IMAT 2012 Q52 [Transport Mechanisms]

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Which of the following transport mechanisms require the use of protein molecules found in membranes and ATP?

  1. Active transport
  2. Diffusion
  3. Facilitated diffusion

A. 1 and 2 only
B. 2 and 3 only
C. 3 only
D. 1 only
E. 1 and 3 only

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The answer to this question is D.

There is a common feature among 2 of the choices, wherein active transport and facilitated diffusion involve protein molecules that are found in the membranes, which serve as the conduits for which molecules and ions can pass through. But to answer this question, you will need to understand the main distinguishing factors among the choices, which revolve around the concept of the concentration gradient.

What is the concentration gradient? To sum it up, the concentration gradient exists if there are 2 regions that differ in concentration of specific particles. Meaning, 1 region has a lower concentration of particles than the other region. These regions are usually separated by the cell membrane. It is also important to remember, that passively, molecules and ions will move along this concentration gradient, where they will move towards the region with the less concentration. This is an important concept to grasp, as it is the main distinguishing aspect among the choices provided to you.

What is the association of the concept of concentration gradient to the choices that were provided?

  1. Active transport: This type of transport of molecules and ions involves movement against their concentration gradient. As mentioned above, passively, particles move along this gradient, thus, moving against it is an active process. What is meant by “active” here? It means that because this transport is not passive, it would need a source of energy, in the form of ATP. The other 2 choices go along their concentration gradients, which proves why this is the only correct choice among the others. There are 2 types of active transport, direct and indirect, but for this question, you only need to be aware of the main concept of “active” transport, which is the utilization of energy.

  2. Diffusion (also called simple diffusion) and facilitated diffusion: In these kinds of transport, energy is not needed to drive the movement of molecules and ions across the membrane, instead, what drives the movement is the concentration gradient itself. As mentioned earlier, particles will move passively along this concentration gradient, which exactly describes these 2 kinds of diffusion. However, do not be confused with diffusion and facilitated diffusion. Thus, it will be useful to remember how they are different. Simple diffusion happens especially if the membrane is permeable to the particle, without the need for channel proteins. Facilitated diffusion on the other hand involves channel proteins that form pores for certain molecules to move across the cell membrane. Simple diffusion involved with the movement of small non-polar molecules, whereas facilitated diffusion involves the movement of large and polar molecules, which explains the need for channel proteins. Simple diffusion is a relatively slow process, but cannot be inhibited by any inhibitor molecules, on the other hand, facilitated diffusion is a relatively faster movement but CAN be inhibited by inhibitor molecules.

Thus, with the different distinguishing factors in mind, it would all support why choice D is the correct answer.