Single-celled Paramecium live in pond water (a hypotonic environment relative to the cytosol). They have a structural feature, a contractile vacuole, which enables them to osmoregulate. If sucrose or saline was added to the pond water in different concentrations (in millimolars, mM), under which conditions would you expect the contractile vacuole to be most active?

A) 0.0 mM sucrose
B) 0.05 mM saline
C) 0.08 mM sucrose
D) 1.0 mM saline

The correct answer is option A) "0.0 mM sucrose".

Explanation:

Since single-celled Paramecium use their contractile vacuole to osmoregulate upon hypotonic environments, it is expected that a condition where no sucrose or salt is added is the condition where the contractile vacuole would be most active. In this extremely hypotonic condition cells tend to absorb high quantities of water, the contractile vacuole pumps water out of the cell to regulate its osmotic pressure.