Lipids Chemistry Notes

Lipids are a heterogeneous group of compounds including fats, oils, steroids, waxes, and related compounds.
Defined more by physical properties (insolubility in water, solubility in nonpolar organic solvents) than chemical properties.
Important dietary constituents due to:
- High energy value.
- Fat-soluble vitamins.
- Essential fatty acids.
Lipoproteins (lipid and protein combinations) are important cellular constituents.
- Occur in the cell membrane and mitochondria.
- Transport lipids in the blood.
Fat is stored in adipose tissue and acts as a thermal insulator.
Nonpolar lipids act as electrical insulators in myelinated nerves.
Knowledge of lipid biochemistry is crucial for understanding:
- Obesity.
- Diabetes mellitus.
- Atherosclerosis.
- The role of polyunsaturated fatty acids in nutrition and health.

Saturated Fatty Acids
- SCFA (Short-Chain Fatty Acids): 2-4 carbons
- MCFA (Medium-Chain Fatty Acids): 6-12 carbons
- LCFA (Long-Chain Fatty Acids): 14-18 carbons
Monounsaturated Fatty Acids
- Examples: 18:1n-9c, 18:1n-7c, 16:1n-7c
Polyunsaturated Fatty Acids
- Omega-6: 18:2, 18:3, 20:3, 20:4
- Omega-3: 18:3, 18:4, 20:5, 22:5, 22:6
Trans Fatty Acids
- Examples: 19-18:1, 19-111 CLA, c9 t11 CLA, 19 +12 18:2
Sterols
- Zoosterols: Cholesterol
- Phytosterols: Sterols, stanols
Vitamins
- A, D, E

Lipid	Primary Functions
Fatty acids	Energy sources, biosynthetic precursors
Triacylglycerols	Storage, transport
Phosphoglycerides	Membrane components
Ketone bodies	Energy sources
Sphingolipids	Membrane components
Eicosanoids	Modulators of physiologic activity
Cholesterol	Membrane component
Steroid hormones	Modulators of physiologic activity

Hydrolyzable: Converted into two or more smaller molecules upon hydrolysis.
Non-Hydrolyzable: Cannot be broken up into smaller units since they do not react with water.
React to base
Cannot react to base

Simple Lipids
- Fats, oils
Compound Lipids
- Phospholipids
  - Phosphoinositol (phosphotidyl)
  - Glycero ionositol
  - Sphingophospholipids (spingomyelins)
- Glycolipids
  - Cerebrosides
  - Gangliosides
  - Globosides
- Sulpholipids
- Amino lipids
- Lipoproteins
Derived Lipids
- Fatty acids, glycerol, cholesterol, steroid, ketone bodies

Esters of fatty acids with various alcohols.
- Fats: Esters of fatty acids with glycerol. Oils are fats in the liquid state.
  - Examples: butter, oil, nuts, meat, fish, and some dairy products
- Waxes: Esters of fatty acids with higher molecular weight monohydric alcohols.
  - Examples: Lanolin, Beeswax, Whale Sperm Oil

Include fatty acids, glycerol, steroids, other alcohols, fatty aldehydes, ketone bodies, hydrocarbons, lipid-soluble vitamins, and hormones.
Acylglycerols (glycerides), cholesterol, and cholesteryl esters are neutral lipids (uncharged).
Derived lipids:
- Terpenenoids & Steroid Terpenes
- Sterols & Cholesterol
- Ergosterol – (7-dehydrocholesterol)
- Androgens & Estrogens, and Adrenal Corticosteroids

Naturally occurring monocarboxylic acids with an unbranched carbon chain and an even number of carbon atoms.
General formula: $R-(CH2)n-COOH$
- $R$ = no. of amino group- part of fatty acids(COOH)
- $n$ = no. of $CH_2$
Occur mainly as esters in natural fats and oils but can be found in unesterified form as free fatty acids (transport form in plasma).
Usually straight-chain derivatives containing an even number of carbon atoms.

Category	Subcategory	Examples
Classification	Chain length	Short, Medium, Long, Very long
	Total carbon atoms	Odd chain, Even chain
	Nature of chain	Saturated, Unsaturated, Branched, Hydroxy
Synthesis		Essential, Non-essential
		Functions

Amphipathic molecules (hydrophobic and hydrophilic components).
Hydrophobic component: hydrocarbon chain made up of C-C and C-H nonpolar bonds.
Hydrophilic component: carboxylic acid (COOH) group that interacts with the surrounding aqueous environment.
Fatty acids with fewer than 8 carbon atoms are LIQUID at physiological temperature.
Those with more than 10 carbon atoms are SOLIDS.
Usually exist as esters or amides.
At physiological pH, fatty acids are ionized:
- $RCOOH ightharpoonup RCOO^- + H^+$ ; $pKa = 4.5$

Boiling and melting points INCREASE with CHAIN LENGTH.
Melting point DECREASES with INCREASING UNSATURATION.
Solubility DECREASES with INCREASE IN CHAIN LENGTH.
Saturated fatty acids have HIGHER MELTING POINTS due to their uniform rod-like shape.
Cis double bonds in unsaturated fatty acids produce kinks, making packing more difficult and preventing the formation of a crystalline lattice.

Water solubility is inversely related to carbon chain length.
- Short-chain fatty acids have slight solubility due to the polarity of the carboxyl group.
- Long-chain fatty acids are essentially insoluble due to the non-polar nature of the hydrocarbon chain.
Melting points are influenced by both carbon chain length and the number of double bonds.

Volatile short-chain fatty acids
- Liquid in nature.
- 1-6 carbon atoms.
- Water-soluble and volatile at room temperature.
- Examples:
 - Acetic F.A. (2C): $CH_3-COOH$
 - Butyric F.A. (4C): $CH3-(CH2)_2-COOH$
 - Caproic F.A. (6C): $CH3-(CH2)_4-COOH$
Non-volatile short-chain fatty acids:
- Solids at room temperature.
- Contain 7-10 carbon atoms.
- Water-soluble and non-volatile at room temperature.
- Examples:
 - Caprylic (8C): $CH3-(CH2)_6-COOH$
 - Capric (10 C): $CH3(CH2)_2-COOH$

Contain more than 10 carbon atoms.
Occur in hydrogenated oils, animal fats, butter, and coconut and palm oils.
Non-volatile and water-insoluble.
Examples:
- Palmitic (16C): $CH3-(CH2)_{14}-COOH$
- Stearic (18 C): $CH3-(CH2)_{16}-COOH$
- Lignoceric (24C): $CH3-(CH2)_{22}-COOH$

Long-chain saturated fatty acids tend to be solids at room temperature.
Long-chain unsaturated fatty acids tend to be liquids at room temperature.
Increasing the degree of unsaturation decreases molecular attractions between carbon chains.
Cis double bonds create