3.1 Significance 2 major functions of lipids – Energy storage by nonpolar lipids – Membrane function by polar lipids Also form micelles Signal molecules

3.2 Fatty Acids Structure – Two distinct parts Large hydrocarbon tail Short carboxyl head group – Delta notation Carboxyl group is carbon one Common names reflect biological origin

3.2 Fatty Acids Unsaturated Fatty Acids – Contains double bonds Saturated Fatty Acids – No double bonds Polyunsaturated Fatty Acids – More than one double bond

3.2 Fatty Acids Stereochemistry of Double Bond – Cis Same side of double bond Appears bent – Trans Opposite sides of double bond Appears straight

3.2 Fatty Acids Stereochemistry of Double Bond cont. – Conjugated Separated by one single bond – Isolated double bonds Separated by at least two single bonds

3.2 Fatty Acids Carboxylic Acid Head Groups – Dissociation properties of weak acid – pKa is 4.8 Usually in anionic form at biological pH

3.2 Fatty Acids Amphipathic – Nonpolar and polar parts Most fatty acids are covalently linked – Triglycerides – Phospholipids

3.3 Triacylglycerol Energy storage form of fatty acids in animals – Lipid storage in plants is confined to see Three fatty acids esterified to glycerol – Ester bond between alcohol and acid Hydrophobic – Ester bond doesn’t form hydrogen bonds – Excludes water

3.3 Triacylglycerol Composition varies – Tail portion have different lengths – Different degrees of saturation Fat – Saturated side chains – Solid Oils – Unsaturated side chains – Liquids

3.4 Phospholipids Polar lipids Principal components of cell membranes Derivatives of phosphatidic acid – Resemble triacyglycerides – Third position is esterified to phosphate

3.4 Phospholipids Polyprotic acid – Multiple equilibria – Only 1 pKa is close to physiological pH Can react with alcohols to form esters Have heterogeneity – Fatty acids in first two positions can vary – Different alcohols esterified to phosphate

3.4 Phospholipids Phosphatidylcholine – Phosphate group is part of a diester – One is formed with glycerol – One is formed with alcohol choline

3.5 Cholesterol Steroid – Fuse flat rings Essential component of animal cell membranes Other steroids are derivatives of – Testosterone, estradiol, aldosterone Amphipathic molecule

3.6 Lipid – Water Interactions of Amphipathic Molecule Amphipathic molecule must satisfy two conflicting natures – Water solubility – Lipid solubility Can be done by forming micelles or liposomes

3.6 Lipid – Water Interactions of Amphipathic Molecules Micelles – Polar portions face aqueous exterior – Nonpolar portion forms interior core – Critical Micelle Concentration Limit of solubility of fatty acids in water Determines how many fatty acids in micelle Dependent on experimental conditions

3.6 Lipid – Water Interactions of Amphipathic Molecule Lipsomes – Three phases Exterior water phase Interior water phase Nonpolar phase – Essence of biological membrane

3.6 Lipid – Water Interactions of Amphipathic Molecule Does Micelle or Liposome Form? – Relative volumes of hydrophobic and hydrophilic portions determine – If unequal, micelle forms – If equal, form bilayer Phosphatidylcholine has roughly equal volumes

3.6 Lipid – Water Interactions of Amphipathic Molecule Bile Salts – Important example of micelles – Polar cholesterol derivative and dietary lipids – Facilitates hydrolysis to fatty acids

3.7 Water Permeability of Membranes and Osmosis Consequence of liposome – Two solutions with distinct compositions Water can cross membrane – Aquaporin can accelerate Channels and transport proteins selectively transports solutes – semipermeable

3.7 Water Permeability of Membranes and Osmosis Diffusion – Movement of molecules due to spatial differences in concentration Osmosis – Water movement

3.8 Effect of Lipids on Membrane Composition Extensive variation in phospholipid molecules – Head groups vary – Diverse fatty acyl chains Fluidity controlled by fatty acyl chains – Double bonds increase fluidity Cholesterol affects fluidity – At low temperatures, decreases fluidity – At high temperatures, increases fluidity