Earlier today, I undertook an exam which featured a question regarding the stickiness of mucus in a person diagnosed with CF's. We had to explain why they had stickier mucus than a 'normal' person, given that CF changes the protein channels used for chloride ions to move through osmosis. This confused me, becuase I thought osmosis was through a permeable membrane, and active transport used a protein channel. I have googled it and I have been proved neither wrong nor right.

Basically, I want to know if osmosis requires or uses a protein channel, or the exam board wrote the question wrong and should have put active transport instead.

(Sorry for the waffle, I just felt some clarification is needed)


2 Answers 2


Normally, epithelial cells of mucous membranes express a chloride ion channel that allows movement of Cl- down its electrochemical gradient and out of the cell. This increase in extracellular electrolytes draws water out of the cells by osmosis, hydrating the mucus.

In cystic fibrosis, this protein is defective, most commonly caused by misfolding and subsequent degradation. Cl- cannot diffuse out of the cell and thus water is not drawn out, leading to thick mucus.

...given that CF changes the protein channels used for chloride ions to move through osmosis.

Water flows by osmosis, not ions.

I thought osmosis was through a permeable membrane...

Osmosis does occur through permeable membranes, but large, bulk flow of water can be mediated by protein channels called aquaporins. Read more here.

...and active transport used a protein channel.

Active transport of solutes does use protein transporters, but they are not strictly "channels": "channel" is the term used for passive transport, also called facilitated diffusion when a channel is involved.

  • $\begingroup$ I hope my edit is okay; I usually try not to edit peoples' posts for content but I thought a little clarification was important. $\endgroup$
    – Bryan Krause
    Jun 9, 2017 at 18:33
  • 3
    $\begingroup$ @BryanKrause How dare you?! But actually, thanks. $\endgroup$
    – canadianer
    Jun 9, 2017 at 18:35

To my present knowledge, osmosis will occur wherever there is a solute concentration difference across a membrane permeable to the solvent. It does not require a protein channel. Not all cells have aquaporins yet all cells have to control their interior concentrations. Water can traverse the cell membrane by moving through the phospholipids. This appears to be the most common form of osmosis in very small microorganisms. My source for this information is an article which I found online on Jan 10, 2019, Verkman A.S. Aquaporins, Current Biology, 2013 January 21, 23(2). The url is ncbi.nlm.nih.gov/pmc/articles/PMC(3590904) I realize your test is long past, but for anyone looking for an answer to the same question (as I was) I hope this helps.


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