STPM Biology Biological Molecules Part 11 Lipid - Phospholipids

1. Phospholipid molecule is formed from the condensation of:

• one glycerol molecule
• two fatty acids molecules
• one phosphoric acid molecule

Structure and symbol of a phospholipid molecule

2. Phospholipid is a major component of cell membrane and hence is distributed all over the body of an organism.

Structure of phospholipid molecule

1. A phospholipid molecule consists of one glycerol molecule attached to two fatty acids chain and one phosphate group.
2. Additional small molecules (usually polar) can be linked to the phosphate group to form a variety of different phospholipids.
3. For instance, choline molecule attaches to the phosphate group of phospholipids to form lecithin. 
4. Lecithin is the most common phospholipid in human bodies. The major phospholipids of cell membrane is lecithin. The fatty acids of lecithin is oleic acid (unsaturated) and stearic acid (saturated).

Structure of lecithin

Physical properties of phospholipid

1. The hydrophilic head of phospholipid molecules is soluble in water, while the hydrophobic tail is insoluble in water.
2. In water, phospholipid molecules assemble into a droplet called micelle. On the surface of water, phospholipid molecules are assemble closely with hydrophilic heads dissolved in water and the tails jutting out of the surface.

   

   Assembly of phospholipid molecules on the water surface and in water

   
   
3. In cells, both the intracellular environment and the immediate external environment are water. This causes phospholipids to form bilayer. In this bilayer, the hydrophilic heads are on the outside (in contact with water); the tails of phospholipids point to the interior. This bilayer is the main structures in the formation of cell membrane.

   

   Cross section of a phospholipids bilayer

Functions of phospholipid

1. Phospholipids are the main structural components for cell membranes.
2. Phospholipid such as lecithin is involved in the production of neurotransmitters such as acetylcholine in the neurons.
3. Phospholipid molecules are usually involved in transporting fat in the body.