Question at position 50 The sodium-potassium pump is an example of which type of membrane transport, and what is its primary characteristic?Osmosis; it involves water moving through aquaporins.Facilitated diffusion; it allows molecules like glucose to enter cells freely.Passive transport; it moves substances down their gradient without energy.Active transport; it uses ATP to move substances against their concentration gradients.

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Here is the question restated with all answer options, followed by analysis of each choice in turn.
Question: The sodium-potassium pump is an example of which type of membrane transport, and what is its primary characteristic?
Options:
- Osmosis; it involves water moving through aquaporins.
- Facilitated diffusion; it allows molecules like glucose to enter cells freely.
- Passive transport; it moves substances down their gradient without energy.
- Active transport; it uses ATP to move substances against their concentration gradients.

Option 1: Osmosis; it involves water moving through aquaporins.
This statement describes osmosis, a process where water moves across a selectively permeable membrane, typically through aquaporin channels, down its water chemical potential gradient. The sodium-potassium pump does not primarily involve water movement or aquaporin-mediated transport, so this description does not apply here. It misattributes the pump’s mechanism to osmosis.

Option 2: Facilitated diffusion; it allows molecules like glucose to enter cells freely.

Facilitated diffusion refers to carrier- or channel-mediated transport that moves substances down their concentration gradient without energy input. While the term diffusion is used, the sodium-potassium pump does not passively move substances down gradients; it actively moves ions against their gradients. Additionally, glucose transport via facilitated diffusion involves specific channels or transporters, not the ATP-driven ion pumping characteristic of this pump.

Option 3: Passive transport; it moves substances down their gradient without energy.

Passive transport covers any movement down a gradient without energy expenditure, including simple diffusion, facilitated diffusion, and osmosis. The key feature here is energy neutrality and gradient-following movement. The sodium-potassium pump, by contrast, requires energy in the form of ATP to move sodium and potassium ions against their concentration gradients, so labeling it as passive transport is inaccurate.

Option 4: Active transport; it uses ATP to move substances against their concentration gradients.

This option correctly identifies the sodium-potassium pump as a classic example of active transport. It employs ATP to power the conformational changes of the pump protein, actively moving three Na+ ions out of the cell and two K+ ions into the cell against their respective gradients. This energy-dependent mechanism distinguishes it from passive or facilitated processes and aligns with foundational concepts of membrane transport.

Summary of why the other options are flawed: options 1–3 mischaracterize the pump's energy requirements or directionality, confusing it with osmosis, facilitated diffusion, or passive transport. Option 4 accurately captures both the transport type (active) and the primary characteristic (ATP-driven, against-gradient movement) that define the sodium-potassium pump.

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