A bacterium is swimming in an isotonic medium. I now add NaCl to a
concentration of 100 mM. Which of the following is true?
The osmotic pressure points inward but the water flows outward
The osmotic pressure points outward so the water flows outward
The osmotic pressure points inward so the water flows inward
The osmotic pressure points outward but the water flows inward
Adding 100 mM NaCl to an isotonic medium surrounding a swimming bacterium makes the external solution hypertonic, increasing solute concentration outside the cell. Osmotic pressure, defined as the pressure exerted by solutes that draws water toward higher solute concentrations, now points outward from the cell’s perspective (toward the exterior). Consequently, water flows outward from the bacterium to dilute the external NaCl, causing the cell to shrink (plasmolysis).
Correct Answer
The osmotic pressure points outward so the water flows outward.
This matches the core principle of osmosis: water moves across a semipermeable membrane from low solute (high water potential) to high solute (low water potential) regions. Here, the exterior’s higher osmolarity after NaCl addition creates an outward osmotic gradient, driving water efflux from the bacterium.
Why Other Options Fail
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The osmotic pressure points inward but the water flows outward: Incorrect—osmotic pressure cannot point inward (toward the cell interior, which has lower solute post-addition); this reverses the gradient direction. Water does flow outward, but the pressure direction is mismatched.
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The osmotic pressure points inward so the water flows inward: Fully wrong—neither occurs. Inward pressure would imply hypotonic exterior (diluting solutes inside), pulling water in and causing swelling, opposite to hypertonic NaCl effects.
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The osmotic pressure points outward but the water flows inward: Half-right on pressure but wrong on flow. Outward pressure drives outward water movement; inward flow defies osmosis fundamentals, as seen in hypertonic scenarios where cells dehydrate.
