Question

a protein called the cystic fibrosis transmembrane conductance regulator (cftr) is a regulated channel that facilitates passive diffusion of negatively charged chloride ions across the cell membrane. cftr is critical for maintaining the balance of salt and water on the surface of your lung cells. people whose genomes are missing the gene encoding cftr or encode a non - functional cftr protein have cystic fibrosis, a disease that causes thick mucus to build up in the lungs, inhibiting lung function.
what happens to chloride ions when cftr is missing or non - functional?
chloride ions can still diffuse through the membrane without functional protein channels, but they will do so just a bit more slowly.
chloride ions will now diffuse across the membrane in the opposite direction.
chloride ions can no longer diffuse across the membrane, unless other ion channels specific to chloride are present and open.
chloride ions can still diffuse across the membrane through channels that normally carry positive ions like sodium or potassium.

Explanation:

Ions, especially charged ones like chloride, typically need specific protein channels (like CFTR in this case) to cross the cell membrane through passive diffusion. Membranes are generally not very permeable to ions without proper channels. If CFTR (the specific chloride - ion channel) is missing or non - functional, chloride ions can't use it. And ion channels are usually specific (a sodium channel won't effectively let chloride ions pass through in significant amounts). So, unless there are other chloride - specific channels (which the problem doesn't mention as being present and open in the context of CFTR deficiency), chloride ions can't diffuse across the membrane.

Answer:

Chloride ions can no longer diffuse across the membrane, unless other ion channels specific to chloride are present and open.