Iíll just give part of the answer, the part that is easiest for a physicist. For short-range motions (say from the inside to the outside of a thin leaf), the plant doesnít need any special mechanism at all. The reason is that the molecules are constantly diffusing around, thanks to random thermal motions. I guess that the typical diffusion coefficients are around 10-5
/s, so that diffusing from the middle to the edge of a leaf thatís 0.02 cm thick would only take about (0.01 cm)2
/s)=10s. Thatís a very rough estimate.
The units may look odd to you. The time required goes as the square of the distance to be travelled, not just proportional to the distance. Thatís because the diffusion is not systematic motion in one direction, but random wandering. Still, it has a systematic effect. If lots of O2
starts in the middle of the leaf and CO2
on the outside, purely random motions will scramble them, bringing O2
out and CO2
Some gradual exchange of molecules with the thicker leaves, stalks, and trunks of plants is also of course needed. Diffusion over long distances is very slow. I believe that there are active liquid flow mechanisms (in some ways like our blood circulation, but much slower) that help out with that. Maybe a botanist could give you some more reliable and detailed information.
Water is a special case in plants, particularly trees. Itís a combination of the capillary effect, osmosis, and just plain hydrostatic pressure from the base of the tree to get water out of the roots in the ground to high up in the tree to get to the leaves. Sap and other nutrients flow in the opposite direction, and other molecules, such as chlorophyll, do complicated, seasonal motions when trees change colors in the fall and sprout new leaves in the spring. Many of these molecular transport mechanisms are specific to the molecules that are being transported and are mediated by specific proteins and enzymes which do exactly the right job at the right time.
(published on 10/22/2007)