Biology 2601A/B Lecture Notes - Stoma, Rubisco, Berlin Outer Ring

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16 Oct 2011
Department
Organismal Lecture 17
Gas Exchange 3: Gas Exchange in Plants
Metabolism and photosynthesis depend on CO₂ /O₂ in the
environment
The loss of water vapour is an important determinant of gas
exchange mechanisms
Leafs tend to lose water vapour (roots tend to absorb it)
How Do Gases Get In and Out of the Leaf?
Stomata: pores in the lead surface that allow access from the
outside to air spaces inside the leaf
o Accounts for >90% of all gas exchange that is occurring in a plant
The stomata is able to open and close, therefore controlling the gas exchange
Although having the stomata open is good for gas exchange, it is bad for water loss (plants must
use strategies to balance both this counteracting forces)
The stomata are located usually on the underside of the leaf; however in aquatic plants the
stomata can be found on the top of the leaf; grasses may have stomata on both top and bottom
Structure of Stomata
Contain an inner ring of guard cells that surround the stoma; these guard cells control the
opening and closing of the stoma
Outer ring is composed of subsidiary cells
Guard cells will have a ledge to prevent particles/spores from falling into the stomata
Below: ellipsoid and graminaceous just refer to the shapes of stomas that different plants have
How Do Plants Open and Close Their Stomata
Plants don’t have muscles; do it through osmotic turgor
Have longitudinal and horizontal microfibrils that are able to bend, but can’t stretch
This results in a stretching along the outside of the guard cells where there are no microfibrils,
causing the guard cells to swell
When guard cells open stomata, have an influx of water and ions into cells, expanding them
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We are not sure how the stomatal opening and closing is regulated
The stomata will respond to the internal PO₂ and light
Will also close in response to desiccation stress
Also has endogenous (diel) rhythms
o Will open in the morning and close at night for example;
has daily rhythms
Why Do the Cells Swell?
Water uptake by the cells is driving by potassium influx
Bean guard cells will
o Open at 552 mM of K
o Closed at 112 mM of K
Will also have malate and Cl ions flowing in; Cl will serve to balance the charge
Water will follow the concentration gradient passively (NOT active); this cause swelling
Photorespiration
Remember that one of the problems of plants doing gas exchange is balancing the amount of
CO₂ to O₂
RuBisCO has oxygenase properties that will decrease the amount of energy going into
photosynthesis
Photorespiration dominates when there is a relatively high *O₂+; therefore RubBisCO will be
exhibiting its oxygenase property
Recall that it is easier to ↑ CO₂ than it is to build a better RuBisCO
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