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BIOLOGICAL MACROMOLECULES: Carbohydrates (Vocabulary)

Carbohydrates: Overview

  • Major class of biomolecules.

  • Play essential roles in energy provision, storage, and structural support in both plants and animals.

  • Glucose:

    • Key energy source in humans; energy converted into adenosine triphosphate (ATP) during cellular respiration.

    • Synthesized by plants from carbon dioxide and water to fuel their energy needs.

    • Excess glucose in humans and other animals stored as starch.

  • Other Monosaccharides:

    • Galactose (part of lactose, milk sugar).

    • Fructose (found in sucrose, fruit).

  • Isomerism:

    • Glucose, galactose, and fructose share the same chemical formula, C6H{12}O_6.

    • They are isomers due to different structural arrangements of functional groups around their asymmetric carbons.

Molecular Formula and Isomerism in Carbohydrates

  • General Stoichiometric Formula: (CH2O)n , reflecting a 1:2:1 ratio of carbon, hydrogen, and oxygen.

  • Origin of Name: "Carbohydrate" derives from carbon and water.

  • Classification Subtypes:

    1. Monosaccharides.

    2. Disaccharides.

    3. Polysaccharides.

  • Monosaccharides (Simple Sugars):

    • Typically 3–7 carbon atoms.

    • Most names end with the suffix -ose.

    • Aldose: Has an aldehyde group (R-CHO); examples include glucose and galactose.

    • Ketose: Has a ketone group (RC(=O)R′); example is fructose.

    • Can exist as linear chains or ring-shaped molecules in aqueous solutions.

  • Hexoses (Six-Carbon Sugars):

    • Glucose, galactose, and fructose are hexoses.

    • Are structural isomers with the same chemical formula, C6H{12}O_6, but distinct atomic arrangements and chemical properties.

  • Ring Form and Anomeric Carbon:

    • In ring form, carbon-1 (anomeric carbon) becomes asymmetric.

    • Alpha (\alpha) form: Hydroxyl group on C-1 is below the ring plane.

    • Beta (\beta) form: Hydroxyl group on C-1 is above the plane.

    • Monosaccharides interconvert between linear and ring forms; rings are predominant in aqueous solutions.

Monosaccharides

  • Single-unit building blocks of carbohydrates.

  • Contain between three and seven carbon atoms.

  • Commonly referred to by the suffix -ose.

  • Classification Factors:

    • By functional group: Aldoses (aldehyde) or Ketoses (ketone).

    • By number of carbons: Trioses (3 C), Pentoses (5 C), Hexoses (6 C).

  • Glucose, Galactose, Fructose: Share the same empirical formula but differ in structure and functional group placement.

  • Glucose in Biological Contexts:

    • Central metabolite.

    • Ring forms (\alpha- and eta-glucose) arise from the orientation of the hydroxyl group on the anomeric carbon.

    • Anomeric carbon (C-1) is a stereogenic center during cyclization.

    • The ring can adopt two stereochemical forms, leading to different physical and chemical properties.

    • Linear form exists, but the ring form is more common physiologically.

Ring Forms and Anomeric Carbon of Glucose

  • Cyclization: Glucose cyclizes from its linear form to a ring form.

  • Anomeric Carbon (C-1):

    • Becomes a new stereocenter during cyclization.

    • Alpha (\alpha) form: Hydroxyl group on C-1 oriented below the plane of the ring.

    • Beta (\beta) form: Hydroxyl group on C-1 oriented above the plane.

  • Mutarotation: The ring-opening and re-closing process (anomeric interconversion).

  • Significance: The distinction between carbon atoms in the ring is essential for subsequent glycosidic bond formation in disaccharides and polysaccharides.

Disaccharides

  • Formation: Two monosaccharides join via a dehydration reaction (condensation/dehydration synthesis).

    • A hydroxyl group from one monosaccharide combines with a hydrogen from another, releasing a water molecule.

    • A covalent bond forms between the two sugar units.

  • Glycosidic Bond (Linkage):

    • The covalent bond linking two monosaccharide units.

    • Alpha (\alpha) glycosidic bond: Formed when the OH group on C-1 of the first sugar is below the ring plane.

    • Beta (\beta) glycosidic bond: Formed when the OH group on C-1 of the first sugar is above the plane.

    • Linkage described by carbon numbers involved (e.g., 1 \to 4 linkage).

  • Common Disaccharides:

    • Lactose: Glucose + Galactose.

    • Maltose: Glucose + Glucose.

    • Sucrose: Glucose + Fructose.

  • Sucrose Linkage: Between C-1 of glucose and C-2 of fructose.

  • Numbering: Carbohydrate units numbered from the carbon nearest the carbonyl (C1 is typically anomeric carbon in cyclic form).

  • Different combinations and linkages yield diverse sugar molecules with distinct properties and digestibility.

Polysaccharides

  • Long chains of monosaccharides linked by glycosidic bonds.

  • Can be branched or unbranched.

  • May consist of different monosaccharide types.

  • Molecular weight can exceed 100,000 daltons.

  • Serve energy storage or structural roles.

Starch

  • Storage carbohydrate in plants.

  • Composed of two components:

    • Amylose: Unbranched chains of glucose connected primarily by 1 \to 4 glycosidic bonds.

    • Amylopectin: Branched, includes branch points formed by 1 \to 6 linkages.

  • Storage: Plants synthesize glucose and store excess in roots and seeds.

  • Function: Nourishment for plant embryo during germination; energy source for humans and animals.

  • Digestion: Enzymes like amylase (e.g., in saliva) catalyze breakdown into maltose and glucose.

Glycogen

  • Storage form of glucose in humans and other vertebrates.

  • Animal equivalent of starch.

  • Highly branched, typically stored in liver and muscle cells.

  • Function: Undergoes glycogenolysis to release glucose into bloodstream when blood glucose levels drop, maintaining energy homeostasis.

Cellulose

  • Most abundant natural biopolymer.

  • Major component of plant cell walls.

  • Composed of glucose monomers linked by eta-1 \to 4 glycosidic bonds.

  • Structure: eta-linkages cause every other glucose monomer to be flipped, forming a linear, fibrous structure.

  • Function: Provides rigidity and high tensile strength to plant cells.

  • Digestion:

    • Humans cannot digest due to lack of cellulase enzymes.

    • Some herbivores (cows, koalas, buffalos) and termites digest cellulose with the help of specialized gut flora or symbiotic microorganisms that secrete cellulases.

    • The appendix in some grazing animals also contains bacteria aiding cellulose digestion.

Chitin

  • Nitrogen-containing polysaccharide.

  • Composed of repeating N-acetyl-eta-D-glucosamine units.

  • Roles:

    • Forms the outer skeleton of arthropods (insects, crustaceans).

    • Major component of fungal cell walls.

  • Function: Contributes to structural integrity.

Functions, Digestion, and Biological Relevance

  • Primary Energy Source: Through glucose, fueling cellular respiration and ATP production.

  • Structural Components: Cellulose in plants, chitin in arthropods.

  • Storage: Plant energy storage and animal metabolism.

  • Key Concepts Highlighted:

    • Classification: monosaccharides, disaccharides, polysaccharides.

    • Monosaccharide naming (suffix -ose), aldoses vs. ketoses.

    • Ring forms and the anomeric carbon (\alpha and eta configurations).

    • Importance of specific glycosidic linkages in determining digestibility and function (e.g., starch vs. cellulose).

Key Relationships and Equations

  • Carbohydrates follow the general empirical formula and ratio: (CH2O)n