Help? What is saturation?

Question

I took this from Raven, Johnson Biology 6e.

"A characteristic feature of transport through selective channels

is that its rate is saturable. In other words, if the concentration

gradient of a substance is progressively increased,

its rate of transport will also increase to a certain

point and then level off. Further increases in the gradient

will produce no additional increase in rate. The explanation

for this observation is that there are a limited number of

carriers in the membrane. When the concentration of the

transported substance rises high enough, all of the carriers

will be in use and the capacity of the transport system will

be saturated."

Can someone clear that up?

Answer 1

It is like going through gates that are big enough for one person to pass through at a time. The number of gates is fixed. If the size of the crowd is increased, at a certain point, the rate of passage reaches a maximum. At that point, the system is said to be saturated.

Answer 2

Saturation is for example when there is an equilibrium between a solute and a solvent.

Like if one puts sugar in water, it's saturated when all sugar and all water molecules are being used.

Hope it makes sense.  

Answer 3

Ok, lets say we have a molecule outside a cell and it's trying to get in.  There are channels in the membrane that allow this, but they can only transport a certain amount of substance through per second or per minute.  Let's say these channels can allow 1 molecule through per second.  Let's also say there's 10 of these channels in the membrane.  If there are 5 molecules trying to get in, then each can choose a different channel and all go through at the same time, so the rate will be 5 molecules per second.  If there's 10 molecules trying to get in, that's also fine, all can go at once, except now there's no spare channels.  The rate will be 10 molecules per second.  Now if there's 20 molecules trying to get in, but only 10 channels, the 10 molecules that can't fit through the first time have to wait around untill a channel becomes free.  So the rate will still only be 10 molecules per second.  The rate is saturated because it cannot go any faster, no matter how many more molecules you try and put through, they will all just have to wait their turn until a channel is free.