The solute potential (also called the osmotic potential) of a solution is a measure of the
effect of dissolved solutes on the osmotic behavior of the solution. The following statement
presents an opportunity for confusion, so study it carefully:
* The greater the solute concentration of a solution, the more negative its solute
potential, and the greater the tendency of water to move into it from another solution of
lower solute concentration (and less negative solute potential).
Owing to the rigidity of the cell wall, plant cells do not burst the way animals cells do when
placed in pure water; instead, water enters plant cells by osmosis until the pressure
potential exactly balances the solute potential. At this point, the cell is turgid; that is, it has
a significant positive pressure potential.
The overall tendency of a solution to take up water from pure water, across a membrane, is
called its water potential and is represented as , the Greek letter psi (pronounced “sigh”)
(Figure 35.2). The water potential of a solution is simply the sum of its (negative) solute
potential (s) and its (usually positive) pressure potential (p):
= s + p
Water always moves across a selectively permeable membrane toward the region of lower
(more negative) water potential.
Osmosis is of great importance to plants. The physical structure of many plants is
maintained by the (positive) pressure potential of their cells; if the pressure potential is lost,
the plant wilts. Within living tissues, the movement of water from cell to cell follows a
gradient of water potential.
Over longer distances, in unobstructed tubes such as xylem vessels and phloem sieve tubes,
the flow of water and dissolved solutes is driven by a gradient of pressure potential, not a
gradient of water potential. The movement of a solution due to a difference in pressure
potential between two parts of a plant is called bulk flow. We’ll see that bulk flow in the
xylem is between regions of differing negative pressure potential (tension) while bulk flow
in the phloem is between regions of differing positive pressure potential (turgidity).
Aquaporins are membrane channel proteins through which water can move without
interacting with the hydrophobic environment of the membrane’s phospholipid bilayer (see
Section 5.3.4). These proteins, important in both plants and animals, allow water to move
rapidly from environment to cell and from cell to cell. Their abundance in the plasma
membrane and tonoplast (vacuolar membrane) varies with environmental conditions,
depending on a cell’s need to obtain and retain water. The permeability of some aquaporins
can be regulated, changing the rate of osmosis across the membrane. However, water
movement through aquaporins is always passive, so the direction of water movement is
unchanged by alterations in aquaporin permeability.
Mineral ions, which carry electric charges, generally cannot move across a membrane
unless they are aided by transport proteins, including ion channels and carrier proteins (see
Section 5.3.4). The ions would otherwise be blocked by the hydrophobic interior of the
membrane, and they are too large to pass through aquaporins.
Electric charge differences also play a role in the uptake of mineral ions. Movement of a
negatively charged ion into a negatively charged region is movement against an electrical
gradient and therefore requires energy. The combination of concentration and electrical
gradients is called an electrochemical gradient. Uptake against an electrochemical gradient
involves active transport, which is fueled by ATP generated by cellular respiration. Active
transport requires specific transport proteins.
Unlike animals, plants do not have a sodium–potassium pump (see Section 5.4.1) for active
transport. Rather, plants have a proton pump, which uses energy obtained from ATP to
move protons out of the cell against a proton concentration gradient (Figure 35.3, step 1).
Because protons (H+) are positively charged, their accumulation outside the cell has two
* An electrical gradient is created such that the region outside the cell becomes positively
charged with respect to the region inside.
* A proton concentration gradient develops, with more protons outside the cell than
Each of these results has consequences for the movement of other ions. Because the inside
of the cell is now more negative than the outside, cations (positively charged ions) such as
potassium (K+) move into the cell by facilitated diffusion through their specific membrane
channels (Figure 35.3, step 2). In addition, the proton concentration gradient can be
harnessed to drive secondary active transport, in which anions (negatively charged ions)
such as chloride (Cl ) are moved into the cell against an electrochemical gradient by a
symport protein that couples their movement with that of H+ (Figure 35.3, step 3). In sum,
there is a vigorous traffic of ions across plant cell membranes, involving specific membrane
transport proteins and both active and passive processes.
The proton pump and the coordinated activities of other membrane transport proteins cause
the interior of a plant cell to be very negatively charged with respect to the exterior; that is,
they build up a significant membrane potential. Biologists can measure the membrane
potential of a plant cell with microelectrodes, just as they can measure similar charge
differences in nerve cells and other animal cells (see Section 44.2). Most plant cells
maintain a membrane potential of at least 120 millivolts (mV).
The movement of ions across membranes can also result in the movement of water. Water
moves into a root because the root has a more negative water potential than does the soil
solution. Water moves from the cortex of the root into the stele (which is where the vascular
tissues are located) because the stele has a more negative water potential than does the
Water and minerals from the soil can pass through the dermal and ground tissues to the
stele via two pathways, the apoplast and the symplast (Figure 35.4):
* The apoplast (Greek apo, “away from”; plast, “living material”) consists of the cell walls,
which lie outside the plasma membranes, and the intercellular spaces (spaces between
cells) that are common to many tissues. The apoplast is a continuous meshwork through
which water and dissolved substances can flow or diffuse without ever having to cross a
membrane. Movement of materials through the apoplast is thus unregulated—until it
reaches the Casparian strips of the endodermis.
* The symplast (Greek, sym, “together with”) passes through the continuous cytoplasm of
the living cells connected by plasmodesmata. The selectively permeable plasma membranes
of the root hair cells control access to the symplast, so movement of water and dissolved
substances into the symplast is tightly regulated.
Osmotic mechanisms govern the movement of water from the soil into the plant stele; this is
a passive process. Uptake of minerals from the soil occurs against an electrochemical
gradient and is therefore an active process requiring energy and membrane transport
proteins. Water and minerals can move through either the apoplast or the symplast, but
must enter and leave the symplast to reach the xylem.
* Living, “pumping” cells were not responsible for the upward movement of the solution,
because the solution itself killed all living cells with which it came in contact.
* The leaves played a crucial role in transport. As long as they were alive, the solution
continued to move upward; when the leaves died, movement ceased.
* The movement was not caused by the roots, because the trunk had been completely
separated from the roots.
In spite of Strasburger’s observations, some plant physiologists hypothesized that xylem
transport was based on root pressure—pressure exerted by the root tissues that would force
The solute potential (also called the osmotic potential) of a solution is a measure of the effect of dissolved solutes on the osmotic behavior of the solution. The following statement presents an opportunity for confusion, so study it carefully: * the greater the solute concentration of a solution, the more negative its solute potential, and the greater the tendency of water to move into it from another solution of lower solute concentration (and less negative solute potential). Owing to the rigidity of the cell wall, plant cells do not burst the way animals cells do when placed in pure water; instead, water enters plant cells by osmosis until the pressure potential exactly balances the solute potential. At this point, the cell is turgid; that is, it has a significant positive pressure potential. The overall tendency of a solution to take up water from pure water, across a membrane, is called its water potential and is represented as (figure 35. 2).