Study Notes on Large Biological Molecules

Chapter 5: The Structure and Function of Large Biological Molecules

Introduction to Macromolecules

  • Macromolecules are large and complex molecules.
  • All living organisms are composed of four classes of large biological molecules:
    • Carbohydrates
    • Lipids
    • Proteins
    • Nucleic acids

Polymers

  • A polymer is a long molecule made up of similar or identical building blocks connected through covalent bonds.
  • Composed of smaller units called monomers.
  • Formation and breakdown of polymers occur through the following reactions:
    • Dehydration Reaction: Joins two monomers or a monomer to a polymer by removing a water molecule.
    • Hydrolysis Reaction: Breaks the bonds between units of a polymer by adding a water molecule.

Diversity of Macromolecules

  • A single cell has thousands of different macromolecules.
  • Types of macromolecules can differ among cells within an organism, vary among different species, and showcase diverse polymers generated from a limited set of monomers.

Carbohydrates

Definition and Function
  • Carbohydrates are sugar molecules that can exist in various forms depending on the type of bonding and molecular length.
  • Functions include offering structure and serving as an energy storage medium.
  • Carbohydrates are composed of Carbon (C), Hydrogen (H), and Oxygen (O).
  • Types of carbohydrates include:
    • Monosaccharides (simple sugars, one monomer)
    • Disaccharides (two monomers)
    • Polysaccharides (three or more monomers)
Specific Carbohydrates
  • Monosaccharides Examples:
    • Glucose
    • Galactose
    • Fructose
  • Disaccharides Examples:
    • Maltose (Glucose + Glucose via a 1,4 glycosidic linkage)
    • Sucrose (Glucose + Fructose via a 1,2 glycosidic linkage)
Storage Polysaccharides
  • Starch:
    • A storage polysaccharide in plants composed of glucose monomers.
    • Plants store surplus starch as granules within chloroplasts and other plastids.
    • Simplest form: Amylose (unbranched)
  • Glycogen:
    • A storage polysaccharide in animals, primarily stored in liver and muscle cells.
    • Upon demand, hydrolysis of glycogen releases glucose.
Structural Polysaccharides
  • Cellulose:
    • A major component of plant cell walls, different from starch in terms of glycosidic linkages.
    • Contains both alpha (α) and beta (β) forms of glucose monomers.
  • Chitin:
    • Found in the exoskeleton of arthropods and provides structural support in fungal cell walls.

Lipids

Characteristics of Lipids
  • Lipids are large biological molecules but are not classified as macromolecules due to their structure.
  • They are hydrophobic, meaning they do not mix with water.
  • The most important biologically relevant lipids include:
    • Fats
    • Phospholipids
    • Steroids
Fat Structure
  • Fats consist of two types of molecules: glycerol and fatty acids.
  • Glycerol:
    • A three-carbon alcohol with a hydroxyl group (-OH) on each carbon.
  • Fatty Acids:
    • Composed of a long carbon skeleton attached to a carboxyl group.
  • The formation of fats involves the process of dehydration reactions, creating an ester linkage between the glycerol and fatty acids.
Types of Fats
  • Saturated Fats:
    • Fatty acids with no double bonds, making them relatively straight and solid at room temperature (e.g., butter).
  • Unsaturated Fats:
    • Fatty acids with one or more double bonds causing kinks, usually liquid at room temperature (e.g., olive oil).
Importance of Fats
  1. Energy storage.
  2. Provide compact fuel reservoirs compared to carbohydrates.
  3. Cushioning for vital organs.
  4. Insulation against heat loss.

Phospholipids

  • Comprised of two fatty acids and a phosphate group attached to glycerol.
  • The tails of fatty acids are hydrophobic, while the phosphate head is hydrophilic, facilitating their role in cell membranes.

Proteins

Overview of Proteins
  • Account for over 50% of the dry mass of most cells.
  • Functions:
    • Enzymatic reactions (e.g., digestive enzymes).
    • Defense against disease (e.g., antibodies).
    • Storage (e.g., casein in milk).
    • Transport (e.g., hemoglobin).
    • Hormonal signaling (e.g., insulin).
    • Structural support (e.g., collagen).
Structure of Proteins
  • Proteins are made of amino acids, with 20 different types:
    • Basic structure consists of an amine group, carboxyl group, and a variable side chain (R group) that determines the properties of the amino acid.
  • Amino acids linked via peptide bonds.
Levels of Protein Structure
  1. Primary Structure: The unique sequence of amino acids in a protein.
  2. Secondary Structure: Coiled and folded arrangements such as alpha helices and beta-pleated sheets.
  3. Tertiary Structure: Determined by interactions among side chains (R groups).
  4. Quaternary Structure: Arrises when two or more polypeptide chains form a functional protein.
Factors Affecting Protein Structure
  • Alterations in pH, salt concentration, temperature, and other factors can lead to denaturation, which is the unraveling of a protein's native structure.
  • Misfolded proteins are associated with various diseases, including Alzheimer's and Parkinson's.

Nucleic Acids

Types of Nucleic Acids
  • Composed of monomers called nucleotides;
  • Two primary types are:
    • DNA (Deoxyribonucleic Acid)
    • RNA (Ribonucleic Acid)
Structure of a Nucleotide
  • A nucleotide consists of a nitrogenous base, a pentose sugar, and a phosphate group.
DNA Characteristics
  • Comprises deoxyribose sugar with two strands forming a double helix.
  • Nitrogenous bases include adenine (A), guanine (G), cytosine (C), and thymine (T).
RNA Characteristics
  • Generally single-stranded and contains ribose sugar.
  • Functions mainly in the synthesis of proteins, with nitrogenous bases of guanine (G), adenine (A), cytosine (C), and uracil (U) (instead of thymine).