(diagram of compressed and enlarged cells) Why has the plant cell shrunk when put in salty water

This occurs due to the phenomenon of osmosis. The basic principle of osmosis is that when two substances of different saline concentration are separated by a permeable divider (plant cell wall) the concentrations on both sides will move towards equilibrium. In this case, the water inside the plant cell does not contain as many salts as the surrounding water, therefor the water inside the plant cell will move outwards to try and reduce the salinity of the surrounding water, causing the cell to shrink in the manner displayed in the diagram.