Lecture 5 Notes

Chapter 5: An Introduction to Carbohydrates

I. Overview of Carbohydrates

Role in Life: The function of carbohydrates is fundamentally based on how their constituent units are linked together.
- Key Areas: They are crucial for cell structure, cell identity, and energy storage.
Molecular Formula: Carbohydrates generally conform to the empirical formula

$n$ represents the number of "carbon-hydrate groups" and can range from $3$ to over a thousand.

They contain a carbonyl group ( $C=O$ ) and numerous carbon-hydrogen bonds.

Distinction: Not all compounds with the molecular formula $(CH2O)n$ are carbohydrates (e.g., formaldehyde, $(CH2O)1$ ).

Polymerization Levels:
- Monosaccharide: "One-sugar" monomer, the simplest form of carbohydrate.
- Oligosaccharide: "Few-sugars" small polymers, typically composed of 2-10 monosaccharide units.
- Polysaccharide: "Many-sugars" large polymers, consisting of many monosaccharide units.

II. Monosaccharides: The Sugar Monomers (Section 5.1)

Importance:
- Provide immediate chemical energy for cells.
- Serve as essential building blocks for larger, more complex compounds.
- Played a significant role in chemical evolution (e.g., ribose is necessary for nucleotide formation).
- also used as feed stock qq for various biological processes, including the synthesis of proteins, lipids, and nucleic acids, enabling cellular function and growth.
Structural Variation: Monosaccharide monomers, or simple sugars, differ structurally in several key ways:
- Location of the Carbonyl Group:
  - Aldose: Carbonyl group is located at the end of the carbon chain (an aldehyde group).
  - Ketose: Carbonyl group is located in the middle of the carbon chain (a ketone group).
- Number of Carbon Atoms:
  - Triose: Contains $3$ carbon atoms.
  - Pentose: Contains $5$ carbon atoms (e.g., ribose, deoxyribose).
  - Hexose: Contains $6$ carbon atoms (e.g., glucose, fructose, galactose).
- Spatial Arrangement of Atoms: Differences in the arrangement of hydroxyl ( $-OH$ ) groups around asymmetric carbons lead to distinct isomers (e.g., glucose, galactose, and mannose are isomers with different $-OH$ configurations).
- Linear and Ring Forms:
  - While often depicted in linear forms, sugars typically exist in stable ring structures when in aqueous solutions.
  - These forms are in dynamic equilibrium.
Functional Consequences: Each structural variation directly impacts the unique function of a monosaccharide.

III. Polysaccharides: Complex Carbohydrate Structures (Section 5.2)

Definition: Polysaccharides, also known as complex carbohydrates, are polymers composed of many monosaccharide monomers.
Disaccharides: Formed when two sugars are linked together.
Glycosidic Linkage Formation:
- Sugars are linked via a condensation reaction (dehydration synthesis), which involves the removal of a water molecule. $\text{H-O-R}1 + \text{H-O-R}2 \rightarrow \text{R}1\text{-O-R}2 + H_2O$
- This reaction forms a covalent bond called a glycosidic linkage between two hydroxyl groups.
Glycosidic Linkage Breaking:
- Glycosidic linkages can be broken by hydrolysis reactions, which involve the addition of a water molecule. $\text{R}1\text{-O-R}2 + H2O \rightarrow \text{H-O-R}1 + \text{H-O-R}_2$
Diversity of Linkages: Glycosidic linkages can form between any two hydroxyl groups of monosaccharides, leading to diverse polymer structures.
- Common Linkages: Two of the most common are the $\alpha \text{-1,4-glycosidic linkage}$ and the $\beta \text{-1,4-glycosidic linkage}$ .
- Location: Both types connect the C-1 carbon of one monosaccharide to the C-4 carbon of another.
- Geometry Difference: The crucial distinction lies in their geometry:
  - $\alpha$ linkage: The hydroxyl group on C-1 is on the same side of the plane as the C-6 $\text{CH}_2\text{OH}$ group in the linear form (or points down in the ring structure).
  - $\beta$ linkage: The hydroxyl group on C-1 is on the opposite side of the plane as the C-6 $\text{CH}_2\text{OH}$ group in the linear form (or points up in the ring structure).
  - This difference in orientation (up vs. down) fundamentally affects the three-dimensional structure and functional properties of the resulting polysaccharide.

IV. Major Polysaccharides and Their Functions

Starch: Energy Storage in Plants
- Composition: Composed of $\alpha \text{-glucose}$ monomers.
- Structure: Primarily forms a helical shape.
- Types of Starch:
  - Amylose: An unbranched form of starch, containing only $\alpha \text{-1,4-glycosidic linkages}$ . This creates a simple helix.
  - Amylopectin: A branched form of starch, containing primarily $\alpha \text{-1,4-glycosidic linkages}$ with occasional $\alpha \text{-1,6-glycosidic linkages}$ at branch points.
    - Branches occur approximately once in every $30$ glucose monomers.
- Role: Used for long-term energy storage in plant cells (e.g., in potatoes, seeds).
Glycogen: Energy Storage in Animals
- Composition: A highly branched polymer of $\alpha \text{-glucose}$ monomers.
- Storage: Primarily stored in liver and muscle cells in animals.
- Structure: Nearly identical to amylopectin but much more highly branched.
  - Branches occur more frequently, about 1 out of every $10$ glucose monomers, allowing for rapid glucose release.
- Role: Can be quickly broken down into glucose monomers to provide energy when needed (e.g., during intense exercise).
Cellulose: Structural Support in Plants
- Composition: A structural polymer made of $\beta \text{-glucose}$ monomers.
- Linkages: Joined by $\beta \text{-1,4-glycosidic linkages}$ .
- Unique Feature: Every other glucose monomer is flipped relative to its neighbors.
  - This arrangement generates a linear, unbranched molecule, unlike the helices of starch and glycogen.
  - The linear molecules allow for extensive hydrogen bonds to form between adjacent, parallel cellulose strands.
- Structure: These hydrogen bonds contribute to the formation of strong fibers or sheets.
- Role: Major component of the plant cell wall, providing rigidity, structural support, and protection to plant cells and many algae.
Chitin: Structural Support in Fungi and Animals
- Composition: A structural polymer derived from $\text{N-acetylglucosamine (NAG)}$ monomers.
- Linkages: Similar to cellulose, it features $\beta \text{-1,4-glycosidic linkages}$ with every other monomer flipped.
- Structure: Forms linear strands that can undergo extensive hydrogen bonding between them.
- Role: Found in the cell walls of fungi and the tough exoskeletons of insects and crustaceans, providing structural support.
Peptidoglycan: Structural Support in Bacteria
- Composition: A structural polymer found in bacterial cell walls.
- Structure: Consists of long backbones of alternating monosaccharides (often N-acetylglucosamine and N-acetylmuramic acid).
- Linkages: The monosaccharides are joined by $\beta \text{-1,4-glycosidic linkages}$ .
- Unique Feature: Short amino acid chains (peptides) extend from the monosaccharide units and form peptide bonds (cross-links) between adjacent parallel strands.
- Role: Provides rigid structural support and protection to bacterial cells, forming a strong, mesh-like layer.

V. Diverse Functions of Carbohydrates (Section 5.3)

Precursors: Serve as starting materials (precursors) for the synthesis of other essential molecules, such as nucleotides (components of DNA/RNA) and amino acids (building blocks of proteins).
Fibrous Structural Materials:
- Cellulose, chitin, and peptidoglycan form long strands with strong bonds (hydrogen or peptide) between adjacent strands.
- These strands are organized into tough fibers or sheets, imparting strength and elasticity to cells and organisms.
- Resistance to Hydrolysis: The $\beta \text{-1,4-glycosidic linkages}$ in these structural carbohydrates are difficult to hydrolyze (break down).
  - Most organisms lack the specific enzymes required to break these bonds.
  - The packed, fibrous structure of these carbohydrates often excludes water, further hindering hydrolysis.
- Dietary Fiber: These indigestible carbohydrates are essential components of dietary fiber, promoting digestive health by aiding waste movement and gut motility.
Cell Identity:
- Carbohydrates display crucial information on the outer surface of cells, acting as