Multiple Choice
How do transporters and channels select which solutes they help move across the membrane?
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9. Transport Across Membranes
Transporters
Multiple Choice
In most mammalian cells, how does glucose cross the plasma membrane into the cell?
A
By primary active transport via a plasma-membrane ATPase that pumps glucose using ATP hydrolysis
B
By facilitated diffusion through a GLUT uniporter down its concentration gradient
C
By simple diffusion directly through the lipid bilayer because glucose is nonpolar
D
By aquaporin channels that selectively conduct glucose along with water
Verified step by step guidance1
Step 1: Understand the nature of glucose and the plasma membrane. Glucose is a polar molecule and relatively large compared to small gases, so it cannot easily cross the hydrophobic lipid bilayer by simple diffusion.
Step 2: Recall the types of membrane transport mechanisms: simple diffusion, facilitated diffusion, primary active transport, secondary active transport, and channel-mediated transport.
Step 3: Recognize that primary active transport involves ATP hydrolysis to move molecules against their concentration gradient, which is not the case for glucose uptake in most mammalian cells.
Step 4: Identify that glucose crosses the membrane via facilitated diffusion, which uses specific carrier proteins called GLUT (glucose transporter) uniporters that allow glucose to move down its concentration gradient without ATP consumption.
Step 5: Note that aquaporins are channels specialized for water transport and do not conduct glucose, so they are not involved in glucose transport.
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