CHE110 Chapter 20-1

Chapter 20: Carbohydrates and Their Functions

  • Overview of carbohydrates as essential biomolecules in biological systems.

20.1 Carbohydrates

  • Definition: Carbohydrates also known as sugars and starches are classified as polyhydroxy aldehydes or ketones.

    • They can undergo hydrolysis to yield monosaccharides.

  • Monosaccharides: The simplest form of carbohydrates.

20.1.1 Types of Carbohydrates

  • Disaccharides: Composed of two monosaccharide units.

    • Example: Lactose is a common disaccharide.

  • Polysaccharides: Comprise three or more monosaccharide units joined together.

    • Example: One form of starch.

20.1.2 Functions of Carbohydrates

  • Synthesized in plants through photosynthesis, converting sunlight into chemical energy stored in carbohydrate form.

  • Serve as a quick source of energy for the body, particularly glucose, during exercise.

20.2 Monosaccharides

  • General Characteristics: Contain 3 to 6 carbon atoms (C) arranged in a chain and possess an aldehyde or ketone group with multiple –OH (hydroxyl) groups.

  • Categories:

    • Aldoses: Monosaccharides with an aldehyde group.

    • Ketoses: Monosaccharides with a ketone group.

20.2.1 Examples of Monosaccharides

  • Simplest Aldose: Glyceraldehyde.

  • Simplest Ketose: Dihydroxyacetone.

  • Classification by Number of Carbons:

    • Triose = 3 carbons

    • Tetrose = 4 carbons

    • Pentose = 5 carbons

    • Hexose = 6 carbons

  • Examples:

    • Glyceraldehyde as an aldotriose, Dihydroxyacetone as a ketotriose.

20.2.2 Properties of Monosaccharides

  • Sweet-tasting with varying levels of sweetness.

  • High melting points due to polar nature and hydrogen bonding, contributing to water solubility.

20.2.3 Chirality in Monosaccharides

  • Fischer Projection Formula: Utilized to represent stereochemistry for carbohydrates.

    • Only one chirality center: Glyceraldehyde has two possible enantiomers (D and L).

    • Glucose has four chirality centers, with configuration based on the orientation of –OH groups.

  • All naturally occurring sugars are classified as D sugars.

20.2.4 Common Monosaccharides

  • Glucose: Blood sugar, most abundant monosaccharide.

    • Normal range: 70-110 mg/dL.

    • Excess glucose is stored as glycogen or fat.

    • Regulated by Insulin, which stimulates glucose uptake.

  • Galactose: Component of lactose, and galactosemia (enzyme deficiency) leads to its accumulation causing cataracts.

  • Fructose: Found in honey, sweeter than glucose, component of sucrose.

20.5 Focus on Health & Medicine

20.5.1 Sucrose and Artificial Sweeteners

  • Sucrose (Table Sugar): Disaccharide of glucose and fructose, high in calories.

    • Artificial sweeteners developed to mitigate caloric intake.

  • Aspartame: Hydrolyzed into phenylalanine, unsuitable for individuals with phenylketonuria.

  • Saccharin: Historically significant sweetener used during WWI.

  • Sucralose: Resembles sucrose structurally.

20.6 Polysaccharides

20.6.1 Cellulose

  • Definition: Unbranched polymer made of repeating glucose units, joined by glycosidic linkages.

  • Function: Provides structural support in plant cell walls; humans cannot digest it (source of insoluble fiber).

20.6.2 Starch

  • Definition: Polymer of glucose, occurring in two forms - amylose and amylopectin.

  • Occurrence: Found in corn, rice, wheat, and potatoes.

20.8 Focus on the Human Body Blood Type

  • Blood Types: Types A, B, and O are defined by the presence of specific monosaccharides in their O-linked glycoproteins.

  • Type A: Contains N-acetyl-D-galactosamine.

  • Type B: Contains D-galactose.

  • Type O: No additional monosaccharide modification.

Conclusion: Carbohydrates play crucial roles as energy sources and structural components in living organisms, with various forms including mono-, di-, and polysaccharides, each serving distinct biological functions.