Question

paramecia are unicellular protists that have contractile vacuoles to remove excess intracellular water. in an experimental investigation, paramecia were placed in salt solutions of increasing osmolarity. the rate at which the contractile vacuole contracted to pump out excess water was determined and plotted against osmolarity of the solutions, as shown in the graph. which of the following is the correct explanation for the data? the contraction rate increases as the osmolarity decreases because the amount of water entering the paramecia by osmosis increases. in an isotonic salt solution, there is no diffusion of water into or out of the paramecia, so the contraction rate is zero. the contractile vacuole is less efficient in solutions of high osmolarity because of the reduced amount of atp produced from cellular respiration. at higher osmolarity, lower rates of contraction are required because more salt diffuses into the paramecia.

Explanation:

The contractile vacuole in paramecia pumps out excess water. In isosmotic solutions, there is no net water movement, so contraction rate is zero. As osmolarity decreases, more water enters by osmosis, increasing the contraction rate. At higher osmolarity, less water enters, so lower contraction rates are required.

Answer:

In an isosmotic salt solution, there is no diffusion of water into or out of the paramecia, so the contraction rate is zero.