Multiple Choice
Which of the following gradients describes an electrical charge difference across a membrane?
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9. Transport Across Membranes
Principles of Transmembrane Transport
Multiple Choice
In the context of transmembrane transport, what is the key difference between simple diffusion and facilitated diffusion across a biological membrane?
A
Facilitated diffusion requires specific membrane proteins (channels or carriers) and can become saturated at high solute concentrations, whereas simple diffusion occurs directly through the lipid bilayer and is not saturable in the same way.
B
Simple diffusion requires a transporter to provide a hydrophilic pathway, whereas facilitated diffusion occurs without any membrane proteins.
C
Facilitated diffusion moves solutes against their concentration gradient using ATP, whereas simple diffusion moves solutes down their concentration gradient without ATP.
D
Simple diffusion occurs only for ions, whereas facilitated diffusion occurs only for nonpolar molecules such as O2 and CO2.
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 membrane, driven by the concentration gradient, without the need for any membrane proteins or energy input.
Step 2: Understand facilitated diffusion. Facilitated diffusion is also a passive transport process, meaning it does not require energy (ATP), but it requires specific membrane proteins such as channels or carrier proteins to help move molecules across the membrane.
Step 3: Identify the key difference related to membrane proteins. Simple diffusion does not involve any proteins and occurs directly through the lipid bilayer, while facilitated diffusion depends on membrane proteins that provide a hydrophilic pathway for molecules that cannot easily cross the lipid bilayer.
Step 4: Consider the saturation aspect. Facilitated diffusion can become saturated because the number of membrane proteins is limited; once all proteins are occupied, increasing solute concentration does not increase the rate of transport. Simple diffusion, however, is not saturable because it does not rely on proteins.
Step 5: Summarize the key difference. The main distinction is that facilitated diffusion requires specific membrane proteins and can be saturated, whereas simple diffusion occurs directly through the membrane lipids without proteins and is not saturable.
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