Study Notes on Organic Molecules and Their Classes

Organic Molecules

• Organic molecules are defined as green organic molecules that do not contain carbon covalently bonded to carbon but bonded to hydrogen.

Major Classes of Biologically Important Molecules

• There are four major classes of biologically important molecules, often referred to as the big four: carbohydrates, lipids, proteins, and nucleic acids.

Overview of Macromolecules

• These classes are often macromolecules, which are large molecules composed of smaller subunits.
• This relationship can be categorized as small-to-big:
  • Carbohydrates: Small units called monosaccharides.
  • These are the building blocks (monomers) that combine to form larger structures (polymers) called polysaccharides.
  • Lipids: Do not fit the polymer model perfectly but consist of building blocks like glycerol and fatty acids, which combine to form larger lipid molecules.
  • Proteins: Formed by linking together small molecules called amino acids to create polypeptides, which are chains of amino acids.
  • Nucleic Acids: Comprised of nucleotides that combine to form large molecules such as DNA and RNA.

Carbohydrates

• Carbohydrates are defined as sugars and sugar polymers.

Monosaccharides

• The smallest form of carbohydrates are called monosaccharides, which means "one sugar" (simple sugars).
• The most pivotal monosaccharide in this course is glucose, also referred to as blood sugar or dextrose in clinical contexts.
• The chemical formula of glucose is C6H{12}O_6:
  • 6 carbons
  • 12 hydrogens
  • 6 oxygens
• Be aware that textbooks may abbreviate the representation of monosaccharides as the chapter progresses.

Disaccharides

• When two monosaccharides bond together, they form a disaccharide. Examples include:
  • Sucrose: Formed from glucose and fructose (common table sugar).
  • Maltose: Formed from two glucose molecules.
• Larger combinations can be referred to as trisaccharides (three) or tetrasaccharides (four), and so on.

Polysaccharides

• Polysaccharides consist of hundreds to thousands of monosaccharides linked together. Important polysaccharides to know include:
  • Starch: The storage form of glucose in plants, with two forms:
  • Amylose: Unbranched chain of glucose.
  • Amylopectin: Branched structure.
  • Glycogen: Storage form of glucose in animals, characterized by a highly branched structure, often referred to as animal starch.
  • Glycogen plays a crucial role when blood sugar levels drop, such as during physical exertion.
  • Cellulose: Found in plant cell walls, is not digestible for humans. Functions as dietary fiber but does not contribute to caloric intake.
  • Chitin: Found in the exoskeletons of crustaceans like crabs and lobsters and in the cell walls of fungi.

Lipids

• Lipids consist of fats and oils, which are generally not soluble in water.
  • Lipids are classified as nonpolar molecules, while water is polar. Therefore, they do not mix well.

Fatty Acids

• Fatty acids are the building blocks of lipids and can be categorized as:
  • Saturated Fatty Acids: Have the maximum number of hydrogen atoms, making them saturated.
  • Unsaturated Fatty Acids: Have one or more double bonds between carbon atoms, thus are not saturated.

Types of Lipids

• Triglycerides: Composed of glycerol and three fatty acids; primarily used for energy storage. One gram of fat contains 9 calories, while one gram of carbohydrate contains 4 calories.
• Phospholipids: Consist of glycerol, two fatty acids, a phosphate group, and a polar molecule. Amphipathic nature (hydrophilic head and hydrophobic tails) is crucial for cell membrane structure.
• Steroids: Characterized by a carbon skeleton with four fused rings, various steroids differ by the chemical groups attached. Key examples include:
  • Cholesterol: A necessary component of cell membranes.
  • Testosterone and Estrogen: Hormones derived from steroid structure.
• Waxes: Such as earwax, serve as protective barriers and minimize water loss. The cuticle of plants also serves a similar function.

Proteins

• Proteins are made up of one or more chains of amino acids, linked by peptide bonds (special covalent bonds).

General Structure of Amino Acids

• There are 20 amino acids used commonly by cells. Each amino acid has a basic structure:
  • Central carbon atom
  • Carboxyl Group (–COOH, gives the molecule its acidic properties)
  • Amino Group (–NH₂)
  • R Group: (side chain that varies among different amino acids)

Functions of Proteins

• Proteins have an enormous variety of functions depending on their structure:
  • Enzymes: Catalysts for biochemical reactions.
  • Structural proteins: Such as keratin in hair and nails.
  • Hemoglobin: Responsible for transporting oxygen in red blood cells.

Protein Structures

• Proteins have distinct structural levels:

  • Primary Structure: Sequence of amino acids in a polypeptide chain.
  • Secondary Structure: Localized folding patterns such as alpha helices and beta-pleated sheets.
  • Tertiary Structure: Three-dimensional folding of the polypeptide chain.
  • Quaternary Structure: Relationship between multiple polypeptide chains.

• A well-known example is hemoglobin, which consists of two different types of chains and is vital for oxygen transport in the bloodstream.

• Misfolding of proteins can lead to diseases, such as sickle cell anemia caused by a single amino acid change, leading to red blood cell deformation.

Protein Denaturation

• Denaturation occurs when proteins lose their structure due to factors like heat or changes in pH. This loss of structure results in loss of function.
  • Example: Cooking an egg results in denaturation, as the egg whites change from translucent to white as proteins uncoil and aggregate.