The image below shows a cell that contains many carrier proteins that facilitates the passive transport of a molecule that is unable to diffuse simply across the membrane.

If the image represents the point immediately before transport starts (eg. when the molecule is added to the liquid surrounding the cell), which of the following images most closely represents the point when no net transport of the molecule occurs?

Question

The image below shows a cell that contains many carrier proteins that facilitates the passive transport of a molecule that is unable to diffuse simply across the membrane.

If the image represents the point immediately before transport starts (eg. when the molecule is added to the liquid surrounding the cell), which of the following images most closely represents the point when no net transport of the molecule occurs?

BlackTom AI · Accepted Answer

To begin, let’s restate what the question is asking: It presents a cell with carrier proteins that facilitate passive transport of a molecule that cannot diffuse directly across the membrane, and asks which image best represents the moment when no net transport occurs, specifically at the instant when transport is about to start (the molecule is just added to the surrounding medium).

Option handling note: The provided answer options in the prompt are empty strings, so there isn’t a labeled set of choices to evaluate one by one. In such a case, we should rely on the scientific concept described—net transport being zero occurs when there is no difference in concentration across the membrane, i.e., the system is at equilibrium for that molecule.

Key concept: Facilitated diffusion through carrier proteins moves molecules down their concentration gradient without using energy. Net transport is the difference between the rate of entry and exit of molecules. When the intracellular concentration equals the extracellular concentration, the driving force for movement in either direction is equal, so the net flux is zero despite ongoing random molecular movement.

First idea to consider: If the molecule is added to the outside medium and the external and internal concentrations are still unequal, there will be a net flow of molecules from the side with higher concentration to the side with lower concentration. This corresponds to a non-zero net transport and is not the state described in the question.

Second idea to consider: As transport proceeds, the carrier proteins cycle, and molecules accumulate on the interior or exterior depending on the gradient. The moment of no net transport occurs precisely when the concentrations have equilibrated across the membrane, yielding equal chemical potential on both sides and no directional preference for movement.

Third idea to consider: In real images of transport processes, one would expect to see either a higher density of molecules on the outside with fewer inside (early stage) or vice versa (late stage), with movement proceeding in the direction of the gradient. The equal-concentration condition is the hallmark of zero net transport, not the presence of a gradient.

Given the scenario and the concept of passive facilitated diffusion, the correct representation is the moment when the outside and inside have approximately the same concentration of the transported molecule, so that the net transport rate is effectively zero.

If you were to evaluate multiple-choice options with non-empty text, you would look for the option that indicates equal concentrations inside and outside, i.e., equilibrium, rather than a state with a concentration difference that would drive net movement. In the absence of explicit option text here, the principle remains that the zero-net-transport state corresponds to equal concentrations on both sides of the membrane.

Summary takeaway: The condition for no net transport during facilitated diffusion is chemical equilibrium across the membrane for the transported molecule, where external and internal concentrations are equal, resulting in no net flux despite ongoing molecular motion and carrier cycling.

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