carrier-mediated transport

[sdm33]

Q If increasing the concentration gradient across the plasma membrane increases the rate of transport until a maximum rate is reached, this would be convincing evidence for
A. simple diffusion.
B. carrier-mediated transport.
C. osmosis.
D. the Fluid Mosaic model.

The answer is B, but I don't understand why?

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[akim6890]

If it was simple diffusion through the membrane, there would be no maximum. Rate would be indefinitely proportional to the gradient. However, since they tell you there is a maximum, something else is providing some sort of rate-limiting factor. If there are only a certain number of membrane channels, rate would have an upper limit proportional to the number of channels while still also proportional to the gradient.

 

[brlin]

If it was simple diffusion through the membrane, there would be no maximum. Rate would be indefinitely proportional to the gradient. However, since they tell you there is a maximum, something else is providing some sort of rate-limiting factor. If there are only a certain number of membrane channels, rate would have an upper limit proportional to the number of channels while still also proportional to the gradient.

Spot on. Simple diffusion displays unsaturable kinetics, but facilitated diffusion displays saturable kinetics.

 

[sdm33]

what you main by maximum ? maximum of diffusion ?

 

[sdm33]

This use of carrier proteins to cross the membrane is known as facilitated diffusion, and can be used by those molecules to cross the membrane in either direction – into or out of the cell, but always down the conc gradient.
Examples, glucose and ions
I still don't see how the Max fits in all this

 

[DNA]

what you main by maximum ? maximum of diffusion ?

Basically what they mean is, after a certain point of constantly adding say calcium ions on one side of the membrane, every calcium gated carrier proteins (for example), would acquire a calcium ion at any point in time. There's a hold process: 1.) Calcium enters protein 2.) Protein conforms to new shape. 3.) Protein releases Calcium to other side of membrane. Eventually when all these ions are in that "hold" state, we call that saturation. In other words, the calcium ions will have to wait to be transported until another carrier protein becomes available. By contrast, with simple diffusion, ions pass either straight through the plasma membrane or through an ion channel directly. There is no "hold" or waiting state where saturation can occur.

If you increase the calcium ions on one side of the membrane, more calcium ions will pass the membrane per second just because they are more available, but after a point in time, that rate will cease to increase once all carrier proteins are full. With simple diffusion, if you increase the gradient on one side, that rate will continue to increase until the ions reach equilibrium. For the sake of simplicity, anytime they ask a question like this -- think of saturation and just ask yourself what form of ion transport this is pertaining to.

 

[akim6890]

Ya a simple analogy is a really 10000 car wide race track with a 50 cars at the start line. The cars are your molecule and the start line is your membrane. When it's wide open, it doesn't matter if you have if 50-10000 cars there is no maximum rate that the cars can go through. However, if there are say 1000 gates that each take 1 second for each car then the rate will be limited to 1000 cars/second.