Multiple Choice
In the context of principles of transmembrane transport, which statement best describes how facilitated diffusion differs from simple diffusion across a cell membrane?
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9. Transport Across Membranes
Principles of Transmembrane Transport
Multiple Choice
Which of the following is an example of active transport across a cell membrane?
A
Movement of water across the membrane through aquaporins down its osmotic gradient
B
Diffusion of oxygen across the lipid bilayer from high to low concentration
C
Uptake of glucose into intestinal epithelial cells via the Na+/glucose symporter (SGLT) driven by the Na+ electrochemical gradient
D
Facilitated diffusion of glucose through a GLUT transporter down its concentration gradient
Verified step by step guidance1
Step 1: Understand the difference between passive and active transport. Passive transport moves substances down their concentration gradient without energy input, while active transport requires energy to move substances against their gradient.
Step 2: Analyze each option to determine if energy is required and if the movement is against the concentration gradient.
Step 3: Recognize that movement of water through aquaporins and diffusion of oxygen across the membrane are passive processes because they follow their concentration or osmotic gradients without energy use.
Step 4: Identify that facilitated diffusion of glucose via GLUT transporters is also passive, as glucose moves down its concentration gradient without energy.
Step 5: Note that the Na+/glucose symporter uses the energy stored in the Na+ electrochemical gradient to transport glucose against its concentration gradient, which is a hallmark of active transport.
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