Question

A single-celled organism which has contractile vacuole to move water from inside to outside of the cell was studied in an experiment in which the organism was placed in water with different salt concentrations. The rate at which the contractile vacuole contracted to pump out excess water was recorded.

Salt conc.	Rate of contractile vacuole contractions per minute
Very high	2
High	8
Medium	15
Low	22
Very Low	30

Which of the following explains the observed relationship between the rate of contractile vacuole contraction and the salt concentration?

• A. When the salt concentration outside the cell is very low, osmosis causes body salts to move outside the cell, increasing water content in body and the contractile vacuole needs to contract rapidly
• B. When the salt concentration outside the cell is very high, diffusion causes water to move outside the cell, and the contractile vacuole does not need to contract as rapidly
• C. When the salt concentration outside the cell is very low, diffusion causes water to move outside the cell, and the contractile vacuole contractions are increased
• D. When the salt concentration outside the cell is very high, osmosis causes water to move outside the cell, and the contractile vacuole does not need to contract as rapidly

Answer 1

The correct option is D When the salt concentration outside the cell is very high, osmosis causes water to move outside the cell, and the contractile vacuole does not need to contract as rapidly

The lower the concentration of the salt solution, the higher will be the water potential of that solution. The higher the water potential, the greater is the tendency for water to enter the cell by osmosis. If there is more water entering by osmosis, then the contractile vacuole will have to fill up and empty more frequently.

So, the correct answer is option D.