Chapter 5: An Introduction to Carbohydrates

Overview of Carbohydrates

Carbohydrates are a diverse group of biomolecules essential for energy storage, cell structure, and cell identity. They are composed of carbon, hydrogen, and oxygen, and include sugars and their polymers. Carbohydrates are classified based on the number of monomeric sugar units they contain.

• Monosaccharide: A single sugar molecule (simple sugar), e.g., glucose.

• Disaccharide: Two monosaccharides joined by a glycosidic linkage, e.g., sucrose.

• Oligosaccharide: Short polymers of 3–50 monosaccharides.

• Polysaccharide: Long polymers of monosaccharides, e.g., starch, glycogen, cellulose.

Carbohydrates have the general formula , where n can range from 3 to over a thousand.

Sugars as Monomers

Monosaccharide Structure and Classification

Monosaccharides are organic compounds with a carbonyl group and several hydroxyl groups. Their structure varies in several ways:

• Location of the carbonyl group:

  • Aldose: Carbonyl group at the end of the carbon chain.

  • Ketose: Carbonyl group in the middle of the carbon chain.

• Number of carbon atoms:

  • Three: Triose

  • Five: Pentose

  • Six: Hexose

• Spatial arrangement of atoms: Different arrangements of hydroxyl groups and alternative ring forms.

Monosaccharides can form ring structures in aqueous solutions, and their functional groups affect their chemical properties and reactivity.

The Structure of Polysaccharides

Formation and Linkages

Polysaccharides are formed by condensation reactions between hydroxyl groups of monosaccharides, resulting in covalent bonds called glycosidic linkages.

• Glycosidic linkage: Covalent bond formed between two sugar monomers.

• Linkages can be broken by hydrolysis reactions.

Types of Polysaccharides

• Starch: Storage polysaccharide in plants, composed of glucose monomers.

  • Amylose: Unbranched, with only α-1,4-glycosidic linkages.

  • Amylopectin: Branched, with some α-1,6-glycosidic linkages.

• Glycogen: Storage polysaccharide in animals, highly branched with α-1,6-glycosidic linkages.

• Cellulose: Structural polysaccharide in plant cell walls, composed of β-1,4-glycosidic linkages. Provides rigidity and strength.

• Chitin: Structural polysaccharide in fungi and animals, composed of N-acetylglucosamine (NAG) monomers.

• Peptidoglycan: Structural polysaccharide in bacterial cell walls, composed of alternating monosaccharides and peptide chains.

Table: Comparison of Major Polysaccharides

What Do Carbohydrates Do?

Functions in Cells

Carbohydrates serve several important functions in biological systems:

• Provide energy and store chemical energy.

• Serve as precursors for other molecules, such as nucleotides and amino acids.

• Provide structural support (e.g., cellulose in plants, chitin in fungi/animals, peptidoglycan in bacteria).

• Cell identity and signaling (e.g., glycoproteins and glycolipids).

Carbohydrates and Energy Storage

Photosynthesis is the process by which plants, algae, and some bacteria convert sunlight into chemical energy stored in carbohydrates.

Photosynthesis equation:

When cells need energy, they break down glucose and capture some of the released energy through synthesis of adenosine triphosphate (ATP):

Enzymes Involved in Carbohydrate Metabolism

• Phosphorylase: Breaks down glycogen to provide glucose in animal cells.

• Amylase: Hydrolyzes starch by catalyzing the hydrolysis of glycosidic linkages.

The energy in ATP is used to drive other cellular reactions, such as polymerization and muscle movement.

Additional Info

• Glycoproteins and glycolipids are important for cell-cell recognition and communication.

• Polysaccharides can be branched or unbranched, affecting their properties and biological roles.

• Cellulose, chitin, and peptidoglycan provide structural support due to their strong β-1,4-glycosidic linkages, which are difficult to hydrolyze.