Carbohydrates: Introduction and Overview

Definition and Importance

Carbohydrates are a major class of biomolecules essential for life. They play critical roles in cellular structure, molecular identity, and energy storage.

• Monosaccharides: Simple sugars, or "one-sugar" monomers (e.g., glucose).

• Oligosaccharides: Short chains of monosaccharide units ("few-sugars").

• Polysaccharides: Long chains of monosaccharide units ("many-sugars").

Key roles: structural support, cell recognition, and energy storage.

Structure of Monosaccharides

General Features

Monosaccharides are the simplest carbohydrates, typically following the general formula (CH2O)n, where n is usually 3 or more.

• Contain at least three carbon atoms.

• Have one carbonyl group (C=O) and two or more hydroxyl groups (–OH).

Naming Monosaccharides

• By Carbonyl Group Position:

  • Aldose: Carbonyl group at the end of the carbon chain (an aldehyde).

  • Ketose: Carbonyl group within the carbon chain (a ketone).

• By Number of Carbon Atoms:

  • 3C = Triose

  • 4C = Tetrose

  • 5C = Pentose

  • 6C = Hexose

Isomers: Monosaccharides with the same molecular formula but different structural arrangements (e.g., glucose and galactose).

Examples

• Glyceraldehyde (Aldotriose, 3C)

• Dihydroxyacetone (Ketotriose, 3C)

• Glucose (Aldohexose, 6C)

Monosaccharide Structure: Linear and Ring Forms

Linear and Ring Structures

Monosaccharides can exist in both linear and ring (cyclic) forms. In aqueous solutions, the ring form is usually predominant.

• Ring formation occurs when the carbonyl group reacts with a hydroxyl group on the same molecule.

• For glucose, the C-1 carbon (anomeric carbon) forms a bond with the C-5 hydroxyl group, creating either α-glucose or β-glucose depending on the orientation of the hydroxyl group at C-1.

Example: Glucose can cyclize to form α- or β-glucose, which differ in the position of the –OH group on the anomeric carbon.

Polymerization of Monosaccharides

Glycosidic Linkages

Monosaccharides polymerize via glycosidic linkages (a type of covalent bond) to form disaccharides and polysaccharides.

• Disaccharide: Two monosaccharides joined by a glycosidic bond (e.g., maltose).

• Polysaccharide: Multiple monosaccharides joined in long chains (e.g., starch, glycogen, cellulose).

Formation: Glycosidic bonds are formed through a dehydration (condensation) reaction, releasing a molecule of water.

Major Polysaccharides and Their Functions

Storage Polysaccharides

• Starch: Main storage polysaccharide in plants; composed of α-glucose monomers joined by α-1,4 and α-1,6 glycosidic linkages.

• Glycogen: Main storage polysaccharide in animals; similar to starch but more highly branched.

Structural Polysaccharides.

• Cellulose: Major component of plant cell walls; composed of β-glucose monomers joined by β-1,4 glycosidic linkages. Parallel strands are joined by hydrogen bonds, providing rigidity.

• Chitin: Found in fungal cell walls and exoskeletons of arthropods; similar structure to cellulose but with N-acetylglucosamine monomers.

• Peptidoglycan: Structural polysaccharide in bacterial cell walls; consists of sugar chains cross-linked by short peptides.

Carbohydrates in Cell Identity and Communication

Glycoproteins and Glycolipids

Carbohydrates attached to proteins (glycoproteins) and lipids (glycolipids) are key molecules in cell recognition and signaling.

• Help identify cells as "self" or "non-self" (important in immune response).

• Mediate cell-cell communication and adhesion.

• Example: ABO blood group antigens are specific glycolipids on red blood cells.

Carbohydrates and Energy Storage

Chemical Energy in Carbohydrates

Carbohydrates store chemical energy in their covalent bonds, which can be released during cellular respiration.

• Plants synthesize carbohydrates via photosynthesis:

• Animals and other organisms break down glucose to produce ATP:

• ATP produced is used to drive cellular processes such as biosynthesis and muscle contraction.

Photosynthesis and Respiration: Complementary Processes

• Photosynthesis: Converts solar energy into chemical energy stored in carbohydrates.

• Cellular Respiration: Releases energy from carbohydrates to produce ATP.

Summary Table: Types and Functions of Carbohydrates

Additional info: The notes above expand on the provided slides and text, adding definitions, examples, and context for clarity and completeness.