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Gas Exchange in Plants summarized lectures notes along with relevant pictures and notes from the assigned readings

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Department
Biology
Course
Biology 2601A/B
Professor
Brent Sinclair
Semester
Fall

Description
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  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 How to Concentrate CO₂  In a hard-working cell, respiration will be using up O₂ and producing CO₂ o But working hard undoes the benefit of photosynthesis  In general, the plant just need to concentrate CO₂ around RuBisCO (so will use its carboxylase properties) o Usually O₂ diffuses away fast enough, and light is not limiting, therefore the CO₂ concentration is not necessary o However thi
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