Premed · Premed · Organic Chemistry 2
Lecture 20: Lipids and Biological Membranes
Organic Chemistry II
Learning Objectives
By the end of this lecture, students will be able to:
- Define lipids and describe their key structural features
- Classify lipids as fatty acids, triacylglycerols, phospholipids, sphingolipids, steroids, and waxes
- Describe the structure of fatty acids and the effect of saturation on physical properties
- Explain saponification and soap action at the molecular level
- Describe the structure and function of phospholipid bilayers
- Recognize the steroid skeleton and key steroids (cholesterol, hormones)
- Describe prostaglandin structure and biological function
Lecture Content
I. Overview of Lipids
Lipids are a structurally diverse group of biomolecules defined not by a common functional group but by their solubility: they dissolve readily in organic solvents yet are insoluble or poorly soluble in water. All lipids are either hydrophobic or amphipathic (possessing both hydrophobic and hydrophilic regions).
Lipids serve a remarkable range of biological functions. They store energy efficiently in the form of fats and triacylglycerols, provide structural integrity as the core components of biological membranes (phospholipids, cholesterol), act as signaling molecules (steroid hormones, prostaglandins, leukotrienes), furnish thermal insulation and mechanical protection, and include several essential vitamins (A, D, E, and K are all lipid-soluble). The major structural classes include fatty acids, triacylglycerols, phospholipids, sphingolipids, steroids, waxes, and terpenes.
II. Fatty Acids
Fatty acids are long-chain carboxylic acids, typically containing 12 to 20 carbons in even numbers (reflecting their biosynthesis from two-carbon acetyl-CoA units). They may be saturated, with no carbon-carbon double bonds, or unsaturated, with one or more C=C double bonds.
Saturated fatty acids adopt a fully extended zigzag conformation that allows adjacent chains to pack tightly through extensive van der Waals interactions. This efficient packing produces higher melting points, and saturated fats are solid at room temperature. Palmitic acid (C16:0) and stearic acid (C18:0) are common examples.
Unsaturated fatty acids, which almost always have the cis (Z) configuration in biological systems, contain one or more double bonds that each introduce an approximately 30-degree kink into the hydrocarbon chain. These kinks disrupt the orderly packing of neighboring chains, weaken intermolecular forces, and lower the melting point. Unsaturated fats are therefore liquid at room temperature (oils). Representative examples include oleic acid (C18:1, one double bond at the delta-9 position), linoleic acid (C18:2), and linolenic acid (C18:3).
Linoleic acid and alpha-linolenic acid are essential fatty acids that humans cannot synthesize. They serve as precursors to the omega-6 and omega-3 families, respectively. Trans fatty acids, produced industrially by partial hydrogenation of vegetable oils, have a geometry that permits tighter packing than the natural cis isomers and are associated with increased cardiovascular disease risk.
<image>Panel A: Three fatty acid structures compared. Stearic acid (C18:0, saturated) shown as a fully extended zigzag chain. Oleic acid (C18:1, cis) shown with a single kink at the delta-9 double bond. Linoleic acid (C18:2, cis,cis) shown with two kinks. Below each structure, a space-filling model illustrates how the kinks disrupt packing. Panel B: A melting point comparison chart: stearic acid (69 C), oleic acid (16 C), linoleic acid (-5 C), showing the correlation between unsaturation and lower melting points.</image>
III. Triacylglycerols (Fats and Oils)
Triacylglycerols are triesters of glycerol with three fatty acid chains. The glycerol backbone (HOCH2-CHOH-CH2OH) has each of its three hydroxyl groups esterified to a fatty acid. The three fatty acids may be identical (simple triacylglycerol) or different (mixed triacylglycerol). Fats, predominantly of animal origin, contain mainly saturated fatty acids and are solid at room temperature. Oils, from plant and fish sources, contain predominantly unsaturated fatty acids and are liquid at room temperature.
Saponification -- the base hydrolysis of triacylglycerols with NaOH -- yields glycerol and three sodium carboxylate salts, which constitute soap. Each soap molecule is amphipathic, with a long nonpolar hydrocarbon tail and a polar carboxylate head. In water, soap molecules organize into micelles, spherical aggregates with the nonpolar tails pointing inward and the polar heads facing the aqueous environment. Grease and oil droplets are solubilized within the hydrophobic interior of these micelles and washed away. In hard water containing Ca2+ or Mg2+ ions, soap forms insoluble precipitates (scum). Synthetic detergents such as sodium alkylbenzenesulfonates avoid this problem.
Catalytic hydrogenation of vegetable oils (H2 over Ni or Pd) converts cis double bonds to single bonds, raising the melting point and producing semi-solid fats like margarine. Partial hydrogenation also generates trans fatty acids as undesirable byproducts.
IV. Phospholipids
Glycerophospholipids (phosphoglycerides) have a glycerol backbone with two fatty acids esterified at C1 and C2 and a phosphate group at C3. The phosphate is typically further esterified to a polar alcohol head group. Common head groups include ethanolamine (phosphatidylethanolamine), choline (phosphatidylcholine, also known as lecithin), serine (phosphatidylserine), and inositol (phosphatidylinositol).
The amphipathic nature of phospholipids -- two hydrophobic fatty acid tails and a hydrophilic phosphate-containing head -- dictates their behavior in water. Single-chain amphiphiles form micelles (spherical structures with tails inward), but phospholipids, with their two bulky tails, preferentially form bilayers in which two leaflets oppose each other with tails facing inward and heads facing the aqueous environment. Closed bilayer vesicles are called liposomes.
Biological membranes are fundamentally phospholipid bilayers. The fluid mosaic model, proposed by Singer and Nicolson in 1972, describes the membrane as a dynamic structure in which lipids and proteins move laterally within the plane. Integral (transmembrane) proteins span the bilayer, while peripheral proteins associate with the surface. Cholesterol is intercalated between the phospholipids, modulating membrane fluidity. The bilayer is selectively permeable: small nonpolar molecules cross freely, but ions and large polar molecules require protein channels or transporters.
<image>Panel A: Structure of phosphatidylcholine (lecithin) with the glycerol backbone, two fatty acid tails (one saturated, one unsaturated with a cis kink), the phosphate group, and the choline head group all clearly labeled. Panel B: Cross-section of a phospholipid bilayer showing two leaflets of phospholipids with hydrophobic tails facing inward and hydrophilic heads facing outward toward the aqueous environment. Cholesterol molecules are intercalated between the phospholipids. An integral transmembrane protein is shown spanning the bilayer, and a peripheral protein is associated with the surface.</image>
V. Sphingolipids
Sphingolipids are built on a sphingosine backbone, an amino alcohol with a long hydrocarbon chain, rather than glycerol. Attachment of a fatty acid to the amino group of sphingosine via an amide bond produces a ceramide. Addition of phosphocholine to a ceramide gives sphingomyelin, a phospholipid found abundantly in the myelin sheath of nerve cells. Addition of one or more sugar residues gives the glycosphingolipids: cerebrosides carry a single sugar (glucose or galactose), while gangliosides carry an oligosaccharide chain that includes sialic acid (N-acetylneuraminic acid). Glycosphingolipids play important roles in cell recognition and signaling.
Sphingolipid storage diseases (lipidoses) result from genetic deficiencies in the lysosomal enzymes responsible for degrading sphingolipids. Tay-Sachs disease is caused by a deficiency of hexosaminidase A, leading to accumulation of ganglioside GM2. Gaucher disease results from deficient glucocerebrosidase, and Niemann-Pick disease from deficient sphingomyelinase.
VI. Steroids
All steroids share a characteristic four-fused-ring skeleton: three six-membered rings (A, B, and C) and one five-membered ring (D), known as the cyclopentanoperhydrophenanthrene system. The carbon atoms are numbered systematically from 1 to 27 or more, and the trans ring junctions typical of natural steroids produce a flat, rigid molecular framework.
Cholesterol is the most abundant steroid in animal tissues. Its structure features the steroid skeleton, a C8 side chain, a 3-beta-hydroxyl group, and a double bond between C5 and C6. Cholesterol serves as the biosynthetic precursor to all other steroids, including bile acids, steroid hormones, and vitamin D. In biological membranes, cholesterol modulates fluidity by broadening the phase transition temperature, reducing the fluidity of the liquid-crystalline phase while preventing crystallization at lower temperatures.
Steroid hormones derived from cholesterol include glucocorticoids (cortisol, involved in stress response and inflammation), mineralocorticoids (aldosterone, regulating sodium retention in the kidneys), and the sex hormones (testosterone, estradiol, and progesterone). Remarkably, small structural differences among these molecules -- sometimes as subtle as the presence of a hydroxyl group versus a ketone -- produce dramatically different biological activities.
Bile acids, such as cholic acid and chenodeoxycholic acid, are cholesterol derivatives conjugated with glycine or taurine to form bile salts. These amphipathic molecules act as biological detergents, emulsifying dietary fats in the intestine to facilitate their digestion by lipases.
VII. Prostaglandins and Related Eicosanoids
Eicosanoids are signaling molecules derived from 20-carbon polyunsaturated fatty acids, primarily arachidonic acid (C20:4). Prostaglandins contain a cyclopentane ring within the 20-carbon chain and mediate a wide range of physiological processes, including inflammation, pain, fever, blood clotting, and smooth muscle contraction. Aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs) work by inhibiting cyclooxygenase (COX), the enzyme that converts arachidonic acid to prostaglandins. The two isoforms, COX-1 (constitutive) and COX-2 (inducible at sites of inflammation), are important drug targets.
Thromboxanes contain an oxane ring and promote platelet aggregation and vasoconstriction. Leukotrienes lack a ring and are involved in asthma and allergic responses; drugs such as montelukast target the leukotriene pathway.
<image>Panel A: The steroid skeleton (four fused rings A, B, C, D) with standard numbering of all carbon atoms. Panel B: Structure of cholesterol with the 3-beta-hydroxyl group, the delta-5 double bond, and the isooctyl side chain clearly labeled. Panel C: Biosynthetic relationship from cholesterol to three classes of steroid hormones: estradiol (estrogen, with aromatic A ring), testosterone (androgen), and cortisol (glucocorticoid), with arrows indicating the enzymatic transformations and key structural differences highlighted in color.</image>
