Macromolecules: Polymers and Monomers

Introduction to Macromolecules

Large biological molecules, or macromolecules, are essential for life and include carbohydrates, proteins, nucleic acids, and lipids. Most macromolecules are polymers, long chains made from repeating units called monomers. The diversity of macromolecules arises from the arrangement of a small set of monomers into complex structures.

• Polymer: A long molecule consisting of many similar or identical building blocks (monomers) linked by covalent bonds.

• Monomer: The repeating unit that serves as the building block of a polymer.

• Enzymes: Specialized macromolecules that speed up chemical reactions, including those that build or break down polymers.

Polymer Synthesis and Breakdown:

• Dehydration Reaction: Two monomers bond together through the loss of a water molecule, forming a polymer.

• Hydrolysis: Polymers are disassembled to monomers by the addition of water, essentially the reverse of dehydration.

Carbohydrates: Structure and Function

Monosaccharides: Simple Sugars

Carbohydrates include sugars and polymers of sugars. The simplest carbohydrates are monosaccharides, which serve as major fuel for cells and as raw material for building other molecules. Monosaccharides are classified by the location of their carbonyl group (as aldose or ketose) and the number of carbons in their skeleton.

• General Formula: Most monosaccharides have formulas that are multiples of CH2O (e.g., glucose is C6H12O6).

• Classification: By carbonyl group (aldose or ketose) and carbon number (triose, pentose, hexose, etc.).

Ring Structures of Sugars

Although often drawn as linear skeletons, many sugars form rings in aqueous solutions. This ring structure is especially important for glucose and other hexoses.

Disaccharides and Glycosidic Linkages

A disaccharide is formed when a dehydration reaction joins two monosaccharides. The covalent bond formed is called a glycosidic linkage.

Polysaccharides: Storage and Structural Roles

Polysaccharides are polymers of sugars and serve storage or structural functions. Their properties depend on the types of monomers and the positions of glycosidic linkages.

• Starch: Storage polysaccharide in plants, composed of glucose monomers. Stored as granules in chloroplasts and plastids.

• Glycogen: Storage polysaccharide in animals, mainly in liver and muscle cells. Hydrolysis releases glucose when needed.

• Cellulose: Structural polysaccharide in plant cell walls. Composed of glucose, but with different glycosidic linkages (β instead of α), resulting in straight, unbranched molecules that can hydrogen-bond to each other.

• Chitin: Structural polysaccharide found in the exoskeleton of arthropods and cell walls of fungi. Contains a modified glucose monomer.

Lipids: Hydrophobic Molecules

Overview of Lipids

Lipids are a diverse group of hydrophobic molecules that do not form true polymers. They consist mostly of hydrocarbon regions and include fats, phospholipids, and steroids.

Fats and Fatty Acids

Fats are constructed from glycerol and fatty acids. Glycerol is a three-carbon alcohol, and each fatty acid consists of a carboxyl group attached to a long carbon skeleton. Fats are formed by joining three fatty acids to glycerol via ester linkages, creating a triacylglycerol (triglyceride).

• Saturated Fatty Acids: No double bonds; solid at room temperature (e.g., animal fats).

• Unsaturated Fatty Acids: One or more double bonds; liquid at room temperature (e.g., plant and fish oils).

• Trans Fats: Created by hydrogenation; associated with health risks.

• Functions: Energy storage, insulation, and cushioning of organs.

Phospholipids

Phospholipids have two fatty acids and a phosphate group attached to glycerol. The fatty acid tails are hydrophobic, while the phosphate group forms a hydrophilic head. In water, phospholipids self-assemble into bilayers, forming the basis of cell membranes.

Steroids

Steroids are lipids with a carbon skeleton consisting of four fused rings. Cholesterol is an important steroid in animal cell membranes and a precursor for other steroids.

Proteins: Structure and Function

Functions and Diversity of Proteins

Proteins are the most structurally sophisticated molecules, accounting for more than 50% of the dry mass of most cells. They serve as enzymes, provide structural support, transport substances, and play roles in cell communication, movement, and defense.

• Enzymes: Proteins that act as catalysts, speeding up chemical reactions without being consumed.

• Polypeptides: Polymers of amino acids; a protein consists of one or more polypeptides folded into a specific shape.

Amino Acids and Peptide Bonds

Amino acids are organic molecules with amino and carboxyl groups, differing in their side chains (R groups). Amino acids are linked by peptide bonds to form polypeptides, which have a unique linear sequence with an amino (N) terminus and a carboxyl (C) terminus.

Protein Structure: Four Levels

The function of a protein depends on its specific structure, which is organized into four levels:

• Primary Structure: Unique sequence of amino acids.

• Secondary Structure: Coils (α helix) and folds (β pleated sheet) formed by hydrogen bonds in the backbone.

• Tertiary Structure: Overall 3D shape stabilized by interactions among R groups (hydrogen bonds, ionic bonds, hydrophobic interactions, van der Waals forces, and disulfide bridges).

• Quaternary Structure: Association of multiple polypeptide chains (e.g., hemoglobin).

Protein Folding and Denaturation

Protein structure can be affected by changes in pH, salt concentration, temperature, or other environmental factors, leading to denaturation (loss of native structure and function). Sometimes, denatured proteins can refold (renaturation), but not always.

Nucleic Acids: Information Molecules

DNA and RNA

Nucleic acids store, transmit, and help express hereditary information. The two types are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA provides instructions for its own replication and directs synthesis of messenger RNA (mRNA), which controls protein synthesis in a process called gene expression.

Nucleotide Structure and Polymerization

• Nucleotide: Monomer of nucleic acids, consisting of a nitrogenous base, a pentose sugar, and one or more phosphate groups.

• Nucleoside: Nitrogenous base plus sugar (no phosphate).

• Nitrogenous Bases: Pyrimidines (cytosine, thymine, uracil) and purines (adenine, guanine).

• Phosphodiester Linkage: Covalent bond joining nucleotides in a polynucleotide, forming a sugar-phosphate backbone.

Structure of DNA and RNA

• DNA: Double helix with antiparallel strands; complementary base pairing (A with T, G with C).

• RNA: Usually single-stranded; uracil replaces thymine; can form complex shapes by pairing within or between RNA molecules.

Genomics and Proteomics

Advances in sequencing technology have enabled the study of entire genomes (genomics) and large sets of proteins (proteomics). These fields use computational tools (bioinformatics) to analyze and compare genetic and protein data, deepening our understanding of evolution and heredity.

Summary Table: Major Classes of Biological Macromolecules