Carbohydrates– Classification, Structures

Carbohydrates are a large and diverse group of organic compounds that are primarily composed of carbon, hydrogen, and oxygen. They serve as a major source of energy for living organisms and play crucial roles in various biological processes. Carbohydrates are commonly found in foods like grains, fruits, and vegetables and are essential for the proper functioning of cells, tissues, and organs.

Classification of Carbohydrates Carbohydrates can be classified into three main categories based on their structure and complexity:

Monosaccharides (Simple Sugars)

• • Monosaccharides are the simplest form of carbohydrates and cannot be hydrolyzed into smaller sugars.
  • They are the building blocks of more complex carbohydrates.
  • Common monosaccharides include:
    • Glucose: A six-carbon sugar (C6H12O6) that is the primary energy source for cells.
    • Fructose: A six-carbon sugar found in fruits and honey.
    • Galactose: A component of lactose found in milk.
    • Ribose and Deoxyribose: Five-carbon sugars found in RNA and DNA, respectively.
  • Properties:
    • They are soluble in water and have a sweet taste.
    • Most monosaccharides have either a six-membered ring (pyranose form) or a five-membered ring (furanose form) in solution.

Disaccharides

• • Disaccharides consist of two monosaccharide units joined by a glycosidic bond.
  • Examples of disaccharides include:
    • Sucrose: Composed of glucose and fructose. It is commonly known as table sugar.
    • Lactose: Composed of glucose and galactose. It is the sugar found in milk.
    • Maltose: Composed of two glucose molecules. It is formed during the digestion of starch.
  • Properties:
    • Disaccharides are also soluble in water and are sweet-tasting.
    • They can be hydrolyzed into their monosaccharide components by enzymes.

Polysaccharides (Complex Carbohydrates)

• • Polysaccharides are long chains of monosaccharide units connected by glycosidic bonds.
  • These molecules can be either branched or unbranched, depending on the specific glycosidic linkages.
  • Examples of polysaccharides include:
    • Starch: A storage form of energy in plants, made of two types of molecules: amylose (unbranched) and amylopectin (branched).
    • Glycogen: A storage form of energy in animals, mainly found in the liver and muscles.
    • Cellulose: A structural component of plant cell walls, made of linear chains of glucose molecules.
    • Chitin: A polysaccharide found in the exoskeletons of arthropods and insects, composed of N-acetylglucosamine units.
  • Properties:
    • Polysaccharides do not have a sweet taste and are generally not soluble in water.
    • They serve as a long-term energy storage and structural materials in living organisms.

Structure of Carbohydrates

Carbohydrates are made up of carbon atoms that are hydrated, meaning they are bonded to hydrogen and oxygen atoms. The general formula for carbohydrates is Cn(H2O)n, where “n” represents the number of carbon atoms.

Monosaccharide Structure

• Monosaccharides have a basic structure that consists of:
  • A carbonyl group (aldehyde or ketone).
  • Multiple hydroxyl groups (-OH).
  • A chiral center at each carbon atom except for the carbonyl carbon.
• Aldoses are monosaccharides with an aldehyde group (e.g., glucose).
• Ketoses are monosaccharides with a ketone group (e.g., fructose).

Monosaccharides can exist in open-chain form (a linear structure) or cyclic form (a ring structure). The cyclic forms are more common in aqueous solutions.

Disaccharide and Polysaccharide Structures

• In disaccharides, two monosaccharide molecules are linked by a glycosidic bond, formed through a dehydration reaction that releases a water molecule.
• In polysaccharides, many monosaccharides are linked by glycosidic bonds to form a long chain. The type of linkage (e.g., α-1,4-glycosidic bond, β-1,4-glycosidic bond) determines the properties of the polysaccharide.
  • Starch and glycogen have α-linkages, making them digestible by humans.
  • Cellulose has β-linkages, which humans cannot digest but some herbivores can due to specific enzymes in their gut.

Key Functional Groups in Carbohydrates

1. Hydroxyl Groups (-OH): Found on each carbon except for the carbonyl carbon in aldoses. These groups allow carbohydrates to form hydrogen bonds and interact with water, making them soluble.
2. Carbonyl Group (C=O): Either an aldehyde (in aldoses) or a ketone (in ketoses), which is key for the chemical reactivity of monosaccharides.
3. Glycosidic Bonds: Covalent bonds formed between two monosaccharides through a dehydration reaction. These bonds link sugar molecules in disaccharides and polysaccharides.

Importance of Carbohydrates

1. Energy Source: Carbohydrates are a major source of energy in living organisms. Glucose, for example, is used in cellular respiration to produce ATP, the primary energy currency of cells.
2. Structural Roles: Polysaccharides like cellulose and chitin provide structural support in plants and animals, respectively.
3. Storage: Starch (in plants) and glycogen (in animals) serve as energy storage molecules.
4. Cell Communication: Carbohydrates on the surface of cells (glycans) are involved in cell recognition, signaling, and adhesion.