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An illustration of a phospholipid shows a hydrophilic head group composed of phosphate connected to a three-carbon glycerol molecule, and two hydrophobic tails composed of long hydrocarbon chains.
A hydrophilic head and two hydrophobic tails comprise this phospholipid molecule. The hydrophilic head group consists of a phosphate-containing group attached to a glycerol molecule. The hydrophobic tails, each containing either a saturated or an unsaturated fatty acid, are long hydrocarbon chains.

Source: OpenStax Biology 2e

OpenStax Biology 2e

The membrane’s main fabric comprises amphiphilic, phospholipid molecules. The hydrophilic or “water-loving” areas of these molecules (which look like a collection of balls in an artist’s rendition of the model) are in contact with the aqueous fluid both inside and outside the cell. Hydrophobic, or water-hating molecules, tend to be non-polar. They interact with other non-polar molecules in chemical reactions, but generally do not interact with polar molecules. When placed in water, hydrophobic molecules tend to form a ball or cluster. The phospholipids’ hydrophilic regions form hydrogen bonds with water and other polar molecules on both the cell’s exterior and interior. Thus, the membrane surfaces that face the cell’s interior and exterior are hydrophilic. In contrast, the cell membrane’s interior is hydrophobic and will not interact with water. Therefore, phospholipids form an excellent two-layer cell membrane that separates fluid within the cell from the fluid outside the cell.

– What is a chemical compound possessing both hydrophilic and lipophilic properties?

A phospholipid molecule consists of a three-carbon glycerol backbone with two fatty acid molecules attached to carbons 1 and 2, and a phosphate-containing group attached to the third carbon. This arrangement gives the overall molecule a head area (the phosphate-containing group), which has a polar character or negative charge, and a tail area (the fatty acids), which has no charge. The head can form hydrogen bonds, but the tail cannot. Scientists call a molecule with a positively or negatively charged area and an uncharged, or non-polar, area amphiphilic or “dual-loving.”

– What is a generic term to designate any group of yellow-brownish fatty substances occurring in animal and plant tissues which are amphiphilic, and are used for smoothing food textures, emulsifying, homogenizing liquid mixtures, and repelling sticking materials?

This characteristic is vital to the plasma membrane’s structure because, in water, phospholipids arrange themselves with their hydrophobic tails facing each other and their hydrophilic heads facing out. In this way, they form a lipid bilayer—a double layered phospholipid barrier that separates the water and other materials on one side from the water and other materials on the other side. Phosopholipids heated in an aqueous solution usually spontaneously form small spheres or droplets (micelles or liposomes), with their hydrophilic heads forming the exterior and their hydrophobic tails on the inside.

The image on the left shows a spherical lipid bilayer, shown as a half sphere whose surface is covered in the spherical polar heads, and thin, strandlike extend inward.  In the core of the sphere is another half sphere, with the same anatomy. The image on the right shows a smaller sphere that has a single lipid layer only, made up of the spherical heads. The image at the bottom shows a lipid bilayer sheet; whose polar heads form the upper and lower surfaces, with tails extending toward each other in the middle.
In an aqueous solution, phospholipids usually arrange themselves with their polar heads facing outward and their hydrophobic tails facing inward. (credit: modification of work by Mariana Ruiz Villareal)
– What is an organic molecule, a sterol, a type of lipid, and is biosynthesized by all animal cells which is an essential structural component of animal cell membranes?

Phospholipids can become immunogenic, and this is surprising since they are major components of cellular surfaces and lipoproteins. The high risk of cardiovascular disease and atherosclerotic plaque development in many lupus patients is related to a combination of traditional risk factors like dyslipidemia and nontraditional ones, including antibodies to anionic phospholipids, increased lipid peroxidation, inflammation. Such phospholipids include cardiolipin and phosphatidylserine, and are also related to lupus anticoagulants.


Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at: