Carbohydrates and their Classification
Carbohydrates refer to neutral compounds of oxygen, hydrogen and carbon. They are mainly starches and sugars. Carbohydrates are the type of nutrients that supply the body with calories. Sugars are the simplest forms of carbohydrates while fiber and starches are the complex forms of carbohydrates. The body breaks most carbohydrates into their simplest forms, such as glucose to supply body cells with energy for cellular processes. There are five groups of carbohydrates: heterosaccharides, polysaccharides, trisaccharides, disaccharides and monosaccharides (Asp, 1996). Glycogen, starch, and cellulose are some examples of polysaccharides. Glucose is a monosaccharide. Polysaccharides range from intermediate to large in length while monosaccharides are the smallest in length. Simple carbohydrates are contained in specific food products. For instance, milk contains lactose, fruit contains fructose and table sugar contains sucrose.
Structure of Glucose
Glucose, chemically written as C6H12O6, is a commonly encountered monosaccharide. It is also known as blood sugar. Glucose consists of 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. Carbon has 4 bonds and forms the backbone of the glucose structure. Hydrogen has one bond and oxygen has 2 bonds. Hydrogen and oxygen are attached to carbon atoms, as illustrated on Figure 1 below (Englyst and Hudson, 1996).
Figure 1: Structure of Glucose
Formation of a Disaccharide
A disaccharide is formed when two single sugar units or monosaccharides are joined. For example, the joining of two molecules of alpha glucose forms maltose. Monosaccharides are formed through a reaction called condensation. The reaction needs energy and results in release of a water molecule. A combination of two alpha glucose molecules through condensation results in the formation of two glycosidic bonds which hold the two glucose molecules together (Englyst and Hudson, 1996). The glycosidic bond forms between carbon 1 and 4, as shown in Figure 2 below.
Figure 2: Formation of Maltose from Two Glucose Molecules
The Structure of Amylose
Amylose is a polymer that is helical in structure. It is composed of several α-D-glucose molecules. More than 300 and often thousands of glucose molecules are joined by α-1, 4-glycosidic bonds to form amylose (See Figure 3). Amylose is one of the two polymers that form starch (the other one is amylopectin). About 20-30% of starch is composed of amylose (Eliasson, 2004). The structure of amylose is tightly packed, making it hard to digest. Amylose is used as a prebiotic because it is a resistant starch. Human intestinal enzymes break down amylose into glucose and maltose. Therefore it functions as an energy source for humans. Amylose’s helical structure makes it possible to pack more glucose molecules into a small volume (Mua and Jackson, 1997). As a result, amylose serves as storage polymer. Amylose functions as energy storage in plants because of its insolubility.
Figure 3: Structure of Amylose
Structure of Cellulose
Cellulose is the polysaccharide that forms the tough cell walls of plant cells. It makes branches, leaves and stems of plants strong due to its rigid structure. Cellulose molecules run parallel to each other. Hydrogen bonds join cellulose molecules to form cable-like strong structures. Cellulose is made up of thousands of D-glucose molecules bound together by β (1→4) glycosidic linkages (See Figure 4) (Eliasson, 2004). It is the most abundant polysaccharide. Its chemical formula is (C6H10O5) n. Cellulose is a component of dietary fiber. It is hard to digest and therefore plays an important role in moving waste along the digestive system and preventing constipation. It is also part of plant fibers, such as hemp, cotton and flax. Cellulose’s strong structure makes it useful in the production of paper, clothing and wood products (O'sullivan, 1997).
Figure 4: Cellulose: β (1→4) Linkages
Saturated and Unsaturated Fatty Acids
Fatty acids are compounds that form fat. They are either unsaturated or saturated (Grundy, 1997). Saturated fatty acids lack a C=C double bond. They are also more solid and stack closely. Capric acid, stearic acid and palmitic acid are examples of saturated fatty acids. On the other hand, unsaturated fatty acids contain one or more C=C double bonds. Monounsaturated fatty acids only have one C=C double bond while polyunsaturated fatty acids have two or more C=C double bonds. The structure of unsaturated fatty acids is fluid like (CHow, 1999). Linoleic acid, oleic acid and palmitoleic acid are examples of unsaturated fatty acids. Differences in the structures of unsaturated and saturated fatty acids are illustrated on Figure 5 below.
Figure 5: Saturated and Unsaturated Fatty Acids
The chemical formula of stearic acid is C18H36O2. The solid structure of stearic acid is attributed to its many uses. It is extracted from vegetable and animal fats and is used as an ingredient of industrial products, cosmetics and food products (Grundy, 1997). It is specifically used to improve the consistency of many products, such as candles, margarine, lotion and soap. The chemical formula of oleic acid is C18H34O2. The fluid-like structure of oleic acid makes it useful as an emulsifying agent in soap manufacturing. It is also used as a moisturizer and a solubilizing agent in the production of aerosols (CHow, 1999).
Formation of a Triglyceride
Triglycerides are types of fats found in human body, plants and in animal fat. Triglycerides are formed through the joining of glycerol and 3 fatty acids in a condensation reaction known as esterification. In the process, an ester bond is formed between glycerol and the three fatty acids (Nye, Kim, Kalhan and Hanson, 2008). Figure 6 below illustrates the formation of a triglyceride from a molecule of glycerol and 3 molecules of stearic acid.
Figure 6: Formation of a Triglyceride
Structure and Properties of Phospholipids
Phospholipids are a type of lipids that have a phosphate group, fatty acids and glycerol. They have one hydrophilic head and two hydrophobic tails. The polar head and phosphate group region of phospholipids is attracted to water. The tail is made of fatty acids and therefore repels water. Phospholipids are found in the cell membrane where they from lipid bilayers (Figure 7) (Cevc, 1993).
Figure 7: Phospholipid Bilayer of a Cell Membrane
Phospholipids form a bilayer in cell membranes to make them impermeable to small polar molecules and large molecules. The phospholipid bilayer of the cell membrane can only allow gases and water to permeate easily into the cell (Van Meer, Voelker and Feigenson, 2008).
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