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In passive transport, how does increasing a cell membrane’s surface area (while keeping the concentration gradient and membrane permeability constant) affect the rate of diffusion across the membrane?
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9. Transport Across Membranes
Passive Transport: Diffusion and Osmosis
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Which of the following statements is true of facilitated diffusion across a cell membrane?
A
It occurs only for small nonpolar molecules that can pass directly through the lipid bilayer.
B
It moves solute against its concentration gradient by using energy from ATP.
C
It is not selective and cannot become saturated because solutes diffuse freely through open space in the membrane.
D
It requires a membrane transport protein and moves solute down its concentration gradient without direct ATP hydrolysis.
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Understand the concept of facilitated diffusion: it is a type of passive transport where molecules move across the cell membrane with the help of specific membrane transport proteins.
Recall that facilitated diffusion moves solutes down their concentration gradient, meaning from an area of higher concentration to an area of lower concentration, without the use of cellular energy (ATP).
Recognize that facilitated diffusion is selective because it requires specific transport proteins that bind to the solute, unlike simple diffusion which can occur freely for small nonpolar molecules.
Note that facilitated diffusion cannot move solutes against their concentration gradient; active transport is required for that, which uses energy from ATP.
Conclude that the true statement must mention the involvement of membrane transport proteins, movement down the concentration gradient, and the absence of direct ATP hydrolysis.
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