Chapter 5: An Introduction to Carbohydrates

Overview of Carbohydrates

Carbohydrates are essential biomolecules that play critical roles in cell structure, cell identity, and energy storage. They are classified based on the number of monomer units they contain.

• Monosaccharide: Single sugar unit

• Oligosaccharide: Short chains of sugar units

• Polysaccharide: Long chains of sugar units

General Structure and Formula of Carbohydrates

Most carbohydrates have the general molecular formula , where n is the number of carbon atoms. Their structure includes a carbonyl group, multiple hydroxyl groups, and many carbon-hydrogen bonds.

• Carbonyl group: , can be at the end (aldose) or in the middle (ketose) of the molecule.

• Hydroxyl groups: , attached to various carbons.

• Carbon-hydrogen bonds: , contribute to energy storage.

• Not all compounds are carbohydrates (e.g., formaldehyde).

Monosaccharides: Sugars as Monomers

Functions and Importance

Monosaccharides are simple sugars that serve as fundamental building blocks for larger carbohydrates and provide chemical energy in cells. They also played a key role in chemical evolution, such as ribose being required for nucleotide formation.

• Energy source: Immediate chemical energy for cellular processes.

• Building blocks: Used to synthesize oligosaccharides and polysaccharides.

• Evolutionary significance: Ribose is essential for the formation of RNA and DNA.

Structural Variations in Monosaccharides

Monosaccharides vary in three main ways: location of the carbonyl group, number of carbon atoms, and spatial arrangement of atoms.

• 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:

  • Triose: 3 carbons

  • Pentose: 5 carbons

  • Hexose: 6 carbons

• Spatial arrangement of atoms:

  • Different arrangement of hydroxyl groups leads to different sugars (e.g., glucose vs. galactose).

Example: D-Glucose vs. D-Mannose

D-Mannose is a six-carbon sugar similar to D-glucose, except for the orientation of the hydroxyl group on carbon 2.

• D-Glucose: Hydroxyl group on carbon 2 is below the plane.

• D-Mannose: Hydroxyl group on carbon 2 is above the plane.

Example: Three-Carbon Sugars

Monosaccharides can also differ by the position of the carbonyl group and the arrangement of hydroxyl groups, as seen in aldoses and ketoses.

Ring Structures of Sugars

In aqueous solutions, monosaccharides often form ring structures. This structural change has important functional consequences.

• Linear form: Rare in solution.

• Ring forms: Most glucose molecules form either α (alpha) or β (beta) ring structures.

The Structure of Polysaccharides

Formation of Polysaccharides

Polysaccharides are formed when monosaccharides are linked together by condensation reactions between hydroxyl groups, resulting in glycosidic linkages. These linkages can be broken by hydrolysis reactions.

• Glycosidic linkage: Covalent bond formed between two monosaccharides.

• Condensation reaction: Water molecule is released during bond formation.

• Hydrolysis: Water molecule is used to break the bond.

Types of Glycosidic Linkages

Glycosidic linkages can form between any two hydroxyl groups, but two of the most common are:

• α-1,4-glycosidic linkage: Found in starch and glycogen.

• β-1,4-glycosidic linkage: Found in cellulose and chitin.

Both linkages are between the C-1 and C-4 carbons, but their geometry is different, affecting the properties of the polysaccharide.

Major Polysaccharides and Their Functions

Comparison of Amylose and Amylopectin

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

• Amylopectin: Branched starch with some α-1,6-glycosidic linkages; branches occur about once every 30 monomers.

Comparison of Chitin and Peptidoglycan

• Chitin: Parallel strands of glucosamines cross-linked by hydrogen bonds.

• Peptidoglycan: Parallel strands cross-linked by peptide bonds; contains alternating monosaccharides.

Functions of Carbohydrates in Cells

Diverse Cellular Functions

Carbohydrates serve multiple roles in cells:

• Precursors: Used to synthesize nucleotides and amino acids.

• Structural materials: Provide fibrous support (e.g., cellulose, chitin, peptidoglycan).

• Cell identity: Display information on cell surfaces via glycoproteins and glycolipids.

• Energy storage: Store chemical energy for cellular processes.

Structural Support

Cellulose, chitin, and peptidoglycan form organized fibers or sheets that give cells and organisms strength and elasticity. Their β-1,4-glycosidic linkages are difficult to hydrolyze, and the fibers exclude water, making them resistant to degradation.

• Aspect 1: Linear, fibrous structure due to β-linkages.

• Aspect 2: Extensive hydrogen bonding or peptide cross-linking between strands.

Cell Identity

Carbohydrates on the cell surface are often attached to proteins (glycoproteins) or lipids (glycolipids), serving as key molecules in cell-cell recognition and signaling.

Energy Storage and Release

Carbohydrates store energy in their chemical bonds. During photosynthesis, plants convert sunlight, water, and carbon dioxide into carbohydrates:

• Photosynthesis equation:

• Carbohydrates have more potential energy than CO2 due to their C-H and C-C bonds.

• Energy is released when polysaccharides are hydrolyzed to glucose.

Bond Energy in Carbohydrates

• C-H and C-C bonds: High potential energy.

• C-O bonds: Lower potential energy.

Enzymatic Hydrolysis of Storage Polysaccharides

Starch and glycogen are easily hydrolyzed due to their α-glycosidic linkages. Specific enzymes catalyze this process:

• Phosphorylase: Hydrolyzes α-glycosidic linkages in glycogen.

• Amylase: Hydrolyzes α-glycosidic linkages in starch; produced by salivary glands and pancreas.

Example: Human digestion of starch begins in the mouth with salivary amylase and continues in the small intestine with pancreatic amylase.

Additional info: The notes have been expanded to include definitions, examples, and comparisons for clarity and completeness.