Chapter 15: Lipids – Structure, Types, and Properties

Chapter 15: Lipids

15.1 Lipids

Lipids are a diverse group of biomolecules that play essential roles in biological systems, including energy storage, cell membrane structure, and signaling. They are characterized by their solubility in organic solvents and insolubility in water.

• Definition: Lipids are biomolecules containing fatty acids or a steroid nucleus.

• Solubility: Lipids are soluble in organic solvents (e.g., ether, chloroform) but not in water due to their nonpolar nature.

• Origin of Name: The term "lipid" comes from the Greek word lipos, meaning "fat."

• Biological Importance: Lipids are crucial components of cell membranes, fat-soluble vitamins, and steroid hormones.

Types of Lipids:

• Hydrolyzable Lipids: Waxes, fats, oils, and phospholipids are esters that can be hydrolyzed to yield fatty acids and other molecules.

• Nonhydrolyzable Lipids: Steroids do not contain fatty acids and cannot be hydrolyzed. They are characterized by a four-ring steroid nucleus.

General Structure of Lipids

Lipids can be classified based on their structural components:

• Fatty acids (long hydrocarbon chains with a carboxylic acid group)

• Waxes (esters of fatty acids and long-chain alcohols)

• Triacylglycerols (esters of glycerol and three fatty acids)

• Glycerophospholipids (glycerol, fatty acids, phosphate, and amino alcohol)

• Sphingolipids (sphingosine, fatty acid, phosphate, and amino alcohol)

• Steroids (four fused carbon rings)

Learning Check 1

• A. Contains a fused four-membered ring system: Steroid

• B. Contains long carbon chains: Fatty acid

• C. Includes carbonyl groups: Fatty acid

Learning Check 2

• Which is NOT a characteristic of lipids? C. soluble in water

• Explanation: Lipids are nonpolar and therefore are NOT soluble in water.

15.2 Fatty Acids

Fatty Acids

Fatty acids are the building blocks of many lipids and are important for energy storage and membrane structure.

• Structure: Long, unbranched carbon chains with a carboxylic acid group at one end.

• Typical Length: 12–18 carbon atoms.

• Solubility: Insoluble in water due to the long nonpolar hydrocarbon chain.

• Saturated Fatty Acids: No C=C double bonds in the carbon chain.

• Unsaturated Fatty Acids: One or more C=C double bonds in the carbon chain.

Drawing Fatty Acids

• In skeletal formulas, ends and bends represent carbon atoms.

• Lauric acid example: can be drawn as a ball-and-stick model, condensed structural formula, or line-angle formula.

Saturated Fatty Acids

Saturated fatty acids contain only single C—C bonds and fit closely together, resulting in higher melting points and solid state at room temperature.

Name	Carbon Atoms: Double Bonds	Present in	Melting Point (°C)	Structures
Lauric acid	12:0	Coconut	44	CH3(CH2)10COOH
Myristic acid	14:0	Nutmeg	55	CH3(CH2)12COOH
Palmitic acid	16:0	Palm	63	CH3(CH2)14COOH
Stearic acid	18:0	Animal fat	69	CH3(CH2)16COOH

Monounsaturated Fatty Acids

Monounsaturated fatty acids have one C=C double bond, resulting in lower melting points and liquid state at room temperature.

Name	Carbon Atoms: Double Bonds	Present in	Melting Point (°C)	Structures
Palmitoleic acid	16:1	Butter	0	CH3(CH2)5CH=CH(CH2)7COOH
Oleic acid	18:1	Olives, pecan, grapeseed	14	CH3(CH2)7CH=CH(CH2)7COOH

Polyunsaturated Fatty Acids

Polyunsaturated fatty acids have two or more C=C double bonds, resulting in even lower melting points.

Name	Carbon Atoms: Double Bonds	Present in	Melting Point (°C)	Structures
Linoleic acid	18:2	Soybean, safflower, sunflower	-5	CH3(CH2)4CH=CHCH2CH=CH(CH2)6COOH
Linolenic acid	18:3	Corn	-11	CH3CH2CH=CHCH2CH=CHCH2CH=CH(CH2)6COOH
Arachidonic acid	20:4	Meat, eggs, fish	-50	CH3(CH2)4(CH=CHCH2)4COOH

Cis and Trans Unsaturated Fatty Acids

• Cis isomers: Bulky groups on the same side of the double bond, causing a bend or kink in the chain.

• Trans isomers: Bulky groups on opposite sides, resulting in a straighter chain.

• Most naturally occurring unsaturated fatty acids are cis.

Essential Fatty Acids

• Humans cannot synthesize sufficient amounts of polyunsaturated fatty acids such as linoleic, linolenic, and arachidonic acids.

• These must be obtained from the diet and are termed essential fatty acids.

Properties of Saturated and Unsaturated Fatty Acids

• Saturated fatty acids: Pack closely, strong dispersion forces, higher melting points, solid at room temperature.

• Unsaturated fatty acids: Cis double bonds cause kinks, less packing, lower melting points, liquid at room temperature.

Learning Check 1: Oleic Acid

• Why is this molecule an acid? It contains a carboxylic acid functional group.

• How many total carbon atoms? 18 carbon atoms.

• Saturated, monounsaturated, or polyunsaturated? Monounsaturated.

• Solid or liquid at room temperature? Liquid.

• Soluble in water? No, due to the long nonpolar chain.

Prostaglandins

Prostaglandins are hormone-like substances derived from arachidonic acid. They play roles in inflammation, pain, and regulation of blood pressure.

• Structure: Derived from arachidonic acid (20:4), with a five-carbon ring.

• Function: Regulate blood pressure, uterine contraction, and inflammation.

• NSAIDs: Nonsteroidal anti-inflammatory drugs (e.g., aspirin, ibuprofen) block prostaglandin synthesis, reducing pain and inflammation.

Chemistry Link to Health: Omega-3 and Omega-6 Fatty Acids

• Omega-6 fatty acids: First double bond at carbon 6 from the methyl end (e.g., linoleic acid, arachidonic acid).

• Omega-3 fatty acids: First double bond at carbon 3 from the methyl end (e.g., linolenic acid, EPA, DHA).

• Health benefits: Omega-3 fatty acids reduce blood clot formation and lower risk of heart disease.

Learning Check 2

• Linolenic acid: Polyunsaturated

• Palmitoleic acid: Monounsaturated

• Stearic acid: Saturated

15.3 Waxes and Triacylglycerols

Waxes

Waxes are esters formed from long-chain fatty acids and long-chain alcohols. They serve as protective coatings in plants and animals.

• Structure: Ester bond between fatty acid and alcohol (14–30 carbons each).

• Function: Prevent water loss and provide waterproofing.

Type	Condensed Structural Formula	Source	Uses
Beeswax	CH3(CH2)14COO(CH2)29CH3	Honeycomb	Candles, shoe polish, wax paper
Carnauba wax	CH3(CH2)24COO(CH2)33CH3	Brazilian palm tree	Waxes for furniture, cars, floors, shoes
Jojoba wax	CH3(CH2)19COO(CH2)21CH3	Jojoba bush	Candles, soaps, cosmetics