Cell water relationships are important topics to be included in cell biology courses. Differences exist in the control of water relationships in plant cells relative to control in animal cells. One important reason for these differences is that turgor pressure is a consideration in plant cells. Diffusion and osmosis are the underlying factors involved in the control of water in plant cells; however, additional attention must be given to osmotic pressure, osmotic potential, and water potential. This discussion shows how these parameters relate to each other, and how they are explained by thermodynamics and the universal gas laws. Detailed laboratory exercises are described that demonstrate these principles. The laboratory exercises include data collection, graphing of data, statistical analysis of data, and calculations of osmotic potential and diffusion pressure deficit from the data. (Contains 5 figures and 2 tables.)