Q: Explain how the structure and properties of triglycerides and phospholipids are related to their functions in living organisms.

Triglycerides are a good source of energy as they have a high ratio of energy storing carbon-hydrogen bonds to carbon atoms. They are also an important source of metabolic water as the triglyceride molecules yield metabolic water on oxidation. This is particularly important to desert animals such as camels. As they are poor conductors of heat, they also function as thermal insulators by preventing excessive heat loss. This is especially important for mammals living in cold climates, such as polar bears. Fourth, triglycerides are a good storage molecule as they possess low mass thus much can be stored in a small volume. This makes them especially useful for animals where locomotion requires mass to be kept to a minimum. As triglycerides are large and uncharged, they are insoluble in water, thus they can be stored in large amounts without having any effect on the water potential of cells. Finally, triglycerides are less dense than waters, hence providing aquatic mammals buoyancy.
Each phospholipid molecule consists of a hydrophilic phosphate head and two hydrophobic fatty acid tails. In aqueous environment, phospholipid molecules form a bilayer in biological membranes. The phosphate heads face outwards and interact with the aqueous medium, while the hydrocarbon tails face inwards and are shielded from the aqueous medium. This allows the formation of a hydrophobic barrier between the interior and exterior of the cell. In addition, carbon-carbon double bonds at fatty acid chains causes a kink and results in greater separation between molecules and membrane fluidity. Hydrophobic interactions occur between the lipid layer and hydrophobic portions of membrane proteins, holding membrane proteins of various functions in place. Hydrophilic heads interact with hydrophilic portions of peripheral proteins, loosely to the holding them at the surface of the membrane. Phospholipids associate covalently with membrane carbohydrates to form glycolipids, which function in cell-cell adhesion and recognition.