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Carbohydrates: Structure, Function, and Types

This summary outlines the key aspects of carbohydrates as a major type of macromolecule, focusing on their composition, structure, and biological roles.

Main Idea

Carbohydrates are essential macromolecules composed of carbon, hydrogen, and oxygen. They are primarily used by the human body for energy production and can exist as single units (monosaccharides), pairs (disaccharides), or long chains (polysaccharides).

Composition and Structure

• Elements: Carbohydrates are made up of carbon (carbo-), hydrogen, and oxygen (hydrate-).
• Basic Unit: They are derived from hydrocarbons and often form ring-shaped structures.
• Classification by Size:
  • Monosaccharides: Single sugar units (e.g., glucose, fructose, galactose).
  • Disaccharides: Two monosaccharides linked together (e.g., sucrose, lactose).
  • Polysaccharides: Many monosaccharides linked together (e.g., starch, cellulose, glycogen).

Role in the Human Body

• Energy Production: The primary functional role of carbohydrates is to provide energy.
• Cellular Components: Carbohydrates are found on cell membranes and associated with genetic material.
• Body Composition: Carbohydrates constitute about 1% of the human body by mass.

Monosaccharides

• Simple Sugars: These are the basic building blocks.
• Key Examples:
  • Glucose: The primary monosaccharide used for energy production.
  • Fructose: Found in fruits.
  • Galactose: Found in milk.
• Isomers: Glucose, fructose, and galactose share the same chemical formula (C6H12O6) but differ in their atomic arrangement.
• Metabolism: The liver converts fructose and galactose into glucose for energy or storage.

Disaccharides

• Formation: Formed by linking two monosaccharides through a dehydration reaction, where a water molecule is removed.
• Examples:
  • Sucrose: Glucose + Fructose (common table sugar).
  • Lactose: Glucose + Galactose (found in milk).
• Breakdown: Disaccharides are broken down into monosaccharides through rehydration reactions, requiring specific enzymes.
  • Lactose Intolerance: Occurs when the enzyme lactase is deficient, preventing the breakdown of lactose. Undigested lactose draws water into the intestines, causing diarrhea due to an osmotic effect.
• Note: A disaccharide formed from fructose and galactose does not naturally occur.

Polysaccharides

• Complex Carbohydrates: Long chains of monosaccharides.
• Types and Sources:
  • Starches: Found in plants (e.g., potatoes, grains). Digestible by humans.
  • Cellulose: Found in plants (e.g., leafy greens, grass). Indigestible by humans due to a lack of necessary enzymes. Forms dietary fiber.
  • Glycogen: Stored in animals (liver and muscles) as a form of glucose storage. Digestible by humans.
• Structure:
  • Cellulose: Linear chains of glucose.
  • Starch: Branched chains of glucose.
  • Glycogen: Highly branched chains of glucose.
• Digestibility: Humans possess enzymes to break down starch and glycogen but not cellulose. Cows, for example, have enzymes to digest cellulose.
• Fiber: Indigestible polysaccharides like cellulose that aid in digestion by adding bulk to stool.