Polysaccharides: Starch and Glycogen

Polysaccharides

• Starch is a homopolymer of glucose, forming an α-glucosidic chain, and is the most important dietary carbohydrate in cereals, potatoes, and other vegetables.
• Starch consists of two main constituents: amylose (13%-20%) and amylopectin (80%-87%).
• Glycogen is the storage polysaccharide in animals and is sometimes called animal starch.
• Dextrins are produced during the hydrolysis of starch.
• Cellulose is the chief constituent of plant cell walls, insoluble, and consists of β-D-glucopyranose units linked by β1 → 4 bonds.
• Human digestive system lacks any enzyme that hydrolyzes the β1 → 4 bonds, and so cannot digest cellulose.
• Cellulose is an important component of human diet as it increases the bulk of intestinal contents, stimulates intestinal contractions, and prevents constipation.

Heteropolysaccharides

• Glycosaminoglycans are the most important members of this group.
• Examples of glycosaminoglycans are hyaluronic acid, chondroitin sulfate, and heparin.
• Glycophorin is a major integral membrane glycoprotein of human erythrocytes.

Carbohydrates

• The basic carbohydrate units are called monosaccharides.
• Carbohydrates are classified into four groups: monosaccharides, disaccharides, oligosaccharides, and polysaccharides.
• Monosaccharides are those sugars that cannot be hydrolyzed into simpler carbohydrates, and contain one saccharide (sugar) unit.
• Examples of monosaccharides are glucose, fructose, and galactose.

Disaccharides

• Disaccharides are condensation products of two monosaccharide units.
• Examples of disaccharides are lactose, maltose, isomaltose, and sucrose.
• Maltose is obtained by the action of amylase enzyme on starch and is hydrolyzed by maltase enzyme into 2 molecules of D-glucose.
• Sucrose is hydrolyzed by sucrase enzyme into D-glucose and D-fructose.
• Lactose is hydrolyzed by lactase enzyme into D-glucose and D-galactose.
• Isomaltose is formed by the action of amylase enzyme on starch.

Lipids

• Lipids have the common property of being relatively insoluble in water and soluble in nonpolar solvents.
• Functions of lipids include:
  • Being important dietary constituents due to high energy value and essential fatty acids and fat-soluble vitamins.
  • Fat storage in adipose tissue.
  • Acting as electrical and thermal insulators.
  • Combination of lipid and protein (lipoprotein) serving as a means of transporting lipids in the blood.
• Lipids are classified into three main groups: simple lipids, complex lipids, and precursor and derived lipids.

Fatty Acids

• Fatty acids are mostly contain even number of carbon atoms.
• Fatty acids are classified into two main groups: saturated fatty acids (SFA) and unsaturated fatty acids (USFA).
• Saturated fatty acids contain no double bonds.
• Unsaturated fatty acids contain one or more double bonds.

Triglycerides

• Triglycerides or triacylglycerols function as energy reservoirs in animals.
• They are not components of cellular membranes.