Main Answer:
Phospholipids in water form a bilayer with polar heads outward and nonpolar tails inward, creating a stable membrane structure. D) The polar heads on the surface and the nonpolar tails in the interior.
Therefore, the correct answer is D) The polar heads on the surface and the nonpolar tails in the interior.
Step-by-step explanation:
Phospholipids are amphipathic molecules, composed of hydrophilic (polar) heads and hydrophobic (nonpolar) tails. When placed in water, phospholipids spontaneously arrange themselves into a bilayer. The hydrophilic heads, being attracted to water, orient themselves towards the aqueous environment, forming the outer surfaces of the bilayer. Meanwhile, the hydrophobic tails avoid water by aligning themselves in the interior, shielded from the surrounding aqueous medium.
This arrangement is driven by the hydrophobic effect, a thermodynamic principle that minimizes the unfavorable interactions between hydrophobic molecules and water. In the context of phospholipids, the hydrophobic tails cluster together, creating a nonpolar region shielded from the polar water molecules. Simultaneously, the polar heads interact with the surrounding water molecules, ensuring a stable and energetically favorable configuration.
This bilayer structure is fundamental to the formation of cell membranes, where phospholipids assemble to create a selectively permeable barrier between the cell's interior and the external environment. The dynamic nature of this bilayer allows for the flexibility and fluidity necessary for various cellular processes.
In summary, the spontaneous formation of a phospholipid bilayer involves positioning the nonpolar heads on the surface and the polar tails in the interior, a configuration dictated by the hydrophobic and hydrophilic properties of the molecule.
Therefore, the correct answer is D) The polar heads on the surface and the nonpolar tails in the interior.