In my book, it is written that when a molecule binds to the carrier protein then the protein rotates and releases the molecule inside the cell. I want to know how is this possible. Wouldn't the surrounding phospholipid molecules or other proteins hinder its rotation?


The rotation axis is in the plane of membrane

Source : http://ncert.nic.in/ncerts/l/kebo111.pdf (page 177, Third paragraph)

  • $\begingroup$ Rotation of carrier protein in which axis? is the axis in the plane of membrane or perpendicular to it? If latter is the case then there shouldn't be any noticeable hindrance as the membrane is quasi fluid in nature. $\endgroup$
    – JM97
    May 19, 2017 at 10:39
  • $\begingroup$ @JM97 The rotation axis is in the plane of the membrane. $\endgroup$ May 19, 2017 at 10:47
  • $\begingroup$ Please cite the source, or upload a photograph showing rotational axis. $\endgroup$
    – JM97
    May 19, 2017 at 10:48
  • $\begingroup$ @JM97 I've added the source in the question. $\endgroup$ May 19, 2017 at 10:52
  • $\begingroup$ Where does it says that axis is in the plane? $\endgroup$
    – JM97
    May 19, 2017 at 10:53

1 Answer 1


"How do transport/carrier proteins rotate during facilitated diffusion?"

They don't. The explanation in the PDF you linked...

Figure 11.1 shows an extracellular molecule bound to the transport protein; the transport protein then rotates and releases the molecule inside the cell.

... is an oversimplification, to say the least (you could simply say that this is plain wrong). What really happens is way more complex.

We call it conformational change.

According to Alberts, Molecular Biology of the Cell (2002):

Each type of carrier protein has one or more specific binding sites for its solute (substrate). It transfers the solute across the lipid bilayer by undergoing reversible conformational changes that alternately expose the solute-binding site first on one side of the membrane and then on the other. (emphasis mine)

This is an image that explains way better the process, from the same chapter in Alberts (the legend provides a detailed explanation):

enter image description here

Figure 11-11A model for the molecular mechanism of action of the bacterial lactose permease (A) A view from the cytosol of the proposed arrangement of the 12 predicted transmembrane helices in the membrane. The loops that connect the helices on either side of the membrane are omitted for clarity. A glutamic acid on helix X binds H+, and amino acids contributed by helices IV and V bind lactose. (B) During a transport cycle, the carrier flips between two conformational states: in one, the H+ -and lactose-binding sites are accessible to the extracellular space (1 and 2); in the other, they are exposed to the cytosol (3 and 4). Unloading of the solutes on the cytosolic face (3 → 4) is favored because the lactose-binding site is partly disrupted and a positive charge contributed by an arginine on helix IX displaces the H+ from the glutamic acid on helix X. (A, adapted from H.R. Kaback and J. Wu, Accts. Chem. Res. 32:805-813, 1999.)

Thus, as you can see, we have conformational states: one of them exposes the binding site to the extracellular space, while the other one exposes the binding site to the cytosol. Just like what happens when an enzyme changes conformation, the 3D changes in the protein structure are quite small, and it's hydrophobic regions keep in the same place in both states.

Source: Alberts, B., Johnson, A. and Lewis, J. (2002). Molecular biology of the cell. 1st ed. New York: Garland Science..

  • 3
    $\begingroup$ +1 but why beat about the bush? Just say it's wrong. $\endgroup$
    – Alan Boyd
    May 19, 2017 at 12:37
  • $\begingroup$ @alanboyd welcome back! (I suppose you've come back today...?) As you see, there are some new faces here (although mine is a bit ancient :P). So, when are we seeing your next answer? Oh, and +1 for Gerardo ;-) $\endgroup$ May 19, 2017 at 12:51
  • $\begingroup$ @AlanBoyd I'm trying to be nice and polite :-) $\endgroup$
    – user24284
    May 19, 2017 at 13:18
  • $\begingroup$ @another'Homosapien' Yes, I've been back for about a week - a sprinkling of answers. I'm trying to be more selective than I used to be. $\endgroup$
    – Alan Boyd
    May 19, 2017 at 18:13
  • $\begingroup$ @alanboyd a week?! (How ignorant of me) anyways, eagerly waiting for some masterpieces :) $\endgroup$ May 19, 2017 at 18:18

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