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Phospholipids [1] are a class of lipids whose molecule has a hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, joined by an alcohol residue (usually a glycerol molecule). Marine phospholipids typically have omega-3 fatty acids EPA and DHA integrated as part of the phospholipid molecule. [2]
The first region on either side of the bilayer is the hydrophilic headgroup. This portion of the membrane is completely hydrated and is typically around 0.8-0.9 nm thick. In phospholipid bilayers the phosphate group is located within this hydrated region, approximately 0.5 nm outside the hydrophobic core. [9]
Phosphorylcholine (abbreviated ChoP) is the hydrophilic polar head group of some phospholipids, which is composed of a negatively charged phosphate bonded to a small, positively charged choline group.
Phospholipids and glycolipids consist of two long, nonpolar (hydrophobic) hydrocarbon chains linked to a hydrophilic head group. The heads of phospholipids are phosphorylated and they consist of either: Glycerol (and hence the name phosphoglycerides given to this group of lipids), or; Sphingosine (e.g. sphingomyelin and ceramide).
The phosphate ester portion ("head") is hydrophilic, whereas the remainder of the molecule, the fatty acid "tail", is hydrophobic. These are important components for the formation of lipid bilayers. Phosphatidylethanolamines, phosphatidylcholines, and other phospholipids are examples of phosphatidates.
Phospholipid bilayer. Each phospholipid consists of a polar hydrophilic head (red) and two hydrophobic fatty acid tails. The fatty acid structure affects the bilayer structure. Fatty acids with an unsaturated tail (blue) disrupt the packing of those with only saturated tails (black).
The hydrophilic phosphate side is outwards and hydrophobic inwards. Carbohydrates Attached to proteins on outside membrane layers It helps in cell-to-cell recognition. Cholesterol Between phospholipids and phospholipid bilayers It helps the plasma membrane to retain its fluidity. Proteins Embedded within or on the surface of phospholipid layers
Other aggregations are also observed and form part of the polymorphism of amphiphile (lipid) behavior. Phase behavior is an area of study within biophysics . [ 60 ] [ 61 ] Micelles and bilayers form in the polar medium by a process known as the hydrophobic effect. [ 62 ]