If a small droplet of triacylglycerol molecules is suspended in water, the fat molecules form a "ball of spaghetti" with no particular orientation. But if a droplet of phospholipid molecules is put in water, all the molecules point outward, toward the water. Phospholipids are forced into this orientation because phospholipids have __________.

• A. three fatty acid molecules, all pointing in different directions
• B. two fatty acid molecules pointing in different directions
• C. both a saturated fatty acid and an unsaturated fatty acid
• D. two charged or polar ends
• E. a charged or polar end and an uncharged or nonpolar end

Answer: (E)

Verified step by step guidance

1. Understand the structure of phospholipids: Phospholipids consist of a glycerol backbone, two fatty acid tails, and a phosphate group attached to the glycerol. The phosphate group is polar, while the fatty acid tails are nonpolar.
2. Recognize the amphipathic nature of phospholipids: Amphipathic molecules have both hydrophilic (water-attracting) and hydrophobic (water-repelling) parts. In phospholipids, the phosphate group is hydrophilic, and the fatty acid tails are hydrophobic.
3. Consider the behavior of phospholipids in water: When phospholipids are placed in water, the hydrophilic phosphate heads orient themselves towards the water, while the hydrophobic tails avoid water, leading to a specific orientation.
4. Compare with triacylglycerols: Triacylglycerols, or triglycerides, consist of three fatty acid chains attached to a glycerol backbone, lacking a polar head group. This structure does not force a specific orientation in water, unlike phospholipids.
5. Conclude the reason for orientation: The orientation of phospholipids in water is due to their amphipathic nature, having a charged or polar end (phosphate group) and an uncharged or nonpolar end (fatty acid tails), which is why they form structures like micelles or bilayers in aqueous environments.