A Level Biology Practice Exam

Which of the following best describes the composition of a phospholipid?

1. Two fatty acids and a pentose sugar

2. One fatty acid and a phosphate group

3. One glycerol, two fatty acids, and a phosphate group

4. Three fatty acids and a phosphate group

The correct answer is: One glycerol, two fatty acids, and a phosphate group

A phospholipid consists of one glycerol molecule, two fatty acids, and a phosphate group. This unique structure is crucial for the formation of cellular membranes. The glycerol acts as the backbone, while the two fatty acids contribute to the hydrophobic (water-repellent) tail of the molecule. The phosphate group is hydrophilic (water-attracting) and forms the head of the phospholipid. This dual nature of the molecule allows phospholipids to spontaneously arrange themselves into bilayers in aqueous environments, which is fundamental to creating cell membranes. This arrangement results in a hydrophilic surface that interacts with the cytoplasm and extracellular fluid, while the hydrophobic tails face inward, away from water. This characteristic is essential for maintaining the integrity and functionality of cell membranes, serving as barriers to the passage of polar substances and compartmentalizing cellular environments. Other combinations listed, like those involving only one fatty acid or three fatty acids, do not accurately represent the structure of phospholipids and thus fail to describe their essential properties or their role in biological membranes.