Multiple Choice
In the context of passive transport across a cell membrane, what is the key difference between simple diffusion and facilitated diffusion?
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9. Transport Across Membranes
Passive Transport: Diffusion and Osmosis
Multiple Choice
In the context of passive transport, how does oxygen (O2) typically cross the plasma membrane into a cell?
A
By simple diffusion directly through the phospholipid bilayer, moving down its concentration gradient
B
By active transport via an ATP-powered pump that moves it against its concentration gradient
C
By receptor-mediated endocytosis in vesicles formed from the plasma membrane
D
By osmosis through aquaporin channels, moving toward higher solute concentration
Verified step by step guidance1
Step 1: Understand the nature of oxygen (O2) as a molecule. Oxygen is a small, nonpolar, and hydrophobic molecule, which influences how it interacts with the plasma membrane.
Step 2: Recall the structure of the plasma membrane, which is primarily composed of a phospholipid bilayer with hydrophobic tails facing inward and hydrophilic heads facing outward. This structure allows nonpolar molecules to pass through more easily than polar or charged molecules.
Step 3: Review the types of passive transport mechanisms: simple diffusion, facilitated diffusion, and osmosis. Simple diffusion involves movement of molecules directly through the membrane without assistance, down their concentration gradient.
Step 4: Analyze the options given: active transport requires energy and is used for moving substances against their gradient; receptor-mediated endocytosis involves vesicle formation and is typically for larger molecules; osmosis specifically refers to water movement through aquaporins, not oxygen.
Step 5: Conclude that oxygen crosses the plasma membrane by simple diffusion directly through the phospholipid bilayer, moving down its concentration gradient, because it is small, nonpolar, and can easily dissolve in the hydrophobic core of the membrane.
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