As we were noticing previously, a cell that is already full may take in additional water because of water potential in hypotonic solutions. When a cell does this, very little water moves in, but the water potential of the cell can change drastically because the pressure potential increases dramatically. This feature is demonstrated in a standard plant physiology diagram, a Höfler plot:
As you can see, as a cell increases in volume (from right to left in this plot) all of the parameters of water potential (ψ = ψs + ψp) change. However you should notice that when the cell volume is less than full (relative cell volume < 0.9) the changes that one sees in ψ can be attributed mostly to ψs. However after the cell is full, most of the changes in ψ can be attributed mostly to ψp. Of course it does make sense that a cell that is not full has no turgor pressure, so changes water will result in dilution or concentration of solutes (Δψs). If a cell is already full, then not much more water can come in to alter the concentration of solutes but each new molecule of water will increase the pressure (Δψp) in the cell.
It is interesting to notice that the slope of the ψp curve in the Höfler diagram is the volumetric elastic modulus of of the cell wall. This "rigidity" is a function of the change in pressure over the relative cell volume:
Because the intake of water at the lower relative volumes is mostly diffusion (driven by Δψs), the rate of water movement will follow Fick's law which is simplified here as:
In this case Lp is the hydraulic conductivity which has membrane and matrix factors. The geometry of the cell is also a factor in hydraulic conductivity.
In typical cells the solute potential (ψs) ranges from -0.5 to -1.2 MPa but can be as high as -2.5 MPa in some sucrose storage tissues: for example in stems of sugar cane or roots of sugar beets. The intercellular spaces between cells typically is about -0.1 MPa.
In typical cells the pressure potential (ψp) ranges from 0.1 to 1 MPa but can take on negative values (about -1.0 MPa) inside the dead xylem cells of a tall tree.
So why are we making such a big deal about water and water movement? Well, it is important to remind you that water impacts the life of plants in major ways. The figure below shows you some of the processes in a plant that are affected with increasing water stress.
As you can easily observe, processes such as these are halted as the water potential of a cell decreases below -1 MPa. The productivity of deserts is among the lowest on earth for this reason. The two arrows that seem to increase in this figure are Abscisic acid...a dormancy-inducing hormone...and solute concentration, which inactivates a range of metabolic enzymes.
Everything we have done up to now in these water problems assumes that cells actually come to equilibrium with a big vat of solution in their environment. While this is a nice state to contemplate in the comfort of our thoughts, it is almost completely unnatural. If you think about life having to gain and lose materials, for materials to flow continuously, then equilibrium (where there is no net movement of materials) is just not compatible with life! An organism therefore usually exists in a state of gradients so that flows can occur, both between and around its cells.
Here is a cell-to-cell example of water movement. Two cells, side by side, share a contact (see below). Through this double-wall, water will flow based upon the water potential difference (Δψ). We call the movement of material from cytoplasm to cytoplasm "symplastic" flow.
As you can easily see above, these two cells are not in equilibrium...at least not yet. Water will flow from the cell where the potential is higher to the cell where the potential is lower. As shown by the arrow, the water will move from the cell on the right into the cell onto the left. Because the cells are about the same size and shape, any movement of fluids will change the two cells in similar but opposite ways. The cytosol gaining water will become more dilute (↑ψs) by just as much as the cytosol in the other cell becomes more concentrated (↓ψs). Similarly since both cells are turgid, the amount of pressure lost by one cell (↓ψp) will be matched by a pressure increase in the other cell (↑ψp).
Notice that, as the water moves in this closed system of two cells, an equilibrium is achieved...or is it? In terms of water movement, it may be...but if the membranes are permeable to solutes, such as sucrose, in the cells are the cells truly in equilibrium? Hint: notice the difference in ψs that still exists. Which way will sugar move? → If sugar can move, then the ψs will change and then what? I think you get an appreciation of what I mean when I say that in life gradients permit flow of water and solutes at almost all times.
To get even closer to reality, let us contemplate what happens to water as it moves through the cells of a root carrying minerals. It enters the xylem and goes up the stem. You might want to review the anatomy of a root shown in the context of water movement:
As you will observe above, and in my sketch below, materials can move from cell to cell in symplastic flow but there is also an intercellular space through which other materials may move in apoplastic flow. There is no barrier to symplastic flow in the pathway of water to get to the xylem, but there is a barrier to apoplastic flow.
The endodermis has radial and transverse walls that are impregnated with suberin, a waxy substance. These are called Casparian strips. They prevent materials from going around the endodermis cells; to get to the inside of the vascular cylinder materials must move into the endodermis cells (join the symplastic flow). This of course requires that the cell membrane have integral or peripheral transport proteins to ferry the materials into the symplast. Materials for which there are no transport proteins are likely to be left outside the endodermis in the cortical apoplast. Now we will dissect this from a water potential point of view:
As you can see from this sketch, a gradient of ψs is maintained across the root by delivery of sugar down from the leaves, diffusion out into the cells, and either use or storage of sugar in the form of osmotically almost-inert polysaccharide. This gradient keeps sugar flowing down the phloem to supply the needs of the various cells in the root. This gradient is essential for the survival of the root. Equilibrium would be fatal.
As you can also see from this sketch a gradient of ψp is maintained across the root. This reduction of pressure at the "sink" end of the phloem provides for movement of fluid in the phloem via bulk flow. The negative ψp in the xylem provides the "lift" for pulling up a cohesive column of water and minerals from the soil up through the xylem to the leaves. Without these pressure differences, bulk flow would cease, the shoot would wilt and the root would starve.
As you can also see from this sketch a gradient of ψ is maintained across the root. This reduction of water potential as you go from the soil water to the xylem provides the potential differences needed to bring water and minerals in from the soil to be lifted up the xylem. If this gradient of decreasing water potential collapsed the plant could not remove water from the soil, it would wilt, and ultimately die.
Hopefully you have concluded that equilibrium is not really
any kind of "natural" phenomenon in a thriving plant. If
so, then this lesson was effective!