Phospholipid bilayer
Phospholipid bilayers form the plasma membrane that surrounds all cells. The molecules that makes up the cell membrane are called phospholipids. Phospholipids are amphipathic molecules. This means that they have a hydrophobic phosphate head and two hydrophilic fatty acid tails. These components of the phospholipids cause them to orientate themselves so the phosphate head can interact with water and the fatty acid tails can't, hence forming a bilayer. This arrangement can also be called a bio-molecular sheet as the hydrophobic tails from each individual lipid sheet interact with one another forming a hydrophobic interior that acts as a permeability barrier. [1] As well as these, the membrane also contains various other types of lipid such as cholesterol and proteins. These molecules contribute significantly to the mass of the membrane. Some of the fatty acids in the phopholipid molecules are unsaturated, with one or more carbon - carbon double bonds in its hydrocarbon chain. These double bonds create a kink in the hydrophobic tails. These kinks prevent adjacent phospholipid molecules from packing too close together, which causes an increase in the fluidity of the bilayer. The bilayer is arranged so that the phospholipid heads face outwards and the fatty acid chains face inwards, with cholesterol and proteins scattered throughout the membrane. This structure is described as fluid because the phospholipids can diffuse along the membrane [2].
References
Aberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P. (2012). Molecular Biology of the Cell - Fifth Edition, New York: Garland Science