D2.3 Water potential Flashcards
D2.3.1—Solvation with water as the solvent
Include hydrogen bond formation between solute and water molecules, and attractions between both
positively and negatively charged ions and polar water molecules.
Solvents are liquids that can dissolve other substances
to make solutions.
Solutes are dissolved substances in solutions.
Solvation is the process of dissolving. Water dissolves
many different types of molecules by forming hydrogen
b o n d s with them. It also dissolves many types of ions,
b e c a u s e t h e p o l e s o f w a t e r m o l e c u l e s a r e a t t r a c t e d t o
both positive and negative charges of ions. The solvent
properties of water are described in Section A7.1.5.
D2.3.2—Water movement from less concentrated to more concentrated solutions
Students should express the direction of movement in terms of solute concentration, not water
concentration. Students should use the terms “hypertonic”, “hypotonic” and “isotonic” to compare
concentration of solutions
Hypertonica higher solute concentration
Hypotonica lower solute concentration
Isotonic-the same solute concentration.
Water is attracted to the solutes in a solution. Because of this, where water molecules are free to move between two solutions with different solute concentrations, more move from the hypotonic solution to the hypertonic solution than from the hypertonic to the hypotonic.
There is therefore a net movement of water up the solute concentration gradient, from the lower to the higher solute concentration.
hypotonic solution
many water molecules move from hypotonic to hypertonic
and some water molecules move from hypertonic to hypotonic
but more move from hypo to hypertonic so that is the net direction of movement
D2.3.3—Water movement by osmosis into or out of cells
Students should be able to predict the direction of net movement of water if the environment of a cell is
hypotonic or hypertonic. They should understand that in an isotonic environment there is dynamic
equilibrium rather than no movement of water.
The process of osmosis is described in Section B2.1.5.
The direction of water movement between cells and
their environment can b e predicted if the relative solute
concentrations are known. The diagrams show three
possibilities, with arrows showing the net direction
of water movement. When the environment of a cell
dynamic equilibrium.
hypertonic hypotonic is isotonic, water molecules do not stop moving, but
the numbers moving into and out of the cell are equal,
so there is no net movement. This is an example of
hypotonic hypertonic i s o t o n i c
i s o t o n i c
D2.3.4—Changes due to water movement in plant tissue bathed in hypotonic and those bathed in
hypertonic solutions
Application of skills: Students should be able to measure changes in tissue length and mass, and analyse
data to deduce isotonic solute concentration. Students should also be able to use standard deviation and
standard error to help in the analysis of data. Students are not required to memorize formulae for
calculating these statistics. Standard deviation and standard error could be determined for the results of
this experiment if there are repeats for each concentration. This would allow the reliability of length and
mass measurements to be compared. Standard error could be shown graphically as error bars.
Method
1. Prepare a series of sugar or salt solutions with a range of solute concentrations, from 0.0 mol dm 3 upwards.
The solute could be glucose, sucrose, sodium chloride or another salt.
2. Cut potato or other plant tissue into samples of equal size and shape, such as cylinders or cuboids.
3. Measure the length of each sample, using a ruler with millimetre graduations.
4. Find the mass of each sample, using an electronic balance.
5. Bathe each tissue sample in one of the solutions for between one and twenty-four hours.
6. Remove the tissue samples from the solutions, dry them and measure their length and mass again.
7. Calculate percentage mass change. Percentage length change can be calculated in a similar way.
% change = (final mass - initial mass)
* × 100
initial mass
8. Plot the results for % mass change or % length change on a graph.
9. Read off the solute concentration which would give no mass change due to being isotonic.
The graph shows results for two tissues, pumpkin (Cucurbita pepo) and sweet potato (pomoea batatas).
* The curve for the sweet potato tissue intercepts the x-axis at 0.37 mol dm-3, so this is the predicted NaCl concentration at which mass would not change, because there would be no net movement of water.
The conclusion is that this concentration of NaCl is isotonic to the sweet potato tissue.
* The curve for pumpkin tissue can be analysed in the same way, with the conclusion that a solution with a concentration of 0.55 mol dm 3 NaCl would be isotonic. The pumpkin cells have a higher solute concentration than the sweet potato cells.
Sample-a sample is a small, representative portion of something. In experiments, the results are a sample of all the results that could possibly have been obtained, so sample size is the number of repeats.
Standard deviation is a measure of variation from the mean.
It is almost always calculated using a computer or calculator, so there is no need to learn the formula. Standard deviation is based on calculating the difference between each individual result and the mean result and then squaring the answers. If some results are far from the mean the standard deviation is high, indicating that the data are widely spread.
Standard error is a measure of how reliably the mean of a sample estimates the mean of the whole population. It is found by dividing the sample standard deviation by the square root of the sample size.
Example: Ten samples of carrot tissue were bathed in concentrations of NaCl solution ranging from 0.0 to 0.8 mol dm 3. All the samples were initially 100 mm long and even in thickness. The graph shows mean length after 24 hours, with bars to show mean ‡ standard error.
These error bars show most variation in length at 0.6 and least at 0.2 mol dm 3. A statistical hypothesis test was performed (Student t-test).
Analysis
103
102
mean length / mm
101
100
0.0
0.4
NaCl
- concentration/
0.6 0.8 mol dm-3
99-
98-
974
20
* Differences between the mean length for
mass change
10
0.1
0.2
0.3
-10
NaCl concentration (moldm 3)
066..
0.7 0.8
—*
pumpkin
0.0 mol dm 3 and means for other concentrations were significant at the 1% level.
* The same was true for differences between the mean for 0.2 mol dm 3 and means for other concentrations.
* Differences between the mean for 0.4 mol dm-3 and
-20-
both 0.6 and 0.8 were significant at the 5% level.
sweet
* No significant difference was found between 0.6 and
potato
0.8 mol dm 3.
There were no repeats in this experiment-only one sample of tissue was placed in each NaCl concentration.
Conclusions
Scientists usually repeat every treatment in their
* Length increased at 0.2 mol dm or less due to water
experiments. This helps to avoid drawing conclusions
uptake and cells swelling slightly.
from atypical or anomalous results. It allows the reliability * Length decreased at 0.4 mol dm * and above, due to of results to be assessed-the closer together repeats
the cells shrinking slightly as they lost water, but there
are, the greater the reliability. This can be assessed in an objective and quantitative way, using the statistics
was no further decrease above 0.6 mol dm 3 because plasmolysis had already occurred and the cell walls
standard deviation and standard error of the mean.
prevented further decreases in length.
D2.3.5—Effects of water movement on cells that lack a cell wall
Include swelling and bursting in a hypotonic medium, and shrinkage and crenation in a hypertonic
medium. Also include the need for removal of water by contractile vacuoles in freshwater unicellular
organisms and the need to maintain isotonic tissue fluid in multicellular organisms to prevent harmful
changes
Animal cells and some unicellular organisms have a plasma membrane but no cell wall. If cells without a cell wall are bathed in a hypotonic or hypertonic solution, movement of water by osmosis into or out of the cell can have harmful consequences.
hypotonic solution
hypertonic solution
water enters
volume of cytoplasm
increases, so the cell swells up and eventually bursts because the plasma memorane is not strong enough to resist the increase, in pressure
dynamic equilibrium with equal| numbers
of water molecules entering and leaving so cell volume does not change
exits
volume reduces but area of
plasma membrane does not change so the cell becomes crenated
isotonic solution
-no net
movement of water
Unicellular organisms that live in freshwater (ponds, lakes and rivers), inevitably take in water by osmosis because their cytoplasm is hypertonic to their environment. To avoid bursting, they must expel water using contractile vacuoles (described in Section B2.1.13). Cells inside animals are surrounded by other cells or by extracellular fluids. To prevent swelling or shrinking extracellular fluids must be kept isotonic to cells in the animal. In humans the kidneys are used to regulate the solute concentration of extracellular fluids, including blood plasma.
D2.3.6—Effects of water movement on cells with a cell wall
Include the development of turgor pressure in a hypotonic medium and plasmolysis in a hypertonic
medium.
Plant cells and some unicellular organisms have both a
plasma membrane and a cell wall. The wall protects cells
from damage due to hypotonic solutions and allows
turgor pressure to develop, but it d o e s not protect
against damage from hypertonic solutions. As water
moves out of a plant cell into a hypertonic medium, any
turgor pressure in the cell is lost and the volume of the
cytoplasm decreases. A gap develops between the
cell wall and the plasma membrane. A cell in this state is
plasmolysed and does not usually recover.
hypotonic
water
e n t e r s
hypertonic
isotonic
n o n e t m o v e m e n t
w a t e r
e n t e r s
i n c r e a s e in
volume of
no change in
cytoplasm,
volume or
so it pushes
pressure of
against the
the
cell wall and
cytoplasm,
h i g h p r e s s u r e s
as water molecules
develop in
the cell
enter and exit
(turgor
pressure,
d e c r e a s e
cytoplasm
D2.3.7—Medical applications of isotonic solutions
Include intravenous fluids given as part of medical treatment and bathing of organs ready for
transplantation as examples
Osmosis can cause cells in human tissues to either swell and burst, or shrink, due to gain or loss of water. To prevent
this, isotonic fluids must be used for some medical procedures.
* Intravenous fluids, injected into veins from a syringe or from an elevated bag, must be close to isotonic to avoid
osmotic damage to blood cells.
* Tissues or organs used in medical procedures such as kidney transplants must be bathed in an isotonic solution or
isotonic slush (crushed ice) to prevent osmotic damage.
D2.3.8—Water potential as the potential energy of water per unit volume
Students should understand that it is impossible to measure the absolute quantity of the potential energy
of water, so values relative to pure water at atmospheric pressure and 20°C are used. The units are usually
kilopascals (kPa).
Water potential is a measure of the potential energy per unit volume.
The symbol for water potential is ‘ (the Greek letter psi) and the
units are kilopascals (kPa) or megapascals (MPa). The absolute
quantity of potential energy cannot be determined, so all values are
relative. Pure water at standard atmospheric pressure and 20°C has
been assigned a water potential of zero.
D2.3.9—Movement of water from higher to lower water potential
Students should appreciate the reasons for this movement in terms of potential energy
Water potential allows us to predict the direction of net movement of water molecules. Water moves from a higher to a lower water potential because this minimizes its potential energy. The range of water potentials found in cells is rather unusual-the maximum value is zero. Cells either have a potential of 0 kPa or a negative value, for example -200 kPa in a leaf cell. Lower water potentials are therefore more negative. For example, water moves from a cell with a water potential of -200 kPa to one with - 300 kPa.
D2.3.10—Contributions of solute potential and pressure potential to the water potential of cells with walls
Use the equation ψw = ψs + ψp
. Students should appreciate that solute potentials can range from zero
downwards and that pressure potentials are generally positive inside cells, although negative pressure
potentials occur in xylem vessels where sap is being transported under tension.
Many factors can influence water potential (t,) but in living systems only two contributors vary in such a way that they need to be considered:
1. Solute potential (V)
Bond formation releases energy. The release of energy during solvation by formation of solute water bonds reduces the potential energy held by water. This reduction is the solute potential (sometimes called the osmotic potential). With no solutes dissolved in water, solute potential is at its maximum of zero. The more solutes that are dissolved, the more negative it becomes.
2. Pressure potential (Y)
Rises or falls in hydrostatic pressure change the potential energy of water. The higher the pressure, the more potential energy water has.
Atmospheric pressure is defined as having a pressure potential of zero. Pressure potential can be negative or positive because it can be greater or less than atmospheric pressure. Plant cells are usually turgid (above atmospheric pressure) so they have a positive pressure potential, but sap in xylem vessels is usually pulled up to the leaves under tension, so pressure potentials there are negative.
Water potential is the sum of solute potential and pressure potential: *=Ф, + *
State
Explanation
Fully turgid
zero
* is positive but is cancelled out by the negative y,
一半；一4
Partially turgid
-ve
vis positive but y, is more negative, so 4, is negative
-*>Ф
Flaccid
—ve
* is zero, so as yand *, are equal. , is very negative, so 4, is also very negative
Xylem vessel
-ve
If xylem sap is under tension, * and v, are negative so 4, is extremely negative
D2.3.11—Water potential and water movements in plant tissue
Students should be able to explain in terms of solute and pressure potentials the changes that occur when
plant tissue is bathed in either a hypotonic or hypertonic solution.
Water movement in plant tissue is always passive, with direction of net movement determined by differences in water potential.
1. Plant tissue bathed in pure water
In pure water, 4, = 0.
Water moves into the plant cells until they
are fully turgid so 4, = 0.
2. Plant tissue in a hypotonic solution
, is more negative in the plant tissue than in the solution, so we expect water to move from the solution to the tissue.
However, if the plant tissue is initially fully turgid (
= 0), some water may move
from the tissue to the solution until their water potentials are equal.
3. Plant tissue bathed in hypertonic solution
* is more negative in the solution than in the plant tissue, so water moves from the tissue to the solution. As water leaves the cells, their pressure potential decreases.
If it reaches zero (atmospheric pressure), the cells become flaccid (limp). If water continues to move from cell to solution, the volume of cytoplasm becomes less than the space inside the cell wall and the plasma membrane is pulled away from the cell wall (plasmolysis). However, this may not happen because loss of water concentrates the cytoplasm, so a dynamic equilibrium may be reached between the cytoplasm and the solution before plasmolysis occurs.
4. Adjacent cells with different water potentials
Water moves by osmosis from the cell with higher water potential to the cell with the lower water potential. The net movement continues until the water potential of the two cells is equal. For example, in the leaf sucrose is pumped into phloem cells, making 4, more negative and therefore lowering 4,, so water moves from adjacent cells into the phloem cells.
