The molecules in a membrane that limit its permeability are __________________. A. phospholipids B. proteins C. water D. negative ions E. carbohydrates

Answer 1

For this question you want to think about what we already know about cell membranes. What is able to pass easily (or diffuse) into or out of a membrane? Well, the smaller something is the less work required to move through the membrane so we definitely want to look at size. So, we can cross off the smallest of the 5 choices (D) because that isn't going to be a limiting factor. The next thing we need to think about is what a membrane is made out of. It is a bilayer made out of phospholipids. It would be really unfortunate if phospholipids limited permeability because then nothing would be able to travel into or out of a cell and that would really cause endless problems. So we can cross A off the list. Next, we need to think about polarity. Our phospholipid bilayer has a structure that has polar and non polar sections to it. The head of the phospholipid is polar. That is why it is facing outwards towards surrounding water. The tail of the phospholipid is hydrophobic, which is why it is hidden in the middle of the bilayer. It is trying to avoid surrounding water. So, to summarize what is going to pass through easily is non polar and small polar molecules. We can now cross water off the list because it is a small polar molecule and does not limit permeability. So now we are down to 2 choices, proteins and carbohydrates. Lets go back and look at membrane structure again because there a few more substances that contribute to the structure that I didn't mention yet and that would be proteins. There are several different protein structures that contribute to a cell membrane. There are integral proteins and transmembrane proteins for example and these are important structures that help the permeability of the cell. So, the only answer we are left with is carbohydrates. In fact carbohydrates can only cross the membrane if they are attached to a protein. I hope this helps. If you have any further questions or would like to schedule a session please feel free to send me a message. I am always happy to help!

Answer 2

Phospholipids limit permeability of the membrane based on charges and size of compounds.