Phospholipids are one of the main parts of cellular membranes. These molecules serve many essential functions in the cell, from forming the organelles, to separating cellular contents, to supporting cellular processes. There are three types of phospholipids, each with unique characteristics and roles to play.
Phospholipids have a unique structure, with a head and double tail. The tails are hydrophobic, repelling water, and the head is hydrophilic, meaning that water easily spreads across it. Head groups can have a positive or negative charge, or they can also be neutral. Phospholipids are arranged in bilayers, with the head facing outward and the tails toward each other.
The bilayer structure of a phospholipid, combined with the hydrophilic head and hydrophobic tails, demonstrates its biological importance. This structure allows phospholipids to create selectively permeable membranes, letting nutrients and ions cross and supporting effective separation of organelles and the cells themselves. All mammalian cells depend on phospholipids to maintain the integrity and functionality of both internal and external membranes.
Phospholipids have other functions beyond providing structure and permeability. Three types of phospholipids exist, and research shows that each has multiple functions. Phospholipids are essential to processes throughout the body, including in the lungs, joints, gastrointestinal tract, and peritoneum.
Phosphatidylcholine (PC) is the most abundant phospholipid in mammalian cells. Two pathways in the body synthesize PC. The first is dependant on choline, a nutrient the body makes in small amounts but which also requires dietary supplementation to avoid deficiencies. PC plays an essential role in the liver for the secretion of very-low-density lipoproteins, which deliver fat energy to other organs. PC also assists with the absorption of fat-soluble nutrients in the intestines. For those with a choline deficiency, the second pathway for synthesizing PC becomes even more important.
Phosphatidylethanolamine or PE is the second most abundant phospholipid in mammals. PE is unique in that the head groups are small, meaning that the cellular membranes it makes up can accommodate protein insertion. PE can also form single-layer structures. One of the most important functions of PE is its support of cellular mitochondria, energy-producing organelles.
Phosphatidylserine or PS makes up only 5 to 10 percent of phospholipids. PS is located primarily in the inner plasma membrane. When it reaches the outside, it signals blood clotting. It facilitates interaction between positively charged proteins and receptors. PS is made from PC and PE, and newly synthesized PS is also a precursor to PE. PS is thought to enter the cellular mitochondria and affect the regeneration of PE.
Because the body naturally produces them and there is a low risk for toxicity, phospholipids have many applications in pharmacology. They can be used for any administration route and function as a wetting agent or emulsifier. Phospholipids make products water-soluble, so they are especially useful in IV medications. Other applications include calming the GI side effects of some oral treatments and increasing the absorption of medications that are not water-soluble.
Some phospholipids used in pharmacology are obtained from natural sources, including sunflowers, soybeans, canola seed, and flaxseed. Animal sources include milk, krill, and egg yolks. In all of these sources, PC is the main phospholipid. Special extraction and formulation are needed to convert these phospholipids into high-quality medical-grade versions that meet regulatory and safety standards.
Synthetic phospholipids are another option for pharmacological use. The use of enzymes to synthesize and modify natural phospholipids has advanced rapidly. There are many benefits to this approach, including less pollution, better customization, and higher quality. Full synthetic phospholipids have also been developed for jobs like improving drug targeting.
Antiphospholipid syndrome (APS) is a rare autoimmune disorder where the body attacks phospholipids. The cause is unknown. Researchers believe that it could be a combination of gene mutations and exposure to something like a virus. Symptoms include seizures, dementia, rash, chronic headaches, and blood clots. There is no cure for APS, and treatment focuses on preventing large clots from forming.
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