Chapter 19: Lipids

Comprehensive vocabulary flashcards covering the structure, classification, reactions, and biological roles of lipids from Chapter 19 of General, Organic, and Biological Chemistry.

Lipid

An organic compound found in living organisms that is insoluble (or only sparingly soluble) in water but soluble in non-polar organic solvents.

Five Functional Categories of Lipids

The classification of lipids based on biochemical function: energy-storage lipids (triacylglycerols), membrane lipids (phospholipids, sphingoglycolipids, cholesterol), emulsification lipids (bile acids), chemical messenger lipids (steroid hormones, eicosanoids), and protective-coating lipids (biological waxes).

Fatty Acid

A naturally occurring monocarboxylic acid with an unbranched (linear) carbon chain, typically containing an even number of carbon atoms.

Long-Chain Fatty Acid

A fatty acid that contains a chain of $C_{12}$ to $C_{26}$ carbon atoms.

Medium-Chain Fatty Acid

A fatty acid that contains a chain of $C_{6}$ to $C_{11}$ carbon atoms.

Short-Chain Fatty Acid

A fatty acid that contains a chain of $C_{4}$ to $C_{5}$ carbon atoms.

Saturated Fatty Acid

A fatty acid in which all carbon-carbon bonds are single bonds.

Monounsaturated Fatty Acid (MUFA)

A fatty acid with a carbon chain in which exactly one carbon–carbon double bond is present.

Polyunsaturated Fatty Acid (PUFA)

A fatty acid with a carbon chain containing two or more carbon–carbon double bonds (up to six double bonds in biochemically important PUFAs).

Oleic Acid

An 18-carbon monounsaturated fatty acid ($18:1$) with the IUPAC name cis-9-octadecenoic acid.

Omega-3 ($\omega$-3) Fatty Acid

An unsaturated fatty acid with its endmost carbon–carbon double bond located three carbon atoms away from its methyl end.

Omega-6 ($\omega$-6) Fatty Acid

An unsaturated fatty acid with its endmost carbon–carbon double bond located six carbon atoms away from its methyl end.

Essential Fatty Acids

Fatty acids that cannot be synthesized by the human body and must be obtained from dietary sources, primarily linoleic acid ($18:2$) and linolenic acid ($18:3$).

Linoleic Acid

An essential omega-6 fatty acid ($18:2$) required for proper membrane structure and production of longer-chain omega-6 fatty acids; deficiency causes skin redness, infections, dehydration, and liver abnormalities.

Linolenic Acid

An essential omega-3 fatty acid ($18:3$) required for proper membrane structure and production of longer-chain omega-3 fatty acids.

Fatty Acid Melting Point Trends

Melting point increases as carbon chain length increases, and decreases as the degree of unsaturation (number of cis double bonds) increases due to chain bending that inhibits tight packing.

Triacylglycerol

A lipid formed by esterifying three fatty acids to a glycerol molecule; the principal energy-storage material in humans, stored predominantly in adipocytes.

Simple Triacylglycerol

A triester formed from the esterification of glycerol with three identical fatty acid molecules.

Mixed Triacylglycerol

A triester formed from the esterification of glycerol with more than one kind of fatty acid.

Fats versus Oils

Fats are predominantly saturated, solid or semisolid at room temperature, and derived from animal sources; oils are predominantly unsaturated, liquid at room temperature, and derived from plants and fish.

Saponification

The hydrolysis of triacylglycerols carried out in basic solution (such as $\text{NaOH}$) to produce glycerol and fatty acid salts (soap).

Hydrogenation (of Triacylglycerols)

The chemical addition of hydrogen across carbon–carbon double bonds in unsaturated triacylglycerols to increase saturation, converting liquid oils into solid or semisolid products like margarine.

Rancidity

The development of objectionable odors in fats and oils caused by the oxidation of alkene double bonds by atmospheric oxygen to yield short-chain aldehydes and carboxylic acids.

Phospholipid

A major class of membrane lipids containing one or more fatty acids, a phosphate group, a platform molecule (glycerol or sphingosine), and an alcohol esterified to the phosphate group.

Glycerophospholipid

A polar membrane lipid containing two fatty acids and a phosphate group esterified to glycerol, with an alcohol (choline, ethanolamine, or serine) esterified to the phosphate, giving it four ester linkages.

Sphingophospholipid

A lipid based on the 18-carbon aminodialcohol sphingosine, containing one fatty acid attached via an amide linkage, one phosphate group, and an alcohol esterified to the phosphate.

Sphingomyelin

A sphingophospholipid in which choline is the alcohol esterified to the phosphate group; found in all cell membranes and abundant in the myelin sheath of nerve cells.

Sphingoglycolipid

A membrane lipid containing a sphingosine backbone linked to a fatty acid by an amide linkage and to a monosaccharide or oligosaccharide through a glycosidic linkage.

Cerebroside

A simple sphingoglycolipid containing a single monosaccharide unit (either glucose or galactose); accounts for approximately $7\%$ of the dry mass of the brain.

Ganglioside

A complex sphingoglycolipid containing a branched chain of up to seven monosaccharide residues, found primarily in the gray matter of the brain and in the myelin sheath.

Steroid

A lipid whose core structure consists of a fused ring system made of three 6-carbon rings and one 5-carbon ring.

Cholesterol

A $C_{27}$ steroid lipid synthesized by the liver ($\sim 1\,\text{g/day}$) and obtained from animal food; regulates cell membrane fluidity by acting as a membrane plasticizer.

Lipid Bilayer

A two-layer sheet of phospholipid molecules measuring $6 - 9\,\text{nm}$ thick, with polar hydrophilic heads on the outer surfaces and nonpolar hydrophobic tails in the interior.

Passive Transport

The movement of a substance across a cell membrane by diffusion from higher to lower concentration without the aid of membrane proteins or expenditure of cellular energy (e.g., $O_2$, $N_2$, $H_2O$, urea, ethanol).

Facilitated Transport

The movement of a substance across a cell membrane from higher to lower concentration aided by specific membrane proteins/transporters without expenditure of cellular energy.

Active Transport

The movement of a substance across a cell membrane against its concentration gradient with the aid of membrane protein pumps, requiring the expenditure of cellular energy (ATP).

Bile Acid

A tri- or dihydroxy cholesterol derivative that acts as an emulsifier in the digestive tract to disperse dietary lipids; bonded to glycine or taurine through an amide linkage.

Steroid Hormones

Cholesterol-derived chemical messengers divided into sex hormones (estrogens, androgens, progestins) and adrenocorticoid hormones (mineralocorticoids and glucocorticoids).

Mineralocorticoids

A class of adrenocorticoid hormones produced in the adrenal cortex that control the balance of $\text{Na}^+$ and $\text{K}^+$ ions in cells.

Glucocorticoids

A class of adrenocorticoid hormones produced in the adrenal cortex that regulate glucose metabolism and counteract inflammation.

Eicosanoids

Hormone-like, short-lived signaling molecules derived from arachidonic acid ($20:4$) that exert localized physiological effects at extremely low concentrations.

Prostaglandins

A principal type of eicosanoid containing a cyclopentane ring and oxygen-containing functional groups; involved in inflammation, fever, pain, smooth muscle regulation, and gastric secretion.

Thromboxanes

A principal type of eicosanoid containing a cyclic ether ring and oxygen-containing functional groups that promotes platelet aggregation.

Leukotrienes

A principal type of eicosanoid containing three conjugated double bonds and hydroxy groups that promotes inflammatory and hypersensitivity (allergic) responses.

Biological Wax

A protective lipid monoester formed from a long-chain fatty acid ($C_{14} - C_{36}$) and a long-chain alcohol ($C_{16} - C_{30}$) that provides water-repellent coatings for skin, fur, feathers, and leaves.