The polysaccharides are the most abundant carbohydrates in nature and serve a variety of functions, such as energy storage or as components of plant cell walls. Polysaccharides are very large polymers composed of tens to thousands of monosaccharides joined together by glycosidic linkages. The three most abundant polysaccharides are starch, glycogen, and cellulose. These three are referred to as homopolymers because each yields only one type of monosaccharide (glucose) after complete hydrolysis. Heteropolymers may contain sugar acids, amino sugars, or noncarbohydrate substances in addition to monosaccharides. Heteropolymers are common in nature (gums, pectins, and other substances) but will not be discussed further in this textbook. The polysaccharides are nonreducing carbohydrates, are not sweet tasting, and do not undergo mutarotation. Show
Starch is the most important source of carbohydrates in the human diet and accounts for more than 50% of our carbohydrate intake. It occurs in plants in the form of granules, and these are particularly abundant in seeds (especially the cereal grains) and tubers, where they serve as a storage form of carbohydrates. The breakdown of starch to glucose nourishes the plant during periods of reduced photosynthetic activity. We often think of potatoes as a “starchy” food, yet other plants contain a much greater percentage of starch (potatoes 15%, wheat 55%, corn 65%, and rice 75%). Commercial starch is a white powder. Starch is a mixture of two polymers: amylose and amylopectin. Natural starches consist of about 10%–30% amylase and 70%–90% amylopectin. Amylose is a linear polysaccharide composed entirely of D-glucose units joined by the α-1,4-glycosidic linkages we saw in maltose (part (a) of Figure 5.1.1). Experimental evidence indicates that amylose is not a straight chain of glucose units but instead is coiled like a spring, with six glucose monomers per turn (part (b) of Figure 5.1.1). When coiled in this fashion, amylose has just enough room in its core to accommodate an iodine molecule. The characteristic blue-violet color that appears when starch is treated with iodine is due to the formation of the amylose-iodine complex. This color test is sensitive enough to detect even minute amounts of starch in solution.  Figure 5.1.1: Amylose. (a) Amylose is a linear chain of α-D-glucose units joined together by α-1,4-glycosidic bonds. (b) Because of hydrogen bonding, amylose acquires a spiral structure that contains six glucose units per turn. Amylopectin is a branched-chain polysaccharide composed of glucose units linked primarily by α-1,4-glycosidic bonds but with occasional α-1,6-glycosidic bonds, which are responsible for the branching. A molecule of amylopectin may contain many thousands of glucose units with branch points occurring about every 25–30 units (Figure 5.1.2). The helical structure of amylopectin is disrupted by the branching of the chain, so instead of the deep blue-violet color amylose gives with iodine, amylopectin produces a less intense reddish brown. Figure 5.1.2: Representation of the Branching in Amylopectin and Glycogen. Both amylopectin and glycogen contain branch points that are linked through α-1,6-linkages. These branch points occur more often in glycogen. Dextrins are glucose polysaccharides of intermediate size. The shine and stiffness imparted to clothing by starch are due to the presence of dextrins formed when clothing is ironed. Because of their characteristic stickiness with wetting, dextrins are used as adhesives on stamps, envelopes, and labels; as binders to hold pills and tablets together; and as pastes. Dextrins are more easily digested than starch and are therefore used extensively in the commercial preparation of infant foods. The complete hydrolysis of starch yields, in successive stages, glucose: starch → dextrins → maltose → glucose In the human body, several enzymes known collectively as amylases degrade starch sequentially into usable glucose units.
Glycogen is the energy reserve carbohydrate of animals. Practically all mammalian cells contain some stored carbohydrates in the form of glycogen, but it is especially abundant in the liver (4%–8% by weight of tissue) and in skeletal muscle cells (0.5%–1.0%). Like starch in plants, glycogen is found as granules in liver and muscle cells. When fasting, animals draw on these glycogen reserves during the first day without food to obtain the glucose needed to maintain metabolic balance.
Note About 70% of the total glycogen in the body is stored in muscle cells. Although the percentage of glycogen (by weight) is higher in the liver, the much greater mass of skeletal muscle stores a greater total amount of glycogen. Glycogen is structurally quite similar to amylopectin, although glycogen is more highly branched (8–12 glucose units between branches) and the branches are shorter. When treated with iodine, glycogen gives a reddish brown color. Glycogen can be broken down into its D-glucose subunits by acid hydrolysis or by the same enzymes that catalyze the breakdown of starch. In animals, the enzyme phosphorylase catalyzes the breakdown of glycogen to phosphate esters of glucose.
Cellulose, a fibrous carbohydrate found in all plants, is the structural component of plant cell walls. Because the earth is covered with vegetation, cellulose is the most abundant of all carbohydrates, accounting for over 50% of all the carbon found in the vegetable kingdom. Cotton fibrils and filter paper are almost entirely cellulose (about 95%), wood is about 50% cellulose, and the dry weight of leaves is about 10%–20% cellulose. The largest use of cellulose is in the manufacture of paper and paper products. Although the use of noncellulose synthetic fibers is increasing, rayon (made from cellulose) and cotton still account for over 70% of textile production. Like amylose, cellulose is a linear polymer of glucose. It differs, however, in that the glucose units are joined by β-1,4-glycosidic linkages, producing a more extended structure than amylose (part (a) of Figure 5.1.3). This extreme linearity allows a great deal of hydrogen bonding between OH groups on adjacent chains, causing them to pack closely into fibers (part (b) of Figure 5.1.3). As a result, cellulose exhibits little interaction with water or any other solvent. Cotton and wood, for example, are completely insoluble in water and have considerable mechanical strength. Because cellulose does not have a helical structure, it does not bind to iodine to form a colored product. Figure 5.1.3: Cellulose. (a) There is extensive hydrogen bonding in the structure of cellulose. (b) In this electron micrograph of the cell wall of an alga, the wall consists of successive layers of cellulose fibers in parallel arrangement. Cellulose yields D-glucose after complete acid hydrolysis, yet humans are unable to metabolize cellulose as a source of glucose. Our digestive juices lack enzymes that can hydrolyze the β-glycosidic linkages found in cellulose, so although we can eat potatoes, we cannot eat grass. However, certain microorganisms can digest cellulose because they make the enzyme cellulase, which catalyzes the hydrolysis of cellulose. The presence of these microorganisms in the digestive tracts of herbivorous animals (such as cows, horses, and sheep) allows these animals to degrade the cellulose from plant material into glucose for energy. Termites also contain cellulase-secreting microorganisms and thus can subsist on a wood diet. This example once again demonstrates the extreme stereospecificity of biochemical processes.
Starch n., plural: starch, starches [stɑɹtʃ] Definition: a polysaccharide carbohydrate made up of glucose Starch is a polysaccharide (C6H10O5)n consisting of a large number of glucose monomers joined together by glycosidic bonds. It occurs especially in seeds, bulbs, and tubers. It belongs to a group of carbohydrates, which are organic compounds made up of carbon, hydrogen, and oxygen, usually in the ratio of 1:2:1. Carbohydrates are one of the major classes of biomolecules. As a nutrient, carbohydrates can be classified into two major groups: simple carbohydrates and complex carbohydrates. Simple carbohydrates, sometimes referred to as simply sugar, consist of one or two saccharide residues. They are readily digested and serve as a rapid source of energy. Complex carbohydrates (such as cellulose, starch, chitin, and glycogen) are those that need more time to be digested and metabolized. They often are high in fiber and unlike simple carbohydrates, they are less likely to cause spikes in blood sugar levels. Glycogen, in particular, is stored in the liver for quick access to energy as it is burnt before fat. Etymology
Chemical Formula Compare Starch has long been known and used as early as 100,000 years ago. It is believed to be used in food preparations, such as in making bread and in porridges. This hypothesis is based upon the stone tools unearthed from old caves. The tools were likely used to scrape and grind starch grains from wild sorghum. This observation led scientists to presume that the inclusion of starch in the prehistoric diet of early humans in the African savannahs and woodlands improved diet quality. The processing of grains into a staple marked the shift of the prehistoric diet and is believed to be a crucial step in human evolution. (Ref.1) The word starch may come from the Old English stearc (“stark, strong, rough”), which in turn might have a Germanic origin, i.e. starchī, meaning “strong”. CharacteristicsStarch is a complex polysaccharide made up of a large number of glucose units joined together by glycosidic bonds. It is white, tasteless, and odorless powder. It has a variable molar mass. It is insoluble in alcohol and in cold water. Its chemical formula is (C6H10O5)n. Two types of molecules comprise pure starch: amylose and amylopectin. Both amylose and amylopectin are polysaccharides comprised of glucose residues. They differ in structure: amylose is a linear chain of glucose molecules connected by α-(1,4) glycosidic bonds whereas amylopectin is a branched-chain of glucose molecules linked linearly with α-(1,4) glycosidic bonds and α-(1,6) bonds at intervals of 24 to 30 glucose subunits. Since starch is a polysaccharide consisting essentially of D-glucose, it, therefore, belongs to a group of α-glucans. Amylopectin is more soluble in water and easier to digest than amylose. Its solubility is due to the many endpoints, which can form hydrogen bonds with water. In general, starch contains 75 -80% amylopectin and 20-25% amylose by weight. Dehydration SynthesisThe chemical process of joining monosaccharide units is referred to as dehydration synthesis since it results in the release of water as a byproduct. Starch is produced by dehydration synthesis. Plants store glucose that is not in use as starch. First, glucose is phosphorylated into glucose-1-phosphate. Starch granules are stored inside the amyloplasts located inside the cells of various plant organs. Starch granules may be found in fruits, seeds, tubers, and rhizomes. Daisies, sunflowers, and Jerusalem artichokes are examples of plants that store inulin (which is a fructan) instead of starch. DegradationIn plants, starch degradation occurs naturally at night. The enzyme glucan water dikinase phosphorylates the starch, particularly at C-6 of one of the glucose residues. Then, another enzyme (phosphoglucan water dikinase) phosphorylates the glucose residue at C-3. After phosphorylation, degrading enzymes can now act on starch to liberate simple sugars. For instance, beta-amylase liberates two glucose residues as maltose. Another degrading enzyme is the disproportionating enzyme-1 that at the end of the degradation process liberates glucose. Starch degradation gives rise to chiefly maltose and smaller amounts of glucose. These simple sugars will then be moved out of the plastid into the cytosol via transporters: maltose transporter for maltose and plastidic glucose translocater for glucose. They may be used later as a substrate for the biosynthesis of sucrose, which is essential in the mitochondrial oxidative pentose pathway that generates ATP at night. (Ref.2) HydrolysisHydrolysis is the process of converting a polysaccharide, such as starch, into simple sugar components. The process of converting polysaccharides into monosaccharides, in particular, is called saccharification. In humans, complex carbohydrates such as starch are digested through a series of enzymatic reactions. These enzymes are salivary amylase, pancreatic amylase, and maltase. Salivary amylase acts on the starch and breaks it down to maltose. When the partially-digested carbohydrates reach the small intestine, the pancreas secretes pancreatic juices that include the pancreatic amylase. This enzyme acts on the partially-digested carbohydrates by breaking them down into simple sugars. The brush border of the small intestine releases digestive enzymes such as isomaltase, maltase, sucrase, and lactase. Isomaltase digests polysaccharides at the alpha 1-6 linkages, and convert alpha-limit dextrin to maltose. Maltase breaks down maltose (a disaccharide) into two glucose units. Sucrase and lactase digest sucrose and lactose into monosaccharide constituents, respectively. The epithelial cells (enterocytes) at the brush border of the small intestine absorb monosaccharides and then release them into the capillaries. The simple sugars are then transported to the cells of other tissues, especially to the liver, from the bloodstream. Glucose in the blood may be utilized by the body to produce ATP. Otherwise, it is transported to the liver, together with the galactose and fructose (which are largely converted into glucose), for storage as glycogen. Resistant starchResistant starch is a form of starch that resists digestion in the small intestine of humans. It is also dietary fiber. It is metabolized instead in the large intestine by the colonic microbiota. The microbes in the colon ferment it and produce metabolic byproducts such as gases and short-chain fatty acids. The short-chain fatty acids, in particular, are absorbed and provide health benefits to the human body. Fermentation of resistant starch also helps promote the growth of beneficial bacteria. Plant starch vs. Animal starchAnimal starch is not a starch per se. It refers to the constituent of the animal’s glycogen owing to the similarity in the structure and composition of amylopectin. While plants store excess glucose in the form of starch, the animals also do so in the form of glycogen. Glycogen is a branched polymer of glucose that is mainly produced in liver and muscle cells, and functions as secondary long-term energy storage in animal cells. Similar to starch, glycogen is a complex carbohydrate that primarily serves as a storage carbohydrate. The difference between the amylopectin in plants and the amylopectin in animals is that the latter has more extensive branching at every 8 to 12 glucose units. Biological ImportanceAll plant seeds and tubers contain starch which is predominantly present as amylose and amylopectin. Plants use starch as a way to store excess glucose, and thus also use starch as food via the mitochondrial oxidative phosphorylation during at night or when photosynthesis is unlikely. Plants store excess starch in amyloplasts, which are leucoplasts that function primarily in storing starch granules through the polymerization of glucose and in converting these reserves back into simpler sugars (e.g. maltose and glucose), especially when light is not available. Chloroplasts, pigmented organelles involved primarily in photosynthesis, are also capable of storing starch. Animals do not store excess glucose as starch; they store them as glycogen. However, certain animals feed on starch-laden food. Health riskToo much starch in the diet is associated with dental caries, obesity, and diabetes mellitus. Starch (especially cooked and contained in processed foods) can cause spikes in blood glucose levels after a meal. Thus, starch consumption is advised to be in moderation. Individuals with celiac disease and congenital sucrase-isomaltase deficiency may need to avoid starchy foods. (Ref.3) Try to answer the quiz below to check what you have learned so far about starch. 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References
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Which of the following is TRUE regarding starches
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