Describe the chemical composition of phospholipids and how they form cell membranes? (10 marks)

Phospholipids are amphipathic molecules i.e. they have a hydrophilic (“water-loving”), polar head end, and a hydrophobic (“water-fearing”), nonpolar tail end (1 mark). The major phospholipids in most animal cell membranes are the phosphoglycerides (1 mark), which means that they are derived from a three-carbon glycerol backbone. The hydrophobic hydrocarbon tails consists of two long-chain fatty acids (1 mark) linked through ester bonds to adjacent carbon atoms of the glycerol (1 mark). The third carbon atom of the glycerol is attached to a phosphate group (1 mark), which in turn is linked to one of several types of head group. The glyerol, phosphate and the head groups together form the polar head. Phospholipids preferentially form bilayers (1 mark), which are sealed compartments formed from two opposing layers of phospholipids. The lipid molecules aggregate to bury their hydrophobic tails into a core region which causes energetically unfavourable rearrangements of the water molecules (1 mark), but also expose their polar, hydrophilic head groups to form favourable electrostatic interactions with the polar water molecules (1 mark). This arrangement is the most energetically favourable (1 mark) when the hydrophobic core protected on both sides from the aqueous environment (i.e. it has no open edges) (1 mark). Thus, both the shape and nature of the polar lipids causes spontaneous bilayer formation in aqueous environments.