Gas Exchange: Stomata &
I. Photosynthesis/Transpiration Paradox (or perhaps more accurately, a
"Compromise" or "Dilemma")
Recall the equation for photosynthesis:
CO2 + H2O � (CH2O) n
+ O2. This equation tells us that:
- Gases are important for the overall energy metabolism of plants;
- Plants must exchange gases with the environment; and
- In order to obtain carbon dioxide plants will necessarily loose water (transpire) or in
short, transpiration is a necessary evil of photosynthesis.
II. Theoretical considerations.
- A large surface area is required for efficient gas exchange (e.g., animals have lungs
and gills; plants have leaves and within the leaf - spongy layer).
- A large surface area for exchanging gases offers a large surface area for desiccation.
Animals solve this problem by placing the absorptive surface inside a humid cavity (lung)
opened with a small exit pore(s). Plants put the absorptive surface (spongy
inside the leaf and cover it with a water impermeable layer (cuticle) peppered with a
series of pores (stomata). The cuticle is comprised of waxes that minimize desiccation.
- Placing the absorptive surfaces inside the organism to reduce desiccation presents a
problem - getting the gases to the absorptive surface. Animals use an active pumping
mechanism (lungs/diaphragm) to move gases inside the organism by bulk flow. The gases are
circulated by another pumping system (heart). The distances needed to move the gases are
too great to be accounted for by simple diffusion. Plants do not have a pumping mechanism
for moving gases. They rely primarily on diffusion (and bulk flow). In either case, plants
do not actively move gases. This is one reason why leaves must be thin - diffusion is not
efficient over long distances (i.e., diffusion is inversely related to the square of
- The Compromise Revisited - In order to obtain carbon dioxide for photosynthesis plants
needed to evolve a large, thin absorptive surface (leaves with spongy layer) and then
protect it from desiccation. Not only is water loss a "necessary evil" of
photosynthesis, but to make matters worse, the tendency to loose water is greater than the
tendency of carbon dioxide to diffuse into the plant. As evidence, let's calculate the
ratio of water loss to the amount of carbon fixed. If carbon dioxide uptake and water loss
are equal, this ratio should be close to one. In reality, experiments show that this ratio
is closer to 200! In other words for every 200 kg of water transpired, 1 kg of dry matter
is fixed by a plant. Fortunately, plants have devised an ingenious compromise
- For more information on gas exchange theory,
III. Stomatal Structure & Function
- Types of guard cells: (1) elliptical or kidney-shaped. These are characteristic of
dicots; and (2) dumb-bell or dog-bone shaped - characteristic of grasses.
For images of stomata from a variety of plants, click
here. In addition, you will see many examples in lab.
- Common features - (1) thickened inner walls; (2) bands of cellulose fibers that radiate
out around the circumference of the pore; and (3) chloroplasts (in fact, guard cells are
the only epidermal cells with chloroplasts)
- Mechanics of Guard Cell Action
Guard cells open because of the osmotic entry of entry
of water into the GC. In turn, this increases the turgidity (water pressure) in the GC and
causes them to elongate. The radial orientation of cellulose microfibrils prevents
increase in girth. Since GC are attached at the ends and because the inner wall is
thicker, the guard cells belly out with the outer wall moving more and pulling open the
guard cell. Guard cell closure essential involves reversing this process. Water entry into
the guard cells is controlled by increasing the solute concentration (osmotic
concentration) in the guard cells. This occurs by: (a) transporting potassium
(and chloride) ions into
the guard cells from surrounding (subsidiary) cells. This process is
mediated by a proton pump; and (b) by sugars produced during photosynthesis or from
starch breakdown (recall that guard cells are the only epidermal cells with chloroplasts).
We can summarize the mechanics of GC action as follows:
stoma closed (GC flaccid) � add solute � lower
water potential � water uptake (osmosis) � increase pressure � stoma open
- Environmental Control of GC Action.
Guard cells respond to their environment, especially
any factors that impact the photosynthesis/transpiration compromise. Thus, we expect any
factor important in photosynthesis to exert regulatory control on GC. And, we also expect
water, a major player in photosynthesis, to have the final word on control since if a
plant dries out too much it's as good as dead!
- Light - exerts strong control. In general: light = open; dark = closed. (reverse
plants). What kind of light is important? Red & blue light these are important
for photosynthesis which (a) produces sugars (sucrose and glucose) for osmotic regulation;
(b) produces ATP (via photophosphorylation) to power ion pumps; (c) reduces internal CO2
levels which stimulates opening (see below). Blue light is also important - There is an
additional effect of blue light on stomatal activity that is irrespective of its role in
photosynthesis. What is blue light doing? Blue light: (a) activates a H+-ATPase
in the membrane; and (b) stimulates starch breakdown.
- Carbon dioxide - intracellular level is most critical. This is an important regulatory
lo CO2 (i.e., during the day, used by photosynthesis) = open
hi CO2 (i.e., at night, produced during respiration) = closed
- Water - protects against excessive water loss. This is the prevailing and overriding
control mechanism. There are two mechanisms by which water loss regulates stomatal
closure, one is active and the other passive.
Hydropassive Control - simply put, as the plant looses water, the turgidity of the leaf
cells, including guard cells, decreases and this results in stomatal closure. The plant is
not "intentionally" closing the stoma, it is simply the consequence of drying
Hydroactive Control - this mechanism is one in which the plant actually seems to
monitor its water status. When the water potential drops below some critical level, it
engages a cascade of events that close the stomata. Presumably the plant is measuring
pressure (turgor) and then synthesizes or releases an anti-transpirant that is
translocated (moved) to the GC to cause closure. The anti-transpirant is abscisic acid
(ABA), one of the major plant growth regulators. It is active in very low concentration
(10-6 M) and appears very rapidly after water stress (within 7 minutes).
- Temperature - increased temperatures usually increase stomatal action, presumably to
open them for evaporative cooling. If the temperature becomes too high the stomata close
due to water stress and increased CO2 that results from respiration.
- Wind - often causes closure because it: (a) brings CO2 enriched air; and (b)
increases the rate of transpiration that causes water stress which causes the stomata to
close. In some cases, wind causes stomatal opening to increase transpiration for cooling.
V. Why does transpiration occur?
- Transport in plants. This is important to a small degree. Transpiration is certainly not
- Heat loss (latent heat of vaporization)
- Carry nutrients in the soil to the plant
Last updated: January 20, 2004
� Copyright by SG Saupe