Multiple Choice
How are osmosis and diffusion alike in passive transport?
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9. Transport Across Membranes
Passive Transport: Diffusion and Osmosis
Multiple Choice
In passive transport, how does increasing the distance a molecule must travel (diffusion path length) affect the rate of diffusion across a medium when all other factors remain constant?
A
It decreases the diffusion rate because a longer path length slows net movement down the concentration gradient.
B
It has no effect on diffusion rate because diffusion depends only on concentration gradient, not distance.
C
It increases the diffusion rate because molecules gain more kinetic energy over a longer distance.
D
It reverses the direction of diffusion so molecules move from low to high concentration.
Verified step by step guidance1
Recall Fick's First Law of Diffusion, which describes the rate of diffusion (J) as proportional to the concentration gradient and inversely proportional to the distance the molecules must travel. The law is expressed as:
\[J = -D \times \frac{\Delta C}{\Delta x}\]
where \(J\) is the diffusion rate, \(D\) is the diffusion coefficient, \(\Delta C\) is the concentration difference, and \(\Delta x\) is the diffusion path length.
Understand that the negative sign indicates diffusion occurs from high to low concentration, following the concentration gradient.
Focus on the term \(\frac{\Delta C}{\Delta x}\), which shows that as the diffusion path length \(\Delta x\) increases, the overall value of the concentration gradient per unit distance decreases, assuming \(\Delta C\) remains constant.
Conclude that increasing the diffusion path length \(\Delta x\) reduces the rate of diffusion \(J\) because molecules have to travel a longer distance, which slows their net movement down the concentration gradient.
Therefore, the correct interpretation is that increasing the distance a molecule must travel decreases the diffusion rate, consistent with Fick's Law.
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