Final answer:
In a molecule of lecithin, a phosphate group and a molecule of choline occupy the third attachment site.
Step-by-step explanation:
When examining a molecule of lecithin, chemists note that a phosphate group and a molecule of choline occupy the third attachment site. Lecithin is a type of phospholipid that consists of a glycerol backbone with two fatty acids attached to carbons 1 and 2, and a phosphate-containing group attached to the third carbon. The phosphate group may be modified by the addition of charged or polar chemical groups. Choline is a common base found in phospholipids, specifically in phosphatidylcholine or lecithin.