CarbohydratesCourse Contents

• Introduction, occurrence and biological significance of carbohydrates.• Nomenclature and classification of carbohydrates.• Structures, chemical and physical properties of monosaccharides,

oligosaccharides and polysaccharides.• Blood groups.• Oligo and polysaccharides and their importance in blood transfusion,

and tissue/organ transplants

Introduction, occurrence and biological significance of carbohydrates.

• Carbohydrates are organic substances, consisting of Carbon, Oxygen,and Hydrogen. Some carbohydrates also contains sulfur and nitrogenas well.

• The Hydrogen to oxygen ratio in most of monosaccharides are similarto that of water molecules, therefore, initially they were named ashydrates of carbon. For example: Glucose C6H12O6

• They have an Empirical formula Cn(H2O)n (where n= 2,3,4 ….).• However, there were certain carbohydrates like deoxyribose present

DNA (C5H10O4 ) where H:O ratio not similar to that of water molecule.• Also Acetic acid is not a carbohydrate but the ratio of H:O is 2:1

• Therefore based on functional groups present carbohydrates were defined as: Polyhydroxy aldehydes or ketones.

• To understand this see the structures of Glucose and fructose:

Occurrence

• They are found in a wide variety of natural and processed foods.• Starch is a polysaccharide. It is abundant in cereals (wheat, maize,

rice), potatoes, and processed food based on cereal flour, such asbread, pizza or pasta.

• Sugars appear in human diet mainly as table sugar (sucrose, extractedfrom sugarcane or sugar beets), lactose (abundant in milk), glucoseand fructose, both of which occur naturally in honey, many fruits, andsome vegetables. Table sugar, milk, or honey are often added todrinks and many prepared foods such as jam, biscuits and cakes.

Significance of Carbohydrates• Carbohydrates perform numerous roles in living organisms.• Monosaccharides are the major source of fuel for metabolism, being used

both as an energy source (glucose being the most important in nature) andin biosynthesis.

• Polysaccharides serve for the storage of energy (e.g. starch and glycogen)and as structural components (e.g. cellulose in plants and chitin inarthropods).

• The 5-carbon monosaccharide ribose is an important component ofcoenzymes (e.g. ATP, FAD and NAD) and the backbone of the geneticmolecule known as RNA.

• The related deoxyribose is a component of DNA.• Saccharides and their derivatives include many other important

biomolecules that play key roles in the immune system, fertilization,preventing pathogenesis, blood clotting, and development.

• The most abundant carbohydrate, cellulose, is a structural component of the cell wall ofplants and many forms of algae. Cellulose, is one of the main components of insolubledietary fiber.

• Although it is not digestible, insoluble dietary fiber helps to maintain a healthy digestivesystem by easing defecation.

• Other polysaccharides contained in dietary fiber include resistant starch and inulin, whichfeed some bacteria in the microbiota of the large intestine, and are metabolized by thesebacteria to yield short-chain fatty acids

• Ribose is a component of RNA. Deoxyribose is a component of DNA.• Lyxose is a component of lyxoflavin found in the human heart.• Ribulose and xylulose occur in the pentose phosphate pathway.• Galactose, a component of milk sugar lactose, is found in galactolipids in plant cell

membranes and in glycoproteins in many tissues.• Mannose occurs in human metabolism, especially in the glycosylation of certain proteins.• Fructose, or fruit sugar, is found in many plants and in humans, it is metabolized in the

liver, absorbed directly into the intestines during digestion, and found in semen.• Trehalose, a major sugar of insects, is rapidly hydrolyzed into two glucose molecules to

support continuous flight.

Reducing and non reducing sugars• Reducing sugars are sugars where the anomeric carbon has an OH group

attached that can reduce other compounds.• In other words: Reducing sugars have a aldehyde group or carbonyl functional

group, which can be reduced to -OH group by chemical reaction• Non-reducing sugars do not have an OH group attached to the anomeric carbon

so they cannot reduce other compounds.• All monosaccharides such as glucose are reducing sugars. Sucrose is the most

common non reducing sugar. The linkage between the glucose and fructose unitsin sucrose, which involves aldehyde and ketone groups, is responsible for theinability of sucrose to act as a reducing sugar

• Some sugars like Fructose get re-arranged in specific conditions like alkaline pH, and form a compound that is reducible.

• Most common reducing agents used to decide this nature are Tollen's reagent,Fehling's reagent, Benedict.

Classification of Carbohydrates

Monosaccharides• Monosaccharides are the simplest carbohydrates in that sense they

cannot be hydrolyzed to smaller carbohydrates. • They are aldehydes or ketones with two or more hydroxyl groups. • The general chemical formula of a monosaccharide is (C•H2O)n,

literally a "carbon hydrate". • Monosaccharides are important fuel molecules as well as building

blocks for nucleic acids. • The smallest monosaccharides, for which n=3, are dihydroxyacetone

and D- and L-glyceraldehydes.

Classification of monosaccharides• Monosaccharides are classified according to three different characteristics:

the placement of its carbonyl group, the number of carbon atoms itcontains, and its chiral handedness.

• If the carbonyl group is an aldehyde, the monosaccharide is an aldose; ifthe carbonyl group is a ketone, the monosaccharide is a ketose.

• Monosaccharides with three carbon atoms are called trioses, those withfour are called tetroses, five are called pentoses, six are hexoses, and so on.

• These two systems of classification are often combined. For example,glucose is an aldohexose (a six-carbon aldehyde), ribose is an aldopentose(a five-carbon aldehyde), and fructose is a ketohexose (a six-carbonketone).

• Note: The chiral handedness based classification, structures and chemicalreactions of monosaccharides has already been explained in the class.

Oligosaccharides

• An oligosaccharide is a carbohydrate containing a small number ofmonosaccharide units.

• Usually they produces from two ten monosaccharide units onhydrolysis.

• Based on number of monosaccharide units produced duringhydrolysis they are named as disaccharides, trisaccharidesm etc

• Oligosaccharides can have many functions including cell recognitionand cell binding. For example, glycolipids have an important role inthe immune response

• They are present as integral part in glycoprotein and glycolipids.

• Glycoproteins have distinct Oligosaccharide structures which have significanteffects on many of their properties affecting critical functions such as antigenicity,solubility, and resistance to proteases.

• Glycoproteins are relevant as cell-surface receptors, cell-adhesion molecules,immunoglobulins, and tumor antigens

• Glycolipids are important for cell recognition, and are important for modulating the function of membrane proteins that act as receptors.

• Glycolipids are lipid molecules bound to oligosaccharides, generally present in the lipid bilayer.

• Additionally, they can serve as receptors for cellular recognition and cell signaling. • The head of the oligosaccharide serves as a binding partner in receptor activity. • The binding mechanisms of receptors to the oligosaccharides depends on the

composition of the oligosaccharides that are exposed or presented above the surface of the membrane.

• There is great diversity in the binding mechanisms of glycolipids, which is what makes them such an important target for pathogens as a site for interaction and entrance. For example, the chaperone activity of glycolipids has been studied for its relevance to HIV infection.

• chaperones are proteins that assist the conformational folding orunfolding and the assembly or disassembly of other macromolecularstructures.

• Chaperones are present when the macromolecules perform theirnormal biological functions and have correctly completed theprocesses of folding and/or assembly.

Functions of oligosaccharides• Cell recognition: All cells are coated in either glycoproteins or glycolipids, both of which help determine cell

types.• Lectins, or proteins that bind carbohydrates, can recognize specific oligosaccharides and provide useful

information for cell recognition based on oligosaccharide binding• Role in determination of Blood Type: An important example of oligosaccharide cell recognition is the role of

glycolipids in determining blood types.• The various blood types are distinguished by the glycan modification present on the surface of blood cells.• These can be visualized using mass spectrometry.• The oligosaccharides found on the A, B, and H antigen occur on the non-reducing ends of the

oligosaccharide.• The H antigen (which indicates an O blood type) serves as a precursor for the A and B antigen.• Therefore, a person with A blood type will have the A antigen and H antigen present on the glycolipids of the

red blood cell plasma membrane.• A person with B blood type will have the B and H antigen present.• A person with AB blood type will have A, B, and H antigens present.• And finally, a person with O blood type will only have the H antigen present.• This means all blood types have the H antigen, which explains why the O blood type is known as the

"universal donor

• Cell adhesion: Many cells produce specific carbohydrate-bindingproteins known as lectins, which mediate cell adhesion witholigosaccharides.

• Selectins, a family of lectins, mediate certain cell–cell adhesionprocesses, including those of leukocytes to endothelial cells.

• In an immune response, endothelial cells can express certain selectinstransiently in response to damage or injury to the cells.

• In response, a reciprocal selectin–oligosaccharide interaction willoccur between the two molecules which allows the white blood cellto help eliminate the infection or damage.

• Protein-Carbohydrate bonding is often mediated by hydrogenbonding and van der Waals forces

• Fructo-oligosaccharides, which are found in many vegetables, areshort chains of fructose molecules. They are considered solubledietary fibre. Fibre help in digestion and prevent human from coloncancer.

• Galactooligosaccharides, which also occur naturally, consist of shortchains of galactose molecules. These compounds cannot be digestedin the human small intestine, and instead pass through to the largeintestine, where they promote the growth of Bifidobacteria, whichare beneficial to gut health.

• Mannan oligosaccharides are widely used in animal feed to improvegastrointestinal health. They are normally obtained from the yeastcell walls of Saccharomyces cerevisiae. Mannan oligosaccharidesdiffer from other oligosaccharides in that they are not fermentableand their primary mode of actions include agglutination of type-1fimbria pathogens and immunomodulation

Sources• Oligosaccharides are a component of fiber from plant tissue. • Fructo-oligosaccharides and inulin are present in Jerusalem artichoke,

burdock, chicory, leeks, onions, and asparagus. • Inulin is a significant part of the daily diet of most of the world’s

population. • Fructo-oligosaccharides can also be synthesized by enzymes of the

fungus Aspergillus niger acting on sucrose. Galactooligosaccharides is naturally found in soybeans and can be synthesized from lactose.

• Fructo-oligosaccharides, Galactooligosaccharides, and inulin are also sold as nutritional supplements

Disaccharides• The most important amongst the oligosaccharides are Disaccharides.• Disaccharides produces two monosaccharides units on hydrolysis.• Examples: Maltose, Sucrose, Lactose etc

Maltose

• A crystalline dextrorotatory fermentable sugar C12H22O11 formed especially from starch by amylase

• Maltose, also known as maltobiose or malt sugar, is a disaccharide formed from two units of glucose joined with an α(1→4) bond.

• In the isomer isomaltose, the two glucose molecules are joined with an α(1→6) bond.

• The two glucose units are in the pyranose form and are joined by an O-glycosidic bond, with the first carbon (C1) of the first glucose linked to thefourth carbon (C4) of the second glucose, indicated as (1→4).

• The link is α because the glycosidic bond to the anomeric carbon (C1) is inthe opposite plane from the CH2OH substituent in the same ring (C6 of thefirst glucose).

• If the glycosidic bond to the anomeric carbon (C1) were in the same planeas the CH2OH substituent, it would be classified as a β(1→4) bond, and theresulting molecule would be cellobiose.

• The anomeric carbon (C1) of the second glucose molecule, which is notinvolved in a glycosidic bond, could be either an α- or β-anomer dependingon the bond direction of the attached hydroxyl group relative to the CH2OHsubstituent of the same ring, resulting in either α-maltose or β-maltose.

• It’s created in seeds and other parts of plants as they break downtheir stored energy in order to sprout.

• Thus, foods like cereals, certain fruits and sweet potatoes containnaturally high amounts of this sugar.

• Maltose is less sweet than table sugar and fructose• Maltose can be made by the breakdown of starch, a long chain of

many glucose units. Enzymes in your gut break these chains ofglucose down into maltose.

• Plant seeds also produce enzymes to release sugar from starch asthey sprout.

• This sugar is important in brewing and as a sweetener.

Properties• Like glucose, maltose is a reducing sugar, because the ring of one of the

two glucose units can open to present a free aldehyde group; the otherone cannot because of the nature of the glycosidic bond.

• Maltose can be broken down to glucose by the maltase enzyme, whichcatalyses the hydrolysis of the glycosidic bond.

• Maltose in aqueous solution exhibits mutarotation, because the α and β isomers that are formed by the different conformations of the anomeric carbon have different specific rotations, and in aqueous solutions, these two forms are in equilibrium.

• Maltose can easily be detected by the Woehlk test or Fearon's test on methylamine.

• It has a sweet taste, but is only about 30-60% as sweet as sugar, depending on the concentration.

• A 10% solution of maltose is 35% as sweet as sucrose

Sources and absorption• Maltose is a component of malt, a substance which is obtained in the

process of allowing grain to soften in water and germinate.• It is also present in highly variable quantities in partially hydrolysed

starch products like maltodextrin, corn syrup and acid-thinned starch.• In humans, maltose is broken down by various maltase enzymes,

providing two glucose molecules which can be further processed:either broken down to provide energy, or stored as glycogen.

Lactose• It is a disaccharide with formula C12H22O11. Also called milk sugar. Its

molecule is made up of two smaller sugar molecules, glucose andgalactose.

• It is a white, water-soluble, non-hygroscopic solid with a mildly sweettaste substance.

• Its systematic name is β-D-galactopyranosyl-(1→4)-D-glucose.

• The glucose can be in either the α-pyranose form or the β-pyranoseform, whereas the galactose can only have the β-pyranose form:hence α-lactose and β-lactose refer to the anomeric form of theglucopyranose ring alone.

• Lactose makes up around 2–8% of milk (by weight).• The lactose present in milk is not absorbed as such from the intestine

and into the body, it must first be split into glucose and galactose.• Lactose is hydrolysed to glucose and galactose, isomerised in alkaline

solution to lactulose, and catalytically hydrogenated to thecorresponding polyhydric alcohol, lactitol. Lactulose is a commercialproduct, used for treatment of constipation.

Sources• Lactose composes about 2–8% of milk by weight.• Several million tons are produced annually as a by-product of the dairy industry.• Whey or milk plasma is the liquid remaining after milk is curdled and strained, for

example in the production of cheese.• Whey is made up of 6.5% solids, of which 4.8% is lactose, which is purified by

crystallisation.• Industrially, lactose is produced from whey permeate – that is whey filtrated for

all major proteins.• The protein fraction is used in infant nutrition and sports nutrition while the

permeate can be evaporated to 60–65% solids and crystallized while cooling.• Lactose can also be isolated by dilution of whey with ethanol.• Dairy products such as yogurt and cheese contain very little lactose, as the

bacteria used to make them consume lactose during the manufacturing process.

Absorption and metabolism• Infant mammals nurse on their mothers to drink milk, which is rich in

lactose.• The intestinal villi secrete the enzyme lactase (β-D-galactosidase) to

digest it.• This enzyme cleaves the lactose molecule into its two subunits, the

simple sugars glucose and galactose, which can be absorbed.• Since lactose occurs mostly in milk, in most mammals, the production

of lactase gradually decreases with maturity due to a lack ofcontinuing consumption.

• When lactose is completely digested in the small intestine, its caloric value is 4 kcal/g, or the same as that of other carbohydrates.

• However, lactose is not always fully digested in the small intestine.• Depending on ingested dose, combination with meals (either solid or

liquid), and lactase activity in the intestines, the caloric value of lactose ranges from 2 to 4 kcal/kg.

• Unidigested lactose acts as dietary fiber. It also has positive effects on absorption of minerals, such as calcium and magnesium.

• Lactose intolerance is a digestive disorder caused by the inability todigest lactose, the main carbohydrate in dairy products.

• It can cause various symptoms, including bloating, diarrhea andabdominal cramps.

• These symptoms typically start thirty minutes to two hours aftereating or drinking milk-based food.

• People with lactose intolerance don't make enough amount of theenzyme lactase, which is needed to digest lactose.

• Many people with of Europe, West Asia, South Asia, West Africa, EastAfrica and a few other parts of Central Africa maintain lactaseproduction into adulthood.

• In many of these areas, milk from mammals such as cattle, goats, andsheep is used as a large source of food.

• Hence, it was in these regions that genes for lifelong lactaseproduction first evolved.

• The genes of adult lactose tolerance have evolved independently invarious ethnic groups.

• In people who are lactose intolerant, lactose is not broken down andprovides food for gas-producing gut flora, which can lead to diarrhea,bloating, flatulence, and other gastrointestinal symptoms.

Sucrose

• Sucrose is common sugar also known as table sugar and is adisaccharide.

• Its molecule is composed of two glucose and fructose. Sucrose isproduced naturally in plants, from which itis refined.

• It has the molecular formula C12H22O11

• Commonly sucrose is extracted and refined from either sugarcane and sugar beet.

Physical and chemical properties:• In sucrose, the components glucose and fructose are linked via an ether bond

(glycosidic linkage) between C1 on the glucosyl subunit and C2 on the fructosylunit.

• In sucrose unlike most disaccharides, the glycosidic bond is formed between the reducing ends of both glucose and fructose, and not between the reducing end of one and the nonreducing end of the other.

• This linkage inhibits further bonding to other saccharide units. Since it contains no anomeric hydroxyl groups, it is classified as a non-reducing sugar.

• Sucrose is crystalline substance at room-temperature with lattice parameters a = 1.08631 nm, b = 0.87044 nm, c = 0.77624 nm, β = 102.938°.

• The purity of sucrose is measured by polarimetry. It has a specific rotation of +66.47° at 20° (589 nm). Commercial samples of sugar are assayed using this parameter.

• Sucrose does not deteriorate at ambient conditions.

• Sucrose does not melt at high temperatures. Instead, it decomposes at 186°C to form caramel.

• Like other carbohydrates, it combusts to carbon dioxide and water. • Mixing sucrose with the oxidizer potassium nitrate produces the fuel

known as rocket candy that is used to propel amateur rocket motors.• C12H22O11 + 6 KNO3 → 9 CO + 3 N2 + 11 H2O + 3 K2CO3

• Sucrose burns with chloric acid, formed by the reaction of hydrochloric acidand potassium chlorate:

• 8 HClO3 + C12H22O11 → 11 H2O + 12 CO2 + 8 HCl• Sucrose can be dehydrated with sulfuric acid (catalyst) to form a black,

carbon-rich solid, as indicated in the following idealized equation: • H2SO4 + C12H22O11 → 12 C + 11 H2O + Heat + some H2O and SO3 (as a result

of the heat).

• Hydrolysis breaks sucrose into glucose and fructose. Hydrolysis is,however, so slow that solutions of sucrose can sit for years withnegligible change. If the enzyme sucrase is added, however, thereaction will proceed rapidly. Hydrolysis can also be accelerated withacids, such as cream of tartar or lemon juice, both are weak acids.Likewise, gastric acidity converts sucrose to glucose and fructoseduring digestion, the bond between them being an acetal bond whichcan be broken by an acid.

• In nature, sucrose is present in many plants, and in particular their roots,fruits and nectars, because it serves as a way to store energy, primarilyfrom photosynthesis. Many mammals, birds, insects and bacteriaaccumulate and feed on the sucrose in plants and for some it is their mainfood source. Seen from a human consumption perspective, honeybees areespecially important because they accumulate sucrose and produce honey,which is an important foodstuff all over the world. The carbohydrates inhoney itself primarily consist of fructose and glucose with trace amounts ofsucrose only.

• As fruits ripen, their sucrose content usually rises sharply, but some fruitscontain almost no sucrose at all. This includes grapes, cherries, blueberries,blackberries, figs, pomegranates, tomatoes, avocados, lemons and limes.Sucrose is a naturally occurring sugar, but with the advent ofindustrialization, it has been increasingly refined and consumed in all kindsof processed foods.

Metabolism of sucrose

• In humans and other mammals, sucrose is broken down into its constituentmonosaccharides, glucose and fructose, by sucrase or isomaltase glycosidehydrolases, which are located in the membrane of the microvilli lining theduodenum. The resulting glucose and fructose molecules are then rapidlyabsorbed into the bloodstream. In bacteria and some animals, sucrose isdigested by the enzyme invertase. Sucrose is an easily assimilatedmacronutrient that provides a quick source of energy, provoking a rapidrise in blood glucose upon ingestion. Sucrose, as a pure carbohydrate, hasan energy content of 3.94 kilocalories per gram. If consumed excessively,sucrose may contribute to the development of metabolic syndrome,including increased risk for type 2 diabetes, weight gain and obesity inadults and children

Polysaccharide

• They are long chain polymers of monosaccharide units boundtogether by glycosidic linkages. They can be hydrolyzed by amylaseenzymes, resulting in the formation of monosaccharides, oroligosaccharides. They range in structure from linear to highlybranched. Examples include storage polysaccharides such as starchand glycogen, and structural polysaccharides such as cellulose andchitin.

• When all the monosaccharides in a polysaccharide are the same type, the polysaccharide is called a homopolysaccharide or homoglycan, but when more than one type of monosaccharide is present they are called heteropolysaccharides or heteroglycans.

Function in diet

• Nutrition polysaccharides are common sources of energy. Many organisms can easilybreak down starches into glucose; however, most organisms cannot metabolize celluloseor other polysaccharides like chitin and arabinoxylans. These carbohydrate types can bemetabolized by some bacteria and protists. Ruminants and termites, for example, usemicroorganisms to process cellulose.

• Even though these complex polysaccharides are not digestible, they provide importantdietary elements for humans. Called dietary fiber, these carbohydrates enhance digestionamong other benefits. The main action of dietary fiber is to change the nature of thecontents of the gastrointestinal tract, and to change how other nutrients and chemicalsare absorbed. Soluble fiber binds to bile acids in the small intestine, making them lesslikely to enter the body; this in turn lowers cholesterol levels in the blood. Soluble fiberalso attenuates the absorption of sugar, reduces sugar response after eating, normalizesblood lipid levels and, once fermented in the colon, produces short-chain fatty acids asbyproducts with wide-ranging physiological activities (discussion below). Althoughinsoluble fiber is associated with reduced diabetes risk, the mechanism by which thisoccurs is unknown.

Starch

• Starch is a glucose polymer in which glucopyranose units are bonded byalpha-glycosidic linkages.

• It is made up of a mixture of amylose (15–20%) and amylopectin (80–85%).• Amylose consists of a linear chain of several hundred glucose molecules,

and Amylopectin is a branched molecule made of several thousand glucoseunits (every chain of 24–30 glucose units is one unit of Amylopectin).

• Starches are insoluble in water.• They can be digested by breaking the glycosidic bonds. Both humans and

other animals have amylases, so they can digest starches.

• The starch is produced by most green plants as energy storage. It isthe most common carbohydrate in human diets and is contained inlarge amounts in potatoes, wheat, maize (corn), rice, etc.

• Pure starch is a white, tasteless and odorless powder that is insolublein cold water or alcohol.

• Amylose is a linear chain of short length while amylopectin branchedchains.

• Amylose is a much smaller molecule than amylopectin. About onequarter of the mass of starch granules in plants consist of amylose,although there are about 150 times more amylose than amylopectinmolecules.

• Starch molecules arrange themselves in the plant in semi-crystallinegranules. Each plant species has a unique starch granular size: ricestarch is relatively small (about 2 μm) while potato starches havelarger granules (up to 100 μm).

• Starch becomes soluble in water when heated. The granules swell andburst, the semi-crystalline structure is lost and the smaller amylosemolecules start leaching out of the granule, forming a network thatholds water and increasing the mixture's viscosity. This process iscalled starch gelatinization. During cooking, the starch becomes apaste and increases further in viscosity. During cooling or prolongedstorage of the paste, the semi-crystalline structure partially recoversand the starch paste thickens, expelling water. This is mainly causedby retrogradation of the amylose. This process is responsible for thehardening of bread or staling, and for the water layer on top of astarch gel (syneresis).

• The enzymes that break down or hydrolyze starch into the constituentsugars are known as amylases.

• Alpha-amylases are found in plants and in animals. Human saliva is rich inamylase, and the pancreas also secretes the enzyme. Individuals frompopulations with a high-starch diet tend to have more amylase genes thanthose with low-starch diets.

• Beta-amylase cuts starch into maltose units. This process is important inthe digestion of starch and is also used in brewing, where amylase from theskin of seed grains is responsible for converting starch to maltose.

• If starch is subjected to dry heat, it breaks down to form dextrin, also called"pyrodextrins" in this context. This break down process is known asdextrinization. (Pyro)dextrins are mainly yellow to brown in color anddextrinization is partially responsible for the browning of toasted bread

Sources

• Starch is the most common carbohydrate in the human diet and iscontained in many staple foods. The major sources of starch intakeworldwide are the cereals (rice, wheat, and maize) and the rootvegetables (potatoes) Many other starchy foods are grown, some onlyin specific climates, including acorns, arrowroot, arracacha, bananas,barley, breadfruit, buckwheat, canna, colocasia, katakuri, kudzu,malanga, millet, oats, oca, polynesian arrowroot, sago, sorghum,sweet potatoes, rye, taro, chestnuts, water chestnuts and yams, andmany kinds of beans, such as favas, lentils, mung beans, peas, andchickpeas.

• Widely used prepared foods containing starch are bread, pancakes, cereals, noodles, pasta, porridge and tortilla.

Glycogen

• Glycogen is a multibranched polysaccharide of glucose that serves as a form ofenergy storage in animals, fungi, and bacteria

• Glycogen functions as one of two forms of energy reserves, glycogen being forshort-term and the other form being triglyceride stores in adipose tissue (i.e.,body fat) for long-term storage. In humans, glycogen is made and stored primarilyin the cells of the liver and skeletal muscle. In the liver, glycogen can make up 5–6% of the organ's fresh weight, and the liver of an adult weighing 1.5 kg can storeroughly 100–120 grams of glycogen. In skeletal muscle, glycogen is found in a lowconcentration (1–2% of the muscle mass) and the skeletal muscle of an adultweighing 70 kg stores roughly 400 grams of glycogen. The amount of glycogenstored in the body—particularly within the muscles and liver—mostly depends onphysical training, basal metabolic rate, and eating habits. Small amounts ofglycogen are also found in other tissues and cells, including the kidneys, redblood cells, white blood cells and glial cells in the brain.

• Approximately 4 grams of glucose are present in the blood of humansat all times; in fasted individuals, blood glucose is maintained constantat this level at the expense of glycogen stores in the liver and skeletalmuscle.

• Glycogen stores in skeletal muscle serve as a form of energy storagefor the muscle itself; however, the breakdown of muscle glycogenimpedes muscle glucose uptake from the blood, thereby increasingthe amount of blood glucose available for use in other tissues.

• Liver glycogen stores serve as a store of glucose for use throughoutthe body, particularly the central nervous system.

• The human brain consumes approximately 60% of blood glucose infasted, sedentary individuals

• It is a white color substance. Its molecules are similar to amylopectin but is more extensively branched and compact.

• Glycogen is a branched polymer consisting of linear chains of glucoseresidues with an average chain length of approximately 8–12 glucoseunits. Glucose units are linked together linearly by α(1→4) glycosidicbonds from one glucose to the next. Branches are linked to the chainsfrom which they are branching off by α(1→6) glycosidic bondsbetween the first glucose of the new branch and a glucose on thestem chain

Function

• Liver: Meal containing carbohydrates is digested which increases bloodglucose level. As a result the pancreas secretes insulin. Blood glucose fromthe portal vein enters liver cells (hepatocytes). Insulin acts on thehepatocytes to stimulate the action of several enzymes, including glycogensynthase. Glucose molecules are added to the chains of glycogen as long asboth insulin and glucose remain plentiful. In this postprandial or "fed"state, the liver takes in more glucose from the blood than it releases.

• After a meal has been digested and glucose levels begin to fall, insulinsecretion is reduced, and glycogen synthesis stops. When it is needed forenergy, glycogen is broken down and converted again to glucose. Glycogenphosphorylase is the primary enzyme of glycogen breakdown. For the next8–12 hours, glucose derived from liver glycogen is the primary source ofblood glucose used by the rest of the body for fuel.

• Glucagon, another hormone produced by the pancreas, in manyrespects serves as a countersignal to insulin. In response to insulinlevels being below normal (when blood levels of glucose begin to fallbelow the normal range), glucagon is secreted in increasing amountsand stimulates both glycogenolysis (the breakdown of glycogen) andgluconeogenesis (the production of glucose from other sources).

• Muscle: Muscle cell glycogen appears to function as an immediatereserve source of available glucose for muscle cells. Other cells thatcontain small amounts use it locally, as well. As muscle cells lackglucose-6-phosphatase, which is required to pass glucose into theblood, the glycogen they store is available solely for internal use andis not shared with other cells. This is in contrast to liver cells, which,on demand, readily do break down their stored glycogen into glucoseand send it through the blood stream as fuel for other organs

Cellulose

• Cellulose is a polysaccharide consisting of a linear chain of several hundredto many thousands of β(1→4) linked D-glucose units. Cellulose is animportant structural component of the primary cell wall of green plants,and many forms of algae. Some species of bacteria secrete it to formbiofilms. Cellulose is the most abundant organic polymer on Earth. Thecellulose content of cotton fiber is 90%, that of wood is 40–50%, and thatof dried hemp is approximately 57%

• Cellulose is mainly used to produce paperboard and paper. Smallerquantities are converted into a wide variety of derivative products such ascellophane and rayon. Conversion of cellulose from energy crops intobiofuels such as cellulosic ethanol is under development as a renewablefuel source. Cellulose for industrial use is mainly obtained from wood pulpand cotton

• Some animals, particularly ruminants and termites, can digest cellulosewith the help of symbiotic micro-organisms that live in their guts, such asTrichonympha. In human nutrition, cellulose is a non-digestible constituentof insoluble dietary fiber, acting as a hydrophilic bulking agent for feces andpotentially aiding in defecation.

• Properties: it has no taste, odor, and a hydrophobic substance that isinsoluble in water while soluble in most organic solvents. It is isbiodegradable. It melts at 467°C. It can be broken down chemically into itsglucose units by treating it with concentrated mineral acids at hightemperature.

• Cellulose is derived from D-glucose units, which condense through β(1→4)-glycosidic bonds. Cellulose is a straight chain polymer. The multiplehydroxyl groups on the glucose from one chain form hydrogen bonds withoxygen atoms on the same or on a neighbor chain, holding the chainsfirmly together side-by-side and forming microfibrils with high tensilestrength. The high tensile strength of plant stems and of the tree woodarises from the arrangement of cellulose fibers intimately distributed intothe lignin matrix.

Heteropolysaccharide