Question 26

[sfarukhi1817]

I don't quite understand why the answer can't be simple diffusion. If a concentration gradient is present, can't there be a maximum rate of diffusion? why is carrier-mediated transport the only answer?

[jellywing_2058]

Simple diffusion is going from a high concentration to a lower concentration until the concentrations on both sides of the membrane are the same.
The bigger the concentration gradient, the bigger the rate of transport across the membrane; there is never a maximum rate.


In carrier-mediated transport, as the concentration gradient increases, the rate of transport increases until a maximum rate is achieved, because the membrane carriers are saturated and cannot act faster.

[mohsinalam5969]

DEFINITION OF DIFFUSION:
The term simple diffusion refers to a process whereby a substance passes through a membrane without the aid of an intermediary such as a integral membrane protein.

The force that drives the substance from one side of the membrane to the other is the force of diffusion.

DEFINITION OF CARRIER MEDIATED TRANSPORT:
Movement which occurs across membranes, such as the blood–brain barrier and the gastrointestinal mucosa. Inherent in the mechanism is a rapidly reversible reaction between the substance being transported and components of the membrane. The membrane component is the ‘carrier’. The mechanism is also characterized by (1) being saturable; (2) like substances being able to compete for the services of the carrier so that competitive inhibition occurs.
There are two forms of carrier-mediated transport, active transport and facilitated diffusion.

As in diffusion the bigger the concentration gradient, the bigger the rate of transport across the membrane; there is never a maximum rate while in carrier mediated transport as the concentration gradient increases, the rate of transport increases until a maximum rate is achieved, because the membrane carriers are saturated and cannot act faster.