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