Multiple Choice
In the principles of transmembrane transport, which molecule is directly required to provide energy for primary active transport across a membrane?
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9. Transport Across Membranes
Principles of Transmembrane Transport
Multiple Choice
In the context of transmembrane transport, which statement best distinguishes active transport from facilitated diffusion?
A
Active transport is not saturable, whereas facilitated diffusion is saturable because it uses membrane proteins.
B
Active transport requires an external energy source (e.g., ATP hydrolysis or coupling to an ion gradient) to move solutes against their electrochemical gradient, whereas facilitated diffusion moves solutes down their electrochemical gradient without direct energy input.
C
Active transport can only move small nonpolar molecules across the membrane, whereas facilitated diffusion is specific to ions.
D
Active transport always occurs through open channels, whereas facilitated diffusion always occurs through carrier proteins that never change conformation.
Verified step by step guidance1
Step 1: Understand the basic definitions of both processes. Facilitated diffusion is a passive transport mechanism that moves solutes down their electrochemical gradient using membrane proteins, without requiring energy input.
Step 2: Recognize that active transport moves solutes against their electrochemical gradient, which requires an external energy source such as ATP hydrolysis or coupling to an ion gradient.
Step 3: Identify that both facilitated diffusion and active transport involve membrane proteins, but the key difference lies in the energy requirement and direction relative to the gradient.
Step 4: Note that facilitated diffusion is saturable because it depends on the availability of specific carrier proteins or channels, and active transport is also saturable but distinguished by its energy dependence.
Step 5: Conclude that the best distinguishing statement is that active transport requires external energy to move solutes against their gradient, whereas facilitated diffusion moves solutes down their gradient without direct energy input.
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