Polysaccharides cellulose and starch

The important difference between a and 3 glycosidic bonds can be seen in the digestibility of the major plant polysaccharides cellulose and starch. 

The monosaccharide D-( + )-glucose, an aldohexose, is formed by plants in photosynthesis and is converted to the polysaccharides cellulose and starch. Simple saccharides are called sugars. Polysaccharides are hydrolyzable to monosaccharides e.g., a mol of trisaccharide gives 3 mol of monosaccharides. 

Bread and jelly are both rich in carbohydrates. The sweet taste of jelly is a result of the disaccharide sucrose, whereas the structure and starchy taste of bread come from the polysaccharides cellulose and starch. Wood also contains cellulose. Check the ingredient label on your bread sometimes, finely ground wood is added to increase the amount of fiber in the bread. This will be listed as cellulose. 

Polymers are substances that have macromolecules composed of many repeating units (known as mers ). A large number of naturally occurring substances are polymers including rubber and many substances based on glucose, such as the polysaccharides cellulose and starch (in plants) and glycogen (in animals). Proteins, nucleic acids, and inorganic macromolecular substances, such as silicates, are other examples. 

Insofar as the monosaccharides do occur as such in nature, it is more common to find the sugars occurring naturally in pairs (disaccharides) or in threes (trisaccharides) and, more likely, as the high-molecular-weight polysaccharides (Table 3.7). It is the polysaccharides which most probably contribute to the source material, especially the two well-known polysaccharides cellulose and starch. The fibrous tissue in the cell wall of plants and trees contains cellulose and starch also occurs throughout the plant kingdom in various forms but usually as a food reserve. The chemical composition of starch vanes with the source but in any one starch there are two structurally different polysaccharides. Both usually consist entirely of glucose units but one is a linear structure (amylose) whereas the other is a branched structure (amylopectin). 

Polysaccharides are macromolecules which make up a large part of the bulk of the vegetable kingdom. Cellulose and starch are, respectively, the first and second most abundant organic compounds in plants. The former is present in leaves and grasses the latter in fruits, stems, and roots. Because of their abundance in nature and because of contemporary interest in renewable resources, there is a great deal of interest in these compounds. Both cellulose and starch are hydrolyzed by acids to D-glucose, the repeat unit in both polymer chains. 

Polysaccharides, which are condensation polymers containing from several hundred to several thousand monosaccharide units. Cellulose and starch are the most common polysaccharides. 

Figure 1.32 The repeating units of cellulose and starch, two of the most common polysaccharides in nature.

Some natural polysaccharides. For cellulose and starch, n and x reflect the different orientations of glucose, which distinguishes these two polymers. 

Two glucose polymers of plant origin are of special importance among the polysaccharides pi 4-linked polymer cellulose and starch, which is mostly al 4-linked. 

Complex Carbohydrate polysaccharides such as cellulose and starch Compound a substance consisting of atoms of two or more different elements chemically bonded... 

Figure 2. The structure of the polysaccharides cellulose, amylose, starch and dextran.

Polysaccharides Polysaccharides are composed of a huge number of monosaccharide units, and the number forming the molecule is often approximately known. For example, cellulose and starch are polysaccharides composed of hundreds of glucose units. 

Carbohydrates are classified as either simple or complex. Simple sugars, or monosaccharides, are carbohydrates such as glucose and fructose that can t be broken down into smaller molecules by hydrolysis with aqueous acid. Complex carbohydrates, or polysaccharides, are compounds such as cellulose and starch that are made of many simple sugars linked together. On hydrolysis, polysaccharides are cleaved to yield many molecules of simple sugars. 

Saccharide molecules can be interlinked while producing a water molecule. In this way polysaccharides are formed. Well-known examples are cellulose and starch. In Fig. 3.21 you can see a small piece of a polysaccharide chain. 

The polysaccharides cellulose and hemicellulose are two major components of lignocellulosic biomass and form an abundant, non-edible and renewable source of carbohydrates. Other sources include starch, chitin, inulin and smaller... 

Cellulose, the most abundant polysaccharide, is the structural component of plant tissues starch is the energy compound stored predominantly in seeds and tubers glycogen is the animal counterpart of starch, but with shorter, more numerous branches. Cellulose and starch cohabit plant tissues with hemicellulose, protoplasm, lipid, and mineral matter in an organization interrupted by intercellular spaces that can amount to more than 50% of the total volume of some fruits and vegetables. A number of useful polysaccharides and their origins are listed in Table I. 

In this experiment you will work with a natural polymer called alginic acid that comes from seaweed. Alginic acid is a polysaccharide as are cellulose and starch. Unlike cellulose and starch, however alginic acid as the name implies contains acidic functional groups called carboxylic acids. Salts of alginic acid are used as food additives, especially as a thickener. 

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