8-4 Discounting the Transverse Carrier Model. At one time, membrane biologists thought that transport proteins might act by binding a solute molecule or ion on one side of the membrane and then diffusing across the membrane to release the solute molecule on the other side. We now know that this transverse carrier model is almost certainly wrong. Suggest two reasons that argue against such a model. One of your reasons should be based on our current understanding of membrane structure and the other on thermodynamic considerations.

The two main reasons are nonpolar core of the bilayer and the active transport.

Explanation:

The membrane is structured to have two outer layers that are polar and an inner layer that is nonpolar.

If a membrane protein is exposed to the solvent, it will also have a polar side. It would be very difficult for the polar face of the membrane to move through the nonpolar core of the bilayer. Therefore, this model is not feasible.

One major form of transport, active transport, moves solutes up the concentration gradient. The binding of a solute and then release on another side of the membrane would only work for facilitated diffusion because it would cause a net movement of solutes down the concentration gradient. It is unclear how energy could be expended to drive this process in the transverse carrier model. Therefore, the transverse carrier model does not explain active transport.