Multiple Choice
In animal cells, what is the primary mechanism by which water enters the cell across the plasma membrane under normal physiological conditions?
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9. Transport Across Membranes
Passive Transport: Diffusion and Osmosis
Multiple Choice
In the context of passive transport across cell membranes, how are simple diffusion and facilitated diffusion similar?
A
Both move substances down their concentration gradient without requiring cellular energy (ATP).
B
Both are limited to the movement of water molecules only.
C
Both can move molecules from low concentration to high concentration when needed by the cell.
D
Both require transport proteins (channels or carriers) embedded in the membrane.
Verified step by step guidance1
Step 1: Understand the concept of passive transport, which is the movement of molecules across the cell membrane without the use of cellular energy (ATP). This process relies on the natural kinetic energy of molecules and their concentration gradients.
Step 2: Define simple diffusion as the direct movement of small or nonpolar molecules (like oxygen or carbon dioxide) through the lipid bilayer of the membrane, moving from an area of higher concentration to an area of lower concentration.
Step 3: Define facilitated diffusion as the movement of molecules across the membrane through specific transport proteins (such as channels or carriers), also moving down their concentration gradient without energy input.
Step 4: Compare the two processes by noting that both simple diffusion and facilitated diffusion move substances down their concentration gradient and do not require ATP, but facilitated diffusion requires transport proteins while simple diffusion does not.
Step 5: Conclude that the similarity between simple diffusion and facilitated diffusion is that both move substances down their concentration gradient without requiring cellular energy, which is the key characteristic of passive transport.
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