Lipids and Membranes: Learning Goals

• Understand the structure, properties, and biological roles of:

  • Storage lipids

  • Structural lipids

  • Signaling lipids

• Comprehend the function of biological membranes and the lipid bilayer.

• Review the structure, composition, and dynamics of membranes.

• Learn the structure and function of membrane proteins.

Classification of Lipids

• Classes of Lipids:

  • 1. Fatty acids

  • 2. Triacylglycerols

  • 3. Wax esters

  • 4. Phospholipids

  • 5. Sphingolipids

  • 6. Isoprenoids

Structural Diversity of Lipids

• Lipids can be categorized into two major groups:

  • Lipids containing fatty acids:

    • Storage lipids

    • Membrane lipids

  • Lipids not containing fatty acids:

    • Examples include cholesterol, vitamins, and pigments.

Classification by Function

• Lipids are chemically diverse and largely insoluble in water:

  • Storage Lipids:

    • High energy content and hydrophobic nature for efficient packing.

  • Structural Lipids:

    • Insulation, surface dryness, and prevention of water loss.

  • Signaling Lipids:

    • Include hormones, steroids, and vitamins.

Functions of Lipids

• Membrane Structure:

  • Fundamental component of cell membranes.

• Cofactors for Enzymes:

  • a substance (other than the substrate) whose presence is essential for the activity of an enzyme.

    • e.g., Vitamin K in blood clotting and coenzyme Q in ATP synthesis.

• Signaling Molecules:

  • Hormones and growth factors (e.g., Vitamins A and D).

• Antioxidants and Pigments:

  • substances that may help prevent or delay cell damage by neutralizing free radicals, unstable molecules that can harm cells

    • e.g., Vitamin E and pigmentation in living organisms.

Storage Lipids

• Fats as fuel storage offer:

  • Denser energy storage compared to polysaccharides (e.g., glycogen).

  • Ideal for long-term energy needs.

Fatty Acids

• Structure:

  • Carboxylic acids with chains of 4 to 36 carbons.

  • C2 is designated as alpha (α), with subsequent carbons noted by Greek letters.

  • Types:

    • Saturated: No double bonds.

    • Monounsaturated: One double bond.

    • Polyunsaturated: Multiple double bonds.

Fatty Acids Nomenclature

• Systematic names (e.g. cis-9-octadecanoic acid).

• Simplified nomenclature format: number of carbons:number of double bonds (e.g., 18:1).

• Positions of double bonds denoted with delta (Δ) notation.

Common Fatty Acids

• Saturated Fatty Acids:

  • Myristic acid (14:0), Palmitic acid (16:0), etc.

• Unsaturated Fatty Acids:

  • Oleic acid (18:1), Linoleic acid (18:2), etc.

Configurations of Fatty Acids

• A conjugated double bond would be directly adjacent to another double bond, but in most fatty acids, the double bonds are separated by a methylene group, preventing conjugation -CH=CH-CH2-CH=CH-

• Cis vs. Trans Configurations:

  • Most unsaturated fatty acids are cis, which leads to kinks in the hydrophobic chain.

  • Trans fats can result from partial hydrogenation processes.

Melting Points of Fatty Acids

• Saturated fatty acids have higher melting points than unsaturated ones.

• Melting point increases with chain length, but decreases with double bonds.

Triglycerides

• The primary storage form of fatty acids in organisms.

• Composed of glycerol bonded to three fatty acids, they are hydrophobic and provide efficient energy storage.

Waxes (?)

• Long-chain fatty acid esters, providing waterproofing and protection.

• Found in biological systems (e.g., on feathers and skin).

Signaling Lipids ??

• Include eicosanoids, derivatives of arachidonic acid affecting processes like inflammation, clotting, and muscle contraction.

• Provide essential signaling functions within and between cells.

Vitamins

• Essential for various biological functions and cannot be synthesized by the body:

• Fat-Soluble Vitamins: A, D, E, K (isoprenoid compounds).

• Water-Soluble Vitamins: Soluble in aqueous solutions.

Membranes Overview

• Composed of lipid bilayers; define the boundaries of cells and organelles.

• Complex structures involved in transport, signaling, and energy production.

• Proteins interact with lipids to fulfill various roles (e.g., receptors, transporters).

Membrane Properties

• Membranes are flexible, sheet-like structures that can spontaneously form in an aqueous environment.

• Asymmetric with distinct lipid compositions.

• Fluid properties allow lateral movement of components, crucial for function.

Fluid Mosaic Model

• Describes the structure of membranes as a fluid combination of lipids and proteins.

• Contains integral and peripheral proteins, influencing transport and communication.

Conclusion on Membranes

• Membrane composition is variable across different cell types and organelles, affecting their specific functions.

• Active transport mechanisms require energy, reflecting the dynamic nature of cellular processes.