Introduction to Cell Biology & Biochemistry

Types of Biomolecules

  • Nucleic Acids: Composed of nucleotides.

  • Proteins: Composed of amino acids.

  • Carbohydrates: Composed of sugars.

  • Lipids: Diverse group of hydrophobic molecules.

Lecture Overview

  • Introduction to lipids

  • Structure and properties of fatty acids

  • Classification of lipids

What are Lipids?

  • Definition: Lipids are a broad class of biomolecules that are insoluble in water; includes waxes, oils, and fats.

Functions of Lipids

  • Energy Storage: Provide long-term energy reserves.

  • Biomembranes: Form cellular membranes that compartmentalize cellular functions.

  • Signaling: Act as signaling molecules in various physiological processes.

  • Enzymatic Co-factors: Serve as essential components in biochemical reactions.

  • Additional roles beyond those specified.

Hydrophobic and Amphipathic Properties

  • Hydrophobic Lipids: Fully water-hating, completely insoluble in water.

  • Amphipathic Lipids: Contain both water-soluble and water-insoluble groups within one molecule.

Classification of Lipids

  • Derived from Fatty Acids: Examples include phospholipids and triglycerides.

  • Derived from Sterols: Examples include estrogen and cholesterol.

Fatty Acids

  • Composed of:

    • Carboxyl Group: Hydrophilic "head" containing -COOH.

    • Hydrocarbon Chain: Long hydrophobic "tail" comprised of 12 to 24 carbons (only even-numbered chains present).

Subdivision of Fatty Acids

  • Saturated Fatty Acids: No double bonds; can be represented by a linear formula without kinks.

  • Unsaturated Fatty Acids: Contains one or more double bonds.

    • Monounsaturated Fatty Acids: One double bond.

    • Polyunsaturated Fatty Acids: Multiple double bonds.

Structural Isomerism in Fatty Acids

  • Cis-Trans Isomerism: Rotation about the C=C double bond is not permitted, leading to two distinct configurations.

    • Cis Configuration: Causes kinks in the fatty acid chain.

Fatty Acids Nomenclature (Standard)

  • Count the number of carbons from the Carboxyl group (COOH).

  • Indicate the number of double bonds following a colon (e.g., 16:1).

  • Indicate the position of double bonds with a delta (D) notation.

    • Example: 16:1D9 corresponds to hexadecenoic acid (palmitoleic acid).

Fatty Acids Nomenclature (Omega)

  • Linoleic Acid: Omega-6 fatty acid, represented as 9,12-octadecadienoic acid.

  • Linolenic Acid: Omega-3 fatty acid, represented as 9,12,15-octadecadienoic acid.

Melting Points of Fatty Acids

  • Chain Length: The longer the fatty acid chain, the higher the melting point due to increased Van der Waals forces.

  • Cis Configuration: Presence of cis double bonds lowers melting point by introducing kinks, preventing close packing of molecules.

Real-Life Examples: Melting Points

  • Margarine: Typically contains a mixture of saturated and unsaturated fatty acids influencing its melting point and physical state.

Classification of Lipids

  • Storage Lipids: Neutral lipids primarily used for energy storage.

  • Membrane Lipids: Polar lipids that form the structural basis of membranes, such as phospholipids and glycolipids.

  • Categories can be defined by:

    • Synthetic Pathway (e.g., steroids vs. fatty acids)

    • Function (energy source, membrane components, signaling messengers)

    • Structure (simple, complex, derived)

    • Chemical Composition (phospholipids, glycolipids).

  • Lipids often cross multiple classification boundaries.

Glycerolipids

  • Composed of glycerol and fatty acids, with glycerol characterized as a 3-carbon polyalcohol.

  • Notably, triglycerides are the most prominent but not the only type of glycerolipids.

Glycerol Esterification Process

  • Formation: A triglyceride + 3 different fatty acids + 3 H₂O molecules. This reaction releases water through ester bond formation.

Roles of Triglycerides

  • Energy Storage: Highly energy-dense, providing more energy content than sugars and can be compacted for storage.

  • Buoyancy: Low density aids in floating and is affected by temperature changes.

  • Thermal Insulation: Fat acts as an insulator, minimizing heat loss.

Phosphoglycerides (Glycerophospholipids)

  • Composed of glycerol, fatty acids, and a phosphate group along with a polar head moiety.

  • Essential for forming cellular membranes and includes different subclasses based on the polar head group (e.g., phosphatidylcholine, phosphatidylethanolamine).

Galactolipids

  • Distinct from phospholipids; composed of galactose attached to glycerol through glycosidic bonds and generally include linoleic acid.

Sphingosine and Sphingolipids

  • Structure: Sphingolipids are derived from sphingosine and can be either phospholipids or glycosphingolipids.

  • Classes include sphingomyelin (phospholipid), glucosylcerebroside (glycolipid), and lactosylceramide as part of their structure and function.

Differences in Lipid Types

  • Storage lipids are categorized as neutral (no charged groups) whereas membrane lipids are polar due to their amphipathic nature (hydrophilic and hydrophobic parts).

Waxes

  • Composed of long-chain alcohols and fatty acids, waxes have higher melting points compared to triglycerides. They serve distinctive roles in energy storage and waterproofing for some marine organisms.