Multiple Choice
Facilitated diffusion differs from simple diffusion because why?
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9. Transport Across Membranes
Principles of Transmembrane Transport
Multiple Choice
In the context of principles of transmembrane transport, which statement best describes how facilitated diffusion differs from simple diffusion across a cell membrane?
A
Facilitated diffusion can transport any small nonpolar molecule without a protein, whereas simple diffusion requires a carrier protein for transport.
B
Facilitated diffusion occurs only for water (osmosis), whereas simple diffusion occurs only for ions.
C
Facilitated diffusion moves solutes against their concentration gradient by using ATP, whereas simple diffusion moves solutes down their concentration gradient without energy input.
D
Facilitated diffusion requires specific membrane proteins (channels or carriers) and is saturable, whereas simple diffusion occurs directly through the lipid bilayer and is not saturable.
Verified step by step guidance1
Step 1: Understand the basic concept of simple diffusion. Simple diffusion is the passive movement of molecules directly through the lipid bilayer of the cell membrane, moving from an area of higher concentration to an area of lower concentration without the need for energy or proteins.
Step 2: Understand facilitated diffusion. Facilitated diffusion also moves molecules down their concentration gradient without energy input, but it requires specific membrane proteins such as channels or carrier proteins to help transport molecules that cannot easily cross the lipid bilayer on their own.
Step 3: Compare the role of proteins in both processes. Simple diffusion does not involve any proteins and is not saturable, meaning the rate of diffusion increases linearly with concentration. Facilitated diffusion depends on specific proteins, which can become saturated when all binding sites are occupied, limiting the rate of transport.
Step 4: Recognize the types of molecules transported. Simple diffusion typically allows small nonpolar molecules (like oxygen and carbon dioxide) to pass freely, while facilitated diffusion is necessary for polar or charged molecules (like glucose or ions) that cannot cross the membrane easily.
Step 5: Summarize the key difference. Facilitated diffusion requires specific membrane proteins and exhibits saturation kinetics, whereas simple diffusion occurs directly through the lipid bilayer without proteins and is not saturable.
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