Final answer:
The lipid raft hypothesis suggests the plasma membrane contains microdomains called lipid rafts, rich in cholesterol and sphingolipids, which function in signaling and trafficking. Phospholipids form membranes spontaneously due to their amphipathic nature.
Step-by-step explanation:
The lipid raft hypothesis posits that the plasma membrane is not a uniform fluid structure, but contains microdomains called lipid rafts that are more ordered and tightly packed than the surrounding bilayer. These rafts can concentrate and segregate proteins and are thought to play a role in various cellular processes.
Lipid rafts are rich in cholesterol and sphingolipids, particularly sphingomyelin. These particular lipids, with their more saturated fatty acid chains and cholesterol's rigid ring structure, contribute to the less fluid, more tightly packed nature of lipid rafts compared to the surrounding membrane, which mostly contains unsaturated phospholipids. These characteristics allow lipid rafts to serve as platforms for cell signaling and trafficking.
Understanding why phospholipids spontaneously organize into membranes is because they are amphipathic molecules with a hydrophilic head and hydrophobic tail. This orientation creates a bilayer that forms the basis of all cell membranes, which can be selectively permeable and allows the cell to maintain a stable internal environment.