Q. 17

[vpteruna3172]

I kinda don't understand the question. Does the question actually ask which the following structures that actually use active transport OR which the following structures that does not use active transport?
The correct answer is B. I was thinking active transport does not occur in alveoli.

Please.. I need some explaination..
Thanks..

[vpteruna3172]

thank you for the reply..

[mcat_premed3832]

To begin with, it's important to recognize that it is not necessary to know specifically how active transport is used in A and C because it should be so clear that the purpose of the alveolus is to provide an environment for simple diffusion of the very important gases of metabolism/respiration.

For C, Nerve and muscle cells: both cell types are able to "signal" because of electrical disturbances of their membranes. The fundamental basis of which is active transport including the all important Na-K ATPase pump.

For A, cells of the large intestine: fluid is absorbed in the large intestine by diffusion or by active transport. Aldosterone increases the diffusion of sodium into the cells and its active transport through the basolateral membrane into the interstitial fluid.

Beyond the MCAT: The active transport of sodium in the large intestine increases the gradient for the diffusion of potassium from cells into the lumen. The substances being absorbed in the large intestine include chains of fatty acids which are produced during fermentation. Neither saccharides, nor amino acids are absorbed in the large intestine.

[smartaleck5326]

Hold on, I dont understand how the large intestine utilizes active transport. The purpose of the large intestine is to absorb water right? Doesn't water follow its concentration gradient from the lumen into the cells of the large intestine and therefore doesn't this mean this is passive diffusion?