Introduction to Biological Molecules

Introduction to Molecules of Cells

• Cells are made up of a vast array of large molecules, derived from smaller molecules.
• These large molecules are called macromolecules and are often composed of thousands of covalently bonded atoms.
• Four major classes of these biologically important molecules are carbohydrates, lipids, proteins, and nucleic acids.

Organic Molecules: The Basics

• Almost all molecules in cells are composed of carbon atoms bonded to each other and other elements, primarily hydrogen, oxygen and nitrogen.
• Carbon forms 4 covalent bonds, enabling the formation of diverse large molecules.
• Organic compounds contain carbon and hydrogen.

Biological Molecules Based on Carbon Skeletons

• Carbon skeletons vary in length and shape.
• Molecular diversity arises from variations in carbon skeletons and their shapes.
• Different shapes lead to different functions.
• Hydrocarbons are organic molecules made up entirely of carbon and hydrogen.

Isomers

• Isomers are compounds with the same molecular formula but different structures and properties.
• Two isomers of a drug may have different effects.
• Isomers are important in biology, as organisms are sensitive to subtle variations in molecules.

Functional Groups

• Functional groups are specific groups of atoms attached to carbon skeletons.
• Number and arrangement of functional groups impact the unique properties of a molecule.
• All functional groups are polar, making them water-soluble and hydrophilic.
• Examples of functional groups include hydroxyl, carbonyl, carboxyl, amino, and methyl.

Dehydration and Hydrolysis Reactions

• Monomers are linked together to form polymers by dehydration reactions.
• One monomer loses a hydrogen atom while another monomer loses a hydroxyl group, releasing a water molecule.
• A new covalent bond is formed between the two monomers to create a longer polymer.
• Polymers are dissembled into monomers by hydrolysis. This reaction is essentially the reverse of dehydration.
• The bond between monomers is broken by the addition of a water molecule.

The Four Classes of Biological Molecules

• Carbohydrates:
  • Include sugars and polymers of sugars.
  • Monosaccharides are the simplest carbohydrates.
  • Polysaccharides are polymers composed of many monosaccharides.
  • Examples include starch, glycogen, and cellulose.
• Lipids
  • Are not made of monomers.
  • Are hydrophobic (do not interact with water).
  • Consist mostly of hydrogen and carbon linked together via nonpolar covalent bonds.
  • Examples include fats, phospholipids, and steroids
    • Fats/Triglycerides: Composed of three fatty acids linked to one glycerol molecule via dehydration reactions. A major function is energy storage
    • Fatty Acids: Vary in length and number of double bonds. Saturated fatty acids lack double bonds and tend to be solid at room temperature; unsaturated fatty acids contain one or more double bonds, and tend to be liquid at room temperature.
    • Cis and Trans Fatty Acids: Unsaturated fatty acids can exist as cis or trans isomers, depending on the arrangement of hydrogen atoms around the double bond.
    • Phospholipids: Similar to fats, but a phosphate group replaces one fatty acid. The addition of this group creates an amphipathic molecule with both hydrophilic (water-loving) and hydrophobic (water-fearing) regions. These are the building blocks of cell membranes
    • Steroids: Are lipids with carbon skeletons bent into four fused rings. Examples include cholesterol, testosterone, and estrogen.
• Proteins:
  • Essential to structure and activities of life.
  • Are the most diverse class of macromolecules.
  • Composed of amino acids linked together by peptide bonds via dehydration reactions.
  • Amino acids: The monomers of proteins. There are twenty different amino acids with different R (side chain) groups.
  • Peptide bonds: A covalent bond that occurs between the carboxyl group of one amino acid and the amino group of another amino acid resulting in a chain of amino acids, called a polypeptide.
  • Protein structure:
    • Primary structure: The sequence of amino acids.
    • Secondary structure: Coiling/folding of the polypeptide chain into local patterns (alpha helix or beta pleated sheet) maintained by hydrogen bonds.
    • Tertiary structure: The 3D shape of a protein, stabilized by interactions between R groups (e.g., hydrogen bonds, ionic bonds).
    • Quaternary structure: Some proteins are composed of multiple polypeptides, and how they associate with each other.
  • Function: Proteins have diverse functions including structural components, movement, storage, transport, defense, signaling, and catalysis (as enzymes).
  • Denaturation: The loss of a protein's native (properly folded) structure; a denatured protein is biologically inactive. pH and temperature changes can cause denaturation.
• Nucleic Acids:
  • Consist of nucleotides.
  • The sequence of nucleotides determines the sequence of amino acids in a protein.
  • There are two main types: DNA and RNA.
  • Nucleotides: Have the same general structure, which includes a phosphate group and a pentose sugar. They differ by having different nitrogenous bases (A, T, C, G, or U).
  • DNA and RNA: DNA is a double helix, with two strands held together by hydrogen bonds according to A-T, and C-G base pairing rules. DNA sequences are unique to each gene. RNA is typically a single strand.
  • Nucleic acids are made by dehydration syntheses.