Chapter 5: Organic Molecules

Biological Organic Molecules

• The four main classes of biological macromolecules:

  1. Carbohydrates – Sugars and polymers of sugars.

  2. Lipids – Fats, phospholipids, and steroids (hydrophobic molecules).

  3. Proteins – Polymers of amino acids.

  4. Nucleic Acids – DNA & RNA.

• Three of these (Carbohydrates, Proteins, and Nucleic Acids) are polymers, meaning they are made up of monomers linked together.

Dehydration Reaction & Hydrolysis

• Dehydration Reaction:

  • Builds polymers by linking monomers through the removal of a water molecule.

  • Example: Forming a disaccharide from two monosaccharides.

• Hydrolysis:

  • Breaks down polymers into monomers by adding a water molecule.

  • Example: Breaking down starch into glucose.

Carbohydrates

Monosaccharides (Simple Sugars)

• Basic formula: (CH₂O)n.

• Example from lecture: Glucose (C₆H₁₂O₆).

• Structure: Can exist in linear or ring form (ring is more stable in water).

• Functions:

  • Fuel for cells.

  • Raw material for building molecules.

Disaccharides & Polysaccharides

• Disaccharides:

  • Formed by a dehydration reaction between two monosaccharides.

  • Bond type: Glycosidic linkage.

  • Example: Sucrose (Glucose + Fructose).

• Polysaccharides:

  • Large polymers of sugars with storage or structural functions.

Polysaccharides & Their Functions

1. Glycogen:

   • Storage polysaccharide in animals.

   • Stored in liver and muscle cells.

2. Cellulose:

   • Structural polysaccharide in plants (major component of cell walls).

   • Special Feature: Humans can’t digest cellulose (passes as fiber).

   • Glycosidic bond difference: Cellulose has β-glucose linkages, while starch has α-glucose linkages.

3. Starch:

   • Storage polysaccharide in plants.

   • Stored in chloroplasts.

   • Example: Amylose (simplest starch).

4. Chitin:

   • Structural polysaccharide in fungal cell walls & arthropod exoskeletons (insects, crabs).

Lipids (Hydrophobic Molecules)

Characteristics & Examples

• Lipids do not mix with water (hydrophobic).

• Three major types:

  1. Fats (Triacylglycerols)

  2. Phospholipids

  3. Steroids

Fats (Triacylglycerols)

• Structure:

  • Made of 1 glycerol + 3 fatty acids.

  • Linked by ester bonds.

• Function in Animals:

  • Energy storage.

  • Cushioning & insulation for organs.

Saturated vs. Unsaturated Fats

• Saturated fats:

  • No double bonds between carbon atoms.

  • Solid at room temperature (ex: butter, animal fats).

• Unsaturated fats:

  • One or more double bonds (causes bending).

  • Liquid at room temperature (ex: oils).

Hydrogenation

• Process of adding hydrogen to unsaturated fats to make them saturated.

• Creates trans fats, which are linked to heart disease.

Phospholipids

• Structure:

  • Glycerol + 2 fatty acids + phosphate group.

  • Has a hydrophilic head (phosphate) and hydrophobic tails (fatty acids).

• Significance:

  • Forms the phospholipid bilayer in cell membranes.

Steroids (Cholesterol & Hormones)

• Structure: Four fused carbon rings.

• Examples:

  • Cholesterol – Essential for cell membranes but high levels lead to heart disease.

  • Hormones – Testosterone & Estrogen are steroid-based.

Proteins

Polypeptides & Amino Acids

• Proteins are made of one or more polypeptides.

• Amino acids:

  • Contain Amino (-NH₂), Carboxyl (-COOH), and R-group (side chain).

  • 20 different types classified as polar, nonpolar, or charged.

Polypeptide Bonds

• Amino acids are linked by peptide bonds through dehydration reactions.

• Form long chains (polypeptides).

Levels of Protein Structure

1. Primary Structure:

   • Sequence of amino acids.

   • Determines final protein shape.

2. Secondary Structure:

   • Hydrogen bonds create α-helices and β-sheets.

3. Tertiary Structure:

   • 3D shape formed by interactions between R-groups.

   • Includes hydrogen bonds, disulfide bridges, hydrophobic interactions.

4. Quaternary Structure:

   • Multiple polypeptide chains combine.

• • Example: Hemoglobin (carries oxygen in blood).