Lipids are a pretty diverse family, generally classified as water-insoluble, organic substances … Like carbs, they’re made up of hydrogen and carbon, but their oxygen is present in significantly smaller amounts, usually tagging along …on the outside looking in *plays world’s smallest violin*. Since they’re mainly made up of non-polar hydrocarbons, it’s not surprising that lipids are hydrophobic. This insolubility in water means that lipids don’t form solutions, however, they do form emulsions (Infoplease.com, n.d.)
Now about that diversity…
Lipids exist as triglycerides- fats (solid at room temp.) & oils (liquid at room temp.)- Phospholipids, steroids and waxes.
More on Lipids…
(We swear there are pictures in there…)
-They are formed by condensation reactions between fatty acids and an alcohol.
-A condensation type of reaction occurs when two molecules or functional groups combine to form one single molecule together with the elimination of a small molecule such as water (H2O).
-Lipids contain fatty acids with the acidic group (-COOH-) AKA, the carboxyl group and an alcohol such as glycerol which contains 3 hydroxyl (-OH-) groups.
-A glycerol molecule can condense with 3 fatty acid molecules to form a triglyceride molecule
-Triglycerides can be further classified as fats or oils depending on whether they are solids (fats) or liquid (oils) at room temperature therefore, how saturated or unsaturated the fatty acids that make up the triglyceride are:
-Fatty acids that contain one or more double bonds (C=C) are said to be unsaturated whilst those that contain single (C-C) bonds are said to be saturated
-Unsaturated fatty acids have much weaker bonds than that of saturated fatty acids; therefore at room temperature the bonds of unsaturated fatty acids are easily broken. This explains why Compounds containing unsaturated fatty acids tend to exist in the form of oils at room temperature whilst those that contain saturated fatty acids tend to exist as solids.
(Champe and Harvey et al. 2005)
There’s always a rebel…
Phospholipids aren’t that unlike their siblings the triglycerides. Phospholipids are formed by a bond between the glycerol of a lipid and a phosphorous molecule (K). However, unlike triglycerides they have two fatty acid chains, (diglyceride). So what about the place where the 3rd triglyceride would’ve been? That’s actually occupied by a phosphate group, attached to a polar region of the molecule. Hold up… Something’s a bit fishy about that?
Well, I’m sure you remember that lipids are generally classified as non-polar, hydrophobic molecules.While that’s still true for the fatty acid region, the phosphate group at the head of the compound is polar *gasp*….and hydrophilic.
That’s why phospholipids are great emulsifiers, they can interact with oil at their non-polar hydrophobic end, and with water at their hydrophilic head. Furthermore, these characteristics (and the tendency of phospholipids to clump together, exposing their polar heads), are precisely why cells utilize phospholipids in their membranes; they get the best of both worlds: functionality, by having their membranes interact with their water rich interior and exterior, and exclusivity, as phospholipids form a continuous barrier around the cell. Think of the Phospholipids as that ever so indifferent bouncer at your favourite club?
Champe, Pamela C, Richard A Harvey and Denise R Ferrier. Biochemistry. Philadelphia: Lippincott/Williams & Wilkins, 2005.
Connexions. “Organic Compounds Essential to Human Functioning.” 2013. http://cnx.org/content/m46008/latest/?collection=col11540/latest (accessed 20 March 2014)
Infoplease.com. “The Chemistry of Biology: Lipids | Infoplease.com.” n.d.. http://www.infoplease.com/cig/biology/lipids.html (accessed 14 March 2014).
Contributors: Thalia, Chris, Roi (editor)