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Lecture 15

Organismal Physiology_Lecture 15.docx

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Western University
Biology 2601A/B
Tamsen Taylor

Organismal Physiology Lecture No. 15: Gas Exchange In Plants Thursday November 1 , 2012t Gas Exchange In Photosynthesis: -Metabolism and photosynthesis are dependent on CO and O exchange with the environment. Gas 2 2 exchange in photosynthesis involves extracting CO out of2the external air medium and into the chloroplasts of the leaf. This usually occurs via Fickian diffusion (relies heavily on the presence of a concentration gradient) through the stomata (or singular stoma). This allows for the diffusion of other gases like water (as vapour or fog), which results in a water saturation inside the plant relative to the dry external environment. The Movement Of Gases In Plants: -In respiration (as well as photorespiration), O di2fuses into the leaf, while CO diffu2es out. In gross photosynthesis, CO dif2uses into the leaf, while O diffu2es out. For all of these diffusions to occur, water is going to diffuse out of the leaf. The cuticle of a leaf`s surface, prevents the diffusion of water. Large intercellular air spaces in between mesophyll cells allow for the diffusion of gases by having saturated water diffuse out of the leaf. The boundary layer acts as an insulation of molecules that stay freely still on the outer surface of the leaf, which provides resistance to diffusion. The two resistance forces present in a leaf are boundary layer resistance (r ) abd leaf stomatal resistance (r ), whish influence the diffusion of water out of and CO in a2leaf. The Stomata: -Stomata are pores or opening on the leaf surface that allow access from the outside to air spaces inside the leaf. Only about 5% loss comes out from the cuticle, but 90-95% of all gas diffusion occurs at stomata. These “mouths” can open or close, but open stomata, though good for CO uptake, can result 2 in water loss. Water loss can be serious for plants as the concentration gradient for water loss is much steeper than it is for CO u2take. For most angiosperms (trees and crops), stomata are found on the underside of the leaf, while floating aquatic plants have their stomata on top of their leaves. Grasses tend to have stomata distributed equally across the leaf’s surface (top and bottom), while conifers have a row of stomata on each of the four sides of a needle. There are two main functional types of stomata and they occur in different types of plants: ellipsoid stomata (look like kidney beans) and graminaceous stomata (dumbbell shape). Guard Cells & The Swelling Of Cells: -Guard cells lie along the epidermal layer of the leaf and surround tiny stomatal airway pores. Guard cells are strengthened by cellulose microfibrils that allow them to bend but not to stretch. The main mechanism of stomatal opening is that water enters the guard cells and causes them to swell in a specific way (due to presence or microfibrils); along the lines of least resistance. The swelling of cells occurs because water uptake is driven by an influx of potassium into guard cells. So, the changes in osmotic potential (due to Ψ chasges) lower cell water potential and water flows into the cell, increasing +
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