Phospholipid

A phospholipid is a component of cell membranes in the human body and other animals. It is a lipid, or fat, in which one of the fatty acids has been replaced by a phosphate. Phospholipids have one part that attracts water and one that repels it, which causes them to align in a specific way. This structure is important in regulating how other molecules enter or leave the cell through the membrane.

Background

The human body is made up of four main classes of molecules. These include proteins, carbohydrates, nucleic acids, and lipids. Lipids are fats that do not combine with water easily. Since much of the body contains water, lipids play an important role in separating some cells from others.

Phospholipids are part of the lipid family. They are made up of two fatty acids connected to a head made of glycerol. Also attached to the glycerol head is a phosphate. The fatty acids are hydrophobic, or repel water, while the phosphate is hydrophilic, or attracts water. In the cell membrane, the phospholipids line up so that all the water-repelling fatty acids are facing one way and the water-attracting phosphates are facing in the opposite direction. Cells that are structured this way are known as amphipathic, meaning they are both water soluble and non-water soluble. It is this unique ability that makes phospholipids vital to the function of cells and living organisms.

The phosphates are on the side of the membrane facing outward, while the fatty acids are facing inward. This bilayer structure attracts water to the cell but controls how much of the water and the molecules with it can get inside the cell. This provides a barrier that allows small molecules to get in and keeps larger ones out. This is the most important function of the phospholipids.

The discovery of the presence and function of phospholipids took the combined efforts of several researchers working decades apart. French chemist Antoine-Francois Fourcroy (1755 – 1809) identified an oily substance in the brain that was later discovered to be phospholipids. Johann Ludwig Jordan (1771 – 1853), a German chemist, identified the same fatty substance and noted that it was unique to the human cells he was studying. French chemist Nicolas-Louis Vauquelin (1763-1829) followed Jordan's research and discovered that the lipids Jordan and Fourcroy had identified also contained phosphoric acid. Another French chemist, Jean-Pierre Couerbe (1805 – 1867), noted that the substance his predecessors had identified was not soluble in water or alcohol. He dubbed it "brain wax." French pharmacist Theodore-Nicolas Gobley (1811 – 1876) conducted additional experiments and identified lecithin in egg yolks. This was the first phospholipid specifically identified. Others would build on the research of these men to further uncover the role phospholipids play in the function of cells.

Overview

Cells are the building block of organic life. It is through the cells that living beings make or absorb food. The reproduction of cells is how life continues and how living things grow. All cells are encircled by a membrane that holds in the important functional parts of the cell and helps to regulate the chemical processes that go on inside. The membranes also protect the internal workings of the cell from outside influences.

The bilayer nature of phospholipids plays an important role in the function of the cell membrane. The cell membrane is fluid and semi-permeable, meaning it is flexible and can be penetrated by some molecules but not others. This is because of phospholipids.

Every cell in the body is surrounded by a phospholipid bilayer. It is because of this bilayer that cells are able to engage in respiration, or breathing by exchanging carbon dioxide for oxygen, and can absorb nutrients from the bloodstream. This protective barrier allows vitamins and other nutrients to circulate throughout the body while helping to gather the oxygen and nutrients needed for the health of the cardiovascular, nervous, and digestive systems as well as providing support for other important bodily functions of the liver and brain. Since they are fats, phospholipids can also be broken down by the body to provide energy in certain circumstances.

Because phospholipids are so important to the function of living organisms, they have the ability to adapt to different conditions. For example, when the weather is colder the body automatically increases the ratio of unsaturated to saturated fatty acids in the bilayer so that the cell membrane remains fluid in colder temperatures. This happens in plant and animal life as well.

Phospholipids work in conjunction with the carbohydrates and proteins that make up the other building blocks of cells. Proteins help the cells to communicate; some are embedded in the phospholipid bilayer while others circulate inside or outside the cell membrane and carry nutrients throughout. Carbohydrates attach to both the membrane and the proteins and help differentiate different cell types.

The human body produces phospholipids but also gets some from dietary sources. Lecithin, the first phospholipid to be identified, remains one of the best food sources of phospholipids. It can be found in egg yolks, soy products, milk, wheat germ, some meats, some fatty foods, and some vegetable oils. Krill oil—which is oil from small marine animals that are part of the food supply for larger animals such as whales, seals, and penguins—and fish oil are also good sources of lecithin and phospholipids for those who choose to supplement their diets. Ingesting sufficient quantities of vitamin B from food sources or supplements will help the body to produce more of its own supply of phospholipids.

Phospholipids are essential to the function and survival of cells. These components of cell membranes enable cells to circulate nutrients and oxygen the body needs to function and maintain health.

Bibliography

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