What specific part of the cell/structure allows the mucus secretions to be thick and viscous rather than fluid (i. e., is it a membrane channel, organelle or something else)? Please be specific and describe the step-by-step mechanism whereby this altered structure/part of the cell causes the secretions to contain less water.

The CFTR behaves like a channel for chlorine. Its dysfunction affects both the transport of this ion and other ions and the transport of water, which causes a thickening of secretions, an alteration of mucociliary transport and local defenses, facilitating bacterial colonization and promoting the release of pro-inflammatory mediators in the airway

Explanation:

CFTR is a protein expressed in the epithelial cells of the respiratory system, pancreas, bile ducts, sweat glands and genitourinary system. It is made up of a single chain made up of 1,480 amino acids. It contains 12 hydrophobic regions embedded in the lipid membrane and acts as a channel for chlorine.The highest levels of expression of the CFTR protein are found in serous cells of the submucosal glands of the proximal airway. In them, Cl- is released to the outside. In addition, there are channels for Na +, through which this ion is also secreted in the same direction. These movements cause the displacement of water and also of mucins, originating in the submucosal glands, allowing their presence on the surface of the airway. For all this to occur normally, a basolateral Na + - K + - ATPase cotransporter must function, another basolateral cotransporter formed by Na +, K + and 2 Cl-, which allows the latter to enter the cell, and an apical CFTR channel through which it exits the Cl- of the cell towards the acinar lumen. Na + leaves the cell following Cl- by a paracellular pathway accompanied by water. When CFTR malfunctions, Cl- does not exit through this channel and this implies a decrease in Na + and water in the canalicular lumen, with the consequent thickening of secretions.