Using a different lipid composition for the liposome preparation can help to elucidate the transport mechanism. To test whether the transporter acts as a channel or a mobile carrier, DPPC (1,2-dipalmitoylsn-glycerophosphatidylcholine) can be used as lipid. The DPPC has a transition temperature between the fluid and gel phase at a temperature (Tm) of 41 °C. The working principle here is that, if the channel is a stable assembly and spans the membrane, the transport process should still function, even in the gel phase. On the other hand, mobile transporters need to diffuse through the membrane to perform transport and hence transport will be hindered by the rigid membrane. Note that care has to be taken when interpreting the results, as certain synthetic transporters can be 'squeezed' out of the membrane at temperatures below Tm. When using DPPC, liposomes must be prepared at a temperature above Tm, and therefore all the operations (sonication, stirring, freeze-thawing and extrusion) are done at elevated temperature (45-50°C