Carbohydrates are made of carbon, hydrogen, and oxygen atoms. Many different monosaccharides, or simple sugars, can combine into polysaccharides, or complex carbohydrates. Even though they have a bad reputation among some diet plans, carbohydrates perform many essential functions for cells. In this chapter, I present the basic structure of carbohydrates and explain their importance to cells.
CH2O: Structure of Carbohydrates
In recent years, due to the comeback of the low- carb diet, carbohydrates have gotten a bad rap. Some people have started thinking that proteins are good, and carbohydrates are bad. However, the idea that carbohydrates aren’t good for you is overly simplified. After all, carbohydrates are an essential component of your cells. What can make a difference is the type of carbohydrates you eat. Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen. The two main types of carbohydrates are as follows:
Monosaccharides are also called simple sugars. (Most diets recommend that you avoid eating too much of this type of carbohydrate.) Glucose is a monosaccharide that is usually available to your cells.
Polysaccharides are also called complex carbohydrates. (Fiber is an example of a complex carbohydrate that is a recommended part of your daily nutrition.)
Keeping it simple: Monosaccharides
Monosaccharides, or simple sugars, are single sugars. (“Mono” means “one” and “sacchar” means sugar.) Many monosaccharides have the generic formula CH2O: For every one carbon atom they have, they have two hydrogen atoms and one oxygen atom. Two monosaccharides that may be familiar to you are glucose (see Figure 5-1A) and fructose (a sugar found in fruit and also in high-fructose corn syrup).
All monosaccharides have certain features in common:
A backbone of 3, 4, 5, 6, or 7 carbons. Sugars are categorized based on the number of carbons: In order of the numbers, they are called trioses, tetroses, pentoses, hexoses, and heptoses. For example, glucose is a hexose, or 6-carbon sugar.
Hydroxyl groups (–OH) attached to every carbon but one. The hydroxyl groups make sugars polar, which is why they dissolve easily in water.
One double-bonded oxygen attached to the carbon backbone. An oxygen double-bonded to a carbon is called a carbonyl group. If the carbonyl group is located at the end of a monosaccharide, the sugar is an aldose. If the carbonyl group is located within the carbon backbone, the sugar is a ketose. Glucose is an aldose because its carbonyl group is at the end of the carbon backbone.
Of the four groups of macromolecules (carbohydrates, proteins, nucleic acids, and lipids), carbohydrates have the greatest number of hydroxyl groups (–OH) attached to their carbon atoms. When you’re trying to distinguish between the four types of macromolecules, a structure with hydroxyl groups attached to almost every carbon is probably a carbohydrate.
Two monosaccharides can have the same numbers of carbon, hydrogen, and oxygen atoms and yet have very different properties. When two monosaccharides have the same atoms, but those atoms are arranged differently, the sugars are isomers of each other (“iso” means same). For example, if the hydroxyl group (–OH) and hydrogen atom (–H) attached to the fourth carbon from the top in glucose (see Figure 5-1A) were swapped with each other, the sugar would be converted to galactose. Glucose and galactose are almost identical, except for that one change in the arrangement of the atoms, and yet they behave very differently in cells.
The way the atoms are bonded together is very important in the structure and function of sugars. Isomers are made from exactly the same atoms, but their atoms are arranged differently.
In the watery environment of the cell, monosaccharides convert into ringshaped structures. A bond forms between two atoms in the backbone of the sugar, causing the sugar to bend around to form the ring. As an example, compare the linear structure of glucose shown in Figure 5-1A with the ring structure shown in Figure 5-1B.
Making it complex: Polysaccharides
Polysaccharides, or complex carbohydrates, are polymers (see Chapter 4) of monosaccharides. (“Poly” means many, and “sacchar” means sugar, so a polysaccharide is “many sugars” strung together.) To make polysaccharides, monosaccharides are joined together by condensation reactions (see Chapter 4). During condensation, a water molecule is removed as a bond is formed between an atom in the growing polysaccharide chain and an atom in
the monosaccharide that is being added to the chain (see Figure 5-1B). The bonds between monosaccharides are called glycosidic linkages.
Polysaccharides are classified based on the number of monosaccharides in the chain:
Disaccharides are chains of two monosaccharides. Sucrose (see Figure
5-1B), or table sugar, is a disaccharide that is probably very familiar to you. Another disaccharide you probably know about is lactose, the sugar
found in milk.
Oligosaccharides are short chains of monosaccharides (see Figure 5-1C). Oligosaccharides are part of receptors in the plasma membranes of your cells.
Polysaccharides are long chains of monosaccharides (see Figure 5-1D). Starch and cellulose, both shown in Figure 5-2, are two polysaccharides that you probably eat every day. Starch is found in bread, potatoes, rice, and pasta; cellulose is referred to as fiber in your diet.
Many cell types produce polysaccharides. Starch and cellulose, which are made by plants, are both polymers of glucose. Glycogen, made by animal cells, is also a polymer of glucose. Chitin, found in the shells of crustaceans and insects, is a polymer of a nitrogen-containing monosaccharide called N-acetylglucosamine. Peptidoglycan, the polysaccharide found in bacterial cell walls (see Chapter 2), is a polymer of two alternating monosaccharides, N-acetylglucosamine and N-acetylmuramic acid.
Polysaccharides can also be different based on how their monosaccharides are strung together. Starch, cellulose, and glycogen are all made entirely of glucose, yet they behave very differently in the body. Starch and glycogen are easily broken down in the human digestive system. Cellulose, or fiber, can’t be broken down at all by humans. Instead, it passes right through your digestive system and exits as part of your wastes.
The difference between starch, cellulose, and glycogen isn’t what they’re made of, but rather in the bonds between the glucose molecules:
The glucose molecules in starch are joined with a bond called a α–1,4–glycosidic linkage.
The glucose molecules in cellulose are joined with a β–1,4–glycosidic linkage.
At approximately every tenth glucose molecule, a branch is joined to the main backbone of glycogen by an α–1,6–glycosidic linkage. Thus, glycogen molecules are highly branched.
The reason humans can digest starch and glycogen, but not cellulose, is that human enzymes can break down some glycosidic linkages, but not others. Human enzymes break down α –1,4–glycosidic linkages and α –1,6–glycosidic linkages, but not β –1,4–glycosidic linkages. Together, starch, cellulose, and glycogen demonstrate how important different types of glycosidic linkages can be to polysaccharide structure and function.
The type of glycosidic linkage between monosaccharides is very important in determining structure and function of polysaccharides.
Functions of Carbohydrates
Carbohydrates are probably most famous for their role in providing energy to bodies (and, of course, cells), but they perform many other important functions for cells as well:
Carbohydrates are an important energy source for cells. The monosaccharide glucose is a rapidly used energy source for almost all cells on planet Earth. In addition, many cell types store matter and energy for later use in the form of polysaccharides. Plants, algae, and bacteria store energy in starch, and animals and bacteria store energy in glycogen.
Carbohydrates are important structural molecules for cells. Polysaccharides are the major components of the cell walls of plants, algae, fungi, and bacteria. The cell walls of plants and algae contain cellulose, the cell walls of fungi contain chitin, and the cell walls of bacteria contain peptidoglycan.
Carbohydrates are important markers of cellular identity. The surfaces of cells are marked with glycoproteins, molecules of protein that have an attached sugar. Different cells have different glycoproteins on their surface, marking the cells with their identity. In your body, liver cells are marked as liver cells, heart cells are marked as heart cells, nerve cells are marked as nerve cells, and so on.
Carbohydrates are important extracellular molecules. Polysaccharides are a major component of the sticky matrix that surrounds cells. They help bacteria stick to surfaces
