Introduction to Biomolecules

Overview of Biomolecules

Biomolecules are organic molecules essential to living organisms, forming the basis of structure and function in cells. There are four primary classes of biomolecules: carbohydrates, proteins, nucleic acids, and lipids. Each class has unique monomers, polymers, and biological roles.

Classes of Biomolecules

• Carbohydrates: Serve as energy sources and structural materials.

• Proteins: Perform a vast array of functions including catalysis, structure, and signaling.

• Nucleic Acids: Store and transmit genetic information.

• Lipids: Provide energy storage, membrane structure, and signaling molecules.

Monomers and Polymers

Definitions and Relationships

Monomers are the individual building blocks that can be repetitively linked together to form polymers, which are long chains of monomers. Carbohydrates, proteins, and nucleic acids are typically polymers, while lipids do not form true polymers.

Polymerization and Depolymerization

• Dehydration Synthesis: Forms covalent bonds to link monomers, releasing water and building polymers.

• Hydrolysis: Cleaves covalent bonds by adding water, breaking polymers into monomers.

Example: Formation of a disaccharide from two monosaccharides via dehydration synthesis; breakdown via hydrolysis.

Carbohydrates

Structure and Classification

Carbohydrates are carbon-based molecules hydrated with many hydroxyl groups (-OH). They are also referred to as saccharides (Greek for "sugars"). Simple carbohydrates fit the formula (e.g., glucose), while complex carbohydrates may include additional elements such as nitrogen or sulfur.

Size Classes of Carbohydrates

• Monosaccharides: Single carbohydrate units (e.g., glucose).

• Oligosaccharides: 2 to ~20 covalently linked monosaccharides.

• Polysaccharides: More than 20 covalently linked monosaccharides (e.g., starch, cellulose).

Formation and Breakdown of Polysaccharides

Polysaccharides are formed by linking monosaccharides via glycosidic bonds through dehydration synthesis. Hydrolysis breaks these bonds to release monosaccharides.

Functions of Carbohydrates

• Structural Support: Cellulose in plants, chitin in arthropods.

• Energy Storage: Starch in plants, glycogen in animals.

Example: Glycogen is the storage form of glucose in animals; starch is the storage form in plants.

Proteins

Structure and Monomers

Proteins are polymers made of amino acid monomers linked by peptide bonds. Each protein has directionality, with an N-terminal (amino group) and a C-terminal (carboxyl group).

Amino Acid Structure

Each amino acid contains a central carbon atom, an amino group, a carboxyl group, a hydrogen atom, and a unique R-group. There are 20 different amino acids, each with a distinct R-group.

Protein-Related Terms

Levels of Protein Structure

• Primary Structure: Sequence of amino acids.

• Secondary Structure: Formation of α-helices and β-sheets via hydrogen bonding.

• Tertiary Structure: Overall 3D shape of a polypeptide chain.

• Quaternary Structure: Association of multiple polypeptide chains.

Denatured Proteins and Chaperones

A protein’s structure is critical for its function. Denatured proteins are non-functional due to altered shape, often caused by changes in pH, temperature, or salt concentration. Chaperone proteins assist in the proper folding or re-naturing of proteins.

Nucleic Acids

Structure and Monomers

Nucleic acids are polymers that store and encode genetic information. Their monomers are nucleotides, each consisting of a phosphate group, a pentose sugar, and a nitrogenous base. DNA and RNA differ in their sugar components and nitrogenous bases.

Nitrogenous Bases

Formation and Structure of Nucleic Acids

Nucleotides are linked by phosphodiester bonds via dehydration synthesis, forming a sugar-phosphate backbone with directionality (5’ phosphate to 3’ hydroxyl end). DNA is typically double-stranded and forms an antiparallel double helix, while RNA is usually single-stranded.

Lipids

Structure and Types

Lipids are hydrophobic biomolecules insoluble in water, highly diverse in structure and function. They are not true polymers and include fats, oils, phospholipids, steroids, and waxes. Some lipids are amphipathic, containing both hydrophobic and hydrophilic regions.

Fatty Acids

• Saturated Fatty Acids: Fully saturated with hydrogens, only single C–C bonds, solid at room temperature.

• Unsaturated Fatty Acids: Contain one or more C=C double bonds, causing kinks, liquid at room temperature.

• Trans Fats: Artificial unsaturated fatty acids that are linear and not kinked.

Triglycerides

Triglycerides are lipids with three fatty acid chains covalently linked to a single glycerol molecule via dehydration synthesis.

Phospholipids

Phospholipids contain a phosphate group and are major components of all cell membranes. They are amphipathic, with a hydrophilic head and hydrophobic tails.

Steroids and Waxes

• Steroids: Lipids with four fused carbon rings (e.g., cholesterol, important for animal cell membranes).

• Waxes: Fatty acids bound to long-chain alcohols, used for protection and prevention of water loss (e.g., beeswax).