Answer: Movement of xylem sap from roots to leaves usually depends on tension, or negative pressure potential.
Step-by-step explanation:
Movement of xylem sap from roots to leaves usually depends on tension, or negative pressure potential. Plants do not have a circulatory system like that of some animals. If a water molecule did "circulate" it would require the activity of transporters (like the circulatory system in animals) i.e help of the xylem and phloem. Xylem sap is a mixture/solution of water and minerals that comes from the roots and moves up the plant through the xylem. It is transported long distances by bulk flow to the veins. What causes it to move upward is the cohension-tension theory along with transpiration. In other words, transpiration provides the pull, cohesion of water molecules transmits this pull along the entire length of the xylem from roots to shoots. The basis of the transpirational pull lies in the fact that the pressure is created by proton pumping of stomatal guard cell. The pressure potencial can be described as the physical pressure on a solution; can be positive or negative relative to atmospheric pressure. In the case of the movement of xylem sap from roots to leaves it is negative.