Carbohydrates

Facts

Overview of Carbohydrates

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically with a hydrogen to oxygen ratio of 2:1. They are a primary source of energy and play structural roles in organisms.

Monosaccharides

Monosaccharides are the simplest form of carbohydrates (simple sugars) and serve as the monomers for building larger carbohydrates.

Key Features

• General formula: (CH₂O)ₙ (where n is the number of carbon atoms)

• Composition: Contain carbon (C), hydrogen (H), and oxygen (O)

• Examples: Glucose, fructose, galactose

• Classification by Carbon Atoms:

  Carbon Atoms	Type	Formula	Example
  3	Triose	C₃H₆O₃	Glyceraldehyde
  5	Pentose	C₅H₁₀O₅	Ribose
  6	Hexose	C₆H₁₂O₆	Glucose

Glucose Isomers

Glucose exists in two isomeric forms:

• α-glucose: Hydroxyl (-OH) group on carbon-1 is below the ring
• β-glucose: Hydroxyl (-OH) group on carbon-1 is above the ring

This slight difference significantly impacts their biological roles and the properties of the polymers they form.

Disaccharides

Disaccharides form when two monosaccharides join via a condensation reaction, creating a glycosidic bond and releasing a molecule of water.

Common Disaccharides

1. Maltose

   • Formation: Glucose + Glucose
   • Source: Germinating seeds
   • Type: Reducing sugar

2. Sucrose

   • Formation: Glucose + Fructose
   • Source: Table sugar, transported in phloem of plants
   • Type: Non-reducing sugar

3. Lactose

   • Formation: Glucose + Galactose
   • Source: Milk
   • Type: Reducing sugar
   • Note: Lactose intolerance occurs when individuals lack the enzyme lactase to hydrolyze lactose.

Polysaccharides

Polysaccharides are large, complex carbohydrates formed by the condensation of many monosaccharides. They serve various functions based on their structure.

Types of Polysaccharides

1. Starch

   • Structure: Made of two polymers of α-glucose:
     • Amylose: Unbranched chains with 1,4 glycosidic bonds, forming a helical structure
     • Amylopectin: Branched chains with 1,4 and 1,6 glycosidic bonds
   • Function: Energy storage in plants
   • Properties:
     • Insoluble (doesn't affect water potential)
     • Compact (stores energy efficiently)
     • Easily hydrolyzed to release glucose

2. Glycogen

   • Structure: Similar to amylopectin but more highly branched
   • Function: Energy storage in animals
   • Properties:
     • Insoluble (doesn't affect osmotic balance)
     • Compact
     • Highly branched (allows rapid glucose release)
     • Stored in liver and muscle cells

3. Cellulose

   • Composition: Made of β-glucose units joined by 1,4 glycosidic bonds
   • Structure:
     • Straight, unbranched chains
     • Chains run parallel and form hydrogen bonds between them
     • Bundled into microfibrils providing high tensile strength
   • Function: Provides structural support in plant cell walls
   • Properties:
     • High tensile strength
     • Insoluble
     • Rigid structure contributes to cell wall rigidity

Reducing and Non-Reducing Sugars

Reducing Sugars

• Definition: Sugars that can donate electrons to another chemical (reducing agent)
• Examples: All monosaccharides (e.g., glucose, fructose), some disaccharides (e.g., maltose, lactose)

Non-Reducing Sugars

• Definition: Sugars that cannot donate electrons (do not have a free aldehyde or ketone group)
• Example: Sucrose

Biochemical Tests

1. Benedict's Test for Reducing Sugars

   • Procedure:
     • Add Benedict's reagent to the sample
     • Heat the mixture in a boiling water bath
   • Positive Result: Color changes from blue to green, yellow, orange, or brick-red precipitate (indicates the presence of reducing sugars)
   • Note: The color intensity correlates with the amount of reducing sugar

2. Test for Non-Reducing Sugars

   • Procedure:
     • If the Benedict's test is negative, boil a fresh sample with dilute hydrochloric acid to hydrolyze the sugar
     • Neutralize with sodium hydrogen carbonate
     • Perform the Benedict's test again
   • Positive Result: Brick-red precipitate after second test indicates the presence of non-reducing sugars (e.g., sucrose broken down into glucose and fructose)

3. Iodine Test for Starch

   • Procedure:
     • Add iodine-potassium iodide solution to the sample
   • Positive Result: Color changes from brown-orange to blue-black

Making and Breaking Polymers

• Condensation Reactions: Build polymers by joining monomers and releasing water
• Hydrolysis Reactions: Break down polymers into monomers using water

Properties of Storage Polysaccharides

Energy Storage: Glycogen vs. Glucose

• Glycogen is more suitable for energy storage in animals compared to glucose because:
  • Insolubility: Doesn't affect cell's osmotic balance
  • Compactness: Stores large amounts of energy in a small space
  • Branched Structure: Allows rapid mobilization of glucose when energy is needed
  • Non-diffusible: Remains within cells

Structural Comparison: Amylose and Cellulose

• Amylose:

  • Composition: Unbranched chains of α-glucose
  • Structure: Helical due to 1,4 glycosidic bonds
  • Function: Energy storage in plants
  • Properties:
    • Compact
    • Insoluble

• Cellulose:

  • Composition: Unbranched chains of β-glucose
  • Structure: Straight chains with every other glucose flipped, forming strong microfibrils via hydrogen bonds
  • Function: Provides structural support in plant cell walls
  • Properties:
    • High tensile strength
    • Insoluble
    • Rigid

Importance of Monosaccharide Structure

• α-glucose polymers (e.g., starch and glycogen) are suitable for energy storage due to their helical and branched structures
• β-glucose polymers (e.g., cellulose) provide structural support due to the formation of straight chains and microfibrils

Recall

Practice