Multiple Choice
Which of the following is an example of active transport across a cell membrane?
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9. Transport Across Membranes
Principles of Transmembrane Transport
Multiple Choice
In the context of principles of transmembrane transport, which statement best describes a key difference between simple diffusion and facilitated diffusion across a plasma membrane?
A
Facilitated diffusion moves solutes from low to high concentration, whereas simple diffusion moves solutes from high to low concentration.
B
Simple diffusion requires ATP hydrolysis to move solutes down their concentration gradient, whereas facilitated diffusion is always ATP-independent.
C
Simple diffusion uses ion channels to move charged solutes rapidly, whereas facilitated diffusion occurs only for small nonpolar molecules such as and .
D
Facilitated diffusion requires a membrane transport protein and can become saturated at high solute concentrations, whereas simple diffusion occurs directly through the lipid bilayer and is not saturable in the same way.
Verified step by step guidance1
Understand the basic definitions: Simple diffusion is the movement of solutes directly through the lipid bilayer from an area of high concentration to low concentration without the assistance of proteins or energy input.
Recognize that facilitated diffusion also moves solutes down their concentration gradient (high to low) but requires specific membrane transport proteins such as channels or carriers to help solutes cross the membrane.
Note that simple diffusion is not saturable because it depends on the solute's ability to dissolve in the lipid bilayer, whereas facilitated diffusion can become saturated when all transport proteins are occupied by solute molecules.
Recall that facilitated diffusion does not require ATP hydrolysis; it is a passive process like simple diffusion, but it differs in the involvement of transport proteins and saturation kinetics.
Summarize the key difference: facilitated diffusion requires membrane proteins and can saturate, while simple diffusion occurs directly through the membrane lipids and does not saturate.
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