13 Carbohydrates
Carbohydrates are macromolecules with which most consumers are somewhat familiar. To lose weight, some individuals adhere to “low-carb” diets. Athletes, in contrast, often “carb-load” before important competitions to ensure that they have sufficient energy to compete at a high level. Carbohydrates are, in fact, an essential part of our diet; grains, fruits, and vegetables are all natural sources of carbohydrates. Carbohydrates provide energy to the body, particularly through glucose, a simple sugar. Carbohydrates also have other important functions in humans, animals, and plants.
Carbohydrates can be represented by the stoichiometric formula (CH2O)n, where n is the number of carbons in the molecule. 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 classified into three subtypes: monosaccharides, disaccharides, and polysaccharides.
Monosaccharides
Monosaccharides (mono- = “one”; sacchar- = “sweet”) are simple sugars, the most common of which is glucose. In monosaccharides, the number of carbons usually ranges from three to seven. Most monosaccharide names end with the suffix -ose.
The chemical formula for glucose is C6H12O6. In humans, glucose is an important source of energy. During cellular respiration, energy is released from glucose, and that energy is used to help make adenosine triphosphate (ATP). Plants synthesize glucose using carbon dioxide and water, and glucose in turn is used for energy requirements for the plant. Excess glucose is often stored as starch that is catabolized (the breakdown of larger molecules by cells) by humans and other animals that feed on plants.
Galactose (part of lactose, or milk sugar) and fructose (found in sucrose, in fruit) are other common monosaccharides. Although glucose, galactose, and fructose all have the same chemical formula (C6H12O6), they differ structurally and chemically (and are known as isomers) because of the different arrangement of functional groups around the asymmetric carbon; all of these monosaccharides have more than one asymmetric carbon. Within one monosaccharide, all of the atoms are connected to each other with strong covalent bonds.
Figure 13.2 Image Description
The image shows several common monosaccharides, which are the simplest type of sugar. Each sugar is represented as a ring of carbon and oxygen atoms with attached hydrogen and other groups. The sugars include glucose, fructose, and galactose. Glucose and galactose have six-membered rings with oxygen and carbon atoms, while fructose has a five-membered ring with oxygen. Labels identify each sugar by name, and the diagram uses lines for chemical bonds and letters for atoms (C for carbon, H for hydrogen, O for oxygen). This picture helps show that these sugars have similar components arranged in slightly different ring structures, which affects how they function in biology.
Disaccharides
Disaccharides (di- = “two”) form when two monosaccharides undergo a dehydration reaction (also known as a condensation reaction or dehydration synthesis). During this process, the hydroxyl (OH) group of one monosaccharide combines with the hydrogen of another monosaccharide, releasing a molecule of water and forming a covalent bond which joins the two monosaccharides together.
Common disaccharides include lactose, maltose, and sucrose (Figure 13.3). Lactose is a disaccharide consisting of the monomers glucose and galactose. It is formed by a dehydration reaction between the glucose and the galactose molecules, which removes a water molecule and forms a covalent bond. connected by a covalent bond. It is found naturally in milk. Maltose, or malt sugar, is a disaccharide composed of two glucose molecules connected by a covalent bond. The most common disaccharide is sucrose, or table sugar, which is composed of the monomers glucose and fructose, also connected by a covalent bond.
Figure 13.3 Image Description
The image shows several common disaccharide sugars, which are molecules made by joining two simpler sugar units. Each disaccharide is drawn as two sugar rings connected by a line that represents the chemical bond between them. The disaccharides shown include maltose, sucrose, and lactose. Each sugar ring is labeled with its name or with chemical symbols for carbon, hydrogen, and oxygen atoms. The diagram helps illustrate that these sugars are built from two monosaccharides linked together, showing the structure and how the individual sugar units are joined.
Polysaccharides
A long chain of monosaccharides linked by glycosidic bonds is known as a polysaccharide (poly- = “many”). The chain may be branched or unbranched, and it may contain different types of monosaccharides. All of the monosaccharides are connected together by covalent bonds. The molecular weight may be 100,000 daltons or more depending on the number of monomers joined. Starch, glycogen, cellulose, and chitin are primary examples of polysaccharides.
Starch is the stored form of sugars in plants and is made up of a mixture of amylose and amylopectin (both polymers of glucose). Basically, starch is a long chain of glucose monomers. Plants are able to synthesize glucose, and the excess glucose, beyond the plant’s immediate energy needs, is stored as starch in different plant parts, including roots and seeds. The starch in the seeds provides food for the embryo as it germinates and can also act as a source of food for humans and animals. The starch that is consumed by humans is broken down by enzymes, such as salivary amylases, into smaller molecules, such as maltose and glucose. The cells can then absorb the glucose.
Glycogen is the storage form of glucose in humans and other vertebrates and is made up of monomers of glucose. Glycogen is the animal equivalent of starch and is a highly branched molecule usually stored in liver and muscle cells. Whenever blood glucose levels decrease, glycogen is broken down to release glucose in a process known as glycogenolysis.
Figure 13.4 Image Description
The image shows three common polysaccharides, which are large carbohydrate molecules made by linking many sugar units together. The sugars are drawn as repeating ring shapes joined in long chains. The polysaccharides shown include cellulose, starch, and glycogen. Cellulose is depicted as a straight chain of glucose units linked together, forming a rigid structure. Starch is shown as a chain that has occasional branches, while glycogen is shown as a highly branched chain with many side branches. Labels identify each structure by name and highlight the repeating units, showing how the sugar rings connect to form these larger molecules. This diagram helps illustrate that while all three are made of glucose units, they differ in how their chains are arranged, which affects their roles in biology.
Cellulose is the most abundant natural biopolymer. The cell wall of plants is mostly made of cellulose; this provides structural support to the cell. Wood and paper are mostly cellulosic in nature. Cellulose is made up of glucose monomers (Figure 13.5).
Carbohydrates serve various functions in different animals. Arthropods (insects, crustaceans, and others) have an outer skeleton, called the exoskeleton, which protects their internal body parts (as seen in the bee in Figure 13.6). This exoskeleton is made of the biological macromolecule chitin, which is a polysaccharide-containing nitrogen. It is made of repeating units of N-acetyl-β-d-glucosamine, a modified sugar. Chitin is also a major component of fungal cell walls; fungi are neither animals nor plants and form a kingdom of their own in the domain Eukarya.
How does carbohydrate structure relate to function?
Energy can be stored within the bonds of a molecule. Bonds connecting two carbon atoms or connecting a carbon atom to a hydrogen atom are high energy bonds. Breaking these bonds releases energy. This is why our cells can get energy from a molecule of glucose (C6H12O6).
Polysaccharides form long, fibrous chains which are able to build strong structures such as cell walls.
Video Transcript
[Narrator]: Carbohydrates are biomolecules that are composed of carbon, hydrogen and oxygen atoms in the ratio of 1:2:1. We can represent the proportion of these elements within carbohydrate molecules with the formula CH2O.
Most carbohydrates are characterized as either monosaccharides, disaccharides or polysaccharides. The term “saccharide” is just another word for sugar. The prefixes mono, di and poly refer to the number of sugars in the molecule. “Mono” means one, so a monosaccharide is a carbohydrate made of one unit of sugar. The prefix “di” means two, so a disaccharide is a carbohydrate made of two units of sugar. And “poly” means many, so a polysaccharide is made of many sugar units bonded together.
Let’s talk about monosaccharides first.
Monosaccharides are the building blocks, or monomers, of all carbohydrates. Common monosaccharides include glucose, fructose, and galactose. Glucose is by far the most abundant monosaccharide. It is water soluble, easily transported through an organism, and is the energy source for cellular respiration and the production of ATP. Fructose is the primary monosaccharide found in fruits and plants, and galactose is the primary monosaccharide found in milk.
All of these monosaccharides are six carbon sugars with the chemical formula C6H12O6. They can be depicted chemically as either straight chains or rings.
Disaccharides are formed when monosaccharides are joined together through dehydration reactions forming glycosidic linkages. Common disaccharides include maltose, which is made up of two glucose molecules; sucrose (also known as table sugar), which is made up of glucose and fructose; and lactose (or milk sugar) which contains glucose and galactose.
Polysaccharides are formed when glucose monomers link together to form long chains. These long chains of glucose units are ideal for storing energy. The chains can be straight or branched.
Plants store energy in the form of amylose, which has straight chains, or amylopectin, which is branched.
Animals differ from plants in that they store energy in the form of glycogen, which is a highly branched polysaccharide that can be broken down quickly to supply energy to tissues.
Other polysaccharides such as cellulose, chitin and peptidoglycan serve as structural molecules in organisms.
The most abundant polysaccharide is cellulose. Cellulose is a straight chain polymer of glucose like amylose, but it differs in the configuration of the bonds between the glucose units. Most organisms are unable to break these bonds and cannot use cellulose as a source of energy. Instead cellulose is used to add strength to plant cell walls.
Chitin is a structural polysaccharide found in animals and fungi. It makes up the exoskeleton of insects and crustaceans. Its unique properties are a result of chitin having amino groups attached to its sugar monomers.
Peptidoglycans are complex polysaccharides found in the cell walls of bacteria. The macromolecule is both flexible and rugged due to its structure. Each monomer of the polysaccharide has a peptide chain attached to it.
Often, we refer to carbohydrates as being either simple sugars or complex carbohydrates. Monosaccharides and disaccharides are commonly referred to as simple sugars. The term complex carbohydrates refers to the polysaccharides.
References
Unless otherwise noted, images on this page are licensed under CC-BY 4.0 by OpenStax.
OpenStax, Biology. OpenStax CNX. May 27, 2016 http://cnx.org/contents/s8Hh0oOc@9.10:QhGQhr4x@6/Biological-Molecules
