Lipid Structure, Digestion, and Metabolism

Comprehensive vocabulary flashcards reviewing lipid classifications, fatty acid nomenclature, isomerism, and the physiological process of lipid digestion and transport.

Fat (Lipid)

A biological substance that is generally soluble in organic solvents such as acetone, ether, or alcohol, insoluble in water, and typically composed of methyl groups, methylene groups, and a carboxylic acid group.

Methyl End (of a fatty acid)

The hydrophobic end of a fatty acid composed of a $CH_3$ group and repeating methylene ($CH_2$) groups that is repelled by water.

Carboxylic Acid End (of a fatty acid)

The polar, hydrophilic end of a fatty acid that interacts favorably with aqueous environments.

Triglyceride (Triacylglycerol)

The primary storage form of dietary fat representing $95\%$ of dietary lipid intake, consisting of a glycerol backbone esterified to three fatty acids at positions 1, 2, and 3.

Short-Chain Fatty Acids

Fatty acids containing fewer than $6$ carbons in length, often produced by colonic microbiota to nourish local gut cells.

Medium-Chain Fatty Acids

Fatty acids characterized by a carbon chain length of $6\text{ to }12$ carbons.

Long-Chain Fatty Acids

Fatty acids containing more than $14$ carbons in length, requiring specialized transport mechanisms in aqueous blood due to high nonpolarity.

Saturated Fatty Acid

A fatty acid hydrocarbon chain that possesses zero carbon-carbon double bonds.

Monounsaturated Fatty Acid

A fatty acid hydrocarbon chain that contains exactly one carbon-carbon double bond.

Polyunsaturated Fatty Acid

A fatty acid chain that contains two or more carbon-carbon double bonds.

Cis Fatty Acid

A naturally occurring isomer configuration where hydrogen atoms and substituent carbon groups flanking a double bond reside on the same side.

Trans Fatty Acid

A thermodynamically stable, more linear fatty acid isomer where large carbon groups are located on opposite sides of a double bond, altering membrane dynamics and signaling.

Partial Hydrogenation

A late nineteenth-century chemical process using hydrogen gas and a nickel catalyst to convert unsaturated double bonds to saturated single bonds in vegetable oils to produce solid margarine.

Delta ($\Delta$) System

A fatty acid naming nomenclature that identifies the locations of double bonds by counting carbons starting from the carboxylic acid end.

Omega ($\omega$) / $n$ System

A fatty acid naming nomenclature that counts carbon atoms starting from the methyl end; the $n$ notation was historically adopted because mechanical typewriters lacked the Greek omega key.

Linoleic Acid

An essential omega-6 fatty acid designated as $18:2\,n-6$ that the human body cannot synthesize de novo and must obtain from dietary sources.

Linolenic Acid

An essential omega-3 fatty acid designated as $18:3\,n-3$ containing three double bonds starting at the third carbon from the methyl end.

Lingual Lipase

A digestive enzyme released in the mouth that functions in slightly acidic environments, preferentially hydrolyzing short- and medium-chain fatty acids at position $3$ of triglycerides.

Gastric Esterase

A digestive enzyme in the stomach that hydrolyzes fatty acids at position $3$; along with lingual lipase, it breaks down roughly $30\%$ of dietary triglycerides.

Lipid Emulsification

The breakdown of large dietary lipid droplets into minute particles through mechanical agitation and bile action, dramatically increasing surface area for hydrophilic digestive enzymes.

Bile

A secretion synthesized in the liver and transported via the bile duct composed of bile salts derived from cholesterol and glycine, pigments such as bilirubin, bicarbonate, and cholesterol.

Pancreatic Lipase

An intestinal enzyme that selectively cleaves fatty acids from position $1$ and position $3$ of a triglyceride molecule, generating free fatty acids and a $2\text{-monoacylglycerol}$.

Cholesterol Esterase

A digestive enzyme that removes a fatty acid from a cholesterol ester to liberate free cholesterol and a free fatty acid.

Micelle

A dynamic, short-lived cluster spontaneously formed in the small intestine with an insoluble hydrophobic core and an outward-facing hydrophilic coat that transports digested lipid products to the brush border.

Chylomicron

A large lipoprotein structure assembled in enterocytes following the re-esterification of absorbed fatty acids and monoacylglycerols, responsible for transporting lipids through the aqueous circulatory and lymphatic systems.