Multiple Choice
In the context of passive transport across cell membranes, what is diffusion?
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9. Transport Across Membranes
Passive Transport: Diffusion and Osmosis
Multiple Choice
In animal cells, what is the primary mechanism by which water enters the cell across the plasma membrane under normal physiological conditions?
A
Active transport via the Na+/K+ ATPase pumping water molecules into the cytosol
B
Receptor-mediated endocytosis of extracellular fluid into vesicles
C
Facilitated diffusion of water through glucose transporters (GLUT proteins)
D
Osmosis, mainly through aquaporin water channels down a water potential (osmotic) gradient
Verified step by step guidance1
Understand that water movement across the plasma membrane in animal cells primarily occurs by osmosis, which is the passive movement of water molecules from an area of higher water potential to an area of lower water potential.
Recognize that the plasma membrane is selectively permeable and that water molecules can cross it more efficiently through specialized protein channels called aquaporins, which facilitate rapid water transport.
Note that active transport mechanisms like the Na+/K+ ATPase pump ions but do not directly transport water molecules; instead, they help establish ion gradients that indirectly influence water movement by creating osmotic gradients.
Exclude receptor-mediated endocytosis and facilitated diffusion through glucose transporters (GLUT proteins) as primary routes for water entry, since endocytosis involves vesicle formation and GLUT proteins specifically transport glucose, not water.
Conclude that the primary mechanism for water entry into animal cells under normal physiological conditions is osmosis through aquaporin channels, driven by osmotic gradients across the plasma membrane.
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