Biology: Translocation of Solutes

Translocation of Solutes

• Mass flow is the phenomenon of passive movement of solutes, highest in the leaves (source) and lowest in the roots (sink).
• The mechanism of mass flow is demonstrated by a model where two reservoirs are immersed in water, and solutes are pushed from the source to the sink by turgor pressure, with water being forced out of the sink by hydrostatic pressure.

Mechanism of Phloem Transportation

• The mass flow hypothesis is limited by the lack of explanation for bidirectional movement of solutes within the same sieve tube.
• The mechanism of cytoplasmic streaming provides further explanation for transportation of solutes, where solutes are transported by streaming along protein filaments of the sieve tubes.
• The energy for the process comes from the sieve tube and companion cells.

Measurement of Water Potential

• Refractive index (RI) is measured quantitatively with a refractometer to determine the water potential (Ψ) of a solution.
• A graded series of sucrose solutions is prepared, and their refractive index (RI) is measured using a refractometer.
• Plant tissues are introduced into the range of solutions, and after equilibration, the refractive index (RI) of each solution is re-determined.
• The ratio of initial/final refractometer readings is plotted against molarities of sucrose solutions to determine the water potential (Ψ).

Concept of Water Potential

• Pressure bomb technique is used to estimate xylem water tension, where a leafy shoot or leaf petiole is quickly cut and placed in a pressure chamber.
• The pressure in the chamber is gradually increased by compressed air until the sap/water just returns to the severed ends of the xylem vessels.

Water Relations of Whole Plant

• Water and dissolved minerals entering the plant through root hairs can follow three possible pathways: apoplast, symplast, and vacuolar.
• The apoplast pathway is along and between cell walls, through intercellular spaces between them.
• The symplast pathway is through the cytoplasm, from cell to cell via plasmodesmata strands.
• The vacuolar pathway is through the sap vacuoles, from one vacuole to the next by osmosis.

The Ascent of Sap

• Primarily, water uptake occurs by osmosis along a water potential gradient from the soil solution into the parenchyma cells of the root cortex.
• The walls of the guard cells that border the stoma pore are thicker and less elastic than the thinner outer walls.

Physiology of the Stomata

• There are no symplastic connections between the subsidiary cells and the guard cells.
• Mechanisms of stomata opening involve changes in turgor in the guard cells, which regulates the opening and closing of the stomata.
• The active transport of ions (principally Potassium K+) into the guard cells from the neighboring epidermal cells alters the solute potential and water potential, causing water to move in by osmosis.

Absorption of Mineral Salts

• The Donnan equilibrium is a state of balance where an electrical potential gradient develops between two solutions due to uneven distribution of ions.
• The passage of some ions across the membrane is promoted, and the net result at equilibrium is that K+ and Cl¯ will be in equal molar amounts together from one side to the other.