From hypotonic to hypertonic?

[mscjulia]

Vibrio alginolyticus is an alkali-tolerant marine bacterium with an internal salt concentration less than that of its normal surrounding medium.

A student transported specimens of V. alginolyticus from their original habitat to a basin of fresh water. With reference to their surrounding medium, they:

a. changed from hypertonic to hypotonic
b. changed from hypotonic to hypertonic
c. changed from isotonic to hypotonic
d. remained isotonic in both environments.

The answer is b. The explanation given is: "the question states that the bacteria are hypotonic in saltwater. Since fresh water contains no solutes, whereas the bacteria do, they must be hypertonic in fresh water."

I kinda understand that in salt water this bacteria is hypotonic, but why when the external salt concentration decreases (to fresh water level), the bacteria change to hypertonic? Shouldn't it be more hypotonic?

Thank you for reading this post and I really appreciate any help/suggestion here

Julia

---

Members don't see this ad.

 

[milski]

Hypotonic means that the concentration is lower than the reference, hypertonic - higher. When the bacterial was in its natural habitat, it had some concentration of salts inside it which was non-zero and smaller than the salt concentration of the water around.

When it was moved to the fresh water, the surrounding concentration became zero, which means that whatever the bacteria salt concentration was, it was higher than the surroundings. That made the bacteria hypertonic with reference to the surrounding water.

If salt water had concentration S and the bacteria B, then:
B < S (hypotonic in salt water) but
B > 0 (hypertonic in fresh water).

 

[MedPR]

Vibrio alginolyticus is an alkali-tolerant marine bacterium with an internal salt concentration less than that of its normal surrounding medium.

A student transported specimens of V. alginolyticus from their original habitat to a basin of fresh water. With reference to their surrounding medium, they:

a. changed from hypertonic to hypotonic
b. changed from hypotonic to hypertonic
c. changed from isotonic to hypotonic
d. remained isotonic in both environments.

The answer is b. The explanation given is: "the question states that the bacteria are hypotonic in saltwater. Since fresh water contains no solutes, whereas the bacteria do, they must be hypertonic in fresh water."

I kinda understand that in salt water this bacteria is hypotonic, but why when the external salt concentration decreases (to fresh water level), the bacteria change to hypertonic? Shouldn't it be more hypotonic?

Thank you for reading this post and I really appreciate any help/suggestion here

Julia

Lets just make up some numbers to simplify things.

Say the normal habitat of this Vibrio species is 5M NaCl. We'll also say that the internal salt concentration is 2M. 2M is less than 5M, so as you say, the bacterium is hypotonic to the surroundings.

Now if you place this bacteria, who has a 2M internal salt concentration, into a beaker of pure water, which has 0M salt concetration, where is the salt more concetrated? In other words, what is hypertonic, the water or the bacterium? The bacterium.

Edit: I think I figured out why you are confused.

When you put the bacterium into the pure water, water will rush into the bacterium down its concentration gradient. Thus decreasing the concentration of the salt inside the bacterium. So yes, the bacterium becomes more hypotonic compared to another bacterium still in its natural habitat. But the question is asking you to compare the bacterium to its surroundings, not the equilibrium change.

 

[mscjulia]

Thank you so much MedPR and milski! I now get it And yes MedPR, I did get confused by the question as you pointed out lol.