|MadSci Network: Chemistry|
I'm not 100% sure what your question is? If you are asking if more than one kind of chemical substance can diffuse (i.e., no transport protein is involved) through a membrane at the same time, the answer is yes. Substances can diffuse through a lipid bilayer if they are sufficiently lipid soluble to overcome the energy barrier created by moving from an aqueous environment to a lipid environment. The more "fat" soluble a molecule, the more likely it is to be able to diffuse through a membrane. This is true of small molecules only however. Molecules/atoms that are charged and/or large are less likely to be able to diffuse through the membrane. More than one substance can be diffusing across at the same time simply because the membrane around a cell has an extremely large surface area compared to the area/volume of a single molecule. If you are asking if a particular transport protein can transport more than one kind of substance/substrate, the answer is again yes, but NOT at the same time. On each cycle of transport, one and only one set of substrate molecules gets transported. Any given transport protein has 1 or sometimes 2 primary substrates. For example, the Na+,K+-ATPase in the plasma membrane of most eukarytic cells in a single cycle of transport moves 3 Na+ ions from the cytosol to the extracellular space and 2 K+ ions in the opposite direction. Other transporters might move a glucose molecular into the cell along with a proton or a Na+ ion. Still other transporters might move Ca2+ into or out of the cell, but nothing else. This does not mean however that each of these transporters cannot move something else if it were present. For example if a rubidium ion (Rb+) were present in the extracellular space, the Na+,K+-ATPase could move Na+ out of and Rb+ into the cell. If this doesn't answer the question, write out a longer question in more detail and I'll try to answer it.
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