Plasma Membrane and Transport
A certain transmembrane protein has its amino terminus at the exterior of the cell, carboxy terminus in the cytoplasm and everything in between embedded in the membrane. Which of the following statements is true?
|A||The amino terminus must be hydrophobic.|
|B||Much of the protein is hydrophobic.|
|C||The protein probably transports sodium.|
|D|| The protein must have a neutral charge.|
This is the best answer of the choices given. The amino terminus is probably not hydrophobic (choice A) as it is located extracellularly, where there is a great abundance of water. We are given no good reason to assume the protein transports sodium, so choice C is not the best. Transmembrane proteins do not necessarily carry any particular charge, and do not have to be neutral, so answer D is not correct. Answer B relies on your knowledge of the composition of the cell membrane, specifically the lipid component. For a protein to span the membrane, it must be compatible with the environment of the membrane, which is hydrophobic.