Lipids:
Fats and Oils
Lipids are a class of naturally-occuring, non-polar organic molecules. Fats and oils, also referred to as triacylglycerols, are two types of lipids that are characterized by the presence of ester functional groups that connect fatty acids to a glycerol backbone (Figure 2). Fatty acids are long chain hydrocarbons(commonly from 12 carbons up to 20 carbons) terminating in a carboxylic acid functional group. Typically, naturally-occurrring fatty acids are not branched and contain an even number carbons. There are two types of fatty acids; saturated fatty acids and unsaturated fatty acids. Saturated fatty acids contain no double bonds. Unsaturated fatty acids contain one or more double bonds that usually have a cis (Z) configuration. Polyunsaturated fatty acids contain multiple double bonds. A saturated fatty acid, palmitic acid (16-carbons) and an unsaturated fatty acid, palmitoleic acid (16 carbons, one double bond) and glycerol are shown in Figure 3. |
|
Figure
2
|
Fatty
Acid |
Number
of Carbons |
Melting
Point (0C) |
Saturated |
||
Lauric |
12 |
44 |
Myristic |
14 |
58 |
Palmitic |
16 |
63 |
Stearic |
18 |
70 |
Arachidic |
20 |
75 |
Unsaturated |
||
Palmitoleic |
16 |
32 |
Oleic (one double bond |
18 |
16 |
Linoleic (two double bonds) |
18 |
-5 |
Arachidonic |
20 |
-50 |
Saturated
(%) |
Unsaturated
(%) |
Other (%) |
|||||
Source |
Lauric (C12) |
Myristic (C14) |
Palmitic (C16) |
Stearic (C18) |
Oleic (C18) |
Linoleic (C18) |
--- |
Fats |
|||||||
Lard |
--- |
1 |
25 |
15 |
50 |
6 |
8 |
Butter |
2 |
10 |
25 |
10 |
25 |
5 |
23 |
Human Fat |
1 |
3 |
25 |
8 |
46 |
10 |
12 |
Oils |
|||||||
Corn |
--- |
1 |
10 |
4 |
35 |
45 |
5 |
Olive |
--- |
1 |
5 |
5 |
80 |
7 |
2 |
Peanut |
--- |
--- |
7 |
5 |
60 |
20 |
8 |
Soaps
and
detergents function as cleaning agents by
organizing into micelles that trap non-polar dirt
and oil
particles.
Carboxylate salts of soaps form micelles,
spherical structures in which
the polar carboxylate is oriented on the outside
diameter of the sphere
and the non-polar hydrocarbon chains of the fatty
acids are oriented
into the center of the sphere. Dirt and oil
associate with the
non-polar tails of the carboxylate salts ("like
dissolves like")
through hydrophobic interactions and are trapped
in the center of the
sphere. Since the outer surface of the micelle is
polar, it is water
soluble and can be washed or rinsed out with water
(polar solvent).
Figure 7 depicts the structure of a micelle
illustrating how dirt and
other non-polar particles are trapped in the
center. Detergents are similar to soaps in their properties but use alternative functional groups as the polar end of the molecule. Detergents are considered to be "synthetic" cleansing agents because they are not dervied from naturally occurring fats and oils. The non-polar part of the detergent molecule may be a branched hydrocarbon or a straight-chain hydrocarbon. Detergents that contain straight chain hydrocarbons are preferred as these are biodegradable. Anionic detergents are derived from sulfonic acids and form sulfonate salts when ionized. Ammonium salts attached to non-polar hydrocarbons are examples of cationic detergents. Figure 8 shows the structures of some synthetic detergents. |
|
Figure 7: Depiction of a Micelle |