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7.2 Carbohydrates

Microbiology7.2 Carbohydrates

Learning Objectives

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

  • Give examples of monosaccharides and polysaccharides
  • Describe the function of monosaccharides and polysaccharides within a cell

The most abundant biomolecules on earth are carbohydrates. From a chemical viewpoint, carbohydrates are primarily a combination of carbon and water, and many of them have the empirical formula (CH2O)n, where n is the number of repeated units. This view represents these molecules simply as “hydrated” carbon atom chains in which water molecules attach to each carbon atom, leading to the term “carbohydrates.” Although all carbohydrates contain carbon, hydrogen, and oxygen, there are some that also contain nitrogen, phosphorus, and/or sulfur. Carbohydrates have myriad different functions. They are abundant in terrestrial ecosystems, many forms of which we use as food sources. These molecules are also vital parts of macromolecular structures that store and transmit genetic information (i.e., DNA and RNA). They are the basis of biological polymers that impart strength to various structural components of organisms (e.g., cellulose and chitin), and they are the primary source of energy storage in the form of starch and glycogen.

Monosaccharides: The Sweet Ones

In biochemistry, carbohydrates are often called saccharides, from the Greek sakcharon, meaning sugar, although not all the saccharides are sweet. The simplest carbohydrates are called monosaccharides, or simple sugars. They are the building blocks (monomers) for the synthesis of polymers or complex carbohydrates, as will be discussed further in this section. Monosaccharides are classified based on the number of carbons in the molecule. General categories are identified using a prefix that indicates the number of carbons and the suffix –ose, which indicates a saccharide; for example, triose (three carbons), tetrose (four carbons), pentose (five carbons), and hexose (six carbons) (Figure 7.8). The hexose D-glucose is the most abundant monosaccharide in nature. Other very common and abundant hexose monosaccharides are galactose, used to make the disaccharide milk sugar lactose, and the fruit sugar fructose.

Diagrams of various monosaccharides. Glyceraldehyde is an aldose because it has a double bonded O attached to an end carbon. Dihydroxyacetone is a ketose because it has a double bonded O attached in the center of the chain. Glyceraldehyde is a triose because it has 3 carbons. Ribose is a pentose because it has 5 carbons. Glucose is a hexose because it has 6 carbons.
Figure 7.8 Monosaccharides are classified based on the position of the carbonyl group and the number of carbons in the backbone.

Monosaccharides of four or more carbon atoms are typically more stable when they adopt cyclic, or ring, structures. These ring structures result from a chemical reaction between functional groups on opposite ends of the sugar’s flexible carbon chain, namely the carbonyl group and a relatively distant hydroxyl group. Glucose, for example, forms a six-membered ring (Figure 7.9).

a) a diagram showing how a linear carbohydrate forms a ring. Glucose has 6 carbons; Carbon 1 has a double bonded O. Carbon 5 has an OH group. After the ring forms, Carbon 1 is attached to the O with a single bond and this O is now also attached to carbon 5. B) shows the final structure which is a hexagon shape. The top right corner is an O, the next 5 corners are Cs and the C at the top left is attached to another C that projects upward from the ring.
Figure 7.9 (a) A linear monosaccharide (glucose in this case) forms a cyclic structure. (b) This illustration shows a more realistic depiction of the cyclic monosaccharide structure. Note in these cyclic structural diagrams, the carbon atoms composing the ring are not explicitly shown.

Check Your Understanding

  • Why do monosaccharides form ring structures?


Two monosaccharide molecules may chemically bond to form a disaccharide. The name given to the covalent bond between the two monosaccharides is a glycosidic bond. Glycosidic bonds form between hydroxyl groups of the two saccharide molecules, an example of the dehydration synthesis described in the previous section of this chapter:


Common disaccharides are the grain sugar maltose, made of two glucose molecules; the milk sugar lactose, made of a galactose and a glucose molecule; and the table sugar sucrose, made of a glucose and a fructose molecule (Figure 7.10).

Maltose is made of 2 glucose molecules linked with O from Carbon 4 of one glucose to carbon 1 of the other. Lactose is made of a glucose linked to a galactose. Carbon 4 of glucose is linked to carbon 1 of galactose. Sucrose is made of a glucose and a fructose. Carbon 1 of glucose is bound to carbon 2 of fructose.
Figure 7.10 Common disaccharides include maltose, lactose, and sucrose.


Polysaccharides, also called glycans, are large polymers composed of hundreds of monosaccharide monomers. Unlike mono- and disaccharides, polysaccharides are not sweet and, in general, they are not soluble in water. Like disaccharides, the monomeric units of polysaccharides are linked together by glycosidic bonds.

Polysaccharides are very diverse in their structure. Three of the most biologically important polysaccharides—starch, glycogen, and cellulose—are all composed of repetitive glucose units, although they differ in their structure (Figure 7.11). Cellulose consists of a linear chain of glucose molecules and is a common structural component of cell walls in plants and other organisms. Glycogen and starch are branched polymers; glycogen is the primary energy-storage molecule in animals and bacteria, whereas plants primarily store energy in starch. The orientation of the glycosidic linkages in these three polymers is different as well and, as a consequence, linear and branched macromolecules have different properties.

Modified glucose molecules can be fundamental components of other structural polysaccharides. Examples of these types of structural polysaccharides are N-acetyl glucosamine (NAG) and N-acetyl muramic acid (NAM) found in bacterial cell wall peptidoglycan. Polymers of NAG form chitin, which is found in fungal cell walls and in the exoskeleton of insects.

Amylose is a chain of hexagons. Starch is a branching chain of hexagons. Glycogen is a highly branching chain of hexagons. Cellulose (fiber) is many rows of hexagons attached into a flat square. Micrographs of starch look like water bubbles, glycogen look like ovals, and cellulose look like long strands.
Figure 7.11 Starch, glycogen, and cellulose are three of the most important polysaccharides. In the top row, hexagons represent individual glucose molecules. Micrographs (bottom row) show wheat starch granules stained with iodine (left), glycogen granules (G) inside the cell of a cyanobacterium (middle), and bacterial cellulose fibers (right). (credit “iodine granules”: modification of work by Kiselov Yuri; credit “glycogen granules”: modification of work by Stöckel J, Elvitigala TR, Liberton M, Pakrasi HB; credit “cellulose”: modification of work by American Society for Microbiology)

Check Your Understanding

  • What are the most biologically important polysaccharides and why are they important?
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