Multiple Choice: Osmoregulation in Fish

[ichigoamu]

Saltwater fish:

a. Concentrate their cells to prevent loss of fluid

b. Concentrate their urine to retain fluid

c. Dilute their urine to excrete more fluid

d. Use a more efficient kidney system

e. Are hypotonic to their environment

 

The answer is listed as A. How do fish concentrate their cells? How does concentrating their cells prevent loss of fluid? Does anybody know of any resources where I can read more about this?

Thank you

[Nicholas Kang]

Ok, though I don't take biology now after high school. I still remember what I learn. This is more of a biochemistry question.

 

The explanation is simple. First, you need to understand what is osmosis, osmotic pressure and how osmoregulation works.

 

Osmoregulation (See:https://en.wikipedia.org/wiki/Osmoregulation) is basically the regulation of an organism's body water content to maintain its osmotic pressure.

 

You also need to understand the term endoosmosis, exosmosis, hypertocnic and hypotonic etc.

 

Ok ,we start with the simple ones first.

 

3 types of solutions exist: solutions hypotonic to cell, solutions hypertonic to cell or solutions isotonic to cell.

 

Say you place an animal cell into a solution that is hypotonic to the cell. The hypotonic solution means that is has less solute in the solution, thus less concentrated and has lower osmotic pressure. So, relatively, the cell has higher osmotic pressure, and water flows across the cell membrane from the outside (solution) to the intracellular space (inside the cell). The cell will accumulate water over time and swells and may eventually burst.

 

The opposite happens to the animal cell when it is placed in a hypertonic solution. It has high solute content and thus more concentrated. So, an animal cell immersed in it tends to lose water to the surrounding, and the animal cell shrinks in size. For an erythrocyte (Red blood cell), it is said to crenate.

 

For plant cell, the condition differs a bit, given that plant cells have cell walls, which is absent in animal cells. Plant wall gives the plant cell strength. So, when a plant cell is immersed in a hypotonic solution, though water enters the plant cell, the plant cell would not burst, but remain turgid, i.e. the vacuoles are large and the plant cell is stiff.

 

However, if immersed in a hypertonic solution, the plant cell will lose water like an animal cell, thus shrinks in size, and we say the plan wilts. If the dry condition is prolonged, it leads to the death of plant.

 

An isotonic solution is the solution which has the same solute concentration as that of the organism's body. Hence, it is the optimum condition for an organism to live in and the cells won't lose or gain any water, and hence no change in the organism's body size is observed.

 

Here is a website that teaches you the 3 concepts.

 

http://www.phschool.com/science/biology_place/biocoach/biomembrane1/solutions.html

 

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So, given such extremes condition, organism (composed of cells) learn to adapt themselves to their surrounding environment. In you question, saltwater fish is the organism in question. Saltwater fish lives in a hypertonic environment, that is their surrounding fluid (sea water) has high salt content, thus hypertonic to the cells in saltwater fish. If no preventive measure is taken, the cells will loss water to its surrounding quickly and they will die due to lack of water.

 

So, one way to overcome this is to increase the organism's body salinity to make them even more concentrated to the surrounding environment (seawater in this situation, or any salty aquatic environment). Now, it is their bodies that are more hypertonic than the surrounding salty water, thus instead of losing water to the surrounding, they gain water from the surrounding.

 

So, the answer to your question is A.

 

Good luck with your studies!

Edited March 8, 2017 by Nicholas Kang