Advanced Placement (AP) Biology Practice Exam

What is the structure of a phospholipid?

1. Glycerol + 3 fatty acids

2. Glycerol + 2 fatty acids + phosphate group

3. Fatty acids + Sterol + Glycerol

4. Glycerol + 2 fatty acids + cholesterol

The correct answer is: Glycerol + 2 fatty acids + phosphate group

The structure of a phospholipid consists of a glycerol backbone bonded to two fatty acid tails and a phosphate group. This unique configuration gives phospholipids their amphipathic nature, which means they contain both hydrophilic (water-attracting) and hydrophobic (water-repelling) portions. The glycerol molecule serves as the main framework, while the two fatty acids are attached to the glycerol, forming a hydrophobic tail that repels water. The phosphate group, which is often attached to another polar or charged group, forms the hydrophilic head, making this region water-attracting. This structural arrangement is crucial for the formation of cell membranes, where phospholipids spontaneously arrange themselves into a bilayer. The hydrophobic tails face inward, shielded from water, while the hydrophilic heads face outward towards the aqueous environment, creating a barrier that separates the interior of the cell from the external environment. The other options do not accurately represent the structure of phospholipids. For instance, the first option includes three fatty acids, which characterizes triglycerides, not phospholipids. The third option mentions sterols, which are a separate class of lipids that do not