Starch in plants

Most carbohydrates exist in the form of polysaccharides. Polysaccharides give stmcture to the cell walls of land plants (cellulose), seaweeds, and some microorganisms and store energy (starch in plants and glycogen in animals). They are important in the human diet and in many commercial apphcations. 

Glycogen is the major storage carbohydrate in animals, corresponding to starch in plants it is a branched polymer of a-D-glucose. It occurs mainly in liver (up to 6%) and muscle, where it rarely exceeds 1%. However, because of its greater mass, muscle contains about three to four times as much glycogen as does liver (Table 18—1). 

Since D-fructose and D-glucose phosphates are amongst the first products of photosynthesis, and since starch (in plants) and glycogen (in animals) are converted by phosphorylase to D-glucosyl phosphate, biosynthesis of carbohydrates revolves around these ubiquitous compounds... 

In contrast to animals, bacteria such as E. coli synthesize glycogen via ADP-glucose rather than UDP-glucose.88 ADP-glucose is also the glucosyl donor for synthesis of starch in plants. The first step in the biosynthesis (Eq. 20-18) is catalyzed by the enzyme ADP-glucose pyrophosphorylase (named for the reverse reaction). 

E2.1 Overview of Laboratory Isolation of Starch from Plant Materials E2.2 Enzymatic Quantitation of Total Starch in Plant Products E2.3 Determination of Total Amy lose Content of Starch... 

E2.2 Enzymatic Quantitation of Total Starch in Plant Products E2.2.1... 

Plants and some algae store food in the form of starch. In plants, starch is mainly found in seeds, roots, and tubers as well as in stems, leaves, fruits, and even pollen. Grain seeds, such as corn kernels, contain up to 75% starch. Therefore, it is an important component of the carbohydrates in the diet and a very good source of energy for the body. Starch is found in foods such as cereal, pasta, and potatoes, and cornstarch is used to thicken sauces. Laundry starch is a liquid form of starch used to stiffen the collars and sleeves of shirts. An advantage of using it is that dirt and sweat will stick to the starch instead of the shirt textile fibers, with the result that collar rings are easily washed away with the starch. 

Polysaccharides Long chains of monosaccharides joined together are collectively called polysaccharides. The major storage polysaccharides are glycogen (in animals), starch (in plants) and dextran (in yeast and bacteria). Cellulose is a structural polysaccharide found in plant cell walls. 

Starch in plants is accompanied by water, metal ions, lipids, proteins, sterols (such as saponins), and alkaloids (as in such exotic plants as Diascoracea).649 Several of these components can be washed out by the isolation of starch, some of them are extractable with organic solvents, and some are volatized by steam treatment. With the exception of metal ions (preceding article, p. 263), the foregoing components form physical mixtures with starch and do not chemically bond with either amylose or amylopectin. Therefore, one may assume that amylose and amylopectin form inclusion complexes with organic components that are similar to those mentioned in the preceding article. 

The starch in plants occurs in small microscopic granules that have a core of starch surrounded by a network of protein. In the granules, amylopectin has an orderly crystalline structure, whereas amylose is somewhat amorphous. The orderly structure of the starch molecules may impair digestion by pancreatic amylase. The occurrence of starch in granules, whether in wheat, potatoes, bananas, or other foods, may prevent the digestion of up to 50% of the starch in these foods... 

INULIN Inulin is a polymer of D-fructose molecules (fructofuranose), which are linked to each other in fi-2.1 bonds. There is a terminal glucose molecule. Inulin occurs as a reserve food instead of starch in plants, mainly within the family Asteraceae for example Inula helenium L., Asteraceae. In the form of a 10% aqueous solution inulin is used in tests of kidney function (like mannitol). Inulin is not absorbed or metabolised. Since there is no reabsorption in the tubules, the rate of excretion is a measure of the function of the glomerulus (inulin clearance). 

---