These macromolecules are classified by type. I have put each type in the table below. All of the types of lipids are at least mostly hydrophobic (meaning that they do not interact well with water).
You will notice that the term "fatty acid" is used quite a bit in the table above. A fatty acid is simply a long-ish hydrocarbon chain with a carboxyl group at one end. It is mostly hydrophobic. Not all lipids are build with fatty acids, but most are. They are not the monomers of lipids, because fatty acids alone do not build any lipids. Even triglycerides, which have three fatty acids in them, are also built using glycerol.
The above table is meant to be a good introduction to lipids and to help you organize your thoughts on them. But a little more explanation is needed to fully understand them.
Hydrophilic vs. hydrophobic
Lipids are hydrophobic. That literally means water ("hydro-") fearing ("-phobic"). They don't do well with water. If you put oil and water together, even if you shake them up together, you know that they separate out. They just cannot tolerate to touch one another, so they separate as much as possible from one another.
Fatty acids, triglycerides, and steroids are all extremely hydrophobic. However, phospholipids have a portion of them that is hydrophilic. That makes phospholipids partly hydrophobic and partly hydrophilic. A bit odd-- right? Well, it is because of that odd property that phospholipids can be the basis for cellular membranes.
Triglycerides vs. phospholipids
Both triglycerides and phospholipids are made up of a glycerol molecule that has combined with fatty acids. Once these smaller molecules (the glycerol and the fatty acids) have bonded together, they are no longer the smaller molecules but are now the larger macromolecule.
Here is a simplified drawing of a triglyceride. In blue is the portion of the triglyceride molecule that was originally a glycerol molecule. In black are the portions of the triglyceride molecule that were originally fatty acid molecules. The way that this drawing is simplified is by not writing in all the atoms-- you can see that the long black squiggly lines seem to be missing lettering. In simplified drawings like this one, every spot where there is a bend in a line, there is a C atom that is there-- it is just not written in. There are other atoms that are not written as well, mainly hydrogen atoms, but we will not worry about that. This is a biology class, after all, not a chemistry class.
People refer to the fatty acid part as the tails of this molecule. There are three tails in a triglyceride molecule.
A phospholipid molecule is also made by combining a glycerol molecule with fatty acids. However, for a phospholipid, there are only two fatty acid tails. Then, where the third tail is absent, a different group gets added on. This different group is hydrophilic. It contains a phosphate group within it, which you have already learned about. The phosphate group is what gives this lipid its name-- phospholipid.
You should be able to see all these components in my drawing to the right. The glycerol and 2 fatty acids are clear. And attached to the glycerol in place of a third fatty acid is a phosphate group, which is part of a large hydrophilic region. Once you consider that everything on top of the glycerol molecule is hydrophilic, and the tails below the glycerol are hydrophobic, you should be able to see how the molecule is part hydrophilic and part hydrophobic. The hydrophilic part of a phospholipid is called its head, while the hydrophobic part is called its tails. A very simplified cartoon of a phospholipid is thus:
© 2006 STCC Foundation Press, content by Dawn A. Tamarkin, Ph.D.
Last changed: January 21, 2007