How to Classify Lipids

Classifying lipids is really easy. In short, lipids are hydrophobic or “water fearing/hating” biomolecules.

Here are some common types of lipids:

  1. fats
  2. oils
  3. steroids
  4. cell membranes

Fatty Acids:

Fatty Acid Structure: a carboxylic acid with a long hydrocarbon chain with even carbons (sometimes odd)

Example of a Fatty Acid:

Palmitic acid:

Palmitoleic acid

Difference between Unsaturated and Saturated Fatty Acid:

  • Unsaturated fatty acids contain a double bond in the structure (see palmitoleic acid above)
  • Saturated fatty acids do not contain a double bond and instead have a long hydrocarbon chain

Effects of Melting Point on Fatty Acids:

  • Saturated fatty acids are solids at room temperature
    • Examples:
      • Lauric acid
      • Myristic acid
      • Palmitic acid
  • Unsaturated fatty acids are liquid at room temperature
    • Examples
      • Palmitoleic acid
      • Oleic acid
      • EPA and DHA (commonly found in krill oil)
    • Unsaturated fatty acids almost always have a cis configuration at the double bond.
    • Double bond makes the structure much more rigid
    • Have a lower melting point than saturated fats

Note: Increasing hydrocarbon chain increases the melting point of saturated fatty acids.

Acylglycerols or Glycerides:

A glycerol (or acylglycerol) contains 1 glycerol group and 1 or more fatty acid groups attached.

Some basic nomenclature:

  • glycerol + 1 fatty acid = monoacylglycerol
  • glycerol + 2 fatty acids = diacylglycerol
  • glycerol + 3 fatty acids = triacylglyerol or triglyceride (FAT)

Esterification of Glycerol:

General Glycerol Reaction Scheme:

Alcohol + Carboxylic Acid <—>(H+) <—> Ester + Water


For example:
Triglycerol


Source: Wikipedia

Functions of Triglycerol:

  • Long term energy storage
    • Stored in adipose tissue
 
  • Release energy upon oxidation
    • 6x the energy for an equal weight of hydrated glycogen

Glycerophospholids:

  • Also known as phosphoglycerides
  • Major lipid in cell membranes
  • Comprised of a Glycerol-3-phosphate + 2 fatty acids and a polar R3group (view image below)
    • R1 is usually a saturated fatty acid
    • R2 is usually an unsaturated fatty acid
    • R3 is a polar group (usually)

Sphingolipids:

  • Contain the alcohol sphingosine instead of glycerol as the backbone
  • One fatty acid connected via an amide linkage to a sphingosine
    • A single fatty acid chain attached is called a ceramide
  • The sphinigolipid may contain one phosphate/alcohol group via an inorganic ester linkage (also known as sphingophospholipids)
  • The sphingolipid may also contain one sugar via a clycosidic linkage (glycosphingolipid)


A sphingosine

Functions of Sphingolipids:

  • Sphingolipids are important components of the membrane, especially in the nervous system.
  • Responsible for protecting nerve cells on myelin sheath
  • Examples:
    • Sphingomyelin
    • Cerebroside
    • Gangliosides

Steroids:

Steroids are all derivatives of cyclopentanoperhydrophenanthrene, which is this thing below:

Most abundant steroid is cholesterol, which is a major component of all animal cell membranes. Cholersterol is also used as a precursor for synthesis of all other steroid.

Steroid are also sometimes hormones:

Example:

  • Glucocorticoids
    • Affect metabolism and inflammatory response (cortisol)
 
  • Mineralcorticoids
    • Affect salt and water excretion in kidneys (aldosterone)
  • Androgens/Estrogens
    • Affect sexual development and function (testosterone and estradiol)

Fat Soluable Vitamins:

  • Vitamin D
    • Comes from the hormone ergosterol
    • Regulates Ca2+ metabolism
 
  • Vitamin A (retinol)
    • From plant beta-carotene
    • Retinal = eye photoreceptor
 
  • Vitamin K
    • From plants and bacteria
    • Important in blood clotting
 
  • Vitamin E
    • An antioxidant in cell membranes

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