The question asks for the most likely mechanism of entry into the cell of this molecule. The answer is simple diffusion.
It seems to me that this molecule would be on the large side for that? If not, is there any limit on size for hydrophobic molecules when passively diffusing through a phospholipid bilayer? I assume larger size means slower diffusion rate? Does the presence of a bunch of fused rings have any effect on this, vs a long saturated fatty acid?
Thanks!
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Hi
@Gurby! Good question about speed, but that is not the Q the AAMC asked. They simply wanted to know considering the STRUCTURE, what is the most likely method by which this molecules (non-polar, very hydrophobic) might cross through the cell membrane. If they had offered facilitated diffusion, diffusion and simple diffusion as an answer choice there may be some cause for nit-picking on the part of the AAMC, but here the only other choices you are given are:
Active transport. Not necessary unless we knew that entering the cell meant going from low to high concentration, which according to the passage, is not the case. In addition, structure has little to do with the need for active vs. passive transport.
Receptor mediated endocytosis. This mechanism allows for specific molecules to be ingested by the cell. The specificity results from a receptor-ligand interaction. Receptors on the plasma membrane of the target tissue will specifically bind to ligands on the outside of the cell. An endocytotic process occurs and the ligand is ingested. There is no mention of ligands in the passage and the simple structure of STN makes this highly unlikely, though not impossible (the metabolism of cholesterol uses RME).
Ion channel. Easiest answer to eliminate as STN is no ion.
As you can see the AAMC will provide some tempting, some not so tempting answers. Your goal is to think like the AAMC and only answer the question they have asked. Sometimes that means eliminating all the unlikely or impossible answers, and going with the only remaining choice. They use phrases like most likely and best in order to avoid having to argue that choice C is impossible, which it is not technically true.
As for speed of diffusion vs. size, that is a relatively simple topic but it is not something the AAMC would ask unless they had you compare 2 molecules of differing size. That is just not how they test the sciences. I know that the surface area (SA) of the organism/cell is directly proportional to the rate of diffusion, so is the concentration difference (d[X]) while the distance the molecule will travel across the membrane is inversely proportional to diffusion rate.
Rate = SA x d[X] / d
You can also think of this in terms of thermodynamics, but on a much smaller scale than we are used to on the exam. You will go through some of this when to get to Pharma in med school but I will keep it MCAT level here.
In any given environment, there are several factors that impact a molecule's mobility: size of the molecule, viscosity of the surroundings, interactions between macromolecules and the specific type of transportation that is used by the molecule. Because diffusion relies on movement and movement relies on the size of the molecule, there is a direct relationship between molecular size and diffusion rate. Larger molecules necessitate larger amounts of energy to engage in the same level of activity as smaller molecules. If there is not sufficient energy to move a larger molecule, it resists the effects of diffusion and is unable to move from one area to the next. Increases in environmental heat or other extraneous factors also have an impact on the energy involved in the diffusion process and the rate at which a large molecule diffuses.
Hope this helps, good luck!
