulting in a covalent bond linking the two sugars together at that point.
Glucose and fructose combine to produce the disaccharide sucrose in a condensation reaction.
Sucrose, commonly known as table sugar, is an example of a disaccharide. A disaccharide is a carbohydrate formed by the joining of two monosaccharides. Other common disaccharides include lactose and maltose. Lactose, a component of milk, is formed from glucose and galactose, while maltose is made from two glucose molecules.
Many simple sugars can combine by repeated condensation reactions until a very large molecule is formed. A polysaccharide is a complex carbohydrate polymer formed from the linkage of many monosaccharide monomers. One of the best known polysaccharides is starch, the main form of energy storage in plants. Starch is a staple in most human diets; foods such as corn, potatoes, rice, and wheat have very high starch contents. Starch is made of glucose monomers and occurs in both straight-chain and branched forms (Figure below). The straight-chain form is referred to as amylose and consists of hundreds of linked glucose molecules. The branched form of starch is called amylopectin.
Amylose and amylopectin are the two most common components of naturally occurring starch. Both consist of many glucose monomers connected into a polymer. Plants store their energy in the form of starch.
Glycogen is an even more highly branched polysaccharide of glucose monomers that is used to store energy in animals (Figure below). Glycogen is made and stored primarily in the cells of the liver and muscles.
Glycogen is a branched polymer of glucose and stores energy in animals.
Cellulose is another polymer of glucose, consisting of anywhere from hundreds to over ten thousand monomers. It is the structural component of the cell walls in green plants and is the single most common organic molecule on Earth. Roughly 33% of all plant matter is cellulose. The linkage structure in cellulose is different than that of starch, and as a result, cellulose is indigestible except by a few microorganisms, some of which live in the digestive tracts of cattle and termites. Figure below shows a triple strand of cellulose. There is no branching and the fibers adopt a very stiff rod-like structure with numerous hydrogen bonds between the fibers, adding to its strength. Cellulose is the main component of paper, cardboard, and textiles made from cotton, linen, and other plant fibers.
Cellulose is composed of very long strands of glucose monomers that are hydrogen bonded to one another. Cellulose is largely indigestible and comprises the cell walls of plants.
- Carbohydrates are used to store energy in plants and animals, and they are a major component of many foods.
- Simple sugars or monosaccharides are organic molecules containing multiple hydroxyl groups and either an aldehyde or a ketone. Glucose and fructose are two important simple sugars.
- Two monosaccharides can be joined together in a condensation reaction to form a disaccharide, such as sucrose, lactose, or maltose.
- Starch is a polysaccharide of glucose monomers that stores energy in plants. Glycogen stores energy in animals.
- Cellulose is another polysaccharide of glucose, but it is indigestible to most organisms and serves as a structural component in plants.
Lesson Review Questions
- What is the origin of the term carbohydrate, and why is it potentially misleading?
- What are some of the main functions of carbohydrates in nature?
- What functional groups are present in glucose? In fructose?
- Describe the reaction by which monosaccharides combine to form a disaccharide.
- Which monosaccharides combine to form the following disaccharides?
- What are the two main components of starch? Describe their differences.
- What is the difference in function between starch and glycogen?
- Starch is digestible while cellulose is not. Explain this difference.
- Write a molecular equation for the formation of sucrose from simple sugars.
- 20.0 g of sucrose is dissolved in 100.0 mL of water.
- What is the molarity of the solution?
- If the sucrose were completely hydrolyzed, what mass of glucose could be isolated?
Further Reading / Supplemental Links
Points to Consider
Proteins are polymers of another class of biomolecules called amino acids.
- How many naturally occurring amino acids are used to form proteins?
- What is the function of an enzyme in biological reactions?