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Lecture 10

Lecture 10 - Gas Exchange II - Plants

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Biology 2601A/B
Graeme Taylor

LECTURE 10 – GAS EXCHANGE II: PLANTS  Metabolism and photosynthesis are dependent on CO and2O with2the environment Gas Exchange in Photosynthesis  Getting CO 2ut of the external medium (air) and into the leaf o Usually via Fickian diffusion through the stomata (or a stoma)  But this allows diffusion of other gases too  The most important is water  Plants lose water through the same pore that allows CO 2n o This is a basic tradeoff that plants have to work with  Cuticle - waxy layer that prevents water loss  Mesophyll cells - photosynthetic tissues  Bottom of the leaf - presence of pores where gases can enter the leaf and diffuse out  High CO 2oncentration in the atmosphere in comparison to the inside of the leaf- concentration gradient which drives diffusion  Pore acts as a conductance for the diffusion  Two resistances - first occurs at boundary layer of the leaf - for CO2molecule to diffuse, there is resistance  High water concentration inside the leaf - cell walls are saturated with water - air inside is fully saturated, air outside of the leaf has the same resistance  For approx. every CO2molecule, it loses about 400 molecules of water - a lot of water is lost in order to get one CO 2olecule that is needed to grow How do Gases get In and Out of Leaves?  Stomata  Pores in the leaf surface that allow access from the outside to air spaces inside the leaf o 90% of all gas exchange  Can open and close  Open is good for CO 2ptake, but bad for water loss  In most angiosperms, stomata can be found on the underside of the plant  In floating aquatic plants, the stomata can be found on the top of the plant  In grasses, stomata are found all over - top and bottom  Structure of stomata o Guard cells – surround stomatal pore o Subsidiary cell on either side of the guard cells o Epidermal cell – covered with cuticle  How do stomata open and close? o Bands of cellulose microfibils - circle around the guard cells - can bend, but are not stretchable o Water enters these cells and causes them to swell, when this happens the cell is forced to twist o Swelling in curvature causes an opening  Why do cells swell? o Water uptake driven by potassium influx o So, solute concentration increases; water flows into cell, which increases turgor pressure o In gu
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