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Phospholipid Molecule
A phospholipid is composed of two fatty acids, a glycerol unit, a phosphate group, and a polar molecule. Ties van Brussel / Wikimedia Commons

What Are Phospholipids?

Phospholipids belong to the lipid family of biological polymers. A phospholipid is composed of two fatty acids, a glycerol unit, a phosphate group, and a polar molecule. The phosphate group and polar head region of the molecule is hydrophillic (attracted to water), while the fatty acid tail is hydrophobic (repelled by water). When placed in water, phospholipids will orient themselves into a bilayer in which the non-polar tail region faces the inner area of the bilayer. The polar head region faces outward and interacts with the liquid.

Phospholipids are a major component of cell membranes, which enclose the cytoplasm and other contents of a cell. Phospholipids form a lipid bilayer in which their hydrophillic head areas spontaneously arrange to face the aqueous cytosol and the extracellular fluid, while their hydrophobic tail areas face away from the cytosol and extracellular fluid. The lipid bilayer is semi-permeable, allowing only certain molecules to diffuse across the membrane to enter or exit the cell. Large organic molecules such as nucleic acids, carbohydrates, and proteins can not diffuse across the lipid bilayer. Large molecules are selectively permitted entrance into a cell through transmembrane proteins that traverse the lipid bilayer.

Phospholipid Function

Phospholipids are very important molecules as they are a vital component of cell membranes. They help cell membranes and membranes surrounding organelles to be flexible and not stiff. This fluidity allows for vesicle formation, which enables substances to enter or exit a cell through endocytosis and exocytosis. Phospholipids also act as binding sites for proteins that bind to the cell membrane. Phospholipids are important components of tissues and organs including the brain and heart. They are necessary for the proper functioning of the nervous system, digestive system, and cardiovascular system. Phospholipids are used in cell to cell communications as they are involved in signal mechanisms that trigger actions such as blood clotting and apoptosis.

Types of Phospholipids

Not all phospholipids are the same as they differ in size, shape, and chemical makeup. Different classes of phospholipids are determined by the type of molecule that is bound to the phosphate group. Types of phospholipds that are involved in cell membrane formation include: phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol.

Phosphatidylcholine (PC) is the most abundant phospholipid in cell membranes. Choline is bound to the phosphate head region of the molecule. Choline in the body is primarily derived from PC phosholipids. Choline is a precursor to the neurotransmitter acetylcholine, which transmits nerve impulses in the nervous system. PC is important structurally to membranes as it helps to maintain membrane shape. It is also necessary for proper functioning of the liver and absorption of lipids. PC phospholipids are components of bile, aid in the digestion of fats, and assist in the delivery of cholesterol and other lipids to body organs.

Phosphatidylethanolamine (PE) has the molecule ethanolamine attached at the phosphate head region of this phospholipid. It is the second most abundant cell membrane phospholipid. The small head group size of this molecule makes it easier for proteins to be positioned within the membrane. It also makes membrane fusion and budding processes possible. In addition, PE is an important constituent of mitochondrial membranes.

Phosphatidylserine (PS) has the amino acid serine bound to the phosphate head region of the molecule. It is typically confined to the inner portion of the cell membrane facing the cytoplasm. PS phospholipids play an important role in cell signaling as their presence on the outer membrane surface of dying cells signals macrophages to digest them. PS in platelet blood cells aid in the blood clotting process.

Phosphatidylinositol is less commonly found in cell membranes than PC, PE, or PS. Inositol is bound to the phosphate group in this phospholipid. Phosphatidylinositol is found in many cell types and tissues, but is particularly abundant in the brain. These phospholipids are important for the formation of other molecules that are involved in cell signaling and help to bind proteins and carbohydrates to the outer cell membrane.