Figure 1.

A schematic presentation of commonly used phospholipids. Most of the commonly used lipids are presented with hydrophobic R₁ and R₂ fatty acyl tail groups and a hydrophilic head group carrying a net charge at neutral pH 7. The head group determines the charge of a phospholipid, whereas the lipid tail group contributes no charge. The lipids with head groups (oval shape shaded area) for sphingomyelin (SPH), phosphatidylcholine, and phosphatidylethanolamine exhibit neutral net charge. Phosphatidylserine and phosphatidylglycerol carry a negative net charge at neutral pH 7. The tail groups (R₁ and R₂) for each phospholipid can have various lengths (typically C14–C18) and degrees of saturation. SPH contains a sphingoid base backbone (unshaded) and the other four phospholipids contain a glycerol backbone (unshaded). In addition, R₁ of SPH is a C15-saturated carbon chain and R₂ is a fatty acid residue connected to the sphingoid base backbone through an amide functional group. The fatty acid residues for the other four phospholipids are attached to the glycerol backbone via an ester functional group. The detailed effects on the physical properties of phospholipids because of charge and variation in R₁ and R₂ are described in Table 3.