Lipids

What are lipids?

Non-polar compounds of biologic origin that are insoluble in water.

What are four lipids that you should be familiar with for the MCAT¨?

1. Steroids
2. Terpenes
3. Triacyl glycerols
4. Free fatty acids

What are steroids?

Very important lipids involved in regulating biological functions, which have a characteristic four-carbon ring system - memorize the shape of steroids and be able to recognize it!

Figure 55.1: The general structure of a steroid.

What steroid is found in cell membranes and contributes to membrane fluidity?

Cholesterol.

What are three other important steroids?

Sex hormones, vitamin D and cortisol.

What are terpenes and terpenoids?

Also collectively called ``essential oils,'' terpenes are hydrocarbons that include carotenes (precursors for vitamin A). Terpenoids are similar to terpenes but are derived from isoprene units and have odoriferous characteristics (e.g. cinammon):

Figure 55.2: Retinol (vitamin A).

What are triacylglycerols?

Esters of glycerols:

Figure 55.3: Triacylglycerols.

What unique physical properties do triacylglycerols have?

Hydrophilic and hydrophobic moieties which can be used to create a barrier between polar and non-polar regions, e.g. cell membranes and micelles in the gut:

Figure 55.4: Triacylglycerols: Hydrophilic heads and hydrophobic tails.

What is created when triacylglycerols undergo hydrolysis?

Free fatty acids and glycerol:

Figure 55.5: Products of triacylglycerols hydrolysis in the presence of a base and water followed by an acid: one glycerol molecule and three free fatty acids.

What are three properties of fatty acids?

1. Unbranched
2. An even number of carbons
3. Unsaturated vs. saturated fats
   • Unsaturated fats have Ònot met their saturation pointÓ of hydrogens and always have $cis$-double bonds
     • $Cis$-double bonds result in lower boiling points because they pack less efficiently

Figure 55.6: Saturated (top) vs. unsaturated fats (bottom).