Transport213-page13 - countered in some plants by the...

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Transport in Plants - 13 To produce these changes in turgor, a ratio of potassium K + and H 2 O is maintained within guard cells that is different in daytime than nighttime. In addition, in contrast to other epidermal cells, guard cells contain chloroplasts, and the process of photosynthesis is used to maintain turgor when stomata are open. Daytime 1. K + moves into guard cells in response to presence of light through membrane channels using the proton gradient generated by hydrogen proton pumps. H + is actively pumped out of guard cells at the time stomata are opening. The pumps, which are activated by blue-light receptors in the guard cell membrane, create a negative water potential in the cell. (More solutes = more negative water potential). 2. The increasing “+” charge (K + may be 4 – 8 time higher within the cell) is
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Unformatted text preview: countered in some plants by the active transport of Cl-into the cell. The Cl-uptake is coupled to a proton pump generated by the release of H + from a number of organic acids within the guard cells. The acids also help increase the “–” balance. 3. K + concentration remains much higher in guard cells than in surrounding epidermal cells when stomata are open. 4. H 2 O moves by osmosis into cell in response to K + solute concentration, increasing the cell volume (a turgor phenomenon) swelling the guard cells which “open” forming the stoma pore. The water is stored in the vacuole. 5. Photosynthesis solute products, along with the H + / K + pump maintain the osmotic gradient needed for turgor, keeping the stomata open during photosynthetic hours....
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