Phospholipids are similar to triglycerides, only the phospholipid has two fatty acid tails and one phosphate group "head." The head is polar and therefore hydrophilic. The tail is nonpolar and is hydrophobic. Because of this, phospholipids can form phospholipid bilayers and vesicles.
And you all know, that can only mean one thing.. cell membranes! According to the fluid mosaic model, our cell membranes are composed of phospholipid bilayers through which small molecules like water and air can diffuse through. For bigger molecules, there are large proteins embedded into the membrane to passively and actively support transport of these materials. It's a good thing that cell membranes are semipermeable; we don't want just anything to enter the cell. We also don't want nothing to enter the cell.
Here's a simple diagram of the cell membrane:
Lipids
- Fats
- Phospholipids
- Waxes and Steroids