What is a phospholipid and how does it form part of the cell membrane?

The cell membrane is a complex material made up of three main components: the phospholipid bilayer, proteins, and cholesterol. The tadpole-shaped phospholipid is made up of a phosphate head attached to a glycerol that has a large fatty acid tail. The polar phosphate head is hydrophilic, and is attracted to a watery environment. The non-polar fatty acid tail is hydrophobic and does not interact with a watery environment. The phospholipid bilayer forms when fatty acid tails from separate phospholipids interact with each other to avoid water, and the phospholipids touch their 'feet' with one another. The phosphate heads then point outwards, interacting with the surrounding water environment. The hydrophobic fatty acid tails are now in a low water environment, and the hydrophilic phosphate heads in a high water content environment. This arrangement extends along the whole cell membrane in a fully closed circle, and remains relatively stable. Gases and some lipids can cross the bilayer, and proteins form pores that are able to selectively transport water soluble molecules into the cell, making the membrane semi-permeable. Cholesterol lines the lipid bilayer and aids its structure, helping to maintain a fluid cell membrane that is not too solid or liquid. This model for the cell membrane is called 'The Fluid Mosaic Model' and is vital for maintaining the environment inside a cell.