Chapter 7: Biomolecules (Organic Molecules)

7.1. Carbohydrates

Learning Objectives

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

  • Discuss the role of carbohydrates in cells and in the extracellular materials of animals and plants.
  • Explain carbohydrate classifications.
  • List common monosaccharides, disaccharides, and polysaccharides.

Carbohydrates are classed in three groups. The single unit building block units (monomers) are called monosaccharides, disaccharides (two monosaccharides joined in a covalent bond called the glycosidic bond), and polysaccharides (‘many’ carbohydrate monomers joined in glycosidic bonds)

Nutrient carbohydrates provide a source of cellular energy. They provide the building block units used by cells to make the energy molecule, ATP.

Molecular Structures

The formula (CH2O)n, where n is the number of carbons in the molecule represents carbohydrates. 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 classed into three subtypes: monosaccharides, disaccharides, and polysaccharides.

Monosaccharides

Monosaccharides (mono- = “one”; sacchar- = “sweet”) are simple sugars and may contain three to seven C atoms. Most monosaccharide names end with the suffix -ose. .

The three most common simple sugars (monosaccharides) are hexoses (6-Carbon monosaccharides:  glucose, fructose, and galactose Figure 7.1.1. shows the structural formulas the molecules as linear chains.

The molecular structures of the linear forms of glucose, galactose, and fructose are shown. Glucose and galactose are both aldoses with a carbonyl group (carbon double-bonded to oxygen) at one end of the molecule. A hydroxyl (OH) group is attached to each of the other residues. In glucose, the hydroxyl group attached to the second carbon is on the left side of the molecular structure and all other hydroxyl groups are on the right. In galactose, the hydroxyl groups attached to the third and fourth carbons are on the left, and the hydroxyl groups attached to the second, fifth and sixth carbon are on the right. Frucose is a ketose with C doubled bonded to O at the second carbon. All other carbons have hydroxyl groups associated with them. The hydroxyl group associated with the third carbon is on the left, and all the other hydroxyl groups are on the right.
Figure 7.1.1: The three hexose simple sugars.
All three hexose sugars are structural isomers because they have the same chemical formula C6H12O6 but show different atom arrangements. The many hydroxyl functional groups in sugars make then highly soluble water. The structures also differ in the positions of the carbonyl-containing functional groups. Note that the carbonyl-containing functional groups of glucose and galactose are is at the end of the chain, an aldehyde. The carbonyl group in fructose is a ketone functional group because it is located between two other C-atoms. Aldehyde sugars are classed as aldoses and ketone sugars are classed as ketoses.
Glucose is an important source of energy. During cellular respiration, energy releases from glucose, and that energy helps make adenosine triphosphate (ATP). Galactose (found in milk sugar) and fructose (present in fruit) are other common monosaccharides.
Another important monomer is a five-Carbon sugar ribose, the sugar component of nucleic acids. Figure 7.1.2. shows the ribose sugar in its ring structure.

Monosaccharides can exist as a linear chain or as ring-shaped molecules. In aqueous solutions, linear and ring forms occur. (Figure 7.1.2.).

The conversion of glucose between linear and ring forms is shown. The glucose ring has five carbons and an oxygen. In alpha glucose, the first hydroxyl group is locked in a down position, and in beta glucose, the ring is locked in an up position. Structures for ring forms of ribose and fructose are also shown. Both sugars have a ring with four carbons and an oxygen.
Figure 7.1.2: Ring forms of five- and six- Carbon monosaccharides.

Disaccharides

Disaccharides (di- = “two”) form when two monosaccharides undergo a dehydration synthesis reaction joining to simple sugars by their glycosidic bond. Figure 7.1.3. shows the formation of sucrose (table sugar) from the two monosaccharides, glucose and fructose

The formation of sucrose from glucose and fructose is shown. In sucrose, the number one carbon of the glucose ring is connected to the number two carbon of fructose via an oxygen.
Figure 7.1.3: Formation of table sugar (sucrose).  This disaccharide is formed from the two monosaccharides glucose and fructose in a dehydration synthesis reaction.

In addition to sucrose, the two other common disaccharides are maltose (malt or grain sugar) and galactose (milk sugar). Figure 7.1.4. shows the monomers; monosaccharides that compose each of the three common disaccharides.

The chemical structures of maltose, lactose, and sucrose are shown. Both maltose and lactose are made from two glucose monomers joined together in ring form. In maltose, the oxygen in the glycosidic bond points downward. In lactose, the oxygen in the glycosidic bond points upward. Sucrose is made from glucose and fructose monomers. The oxygen in the glycosidic bond points downward.
Figure 7.1.4:  Disaccharides maltose (grain sugar), lactose (milk sugar), and sucrose (table sugar).  Note the different monomers that form each disaccharide.

Polysaccharides

Starch, glycogen and cellulose are long-chain repeating units (polymers) of glucose units held by glycosidic bonds (Figure 7.1.5.)

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 7.1.5: Polysaccharides: Starch, glycogen and cellulose.

Starch (Figure 7.1.5.(a)) is the storage form of energy in plants and provides a source of human energy when the ingested starch is hydrolyzed to its monomers, glucose. Starch exists in two forms: amylose and amylopectin which vary by their branching patterns. The ratio of the two forms in our diet are of interest to nutritionists because they differ in their glycemic index, a topic of concern to people who must watch their blood sugar glucose levels.

Glycogen (Figure 7.1.5.(b)), also a highly branched polysaccharide, is the form in which humans store glucose energy.   It is stored in organs whose cells  have a high metabolic rate and there high energy needs, liver and muscle. When cell uptake of glucose increases (reflecting energy demand) blood glucose levels decrease. To keep the blood glucose supply available for cell needs, hormones are released stimulating glycogen breakdown with release the simple sugar into the blood. 

Cellulose (Figure 7.1.5.(c)), is a structural polysaccharide in plants, giving strength to its cell wall. The orientation of glycosidic bonds in cellulose differ from the orientation occurring in starch. As a result, human cannot digest cellulose to its monomers. Even though cellulose is not used for energy production by cells, it is important as nondigestible bulk or dietary fiber.

License and attributions:

  • Biology for AP Courses, 2018, Zedalis, J. et al. License: CC BY 4.0. Located at https://openstax.org/books/biology-ap-courses/pages/3-2-carbohydrates
  • Microbiology, 2016, Parker, N. et al. License: CC BY 4.0. Located at https://openstax.org/books/microbiology/pages/7-2-carbohydrates

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BIO130: Introduction to Physiology Copyright © 2024 by Dinor Dhanabala; Sandra Fraley; and Gordon Lake is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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