  We revise three common models accounting for water exchange in
pulsed-gradient spin-echo measurements: a bi-exponential model with
time-dependent water fractions, the K\"arger model, and a modified K\"arger
model designed for restricted diffusion, e.g. inside cells. The three models
are compared and applied to experimental data from yeast cell suspensions. The
K\"arger model and the modified K\"arger model yield very close results and
accurately fit the data. The bi-exponential model, although less rigorous, has
a natural physical interpretation and suggests a new experimental modality to
estimate the water exchange time.
