Polymers, Monomers, and Biological Macromolecules: Structure and Function

Polymers and Monomers

Dehydration Synthesis and Hydrolysis

Biological macromolecules are formed by linking monomers through dehydration synthesis and are broken down by hydrolysis. These processes are fundamental to the formation and breakdown of carbohydrates, proteins, and nucleic acids.

• Dehydration synthesis: Removal of water to join two monomers.

• Hydrolysis: Addition of water to break a polymer into monomers.

Carbohydrates

Glucose Monomers and Structures

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically in a 1:2:1 ratio. Their structure and function depend on the type of glycosidic linkage between glucose monomers.

• Shape depends on glycosidic linkage type: The orientation of glycosidic bonds determines the structure and digestibility of polysaccharides.

Examples and Uses

• Glycogen: Storage form of glucose in animals.

• Amylose: Unbranched starch in plants.

• Amylopectin: Branched starch in plants.

• Cellulose: Structural polysaccharide in plant cell walls.

Uses

• Energy storage (glycogen, starch)

• Structural support (cellulose)

• Why can humans not digest cellulose? Humans lack the enzyme cellulase needed to break β(1→4) glycosidic bonds in cellulose.

Lipids

General Structure and Function

Lipids are hydrophobic molecules that serve as energy storage, structural components of membranes, and signaling molecules.

• Phospholipids: Major component of cell membranes; form bilayers due to hydrophilic heads and hydrophobic tails.

• Micelles: Spherical lipid structures formed in water.

• Bilayers: Double-layered structures forming the basis of biological membranes.

• Cholesterol: Steroid that stabilizes membrane fluidity and serves as a precursor for steroid hormones.

Proteins

General Amino Acid Structure and Polymerization

Proteins are polymers of amino acids linked by peptide bonds. Their structure determines their function.

• General amino acid structure: Central carbon, amino group, carboxyl group, hydrogen, and variable R group.

• Polymerization: Amino acids are joined by peptide bonds through dehydration synthesis.

Four Levels of Protein Structure

• Primary: Sequence of amino acids.

• Secondary: Local folding (α-helix, β-sheet) stabilized by hydrogen bonds.

• Tertiary: Overall 3D shape due to interactions among R groups.

• Quaternary: Association of multiple polypeptide chains.

Forces that Contribute to Protein Conformation

• Hydrogen bonds

• Ionic bonds

• Hydrophobic interactions

• Disulfide bridges

Denaturation

• Denaturation: Loss of protein structure due to heat, pH, or chemicals.

• Conditions that contribute to denaturation: Aggregates and disease examples involving aggregates (e.g., Alzheimer's disease).

Role of Chaperones in Protein Folding

• Chaperone proteins assist in proper folding and prevent aggregation.

Enzyme Structure and Function

Active Sites and Models

Enzymes are biological catalysts that speed up reactions by lowering activation energy.

• Active sites: Region where substrate binds and reaction occurs.

Reaction Coordinate Diagrams

Reaction coordinate diagrams illustrate the energy changes during a chemical reaction.

• Enzymes lower activation energy: Enzymes reduce the energy barrier for reactions.

• Enzymes don't alter ΔG: The overall free energy change ($\triangle G$) of a reaction remains unchanged.

Spontaneous vs. Non-spontaneous Reactions

• Spontaneous: $\triangle G < 0$

• Non-spontaneous: $\triangle G > 0$

Linked / Coupled Reactions

ATP Hydrolysis and Cellular Power

Cells use energy from spontaneous reactions to drive non-spontaneous ones through coupling.

• Energy coupling: Energy released from ATP hydrolysis ($\mathrm{ATP} \rightarrow \mathrm{ADP} + P_i$) is used to power cellular processes.

• Example: ATP hydrolysis drives active transport, muscle contraction, and biosynthesis.

Additional info: Coupled reactions are fundamental to metabolism, allowing cells to perform work that would otherwise be energetically unfavorable.