Chapter 32, pages 692-699
I. Water Movement in Plants
A. Terminology dealing with the movement of water
1. Diffusion
a. The even dispersion of one substance into another
b. Occurs when water passes through a semipermeable membrane
c. Occurs when a solute dissolves in a solvent
2. Forces between molecules
a. Cohesion
(1) Clumping of like molecules together when the
molecules are not
compressed
(2) These molecules clump because they are
polar
(contains
positive
and
negative regions)
(3) Opposite charges attract and like charges repel
(4)
Hydrogen bonds - a positive region of one molecule is attracted
to the
negative charge
of another
like
molecule
(5) Examples:
(a) Water is polar and evaporates more slowly
(b) Gasoline is nonpolar and evaporates easily
b. Adhesion
(1) Clumping of unlike molecules
(2) Only occurs between polar molecules
(3) Water and cellulose strongly attracted to each other
(4) Reduces water's mobility and thus its Water Potential
3. Osmosis - diffusion of water across a differentially permeable membrane
4. Turgor Pressure - the hydrostatic pressure within a cell due to osmosis
5.
Plasmolysis
- a greatly reduced turgor pressure to the point that the
protoplast
pulls
away
from the
cell
wall
6. Hypoosmotic - a solution that has a lower solute concentration than the cells that the solution surrounds
7.
Hyperosmotic
- a solution that has a higher solute concentration than the cells that
the solution surrounds
8. Halophyte - plants that can tolerate high salt concentrations
9. Xerophyte - plants that can tolerate dry conditions
10. Mesophyte - plants that must have moderate moisture conditions
B. Passive and Active Transport
a. Definition - solutes that diffuse down their gradient
b. No direct expenditure of metabolic energy by the cell occurs
c. Very slow process
d. Process speeded up by:
(a) Carrier proteins bind selectively to a solute
(b) Proteins carry the solute molecule
across the
membrane
and release it
on the other side
(c) Requires a shape change by the carrier protein for this to happen
(a) A passageway or canal through the membrane
(b) Each channel is designed for
only a particular
ion
and are thus
selective
for that ion only
(c) Potassium channels allow only potassium to pass through
(d) Some channels are gated, using environmental stimuli to open the gate
a. Definition - pumping solutes across a membrane against the solute's gradient
b. Proton Pump - example in plant cells
(1)
Hydrolyzes
ATP,
using
energy
released to pump hydrogen ions
(H+)
out of
the cell
(2) Creates a higher concentration of H+ ions on
the outside of the cell and thus
a
membrane
potential which is a form of stored energy
(3) Plants use this energy is used to drive the transport of many solutes
(4) Example - Potassium Transport
(a) Potassium ions are positively charged
(b) Inside cell is negatively charged
(c) Potassium is diffusing down its electrochemical gradient
(5) Example - Nitrate (NO3-) Transport
(a) Nitrate is negatively charged
(b) Carriers allow nitrate and H+ to enter together - called Cotransport
(c) Nitrate is diffusing against its electrochemical gradient
(d) Sucrose is transported in the same way
1. Definition: The
flow of water from a wet place to a dry place and the
physical
force that
drives that flow
a. This flow is the result of passive transport
b. Wetness and dryness are relative
c. Pure water is perfectly wet
d. A completely dry substance is perfectly dry
2. Pure water
a.
Pure water has the highest
water
potential which is 0
bars
at 25oC and one
atmosphere of pressure
b. 1 bar is approximately equal to one atmosphere of pressure
c. 1 megapascal (MPa) = 10 bars
3. A completely dry object has the lowest water potential = negative infinite bars
a. Water with anything dissolved in it has a negative water potential
b.
If two sides of a
semipermeable membrane have different water potentials, water
will flow from the side of higher water potential through the
membrane
to the
side of lower water potential
c. WATER FLOWS FROM WET PLACES TO DRY PLACES
(1) Water flows from places of high water potential to places
of lower water
potential
(2) Water flows from hypoosmotic regions to hyperosmotic regions
4. Factors affecting water potentials
a. Temperature - directly proportional
b. Solutes
(1) When solutes are
dissolved
in water, it causes the water to be drier and thus
the
water potential to be lower
(2) The contribution of solutes to the over all water potential
is called
osmotic
potential
(3) The osmotic potential is always negative when solutes are present
c. Pressure
(1) Pressure increases the water potential of the cell as the pressure in the cell increases
(2) The contribution of pressure in the cell to the over all
water potential of the
cell is called the
pressure potential
(3) The pressure potential is either zero or positive
d. Matrix
(1) A matrix is any particle that is too large to go into solution
(2) Examples of a matrix: solid particles, sand grains,
soil particles, cell wall
microfibrils
(cellulose), and starch
(3) Decreases water potential
(4) The contribution of the matrix to the in the cell to the
overall water potential
is called the
matrix
potential
(4) The matrix potential is either zero or negative
5. Water Potential = Osmotic Potential + Pressure Potential + Matrix Potential
6. How solutions behave under the following conditions:
a. Pressure
(1) The effect of increased pressure within a cell is to decrease
the
water
potential of the cell.
(2) Thus, increased
turgor
pressure effectively makes a
concentrated solution
behave like a
dilute
solution
b.
Adhesion
- adhesion hold water to the cell walls and
cohesion
holds the mass of
water together. Thus, the adhesion forces exerted by
wall
polymers tend to make a
dilute
solution behave
osmotically
as
if
it were more
concentrated in solutes because the water is
less
mobile
when it is stuck to the
wall
polymer.
II. Movement of Substances in Plants
A. Four types of movement
1. Slow diffusion of molecules and ions across membranes
2. Moderate movement of materials carried by cytoplasmic streaming
3. More rapid flow of material in sieve tubes
4. Very rapid conduction of water and mineral solutes in xylem
B. Diffusion and protoplasmic streaming
1. Diffusion is very slow and is the slowest movement of all types
a. Chief method of movement of materials within cells
b. Can be a few mm/hour to several hundred mm/hour
c. The highest rate of cytoplasmic streaming is in slime molds - 486 mm/hr
3. Phloem conduction at its maximum is 100 cm/hr
4. Xylem conduction is 15 m/hr or faster
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