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As far as I know, lipids are defined as biomolecules which are hydrophobic.

Triglycerides are composed of fatty acids and glycerol and are considered lipids but, are fatty acids alone or glycerol alone considered as lipids ?

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From IUPAC Goldbook:

A loosely defined term for substances of biological origin that are soluble in nonpolar solvents. They consist of saponifiable lipids, such as glycerides (fats and oils) and phospholipids, as well as nonsaponifiable lipids, principally steroids.

That means that lipid does not mean any hydrophobic molecule. Lipids can be amphipathic as you already know about fatty acids.

Glycerol is not a lipid; neither is it hydrophobic. Glycerol, if to be classified into one of biomolecular classes, it would be that of monosaccharides (in the form of sugar alcohol like sorbitol and xylitol).

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    $\begingroup$ I would rather classify glycerol as an alcohol, but this can be debated. $\endgroup$ – Chris Nov 14 '14 at 19:05
  • $\begingroup$ I dropped my answer and fully support this one. The wiki page on glycerol says it is a poly-alcohol and thus a sugar. $\endgroup$ – AliceD Nov 15 '14 at 2:51
  • $\begingroup$ @Chris. Yes it is alcohol no doubt. I was talking about a biochemical class and it is one of the simplest sugar alcohol (like sorbitol, xylitol etc). $\endgroup$ – WYSIWYG Nov 15 '14 at 5:00
  • $\begingroup$ Instead of calling glycerol a monosachharide, shouldn't it be called derivative of a monosachharide as it doesn't have a ketone / aldehyde grp ? $\endgroup$ – biogirl Nov 17 '14 at 5:37
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    $\begingroup$ @biogirl I didn't call it a monosaccharide. I said if at all it is to be grouped somewhere it would be in that of monosaccharides. And yes it is a reduced form of glyceraldehyde and dihydroxy acetone (which are the simplest aldose and ketose resp.). It would be called a sugar-alcohol (so yes it is a derivative). $\endgroup$ – WYSIWYG Nov 17 '14 at 6:08
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Glycerol is not lipid (short chain polyol, so it is polar and water soluble), but fatty acids (amphiphilic) and triglycerides (hydrophobic) are.

Lipids may be broadly defined as hydrophobic or amphiphilic small molecules; the amphiphilic nature of some lipids allows them to form structures such as vesicles, multilamellar/unilamellar liposomes, or membranes in an aqueous environment.

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