Biology Reference
In-Depth Information
B.
WHY ARE THERE SO MANY DIFFERENT LIPID
S?
Lipids have proven to be very difficult to study due to their overwhelming diversity
[1]
.
Although the human body may contain about 1,000 major lipids, countless minor lipids also
exist. As a result, lipid studies lag far behind protein and nucleic acid studies and it is often
considered that lipids are the poor relations among wealthier biological macromolecules. At
first glance there may appear to be an almost infinite number of lipid species, and new ones
are being discovered each day. It has been estimated that a typical animal cell contains about
1,000 different major lipid species, which is a large number. But this is a pittance when minor
lipids are added into the calculation. For example, it has been reported that human tears
contain
30,000 lipid molecular species
[2,3]
! And this is undoubtedly just the tip of the lipid
iceberg. We probably know about only a fraction of the total lipids in the biosphere. Compre-
hending such large numbers of very different and often strange molecules makes scientists
uncomfortable. The question has often been raised, why are there so many different lipids
[1]
, and can such an array of structures be organized into an understandable and useful clas-
sification system?
>
Lipid Functions
It is clear that after ~4 billion years of chemical and biological evolution, nature does not
make mistakes. There must be a reason, and hence a discoverable function, for each and
every lipid species. These functions can be roughly divided into three general categories:
storage lipids, structural lipids and active lipids. Storage lipids are generally non-polar
and so are normally found sequestered away from water in lipid droplets and are not
commonly found in membranes. They are storage forms of fatty acids (triacyl glycerols,
see Chapter 5) that are densely packed biochemical energy sources. Triacyl glycerols contain
9 kcal/g of energy compared to only 4 kcal/g for sugars. Structural lipids form the amphi-
pathic matrix of membranes and are the focus of this topic. Active lipids come in many
shapes and sizes and possess potent biochemical activities. Often they are found in
membranes at much lower levels than structural lipids. Membrane lipids make up 5% to
10% of the dry mass of most cells and storage lipids account for more than 80% of the dry
mass of fat storing adipocytes. Lipids exhibit a wide diversity of biological functions
including:
1.
Energy source (fats and oils).
2.
Membrane bilayer component defining the cell permeability (lipid bilayer) barrier.
3.
Providing the matrix for assembly and function of many catalytic processes.
4.
Chaperones for protein folding.
5.
Light absorber (e.g. chlorophyll).
6.
Energy transduction (e.g. component attached to rhodopsin).
7.
Electron carrier (e.g. coenzyme Q).
8.
Hormone (e.g. testosterone, progesterone).
9.
Vitamins (e.g. vitamin A, E, D).
10.
Anti-oxidant (e.g. vitamin E).
11.
Signaling molecules (e.g. ceramide).
