Lipids are a group of hydrophobic biomolecules that play
important roles in living organisms.
While the primary function of lipids is long-term energy storage, lipids are also used for protection,
insulation, and lubrication. They also act as precursors for some hormones,
and are a key component of cell membranes.
There are four basic groups of lipids. These are triglycerides,
phospholipids, steroids, and waxes.
Although these groups differ in many respects,
they all have one characteristic in common:
They are all insoluble in water.
You may have noticed that lipids and water do not mix.
For example, notice the yellow colored oil in the beaker of water shown here?
Even if we stirred this for several minutes, or even several hours,
the oil would still separate out from the water. This is because lipids are hydrophobic.
From Latin, the prefix “hydro” means “water” and “phobic” means “fear of”.
So when you hear that lipids are “hydrophobic”, this basically mean that water
and lipids do not mix.
Let’s take a closer look at the category of lipids known as triglycerides.
Triglycerides include the fats and oils. Fats (such as lard and butter) are
solid at room temperature and are used by animals for insulation,
protection and long-term energy storage.
Oils (such as corn oil and olive oil) are liquid at room temperature
and are used by plants for long-term energy storage.
At the molecular level, triglycerides contain two types of subunit molecules:
glycerol and fatty acids. Let’s take a quick look at fatty acids.
Let’s take a quick look at fatty acids.
A fatty acid has three main parts: a chain of carbon and hydrogen atoms called
the “hydrocarbon chain,” a methyl group at one end,
and an acid group at the other end.
Fatty acids can be either saturated or unsaturated.
A fatty acid that has only single carbon to carbon bonds
is known as a saturated fatty acid. This is because the carbon chain is
“saturated” with all the hydrogen atoms it can hold.
Unsaturated fatty acids have one to several double bonds.
Double bonds result in kinks in the
fatty acid chain which affects the melting point of the fat.
Animal fats have saturated fatty acids and are solid
at room temperature while vegetable oils have one or many double
bonds and are liquid at room temperature.
A trans-fat is an example of an unsaturated fatty acid where the hydrogen atoms are on
opposite sides of the double-bond. Trans-fats are usually formed during the production of processed foods
and are also common in partially hydrogenated oils.
In order to increase shelf life and melting point of the fat, excess hydrogen atoms are
introduced to a unsaturated oil. This causes the
formation of trans-fat bonds in the fatty acid chain. Unfortunately the consumption of trans
fats has been associated with cardiovascular disease and its use has fallen from favor.
Now that you understand a little bit fatty acids,
let’s zoom back out and look at how the triglyceride subunits fit together.
Remember, a fatty acid is only a small part of a triglyceride.
To become a triglyceride, 3 separate fatty acids have to
bond with a glycerol molecule through the process of dehydration synthesis.
Let’s move on to the next category of lipids, which is phospholipids.
Phospholipids are similar to triglycerides
in that they contain glycerol and two fatty acids.
What’s different is that a phosphate group rather than a third fatty
is attached to the third carbon of glycerol.
Phospholipids are extremely important, mainly because of their unique properties in regard to water.
The phosphate head of the molecule is hydrophilic (or water-loving).
This means that it mixes well with water. The fatty acid tails, however,
are hydrophobic (or water-hating) and do not mix well with water.
Because of these unique properties, phospholipids tend to arrange themselves so that
only the hydrophilic heads interact with a watery environment,
and the hydrophobic tails crowd inward away from the water.
This structure is the major component of plasma membranes of the cell.
Steroids are the next category of lipids.
Steroids are composed of four fused rings of carbon to which different functional
groups are attached. One well-known steroid
molecule is cholesterol.
Cholesterol serves as a precursor for the synthesis of
other steroids such as testosterone, estrogen, vitamin D, and cortisone.
Cholesterol is present in plasma membranes where it stabilizes the membrane.
The hormones testosterone and estrogen have small differences in their functional groups
but large differences on their effects on an organism.
Waxes are the final group of lipids. Waxes are non-polar and repel water.
They are found in protective coatings on leaves and on outer surfaces of animals.
Wax is produced in the ears of some animals to protect the eardrum.
In addition, bees construct honey combs from wax.
Now that we’ve covered all four categories of lipids, Let’s do a quick recap.
The four categories of lipids are triglycerides, phospholipids,
steroids and waxes. All lipids are insoluble in water.
While the primary function of lipids is long-term energy storage,
lipids are also used for a multitude of other purposes, such as protection
and insulation, and as key component of hormones and cell membranes.