Essay About Unit Of Pressure And Actual Resistance

Water and Plant Cells
Water and Plant Cells
THERMAL MOTION LEADS TO DIFFUSION AND MIXING
DIFFUSION IS RAPID OVER SHORT DISTANCES BUT EXTREMELY SLOW OVER LONG DISTANCES.
THE UNIT OF PRESSURE USED TO DESCRIBE THE STATE OF WATER IN PLANT CELLS IS THE MEGAPASCAL (MPa)
WHAT DRIVES THE MOVEMENT OF WATER?
DIFFUSION OR BULK FLOW
WATER MOVEMENT CAN OCCUR BY TWO BASIC MECHANISMS:
WATER POTENTIAL EQUATION ?w = ?s + ?p + ?g
VALUES OF RT AND ?S AT VARIOUS TEMPERATURES AND CONCENTRATIONS
APPLYING THE WATER POTENTIAL EQUATION TO WATER IN BEAKERS AND CELLS
SMALL CHANGES IN PLANT CELL VOLUME CAUSE LARGE CHANGES IN TURGOR PRESSURE
WATER MOVEMENT THROUGH THE XYLEM REQUIRES LESS PRESSURE THAN MOVEMENT THROUGH LIVING CELLS
TO DRIVE WATER THROUGH AN IDEAL VESSEL ELEMENT WITH A 40 ?m RADIUS AT A VELOCITY OF 4 mm s-1 REQUIRES 0.02 MPa m-1.
BECAUSE OF PERFORATION PLATES AND ROUGH INNER WALLS, ACTUAL RESISTANCE IS DOUBLE THIS ABOUT AMOUNT (~0.04 MPa m-1).
RESISTANCE INCREASES BY TEN ORDERS OF MAGNITUDE IF WATER HAS TO CROSS MEMBRANES BETWEEN CELLS.
WHAT PRESSURE GRADIENT IS NEEDED TO LIFT WATER TO THE TOP OF A 100 METER TREE?
IF WE ASSUME A RESISTANCE OF 0.02 MPa m-1, THE TOTAL RESISTANCE IS 0.02 MPa m-1 x 100 m = 2 MPa.
MUST ALSO CONSIDER GRAVITY POTENTIAL (100 m x 0.01 MPa m-1 = 1 MPa.
THUS A PRESSURE GRADIENT OF AT LEAST 3 MPa