Lipids Comprehensive Study Notes

What is a lipid?

  • Lipids are defined by their solubility: not soluble in water; soluble in organic solvents like benzene, cyclohexane, and ether (e.g., gasoline, paint thinner).

  • Lipids include fats and oils (fats are solid at 25°C; oils are liquid at 25°C).

Types of lipids (saponifiable vs non-saponifiable)

  • Saponifiable lipids (can be used to make soap):
    • Fatty Acids
    • Triglycerides
    • Phosphoglycerides
    • Waxes
    • Sphingolipids
    • Sphingomyelins
    • Glycolipids
    • Cerebrosides
    • Gangliosides
  • Steroids are not saponifiable (cannot be made into soap).

Note: A broad taxonomy lists fatty acids, triglycerides, phosphoglycerides, waxes, sphingolipids, sphingomyelins, glycolipids, cerebrosides, and gangliosides as major lipid classes; steroids stand apart as non-saponifiable.

Lipids in a quick overview

  • Major lipid classes: Fatty Acids, Steroids, Triglycerides, Phosphoglycerides, Waxes, Sphingolipids, Sphingomyelins, Glycolipids, Cerebrosides, Gangliosides.

Fatty Acids: structure and notation

  • General structure: a long hydrocarbon chain with a terminal carboxyl group –COOH; often written as
    ext{CH}3( ext{CH}2)_{n} ext{-COOH}
  • The end CH3- is the omega (ω) end (also called w- or ω); the first –CH$_2$ after the –COOH is designated as the a carbon.
  • Common notations:
    • Stearic acid: ext{C}_{18:0} (18 carbons, 0 double bonds)
    • Palmitic acid: ext{C}_{16:0}
    • Myristic acid: ext{C}_{14:0}

Saturated fatty acids

  • Definition: all C–C bonds are single bonds.
  • Examples:
    • Stearic acid: ext{C}_{18:0}
    • Structure: ext{CH}3 ext{CH}2 ext{CH}2 ext{CH}2 ext{CH}2 ext{CH}2 ext{CH}2 ext{CH}2 ext{CH}2 ext{CH}2 ext{CH}2 ext{CH}2 ext{CH}2 ext{CH}2 ext{CH}_2 ext{COOH}
    • Palmitic acid: ext{C}_{16:0}
    • Myristic acid: ext{C}_{14:0}

Monounsaturated fatty acids

  • Definition: contain exactly one C=C double bond.
  • Example: Oleic acid: ext{C}_{18:1} ext{ω-9}
    • Structure: ext{CH}3 ext{CH}2 ext{CH}2 ext{CH}2 ext{CH}2 ext{CH}2 ext{CH}2 ext{CH}2= ext{CH} ext{CH}2 ext{CH}2 ext{CH}2 ext{CH}2 ext{CH}2 ext{CH}2 ext{COOH}

Polyunsaturated fatty acids

  • Definition: have 2 or more C=C double bonds.

  • Examples:

    • Linoleic acid; ω-6; ext{C}_{18:2} ext{ ω-}6
    • Δ double bonds at 9 and 12: Δ9,12
    • Alpha-linolenic acid; ω-3; ext{C}_{18:3} ext{ ω-}3
    • Δ double bonds at 9, 12, 15: Δ9,12,15

  • Common long-chain PUFA structures given in many dietary resources; health implications discussed elsewhere (see omega-3/6 guidelines).

  • Arachidonic acid (C20:4) is a PUFA and a precursor to prostaglandins.

    • Representation: ext{C}_{20:4}
    • Prostaglandins are bioactive lipids derived from AA; see additional resources for detailed signaling roles.

Detailed examples and notes

  • Arachidonic acid (C20:4):

    • ext{CH}3( ext{CH}2)4( ext{CH}= ext{CH})4( ext{CH}2)2 ext{COOH}
    • Precursor to prostaglandins; see Saylordotorg resources for fatty acids and prostaglandins.

  • Cis vs Trans fatty acids:

    • Oleic acid: cis form; Elaidic acid: trans form.
    • Cis form causes backbone to bend; trans form is straighter.
    • Representations:
    • Cis: Oleic acid, ext{C}_{18:1} ext{ cis}
    • Trans: Elaidic acid, ext{C}_{18:1} ext{ trans}

Summary of common fatty acids by chain length

  • Long chain: >12 carbons; typical in most oils and fats.
  • Moderate chain: 7–12 carbons; common in coconut oils.
  • Short chain: 1–6 carbons; typical in dairy fats.

Problems with rancidity and its prevention

  • Rancidity arises when unsaturated fatty acids oxidize in air, producing unpleasant smell and taste.
  • Health impact is not typically a chronic issue (smell/talet is due to spoilage); packaging and shelf-life are major concerns.
  • Prevention methods:
    • Hydrogenation of double bonds to reduce oxidation susceptibility.
    • Use of antioxidants (e.g., vitamin E).
    • Other preservatives: butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT).

Lipids in digestion and energy storage

  • Triglycerides (triacylglycerols):

    • Structure: three fatty acids esterified to glycerol.
    • Digestion: enzymes hydrolyze triglycerides in the lumen of the intestine to monoglycerides and free fatty acids.
    • Absorption: monoglycerides and free fatty acids are absorbed in intestinal cells and usually re-esterified.
    • Turnover: continual breaking and rebuilding of triglycerides occurs as lipids move from cell to cell.

  • Adipose tissue: maps various depots and cell types involved in fat storage and metabolism

    • Brown adipose tissue; mesenteric, omental, abdominal subcutaneous, retroperitoneal, gluteal, femoral depots
    • Cellular components: adipocytes, preadipocytes, leukocytes & lymphocytes, macrophages & immune cells, blood vessels
    • References illustrate structural context of adipose tissue in biology resources.

  • Triglyceride example structures (illustrative):

    • Tripalmitin (a simple triglyceride):
      ext{H}2 ext{C-O-C}( ext{CH}2)_{14} ext{COOH}
      ightarrow ext{palmitoyl chains across glycerol}
    • A mixed triglyceride: mix of different fatty acids esterified to glycerol.

Saponification: hydrolysis of triglycerides

  • Modern method schematic:

    • Triglyceride + NaOH (or KOH) hydrolyzed to glycerol + fatty acid salts (soap).
    • Reaction (general):
      ext{TAG} + 3 ext{NaOH}
      ightarrow ext{Glycerol} + 3 ext{RCOO}^{-} ext{Na}^{+}
    • In practice: base-catalyzed hydrolysis yields fatty acid salts (soap) and glycerol.
    • The hydrolyzed products are used to form soaps via fatty acid salts.

  • Hydrophilic vs hydrophobic parts in triglycerides support soap formation and micelle behavior in solution.

Lipids in membranes: phosphoglycerides and phospholipids

  • Phosphoglycerides contain glycerol, fatty acids, and a phosphoric acid residue.

  • They are essential components of cell membranes and form a lipid bilayer.

  • Polar (hydrophilic) head group interacts with water; nonpolar (hydrophobic) fatty acid tails form the interior of the bilayer.

  • Structure highlights:

    • Glycerol backbone with two fatty acid chains and a phosphoric acid-containing head group.

    • Common phospholipid heads include ethanolamine and choline.

    • Examples:

    • Phosphatidylethanolamine (cephalin)

    • Phosphatidylcholine (lecithin)

    • Diagrammatic representation (textual):

    • Phospholipid with hydrophilic head (phosphate + amino alcohol) and hydrophobic tails; forms bilayer with protein components.

Phosphoglycerides: specific components and common variants

  • Heads and backbones:

    • Glycerol unit with two fatty acid chains (R and R') and a phosphate group.
    • Phosphoric acid unit connects to an amino alcohol head (e.g., ethanolamine or choline).
    • Common variants:
    • Phosphatidylethanolamine (cephalin)
    • Phosphatidylcholine (lecithin)

  • Functional note: Some phospholipids function as emulsifiers due to dual hydrophilic/hydrophobic properties.

  • Lecithins (phospholipids) are important emulsifiers:

    • Found in egg yolk, liver, peanuts, wheat germ; present in most organisms.
    • Emulsifiers help fat and water mix; they have both hydrophobic and hydrophilic characteristics.

Sphingolipids: a different backbone

  • Instead of glycerol, sphingolipids have a sphingosine backbone.

  • Major categories include ceramides, cerebrosides, gangliosides, and sphingomyelins.

  • Structure notes:

    • Sphingosine backbone with a fatty acid covalently attached (as amide) and various head groups.
    • Sphingomyelin is a phosphosphingolipid (has a phosphocholine head group).

  • Examples from the figures:

    • Sphingosine unit: CH? chains and OH groups; fatty acid unit attached via amide; phospho group in sphingomyelin.

  • Cerebrosides and gangliosides:

    • Cerebrosides: sphingolipids that contain a sugar unit.
    • Gangliosides: sphingolipids with more complex sugar chains (often including sialic acid).

Steroids and sterols

  • Steroids have a characteristic multi-ring hydrocarbon framework (four fused rings: A, B, C, D).

  • Cholesterol is a sterol (a type of lipid).

  • Cholesterol is a precursor to:

    • Bile acids
    • Sex hormones (estrogens, testosterone)
    • Corticosteroid hormones

  • Properties:

    • Cholesterol is synthesized by the human body; dietary intake is not strictly required.
    • Cholesterol is not found in plant sources.
    • Labels like “cholesterol-free” on plant-based foods can be misleading.

  • Steroids as emulsifiers:

    • Bile acids are steroid derivatives made from cholesterol and help to emulsify fats during digestion.

Transport of lipids in the body

  • Lipid transport is accomplished by lipoprotein particles:

    • Chylomicrons
    • Very-low-density lipoprotein (VLDL)
    • Low-density lipoprotein (LDL)
    • High-density lipoprotein (HDL)

  • Lipoprotein structure:

    • A core containing triacylglycerols and cholesteryl esters
    • A surface of phospholipids and cholesterol
    • Apolipoproteins on the surface to mediate receptor interactions and enzyme activity

  • These particles balance lipid transport between tissues and the bloodstream, delivering triglycerides and cholesterol where needed.

Connections and practical notes

  • Lipids serve as energy storage (triglycerides in adipose tissue) and structural components (cell membranes) and signaling molecules (eicosanoids from arachidonic acid; steroid hormones).
  • The physical state of lipids (solid vs liquid) is temperature-dependent and influences dietary properties and food science.
  • Health implications: saturated vs unsaturated fatty acids influence membrane fluidity and metabolic risk; omega-3 and omega-6 fatty acids have distinct physiological roles.
  • Processing considerations: rancidity affects shelf-life of unsaturated fats; strategies like hydrogenation and antioxidants mitigate spoilage.

References and additional notes

  • Arachidonic acid (C20:4) is documented as a precursor to prostaglandins; see additional resources for detailed biology and signaling.
  • Table 17.2 in the texts lists the average fatty acid composition of common fats and oils (with components 12:0, 14:0, 16:0, 18:0, 18:1, 18:2, 18:3).
  • Useful resources for expanded details on fatty acids, prostaglandins, and fats-and-oils chemistry are linked in the transcript references.

Quick reference notations used in this material

  • Fatty acid general formula: ext{R-CH}_2 ext{()} ext{COOH} (representing long hydrocarbon chain R attached to –COOH)
  • Saturated: ext{C}_{n:0}
  • Monounsaturated: ext{C}_{n:1} (one C=C)
  • Polyunsaturated: ext{C}_{n:m} where m ≥ 2 (multiple C=C bonds)
  • Omega notation: ext{C}_{n:m} ext{ ω-}k indicating the position of the last double bond relative to the methyl end