The figure below represents a component of all cell membranes. Which of the following best describes the nature of the part labeled X? (A) A hydrophilic phosphate group. (B) A hydrophilic saturated fatty acid. (C) A hydrophobic phosphate group. Or (D) a hydrophobic unsaturated fatty acid.
Let’s look at the main component of a cell membrane, the phospholipid. All cell membranes consist of a structure called the phospholipid bilayer. A bilayer means that there are two layers next to each other. The provided figure shows one of these phospholipids.
If we break down the word “phospholipid,” it’s made up of two words: phosphate and lipid. The phospholipid has a hydrophilic, or water-loving, phosphate head and a hydrophobic, or water-fearing, fatty acid tail. The fatty acid tail can be either saturated or unsaturated. Saturated fatty acids consist of single bonds between all carbon atoms. This forms a straight chain of carbons that are saturated with hydrogen atoms. Unsaturated fatty acids contain at least one double bond between two carbons. So it is not fully saturated with hydrogen atoms. This forms a kink in the chain, as we could see here.
The part labeled X is a fatty acid tail with a kink. This means that it’s an unsaturated fatty acid that’s hydrophobic. So the correct answer is (D), a hydrophobic unsaturated fatty acid.