Biomolecules: Structure, Function, and Diversity in Cells

Introduction to Biomolecules

Biomolecules are essential organic compounds that form the foundation of all living organisms. Cells synthesize large, complex molecules from a limited set of small molecules, primarily through the assembly of macromolecules. These macromolecules are typically polymers, which are long chains of repeating subunits called monomers. The four major classes of biomolecules are nucleic acids, proteins, carbohydrates, and lipids, each with distinct structures and functions.

• Nucleic acids (~15% of cell dry mass): DNA and RNA, responsible for genetic information storage and transfer.

• Proteins (~50%): Enzymes, structural components, and signaling molecules.

• Carbohydrates (~15%): Energy storage and structural support.

• Lipids (~15%): Membrane structure and energy storage.

Cells are composed of about 80% water, with the remaining 20% consisting of these biomolecules in the proportions listed above.

Macromolecules: Polymers and Monomers

Macromolecules are large molecules formed by the polymerization of smaller subunits called monomers. The process of building and breaking down polymers involves two key reactions:

• Dehydration Synthesis (Condensation): Monomers are joined together by removing a water molecule, forming a covalent bond.

• Hydrolysis: Polymers are broken down into monomers by the addition of a water molecule, breaking the covalent bond.

Major Classes of Biomolecules

The four major classes of biomolecules differ in their monomers and polymers:

Carbon: The Backbone of Life

Properties of Carbon

Carbon is the central element in organic molecules due to its ability to form four covalent bonds, allowing for a diversity of stable structures. Carbon atoms can bond to other carbons and various elements, creating chains, branches, and rings. This versatility underlies the molecular diversity of life.

• Can form single, double, or triple bonds

• Can create linear, branched, or ring structures

• Each bond can rotate freely, allowing for complex 3D shapes

Functional Groups in Biomolecules

Functional groups are specific groups of atoms attached to the carbon skeleton that determine the chemical reactivity and properties of organic molecules. Only a few functional groups are responsible for the diversity of biological molecules.

Carbohydrates

Structure and Function

Carbohydrates are composed of monosaccharides (simple sugars) that can be linked to form disaccharides and polysaccharides. They serve as energy sources and structural materials in cells.

• Monosaccharides: Glucose (C6H12O6) is a common example.

• Disaccharides: Formed by dehydration synthesis (e.g., maltose).

• Polysaccharides: Long chains such as starch, glycogen, and cellulose.

Lipids

Structure and Function

Lipids are hydrophobic molecules that include fats, oils, and phospholipids. They are not true polymers but are assembled from glycerol and fatty acids. Lipids function in energy storage, insulation, and membrane structure.

• Fats (triglycerides): Formed by joining glycerol and three fatty acids via dehydration synthesis.

• Phospholipids: Major components of cell membranes.

Proteins

Structure and Function

Proteins are polymers of amino acids linked by peptide bonds. They perform a vast array of functions, including catalysis (enzymes), structural support, transport, and signaling. The structure of a protein determines its function and is organized into four levels:

• Primary structure: Sequence of amino acids.

• Secondary structure: Local folding into alpha helices and beta sheets via hydrogen bonds.

• Tertiary structure: Overall 3D shape due to side chain interactions.

• Quaternary structure: Association of multiple polypeptide subunits.

Changes in temperature or pH can denature proteins, causing loss of structure and function.

Amino Acids and Side Chains

Amino acids have a central carbon atom bonded to an amino group, a carboxyl group, a hydrogen atom, and a variable side chain (R group). The properties of the side chain determine the behavior and folding of the protein.

• Hydrophobic (nonpolar) side chains

• Hydrophilic (polar) side chains

• Acidic and basic side chains (charged at cellular pH)

Nucleic Acids

Structure and Function

Nucleic acids (DNA and RNA) are polymers of nucleotides. They store and transmit genetic information and direct protein synthesis. Each nucleotide consists of a sugar, a phosphate group, and a nitrogenous base.

• DNA: Double-stranded helix, stable, stores genetic information.

• RNA: Single-stranded, less stable, involved in protein synthesis and regulation.

Nucleotide Structure and Polymerization

Nucleotides are joined by phosphodiester bonds between the 5' phosphate of one nucleotide and the 3' hydroxyl of the next. DNA strands are antiparallel and complementary, with base pairing (A-T, G-C in DNA; A-U, G-C in RNA).

• Purines: Adenine (A), Guanine (G)

• Pyrimidines: Cytosine (C), Thymine (T, in DNA), Uracil (U, in RNA)

Base Pairing and Replication

Base pairing rules ensure accurate replication and transcription:

• %A = %T (or %U in RNA)

• %G = %C

During DNA replication, each strand serves as a template for the synthesis of a new complementary strand.

pH and Biomolecules

Impact of pH on Biomolecules

pH is a measure of hydrogen ion concentration in a solution. It affects the ionization state of functional groups in biomolecules, influencing their structure and function. For example, amino and carboxyl groups can gain or lose protons depending on the pH, altering protein folding and activity.

• Low pH (acidic): More H+ ions, can protonate side chains.

• High pH (basic): Fewer H+ ions, can deprotonate side chains.

Summary Table: Major Biomolecules

Additional info: This guide covers foundational concepts from General Biology, including the chemical context of life, the structure and function of macromolecules, and the importance of carbon and functional groups. It also addresses the impact of pH on biomolecular structure and function.