Active Absorption

b. Active absorption : Active absorption is water uptake driven directly by the metabolic activity of living root cells, rather than by any pull from the shoot. Its driving force is root pressure — the hydrostatic pressure that living root cells build up — and because water is here being moved against the DPD (water potential) gradient rather than along it, active absorption does require the expenditure of ATP, generated through the root's own respiratory activity. Active absorption is most significant at night, when the stomata close, transpiration stops, and passive absorption (which depends on transpiration) is unavailable.

Active absorption itself has been explained by two proposed mechanisms:

1. Osmotic absorption. Atkins and Priestley (1922) proposed that water is absorbed from the soil into the root xylem in line with an osmotic gradient that the living root cells actively maintain. Creating and sustaining this osmotic gradient does require an expenditure of energy by the cells, even though the actual movement of water down that gradient is not itself a direct energy-consuming step. As water absorption continues, a gradient of DPD builds up continuously from the epiblema to the pericycle because of this ongoing cellular activity, and the resulting hydrostatic pressure — root pressure — forces water from the pericycle into the xylem and onward up the stem. …