IMAT 2017 Q31 [Concentration Net Movement]

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Two adjacent healthy human cells, X and Y, contain the two molecules Q and R.

The concentration of molecule Q in cell X is 4 arbitrary units (a.u.) and in cell Y it is 6 a.u.

The concentration of molecule R in both cell X and cell Y is 7 a.u.

Which of the following statements about the net movement of ONLY molecules Q and R between cells X and Y could be correct?

  1. Molecules of Q move by facilitated diffusion from cell X.
  2. Molecules of Q move by active transport into cell Y.
  3. Molecules of R move into and out of both cell X and cell Y.

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

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The correct answer is A.

To answer this question, we first have to know what a concentration gradient is.

It is pretty simple : it occurs when the concentration of a molecule (or particle) is higher in one compartment compared to another compartiment.

  • In the case of molecule Q, it is said that “the concentration of molecule Q in cell X is 4 arbitrary units (a.u.) and in cell Y it is 6 a.u.”

    Therefore, there is a concentration gradient of molecule Q between cell X [LOW] and in cell Y [HIGH].

  • Now, onto molecule R : it is said that the concentration of molecule R in both cell X and cell Y is 7 a.u.

    Because the concentration of molecule R in both cells is the same, there is NO concentration gradient between them.

What is active transport ?

Active transport is the movement of a molecule through the cell membrane against its concentration gradient. Like the name suggests, it uses energy, as opposed to passive transport which doesn’t.

What is facilitated diffusion ?

Facilitated diffusion is, as opposed to active transport, a passive and spontaneous movement of a molecule through the cell membrane.

It is to be noted that movement of particles go from the compartiment of higher concentration into the compartment of lower concentration to restore equilibrium.

Now that we know that, let’s answer the question:

  • Item 1 suggests that molecules Q move by facilitated diffusion from cell X.

    That is NOT true because molecule Q is in higher concentration in cell Y than cell X.

    Movement of particles go from the compartiment of higher concentration into the compartment of lower concentration.

  • Item 2 suggests that molecules of Q move by active transport into cell Y.

    That is true because the movement suggested is against the concentration gradient, which WOULD require active transport.

  • Item 3 suggests that molecules of R move into and out of both cell X and cell Y

    That is true because the molecule is in equal concentration in both cells. A cell being a dynamic structure, molecule R would move freely in and out of both cell X and cell Y.

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