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