Lipids are macromolecules composed of mostly carbon and hydrogen chains. Lipids also contain oxygen and sometimes phosphorous. Examples of lipids include fats, waxes, oils and steroids. They are a heterogeneous group of biochemicals, grouped together because of two characteristics:
- are insoluable (do not dissolve) in water
- store high amounts of energy
Nutrition-wise, we are told to stay away from too many lipids (fats), but it is important to realize that biochemically, lipids play an important role in living things. They can:
- act as waterproofing (waxes)
- act as chemical messengers (steroids)
- store energy (fats and oils)
- form cell membranes (phospholipids)
Let’s take a closer look at a few of the many types of lipids in more detail:
Triglycerides are used by organisms for high energy storage. Vegetable oils (peanut oil, olive oil) and animal fats (such as the fat in butter) are examples.
Triglycerides are composed of two monomers: 1 glycerol, and from 1 to 3 fatty acids. Examine a picture of a triglyceride below that has one glycerol and three fatty acids. It sort of looks like a capital E, right?
There is a lot of energy located in the carbon-hydrogen bonds of the fatty acids attached to the glycerol. When broken down this energy can then be used by the body or these molecules can be synthesized for storing energy for later use.
The fatty acids in a triglyceride can be saturated or unsaturated. You’ve probably heard those terms before, so let’s take a closer look at what they mean at the chemical level:
Phospholipids are important to every cell on earth. This is because they are the primary component of cell membranes, the outer boundary of our cells. Phospholipids are polymers that are built of a specific set of monomers:
- one glycerol molecule
- two fatty acids
- one phosphate group
Take a look at the phospholipid pictured. Notice that one of the ends has a charge. This means that the molecule is polar at one end, and non-polar at the other end. The polar end is hydrophilic (loves water) and the non-polar end is hydrophobic (avoids water).
Now examine these polymers within a cell membrane. Notice the unique features of phospholipids that allow cell membranes to form: hydrophilic heads point toward water (inside and outside cell) and hydrophobic tails cluster together to avoid water.
Steroids and Waxes
Steroids serve as signaling molecules or hormones. Cholesterol is a steroid that plays an important role in cell membrane stability. Though we are told to keep our cholesterol levels in check, our bodies still require it. Steroids are typically flat molecules composed of four rings. Waxes play an important role in animals and other organisms. They can waterproof surfaces (plant leaves or duck’s feathers) or form structures (beeswax in a hive). Their structure is based on two fatty acids that are linked together.