A phospholipid is an amphiphilic molecule consisting of a polar head region, a unit of glycerol, and two or more non-polar fatty acid tails, typically found in a cell membrane. A bilayer of phospholipid molecules forms a plasma membrane.
When the phospholipid molecules are joined by other lipids and integral proteins, the surface can function as a cellular membrane. This semi-permeable membrane blocks the flow of polar substances, allowing the cell to control the concentration of various substances through the use of protein channels.
A phospholipid consists of two basic parts: the head and the tail. The hydrophilic head consists of a glycerol molecule bound to a phosphate group. These groups are polar and are attracted to water. The second group, the hydrophobic tail, consists of two fatty acid chains. Some species use three fatty acid chains, but two is most common. The fatty acid chains can be saturated, or unsaturated. Unsaturated fatty acid chains have less hydrogen, forcing the molecule to form double carbon-carbon bonds. These bonds create bends in the tail, as seen in the image below.
A phospholipid starts as all of these constituent parts, floating around the cytosol. When they come together near the endoplasmic reticulum, special enzymes bind all the parts together, forming a single phospholipid. As many enzymes are functioning together, many phospholipid molecules can be created quickly. As they are created, the phospholipid molecules arrange into a bilayer. This bilayer is eventually large enough to form a small, empty vesicle. These small round balls of phospholipid bilayer are transported to the cell membrane. Here, they can fuse to the cell membrane, growing the cell.
The amphiphilic nature of a phospholipid is extremely important for the functioning of cells. Every phospholipid, because of this dual relationship with water, is self-arranging when grouped together. The hydrophilic tails are drawn together via hydrophobic interactions, such as van der Waals forces. The hydrophilic heads are drawn toward the aqueous solution on either side of the phospholipid bilayer.
This creates one of the most important structures in biology: a semi-permeable membrane. Each phospholipid molecule pushes closely into its neighbor. The hydrophobic core of the membrane helps exclude ions and water. This is extremely important for cells, which must protect themselves from changes in the environment, including excess ions, water, or other substances. Every species of cell has a different arrangement of surface and integral membrane proteins. These channels and enzymes allow the cell to control what goes in and out of the cell, and at what rate.
As part of this important cell membrane, each phospholipid plays an important role. The heads can have various additions and attachments, changing how they interact with water and molecules. The tails have a very important function in establishing the fluidity and strength of the membrane. Some phospholipid molecules have two straight tails, while others have crooked tails. The ratio of straight-to-bent phospholipids determines how closely each phospholipid can get to its neighbor. The closer the phospholipid molecules, the tighter and more rigid the membrane. Other lipid molecules, known as sterols, are also embedded in the membrane and can increase or decrease membrane fluidity.
The composition of cell membranes varies greatly, but in general, animals that live in the cold have more fluid membranes, while those living in hot environments have less fluid membranes. As the fluidity also changes with temperature, this ensures that animals in both environments have adequately fluid membranes. Too stiff, and the cell cannot function properly. Too loose, and the cells will fall apart. That is why organisms adapted to heat cannot survive cold environments and vice versa.
Cholesterol and Phospholipids
Within human cells, the balance of straight to bend phospholipid molecules, as well as sterol molecules, are important to the fluidity of cells. Cholesterol is especially important, helping to make cell membranes more rigid. Humans naturally produce cholesterol and do not need additional cholesterol in our diets. Our bodies will naturally regulate the cholesterol within our cell membranes, but this process can be overridden by diet.
Only animals produce cholesterol, as plants produce other fats and oils to store energy and support their cell membranes. Cholesterol, in humans and animals, is similar enough that our bodies can easily incorporate it into their cells. A diet high in animal tissue, eggs, and milk will add tons of cholesterol to your bloodstream. Here, it works its way into the cell membranes of your arteries, making them more rigid. Rigid arteries are much more likely to clog and burst, leading to heart attacks, strokes, and aneurysms. Luckily, this process can be combated by eating a plant-based diet and limiting cholesterol intake.
Drug Delivery using Phospholipid Micelles
The phospholipid molecules of the cell membrane are great at keeping substances out, but sometimes doctors want to get substances into a cell, to deliver a medicine or treatment. Many drugs now have phospholipid delivery systems. The drugs are either bound to the phospholipid molecule or enclosed in a micelle. A micelle is a small ball of phospholipids. These can easily fuse with the cell membrane, allowing the medicine to be deposited within the cell as this happens. As the science progresses, scientists are even planning to directly engineer these small capsules. By attaching specific proteins to the surface, they can target receptors on tissue-specific cells, allowing the drug to be delivered specifically to a single organ or place in the body.
Lecithin, A Phospholipid-Based Food Additive
Lecithin is a common food additive, made mostly from phospholipid molecules packed together. These phospholipids are extracted from plant and animal cells, and the rest of the cell is removed. The fatty, but polar nature of these proteins allows them to be used as an emulsifier. These types of culinary additives help dissolve fatty substances in aqueous solutions. Each phospholipid molecule can bind to both the non-polar fatty substances, as well as interact with water molecules and polar substances. This can help dissolve powders into non-polar or fat-laden dishes.