On average, plants possess about three times more aquaporin genes than humans. One explanation is that for each tissue and for different physiological situations a specific set of aquaporins is required. Another or an additional reason could also be the different transport specificity of individual aquaporin-proteins. Functional analysis of plasma membrane aquaporins in heterologous expression systems like yeast and Xenopus oocytes as well as the behaviour of plants modified in aquaporin expression, lead to the suggestion that some aquaporins were highly permeable, others almost impermeable for water. The latter facilitate the membrane transport of other small substances including gases like CO₂ of NH₃. In vitro functional assays indicated for the formation of heterotetramers and a concomitant change in transport properties. Data from physiological studies on plants indicate for features of aquaporin-mediated transport, which could be relevant for aquaporin function in other living organisms, too.