In the lungs, oxygen and carbon dioxide pass through cell membranes by diffusion. Which row is correct? enter image description here

The correct answer is D, but I think it should be B. I can only think about three layers as maximum which are; epithelium of alveolus, endothelium of capillaries and the membrane of red blood cell. I don't know what are remainings.

Any help would be much appreciated!

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    $\begingroup$ Assuming that user24284 /Jam's answer is the correct way to think about it (I think it is), the question is a bit ambiguous. We don't know where to stop: should we track O2 until it reaches blood? or the final destination cells (where O2 is actually consumed)? In the latter case, the correct answer is not actually more than 5, since the oxygen also has to exit the erythrocyte (+1), enter the destination cell (+1), cross mitochondrial membranes... $\endgroup$ – Mowgli Dec 12 '18 at 2:47

Oxigen goes from the alveolar's lumen to the cytoplasm of the erythrocyte, and that's 5 membranes:

  • the "top" of the alveolar epithelial cell
  • the "bottom" of such cell
  • the "top" of the endothelial cell (capillary)
  • the "bottom" of such cell
  • the erythrocyte membrane

You got all the cells right, but your only problem was this: oxygen diffuses through the cell membrane entering the cell, moves through the cytoplasm, and diffuses through the membrane again exiting the cell. So, for each cell, you have to count 2 membranes. For the last one, the erythrocyte, you have only 1 membrane (because it is $\ce{O2}$ final destination).

For the $\ce{CO2}$ the situation is a little bit more tricky. We have the same 4 membranes (2x epithelial cell and 2x capillary), but $\ce{CO2}$ can come from 2 locations:

  • from the erythrocyte, where it is formed from $\ce{H2CO3}$ (by the reaction $\ce{H2CO3 -> H2O + CO2}$) or released from the N-terminal group of proteins, like haemoglobin (where it has previously bound)
  • from the plasma (around 9% of the $\ce{CO2}$).

In the first case we have 5 membranes, and in the second case just 4.

So, the correct answer is D.


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