Which description best explains how phospholipids are arranged in the cell membrane lipid bilayer?

• A. They form a bilayer with hydrophobic tails facing the extracellular fluid and cytosol, and hydrophilic heads facing inward toward each other.
• B. They form micelles in which the hydrophilic heads cluster at the center and the hydrophobic tails point outward toward water.
• C. They form a single layer with all hydrophilic heads facing the cytosol and all hydrophobic tails facing the extracellular fluid.
• D. They form a bilayer with hydrophilic (polar) heads facing the extracellular fluid and cytosol, and hydrophobic (nonpolar) fatty acid tails facing inward toward each other.

Answer: (D)

Verified step by step guidance

1. Understand the basic structure of phospholipids: each molecule has a hydrophilic (water-attracting) 'head' and two hydrophobic (water-repelling) 'tails'.
2. Recall that in an aqueous environment, phospholipids spontaneously arrange themselves to minimize the exposure of hydrophobic tails to water, while maximizing the exposure of hydrophilic heads to water.
3. Recognize that this arrangement leads to the formation of a bilayer, where the hydrophilic heads face outward toward the aqueous environments (extracellular fluid and cytosol), and the hydrophobic tails face inward, away from water, toward each other.
4. Eliminate incorrect options by noting that micelles have hydrophilic heads inward and tails outward, which is opposite to the bilayer structure of cell membranes, and that a single layer with heads and tails facing opposite aqueous environments is unstable.
5. Conclude that the correct description is the phospholipid bilayer with hydrophilic heads facing the extracellular fluid and cytosol, and hydrophobic tails facing inward toward each other.