Multiple Choice
In mammalian cells, how does cytosolic pyruvate enter the mitochondrial matrix for oxidation by the pyruvate dehydrogenase complex?
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9. Transport Across Membranes
Principles of Transmembrane Transport
Multiple Choice
In the context of transmembrane transport, which statement best describes how active transport differs from simple diffusion across a lipid bilayer?
A
Active transport is limited to small nonpolar molecules, whereas simple diffusion is used primarily for ions and large polar solutes.
B
Active transport occurs only through the phospholipid bilayer without proteins, whereas simple diffusion requires a specific membrane transporter.
C
Active transport always moves solutes down their concentration gradient, whereas simple diffusion moves solutes against the concentration gradient.
D
Active transport requires an energy input (directly or indirectly, e.g., via ATP) and can move solutes against their electrochemical gradient, whereas simple diffusion is passive and occurs down the gradient.
Verified step by step guidance1
Understand the basic definitions: Simple diffusion is the passive movement of molecules across the lipid bilayer from an area of higher concentration to an area of lower concentration without the use of energy or transport proteins.
Recognize that active transport differs because it requires energy input, often in the form of ATP or an energy gradient, to move molecules against their concentration or electrochemical gradient.
Note that simple diffusion typically involves small, nonpolar molecules that can freely cross the lipid bilayer, while active transport often involves specific transporter proteins embedded in the membrane.
Recall that active transport can move solutes from low to high concentration (against the gradient), which is essential for maintaining cellular homeostasis, whereas simple diffusion cannot move substances against their gradient.
Summarize that the key difference is energy usage and directionality relative to the concentration gradient: active transport requires energy and can move solutes against the gradient, while simple diffusion is passive and moves solutes down the gradient.
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