Question

why is permeability dependent on size and charge? the answer is based on a substances ability to interact with both the polar heads of phospholipids on the outside of the membrane and their nonpolar hydrocarbon tails inside. sodium and chloride ions, for example, cannot interact with the nonpolar tails at all, so they rarely pass through. but heres a catch: life also requires the ability for cells to move large and charged things across their membranes. your ability to think and move depends on sodium and other ions moving across nerve cell membranes to conduct electrical signals. how does this happen? enter the role of transport proteins. some integral membrane proteins have a structure that allows them to act as a channel for specific ions or molecules. when these membrane proteins are present and functioning, a phenomenon called facilitated diffusion occurs. the sodium channel protein in your nerve cell membranes \facilitates\ passage of na+ ions across the lipid bilayer, whenever a concentration gradient exists to support diffusion of na+ (side note: most transport proteins can open or close depending on signals they receive, but more on that later.) in almost all cases, cell membranes contain specialized integral proteins that facilitate diffusion of all the polar, charged, or larger substances that are helpful to the cell but do not cross the lipid bilayer readily on their own. lets apply the concepts you just learned about selective permeability. predict which of the following substances will cross a lipid bilayer that lacks transport proteins more readily than others. then, move the options in order from most likely to cross at the top to least likely to cross at the bottom. potassium ions (k+), nitrogen molecule (an n atom triple - bonded to an n atom, written n2), water (h2o), glycerol, ch2—oh, ch—oh, ch2—oh

Explanation:

Small, non - polar molecules cross lipid bilayers most readily without transport proteins. Nitrogen is a small non - polar molecule. Water is a small polar molecule. Glycerol is a larger polar molecule. Potassium ions are charged and large relative to the others. So, the order is based on size and polarity/charge.

Answer:

1. Nitrogen molecule (an N atom triple - bonded to an N atom, written N2)
2. Water (H2O)
3. Glycerol
4. Potassium ions (K+)