Carbohydrates

Overview of Carbohydrates

• Carbohydrates are a class of organic macromolecules.

• Commonly associated with dietary components such as sugars and starches.

Chemical Structure of Carbohydrates

• Carbohydrates consist of three types based on molecular size:

  • Monosaccharides

  • Disaccharides

  • Polysaccharides

• Monosaccharides serve as the building blocks for disaccharides and polysaccharides.

Monosaccharides

• Defined as single sugar molecules.

• General chemical formula: For each carbon atom (C), there are twice as many hydrogens (H) and the same number of oxygens (O): CnH{2n}O_n

  • Examples of monosaccharides include:

  • Glucose (C6H{12}O_6)

  • Fructose (C6H{12}O_6)

  • Galactose (C6H{12}O_6)

• These monosaccharides are classified as hexoses due to having six carbon atoms.

• Despite having the same chemical formula, glucose, fructose, and galactose are isomers (different structures).

Functional Roles of Monosaccharides

• Monosaccharides can also serve as starting materials for synthesizing other organic macromolecules, including:

  • Amino acids

  • Fats

• Carbohydrates act as the main source of energy for organisms.

• Additionally, they can have structural roles in some cases.

Structural Forms of Monosaccharides

• Monosaccharides exist in two forms:

  • Linear form

  • Ring form

  • In aqueous solutions (such as the cytoplasm of cells), the ring form is more stable.

Disaccharides

• Formed by the joining of two monosaccharides through a dehydration reaction:

  • When two monosaccharides join, they lose water (the components of the molecules combine to form water).

  • Example: When glucose and fructose combine, they form sucrose (table sugar, C{12}H{22}O_{11}).

  • These links are called glycosidic bonds.

• Common disaccharides include:

  • Sucrose

  • Maltose (formed from two glucose molecules, involved in starch digestion)

  • Lactose (found in milk)

• Isomeric relationship: Sucrose, maltose, and lactose all share the same chemical formula C{12}H{22}O_{11} but have different structural arrangements.

• The reason the expected ratio C{n}H{2n}O_{n} does not hold true for disaccharides is that during their formation, two hydrogens and one oxygen are removed when water is released.

Polysaccharides

• Polysaccharides are long chains formed by repeating monosaccharides.

• Understandably categorized based on their function:

  • Storage polysaccharides

  • In plants, energy storage is primarily through starch and amylose (composed of long chains of glucose).

    • Example: Potatoes are known for their starchy content, which is stored glucose.

  • In animals, carbohydrates are stored as glycogen, which features numerous branches, making it easier to access glucose for energy.

  • Structural polysaccharides

  • These polysaccharides provide structural integrity.

  • Cellulose contributes rigidity to plant cell walls, analogous to a skeleton's function in providing structure.

    • Composed of glucose in a mesh-like structure as opposed to linear chains.

  • Chitin is another structural polysaccharide found in arthropod exoskeletons, distinguished by the presence of nitrogen in its su gar units.

Structural Differences Between Polysaccharides

• Alpha Glucose vs. Beta Glucose

  • Alpha glucose: Hydroxyl group on the first carbon points down.

  • Beta glucose: Hydroxyl group on the first carbon points up.

• The alternating orientation of glucose molecules in cellulose gives its fibrous structure, making it resistant to digestion by most animals.

• Some organisms, such as certain ruminants, digest cellulose due to symbiotic relationships with microflora in their guts that can break the beta-1,4 bonds found in cellulose.

• Fungi also play a role as decomposers by breaking down cellulose, recycling nutrients back into ecosystems.

Carbohydrates in Diet and Digestion

• Cellulose serves as insoluble fiber in the human digestive system, helping to facilitate bowel movements and overall gut health.