Chapter 32, pages 701-705
I. Stomatal Opening/Closing
A. Structure of the guard cells
1. Crescent-shaped or semicircular in form
2. Walls adjacent to pore are thicker and less elastic
B. Physics of opening and closing
1. Depends upon the relative turgor pressure of the guard cell
2. Turgor controlled by adding and removing solutes
3. Water moves in from adjacent cells to causes a buildup of pressure in the guard cell
4. Guard cell changes shape (outer wall of guard cell expands)
5. Results in the bowing of the thickened cell wall adjacent to the pore
6. When this occurs to both guard cells, it produces an elliptical aperture in the stoma
C. Physiology of opening and closing (adding and removing solutes)
1. Potassium ions, protons and Cl-
a.
Sun shining on the
guard
cell causes these ions to be taken up by the guard cell
from the surrounding cells
b. These ions are released to the adjacent cells when the sun goes down
c. Active transport is what causes these ions to be taken up by the guard cells
2. Starch
a. Starch is a polymer of glucose molecules
b. An enzyme cuts up the starch molecule into separate glucose molecules
c.
Glucose molecules are hooked back up into a polymer by another enzyme when
the stoma closes
3. Theoretical physiological mechanism
a. CO2 dissolved in water makes a weak acid called carbonic acid
b.
In sunlight, the cell sap becomes less acid
(pH
increase) because the cell is using
CO2 to make sugar
c.
In the presence of
potassium
ions
and increased pH, an
enzyme
is activated to
break down
starch
to
glucose
d.
The
water
potential decreases as more solute (glucose) is added to the
cell
sap
of the guard cell
e.
As the water potential decreases in the guard cell, the guard cell becomes
more
turgid
because water from the surrounding cells moves in by
osmosis
f. When the cells become fully turgid the stomata are fully open
g. The reverse of this occurs when the sun goes down at night
D. Factors affecting stomatal opening
1. Time of day
a. Majority of plants have stomates open during the day time
b. A few plants open their stomates at night
(1) Carrier molecule picks up the CO2 and holds it until daylight
(2) Carrier molecule releases its CO2 for photosynthesis during the day
2. Dehydration and water stress
a.
Extreme water loss causes the stomata to close because there is not enough
water in the plant to maintain
turgor
pressure in the
guard
cells
b.
Moderate water loss causes
abscisic
acid to be released which causes stomatal
closure
3. Carbon dioxide inside of the leaf
a. High concentrations of CO2 cause the stomata to close
b. Low concentrations of CO2 cause the stomata to open
E. Rate of transpiration is affected by the following factors:
1. Humidity
a. The higher the humidity the slower transpiration goes
b. The lower the humidity the faster transpiration goes
2. Air temperature
a. As air temperature increases the relative humidity decreases
b. As air temperature decreases the relative humidity increases
c.
Air temperature heats up the leaf and causes
evaporation
on the surface of the
cell
walls of the
spongy
parenchyma to increase
3. Midday
a. Midday is the time of day when transpiration is at it highest rate
b.
If moderate temperatures and humidity occurs during this time of day, the
transpiration rates do not go beyond what the plant can handle
c.
High temperatures and low humidity causes transpiration to occur faster than
the
plant can handle which begins to deplete the
protoplast
of water causing the
plant to temporarily
wilt
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