Concept 3.3: Carbohydrates Are Made from Simple Sugars

Concept 3.3 Carbohydrates Are Made from Simple Sugars

  • General formula and classification

    • Carbohydrates have the empirical formula that can be summarized as (CH2O)n, i.e., carbon, hydrogen, and oxygen in a simple ratio.
    • They are built from simple sugars (monosaccharides) and organized into larger units: monosaccharides, disaccharides, oligosaccharides, and polysaccharides.
    • Major categories mentioned:
    • Monosaccharides
    • Disaccharides
    • Oligosaccharides
    • Polysaccharides

  • Monosaccharides: simple sugars

    • Pentoses (five-carbon sugars)
    • Ribose and deoxyribose each have five carbons.
    • They have different chemical properties and biological roles.
    • Hexoses (six-carbon sugars)
    • Examples include glucose, fructose, and mannose.
    • All hexoses share the formula C6H{12}O_6 and are structural isomers with distinct biochemical properties.
    • Structural representations shown in figures (not reproduced here) include the straight-chain form vs. ring forms and the formation of ring structures from linear forms.

  • Glucose: straight-chain and ring forms; anomers

    • Straight-chain form of glucose has an aldehyde group at carbon 1 (an aldose).
    • The ring (cyclic) form arises when the aldehyde reacts with the hydroxyl group at carbon 5 to form a hemiacetal.
    • Depending on the orientation of the aldehyde group during ring closure, two anomers are formed:
    • α-D-Glucose
    • β-D-Glucose
    • The pictured forms show α vs β orientations in the ring structure.

  • Disaccharides: formed by glycosidic bonds

    • Glycosidic bonds link two monosaccharide units.
    • Sucrose: glucose + fructose linked by an α-1,2 glycosidic bond.
    • Maltose: two glucose units linked by an α-1,4 glycosidic bond.
    • The hydroxyl group on carbon 1 of one D-glucose in the α (down) position reacts with the hydroxyl group on carbon 4 of the other glucose.
    • Cellobiose: two glucoses linked by a β-1,4 glycosidic bond.
    • Key notations:
    • α-1,2 glycosidic bond
    • α-1,4 glycosidic bond
    • β-1,4 glycosidic bond

  • Polysaccharides: large polymers of monosaccharides

    • Polysaccharides are long chains of monosaccharides connected by glycosidic bonds; many are branched.
    • Major examples:
    • Starch
    • Glycogen
    • Cellulose
    • Structural features and bonding:
    • Cellulose: unbranched polymer of glucose with β-1,4 glycosidic bonds; highly stable chemically.
    • Starch and glycogen: polymers of glucose with α-1,4 glycosidic bonds; branching occurs via α-1,6 glycosidic bonds.
    • Macromolecular organization:
    • Linear cellulose chains form parallel arrangements that can hydrogen-bond to form thin fibrils.
    • Branching in starch and glycogen affects hydrogen-bonding networks and compactness.
      • Branching limits the number of hydrogen bonds in starch, making it less compact than cellulose.
      • Very high branching in glycogen leads to more compact deposits.
    • In cells:
    • Potato cells show starch as granules deposits.
    • Liver cells show glycogen deposits as dark clumps.

  • Chemically modified carbohydrates (illustrated in Figure 3.20)

    • (A) Sugar phosphates
    • Example: Fructose 1,6-bisphosphate is involved in reactions that liberate energy from glucose.
    • The numbers in its name refer to the carbon sites bearing phosphate groups; "bis" indicates two phosphate groups present.
    • Notation: Fructose 1,6-bisphosphate
    • (B) Amino sugars
    • Glucosamine and galactosamine are amino sugars with an amino group replacing a hydroxyl group.
    • Structures show the amino group substituting for an OH group.
    • Galactosamine is an important component of cartilage (a connective tissue in vertebrates).
    • Notable structures include NH2 groups (amino) in place of OH.
    • (C) Chitin
    • Chitin is a polymer of N-acetylglucosamine; N-acetyl groups provide extra sites for hydrogen bonding between polymer chains.
    • External skeletons of insects are made up of chitin.
    • Structural motif: N-acetylglucosamine units with N-acetyl groups enhancing intermolecular bonding.

  • Key concepts and connections

    • Monosaccharides serve as the building blocks for disaccharides and polysaccharides.
    • Glycosidic bond type and position (e.g., α vs β, 1,4 vs 1,6, 1,2) determine the properties and digestibility of the carbohydrate.
    • Isomerism among hexoses (glucose, fructose, mannose) leads to different biochemical roles despite identical molecular formula.
    • Ring closure of glucose (pyranose form) is a key feature of monosaccharide chemistry and underpins anomeric specificity (α vs β).
    • Polysaccharide structure (linear vs branched) affects mechanical properties (e.g., cellulose fibrils vs starch/glycogen granules) and biological roles (plant cell walls, energy storage).
    • Chemical modification of carbohydrates expands function (energy transfer via phosphates, structural components via amino sugars and chitin).

  • Summary of significance

    • Carbohydrates provide immediate energy (glucose), store energy in polymers (starch in plants, glycogen in animals), and provide structural materials (cellulose in plants, chitin in arthropod exoskeletons).
    • Their chemistry—monosaccharide diversity, glycosidic linkage variety, and capacity for chemical modification—underpins a wide range of biological processes and practical applications in biology, medicine, and industry.