W3

• Sugars in human diet

  • Table sugar (sucrose), lactose, glucose, fructose
• Objectives

  1. Define monosaccharides

  2. Distinguish a monosaccharide from a disaccharide and from an oligosaccharide

  3. Identify the structural formula of carbohydrates and its name
• Carbohydrates


  • Central to nutrition, found in a wide variety of natural and processed foods
  • Starch is a polysaccharide abundant in cereals, potatoes, and processed food based on cereal flour
• Examples of foods with sugars

  • Table sugar, milk, honey, drinks, jam, biscuits, cakes
• Monosaccharides are polyhydroxy aldehydes or ketones with more than one hydroxyl group and a carbonyl group
• Fructose is a ketohexose
• Major dietary carbohydrates

  • Sugars (Monosaccharides, Disaccharides, Polyols), Oligosaccharides, Polysaccharides
• Carbohydrate

  A biomolecule consisting of carbon, hydrogen, and oxygen atoms with a hydrogen-oxygen atom ratio of 2:1
• All carbohydrates consist of carbon, hydrogen, and oxygen atoms and are polyhydroxy aldehydes or ketones or compounds that can be broken down to form such compounds
• Monosaccharides are the building blocks of carbohydrates, classified by the number of carbon atoms in the molecule
• Not all carbohydrates conform to the precise stoichiometric definition of Cm(H2O)n
• Cyclic structure of monosaccharides involves the aldehyde reacting with the OH group on the fifth carbon atom
• Classification of monosaccharides by carbon atoms

  • Dioses, Trioses, Tetroses, Pentoses, Hexoses, Heptoses
• The term carbohydrate originated from a misinterpretation of the molecular formulas of many substances
• Important aldohexoses

  • Glucose, mannose, galactose
• Important pentoses

  • Xylose, arabinose, ribose, deoxyribose
• Breaking down a disaccharide into its two monosaccharides

  Is accomplished by hydrolysis with the help of a disaccharidase enzyme
• Fructose


  • Ketohexose
• Reduction

  Sodium borohydride reduction of an aldose makes the ends of the resulting alditol chain identical
• Oligosaccharides are saccharide polymers containing a small number of monosaccharides, typically three to ten
• Cyclic structure of monosaccharides

  Cyclic alkanes containing five or six carbon atoms in the ring are the most stable, similar to monosaccharides forming cyclic structures
• Disaccharides are simple sugars formed when two monosaccharides are joined by glycosidic linkage
• Oligosaccharides have functions including cell recognition and cell binding
• Ester and Ether Formation

  1. -OH groups on a monosaccharide can be converted to esters and ethers using acid chloride, acid anhydride, or alkyl halide

  2. Acetal derivatives formed when a monosaccharide reacts with an alcohol in the presence of an acid catalyst are called glycosides

  3. Phosphorylation of alcohol groups is a critical metabolic step, often involving ATP as the phosphate donor

  4. Sugars may be classified as reducing or non-reducing based on their reactivity with certain reagents

  5. Sodium borohydride reduction of an aldose makes the ends of the resulting alditol chain identical

  6. Disaccharides are formed when two monosaccharides are joined by glycosidic linkage
• Disaccharides are soluble in water and differ in their monosaccharide constituents and the specific type of glycosidic linkage connecting them
• Oxidation

  1. Sugars may be classified as reducing or non-reducing based on their reactivity with certain reagents

  2. If a sugar is oxidized by these reagents, it is called reducing

  3. Sodium borohydride reduction of an aldose makes the ends of the resulting alditol chain identical
• Monosaccharides

  • glucose
  • fructose
  • galactose
• Glycoproteins are membrane proteins often covalently linked to oligosaccharides, which are branched glycoside-linked sugars
• Joining of monosaccharides into a disaccharide

  Occurs by a condensation reaction involving the elimination of a water molecule
• Starch


  • Is the most important source of carbohydrates in the human diet
  • Accounts for more than 50% of our carbohydrate intake
  • Is a mixture of two polymers: amylose and amylopectin
  • Consists of about 10%–30% amylose and 70%–90% amylopectin
  • Amylose is a linear polysaccharide composed entirely of D-glucose units joined by the α-1,4-glycosidic linkages
• Lipids


  • Have an important role in the immune response
• Glycosylation

  1. Sugars are commonly attached to proteins typically to a hydroxyl or other functional group

  2. The majority of proteins synthesized in the endoplasmic reticulum are glycosylated
• Cellulose


  • Is an important structural component of the primary cell wall of green plants, many forms of algae, and the oomycetes
  • Consists of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units
• Glycoproteins


  • Membrane proteins are often covalently linked to oligosaccharides, which are branched glycoside-linked sugars
  • Glycoproteins are rare in the cytosol, but common on secreted and membrane proteins
• Glycogen


  • Is the energy reserve carbohydrate of animals
  • Practically all mammalian cells contain some stored carbohydrates in the form of glycogen, especially abundant in the liver and skeletal muscle cells
  • Found as granules in liver and muscle cells
  • Used during fasting to obtain glucose needed to maintain metabolic balance
• Polysaccharides

  • Are the most abundant carbohydrate found in food
  • Are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages
  • Can react with water using amylase enzymes as catalyst, producing constituent sugars (monosaccharides, or oligosaccharides)
  • Range in structure from linear to highly branched
  • Examples include storage polysaccharides such as starch, glycogen, and galactogen, and structural polysaccharides such as cellulose and chitin
• Chitin


  • Forms a structural component of many animals, such as exoskeletons
  • Over time, it is bio-degradable in the natural environment
  • Breakdown may be catalyzed by enzymes called chitinases, secreted by microorganisms such as bacteria and fungi, and produced by some plants
  • Some microorganisms have receptors to simple sugars from the decomposition of chitin, producing enzymes to digest it by cleaving the glycosidic bonds to convert it to simple sugars and ammonia