Final answer:
The polar head of a molecule like a phospholipid is hydrophilic due to its polar nature and dissolves in water, while the nonpolar tail is hydrophobic, does not dissolve in water, and is made up of two fatty acids. Together, these features make the molecule amphiphilic, crucial for forming cellular membranes.
Step-by-step explanation:
The difference between the polar head and the nonpolar tail of a molecule, such as those found in phospholipids, lies in their affinity for water and their structure. The polar head is hydrophilic, meaning it has an affinity for water, and usually contains a phosphate group. This part of the molecule can dissolve in water due to its polar nature, meaning it has a separable charge with the ability to form hydrogen bonds. In contrast, the nonpolar tail is hydrophobic and repels water. It is made up of two fatty acid chains that are nonpolar, meaning they have no separable charge and cannot form hydrogen bonds. These tails do not dissolve in water. In a phospholipid bilayer, such as those that make up cellular membranes, the polar heads face the aqueous exterior while the nonpolar tails face each other on the inside, creating a barrier that regulates the movement of substances into and out of the cell. The dual character of these molecules, being part hydrophilic and part hydrophobic, makes them amphiphilic, or 'dual-loving', allowing them to form the structural matrix of a cell membrane.