Advanced Placement (AP) Biology Practice Exam

Which type of amino acids are known to anchor proteins in the cellular membrane?

1. Polar

2. Non-polar

3. Charged

4. Neutral

The correct answer is: Non-polar

Non-polar amino acids play a crucial role in anchoring proteins within the cellular membrane due to their hydrophobic nature. Cellular membranes are composed predominantly of a lipid bilayer, which acts as a barrier to water-soluble substances. The non-polar (or hydrophobic) side chains of these amino acids tend to interact favorably with the lipid components of the membrane, effectively integrating the protein into the bilayer. This hydrophobic characteristic allows non-polar amino acids, which include residues such as alanine, valine, leucine, isoleucine, and phenylalanine, to be embedded within the membrane's core, thereby stabilizing the protein's structure and facilitating its functional role in various cellular processes. Their presence ensures the proper positioning and orientation of membrane proteins, which is essential for activities like signaling, transport, and acting as receptors. In contrast, polar, charged, and neutral amino acids are generally found on the exterior surfaces or in regions where the protein interacts with the aqueous environment, as these amino acids can form hydrogen bonds or ionic interactions with water and other polar molecules. This difference in chemical nature highlights the specific role that non-polar amino acids play in anchoring proteins within membranes.