Multiple Choice
Which of the following best describes the process of diffusion?
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6. The Membrane
Concentration Gradients and Diffusion
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Which of the following best describes facilitated diffusion?
A
Random movement of molecules directly through the phospholipid bilayer without assistance
B
Movement of water molecules through a semipermeable membrane from low to high solute concentration
C
Movement of molecules against their concentration gradient using ATP
D
Movement of molecules across a membrane with the help of transport proteins, without energy input
Verified step by step guidance1
Facilitated diffusion is a type of passive transport, meaning it does not require energy input from the cell. Begin by understanding that passive transport relies on the natural movement of molecules down their concentration gradient (from high to low concentration).
Recognize that facilitated diffusion specifically involves the use of transport proteins embedded in the cell membrane. These proteins assist molecules that cannot pass directly through the phospholipid bilayer due to their size, polarity, or charge.
Understand the role of transport proteins, which can be either channel proteins or carrier proteins. Channel proteins form pores that allow specific molecules or ions to pass through, while carrier proteins bind to the molecule and change shape to shuttle it across the membrane.
Note that facilitated diffusion does not involve ATP or any other form of energy input. The movement is driven solely by the concentration gradient of the molecules being transported.
Finally, compare facilitated diffusion to other transport mechanisms, such as simple diffusion (movement directly through the bilayer without assistance) and active transport (movement against the concentration gradient requiring energy). This will help clarify why facilitated diffusion is distinct and fits the description provided in the problem.
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