4.2 - Carbohydrates

Types of Carbohydrates

• Carbohydrates are organic compounds consisting of carbon (C), hydrogen (H), and oxygen (O) in the ratio $1:2:1$, with the chemical formula $(C{H}_{2}O{)}_n$.
• The three main types are monosaccharides, disaccharides, and polysaccharides.

Monosaccharides (Simple Sugars)

• Monosaccharides are carbohydrate monomers and the simplest carbohydrate units.
• Most taste sweet, form crystals, and dissolve in water.
• Monosaccharides are reducing sugars: when heated in Benedict’s solution, they reduce blue copper(II) sulphate to a brick-red, water-insoluble precipitate of copper(I) oxide.
• Glucose: Found in rice, wheat, and fruits such as grapes; it is the most common monosaccharide and forms most polysaccharides.
• Fructose: Found in honey and sweet fruits.
• Galactose: Found in milk.

Disaccharides

• Two monosaccharides combine by condensation to form a disaccharide; one water molecule is removed.
• A disaccharide is broken down into its monosaccharides by hydrolysis; one water molecule is added.
• Formation by condensation:
  • $\text{Glucose}+\text{glucose}\to \text{maltose}+\text{water}$
  • $\text{Glucose}+\text{fructose}\to \text{sucrose}+\text{water}$
  • $\text{Glucose}+\text{galactose}\to \text{lactose}+\text{water}$
• Breakdown by hydrolysis:
  • $\text{Maltose}+\text{water}\to \text{glucose}+\text{glucose}$
  • $\text{Sucrose}+\text{water}\to \text{glucose}+\text{fructose}$
  • $\text{Lactose}+\text{water}\to \text{glucose}+\text{galactose}$
• Maltose and lactose are reducing sugars; sucrose is a non-reducing sugar.
• Maltose: Found in grains. Sucrose: Found in sugar cane, sweet fruits, and sugar beet. Lactose: Found in milk.

Polysaccharides

• Polysaccharides are sugar polymers consisting of monosaccharide monomers.
• Hundreds of monosaccharides combine by condensation to form long molecular chains.
• Polysaccharides can be broken down by hydrolysis with the help of dilute acids, boiling, and enzyme action.
• They are insoluble in water because of their large molecular size, do not taste sweet, and do not crystallise.
• Starch: Main storage polysaccharide in plants; also found in chloroplasts. Sources include grains, potatoes, and legumes.
• Glycogen: Main storage polysaccharide in animal liver and muscle cells.
• Cellulose: Main structural component of plant cell walls.
• Importance in cells: Glucose supplies energy, glycogen and starch act as food reserves, and cellulose provides support.