Transpiration is the loss of water and water vapour through the epidermal cells and the sub-stomatal pores of the leaves by evaporation. The loss of water is what leads the water to move through the plant.
Translocation is a generalised term that refers to the movement of substances around the plant.
Transpiration is extremely important and there is a cohesion-tension theory that explains how the transpiration stream works.
1) Water evaporates from the surface of the epidermal cells.
2) Water diffuses from sub-stomatal chamber into the atmosphere.
3) Water evaporates from the cellulose cells walls of the mesophyll cells into the sub-stomatal air spaces.
4) Water moves through the leaf tissues in three different ways - apoplast, symplast or vacuolar
5) Water leaves the xylem via the unlignified vessel areas known as pits that allow lateral movement of water down the concentration gradient into the cell walls of the mesophyll cells
6) Water molecules have extremely high cohesive strength due to their dipole nature allowing many hydrogen bonds being formed. This cohesion along with the adhesion of water molecules to the walls of the xylem vessels creates tension that pulls the stream up.
7) More water is continuously moved in from the roots because of osmosis
Root pressure uses active transport to move water in transpiration. By pumping ions from root cells into the xylem, it creates a concentration gradient, and water follows by osmosis pushing the water slightly.
