Carbohydrates

Alpha and Beta Glucose

• Both are isomers of glucose, differing in the orientation of the hydroxyl (-OH) group on carbon 1.

  • Alpha-glucose: OH group on C1 is below the plane of the ring.

  • Beta-glucose: OH group on C1 is above the plane of the ring.

• This small difference changes how glucose molecules link together, which in turn affects the structure and digestibility of polysaccharides.

Type of Linkages

• Glycosidic linkage = covalent bond between two monosaccharides, formed by a dehydration reaction.

  • Alpha (1→4) linkage: found in starch & glycogen; easily digested by animals.

  • Alpha (1→6) linkage: causes branching in glycogen and amylopectin (a form of starch).

  • Beta (1→4) linkage: found in cellulose; forms strong, rigid fibers; not digestible by most animals.

Isomers of Carbohydrates

• Structural isomers: same formula, different arrangement (e.g., glucose, fructose, galactose all = C₆H₁₂O₆).

• Ring isomers: alpha vs. beta glucose (different orientation at carbon 1).

• Functional differences: even tiny structural changes lead to huge functional differences (e.g., digestible starch vs. indigestible cellulose).

Structure and Function

• Cellulose:

  • Polymer of beta-glucose with beta (1→4) linkages.

  • Linear, unbranched chains form hydrogen bonds → strong microfibrils.

  • Function = structural support in plant cell walls.

  • Bond type: Linked by β-1,4 glycosidic bonds, which create straight chains.

  • Structure: Chains run parallel and are held together by hydrogen bonds, forming strong fibers called microfibrils.

  • can’t be digested by humans because we don’t contain the right enzymes to break it down

• Starch (plants):

  • Polymer of alpha-glucose.

  • Starch is a polysaccharide (a polymer of sugars) composed of many glucose units.

  • made of alpha glucose monomers

    • Amylose: unbranched, alpha (1→4) linkages.

    • Amylopectin: branched, alpha (1→4) and alpha (1→6) linkages.

  • Function = energy storage in plants.

  • Insoluble (cannot dissolve) in water (large polymer).

  • Starch = plant energy storage

• Glycogen (animals/fungi):

  • Polymer of alpha-glucose.

  • Highly branched (alpha 1→4, 1→6 linkages).

  • Function = rapid energy storage in animals, stored in liver/muscle cells.

  • Glycogen = animal energy storage (similar function, different structure)

Starch vs. Cellulose

• Type of bond in starch: alpha (1→4) glycosidic bonds, with some alpha (1→6) at branch points.

• Type of bond in cellulose: beta (1→4) glycosidic bonds.

• Which bond can be broken by animals?:

  • Animals have enzymes (amylase) to break alpha linkages in starch & glycogen.

  • Most animals cannot break beta linkages in cellulose (except some herbivores with symbiotic gut microbes).