Phospholipids are major constituents of the plasma membrane, the outermost layer of animal cells. Like fats, they are composed of fatty acid chains attached to a glycerol or sphingosine backbone. Instead of three fatty acids attached as in triglycerides, however, there are two fatty acids forming diacylglycerol. Also, the third carbon of the glycerol backbone is occupied by a modified phosphate group (see image below).
A phosphate group alone attached to a diaglycerol does not qualify as a phospholipid. It is phosphatidate (diacylglycerol 3-phosphate), the precursor of phospholipids. The phosphate group is modified by an alcohol. Phosphatidylcholine and phosphatidylserine are two important phospholipids that are found in plasma membranes.
Phospholipids as amphipathic molecules
A phospholipid is an amphipathic molecule, meaning it has a hydrophobic and a hydrophilic part. The fatty acid chains are hydrophobic and cannot interact with water, whereas the phosphate-containing group is hydrophilic and interacts with water (see image below).
The head is the hydrophilic part, and the tail contains the hydrophobic fatty acids. In a membrane, a bilayer of phospholipids forms the matrix of the structure, the fatty acid tails of phospholipids face inside, away from water, whereas the phosphate group faces the outside, aqueous side (see image above).
Phospholipids are responsible for the dynamic nature of the plasma membrane. If a drop of phospholipids is placed in water, it spontaneously forms a structure known as a micelle, where the hydrophilic phosphate heads face the outside and the fatty acids face the interior of this structure. We will talk more about the cell’s plasma membrane in another tutorial.
Unlike the phospholipids and fats discussed earlier, steroids have a fused ring structure. Although they do not resemble the other lipids, they are grouped with them because they are also hydrophobic and insoluble in water. All steroids have four linked carbon rings and several of them, like cholesterol, have a short tail (see image below). Many steroids also have the –OH functional group, which puts them in the alcohol classification (sterols).
Cholesterol is the most common steroid. Cholesterol is mainly synthesized in the liver. In fact, it is the precursor to many steroid hormones such as testosterone and estradiol, which are secreted by the gonads and endocrine glands. It is also the precursor to Vitamin D. In addition, cholesterol is also the precursor of bile salts, which help in the emulsification of fats and their subsequent absorption by cells.
Although cholesterol is often spoken of in negative terms by lay people, it is necessary for proper functioning of the body. As a matter of fact, it is a component of the plasma membrane of animal cells and is found within the phospholipid bilayer. Since the plasma membrane is the outermost structure in animal cells, it is responsible for the transport of materials and cellular recognition. Also, it is involved in cell-to-cell communication.
Video Summary of Lipids
The short video below by Ricochet Science gives an overview of the four basic groups of lipids: triglycerides, phospholipids, steroids, and waxes.