Chapter 16 lecture 3


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Chapter 16 lecture 3

  1. 1. Chapter 16 2005-2006 Photosynthesis Leaf Structure and Function & Factors necessary
  2. 2. Leaves <ul><li>Function of leaves? </li></ul><ul><ul><li>photosynthesis </li></ul></ul><ul><ul><ul><li>energy production </li></ul></ul></ul><ul><ul><ul><li>CH 2 O production </li></ul></ul></ul><ul><ul><li>gas exchange </li></ul></ul><ul><ul><li>transpiration </li></ul></ul>2005-2006 simple vs. compound
  3. 3. 2005-2006
  4. 4. Which cells appear to contain chloroplasts?
  5. 5. Collenchyma
  6. 6. 2005-2006 Summary of stages of photosynthesis Summary of stages of photosynthesis
  7. 7. Stomates 2005-2006 Function of stomates?
  8. 8. <ul><li>blue-light wavelengths of daylight, detected by zeaxanthin (a carotenoid) </li></ul><ul><li>activate proton pumps in the guard cell membranes, which proceed to extrude protons from the cytoplasm of the cell; </li></ul><ul><li>this creates a &quot;proton motive force&quot; (an electrochemical gradient across the membrane) which; </li></ul><ul><li>opens voltage operated channels in the membrane, allowing positive K ions to flow passively into the cell, from the surrounding tissues. </li></ul><ul><li>Chloride ions also enter the cell. </li></ul>2005-2006
  9. 9. <ul><li>Water passively follows these ions into the guard cells, and as their tugidity increases so the stomatal pore opens, in the morning. </li></ul><ul><li>As the day progresses the osmotic role of potassium is supplanted by that of sucrose, which can be generated by several means, including starch hydrolysis and photosynthesis. </li></ul><ul><li>At the end of the day (by which time the potassium accumulation has dissipated) it seems it is the fall in the concentration of sucrose that initiates the loss of water and reduced turgor pressure, which causes closure of the stomatal pore. </li></ul>2005-2006
  10. 10. Regulating Stomatal Opening:-the potassium ion pump hypothesis Guard cells flaccid Stoma closed K + K + K + K + K + K + K + K + K + K + K + K + K + ions have the same concentration in guard cells and epidermal cells Light activates K + pumps which actively transport K + from the epidermal cells into the guard cells
  11. 11. Regulating Stomatal Opening:-the potassium ion pump hypothesis K + K + K + K + K + K + K + K + K + K + K + K + Increased concentration of K + in guard cells Lowers the  (water potential) in the guard cells Water moves in by osmosis, down  gradient H 2 O H 2 O H 2 O H 2 O H 2 O
  12. 12. Stoma open Guard cells turgid K + K + K + K + K + K + K + K + K + K + K + K + Water moves in by osmosis, down  gradient H 2 O H 2 O H 2 O H 2 O H 2 O H 2 O
  13. 13. Factors affecting the rate of photosynthesis <ul><li>The main factors are </li></ul><ul><ul><li>light intensity, </li></ul></ul><ul><ul><li>carbon dioxide concentration and </li></ul></ul><ul><ul><li>temperature, </li></ul></ul><ul><ul><li>known as limiting factors. </li></ul></ul>2005-2006
  14. 14. Factors affecting the rate of photosynthesis <ul><li>As light intensity increases, the rate of the light-dependent reaction, and therefore photosynthesis generally, increases proportionately. </li></ul><ul><li>As light intensity is increased however, the rate of photosynthesis is eventually limited by some other factor. </li></ul><ul><li>Chlorophyll a is used in both Photosystems. The wavelength of light is also important. PSI absorbs energy most efficiently at 700 nm and PSII at 680 nm. Light with a high proportion of energy concentrated in these wavelengths will produce a high rate of photosynthesis. </li></ul>2005-2006
  15. 15. Factors affecting the rate of photosynthesis <ul><li>An increase in CO 2 increases the rate at which carbon is incorporated into carbohydrate in the light-independent reaction and so the rate of photosynthesis generally increases until limited by another factor. </li></ul><ul><li>Photosynthesis is dependent on temperature. It is a reaction catalysed by enzymes. As the enzymes approach their optimum temperatures the overall rate increases. Above the optimum temperature the rate begins to decrease until it stops. </li></ul>2005-2006
  16. 16. C3/C4 Plants? <ul><li>The photosynthetic reactions we have been discussing related to C3 plants </li></ul><ul><ul><li>CO 2 combines with RuBP to form a 6C molecule that immediately splits into two 3C molecules </li></ul></ul><ul><li>In some plants the first thing produced is a 4C molecule </li></ul>2005-2006
  17. 17. Avoiding photorespiration <ul><li>Rubisc o catalyzes the CO 2 combining with RuBP </li></ul><ul><li>But, can also catalyze O 2 combining with RuBP – photorespiration </li></ul><ul><li>RuBP is being wasted </li></ul><ul><li>Photorespiration occurs more readily in high temperature/high light conditions </li></ul>2005-2006
  18. 18. Avoiding photorespiration <ul><li>Tropical grasses evolved a method of avoiding photorespiration </li></ul><ul><li>Bundle sheath cells – keep RuBP and rubisco arranged in vascular bundles away from high oxygen concentration areas </li></ul><ul><li>Mesophyll cells containing the enzyme PEP carboxylase absorb CO 2 </li></ul>2005-2006
  19. 19. Avoiding photorespiration <ul><li>CO 2 + PEP (phosphoenolpyruvate) yields oxaloacetate converted to malate </li></ul><ul><li>Malate passed to bundle sheath cells where the CO 2 is removed </li></ul><ul><li>Light independent reactions then proceed normally </li></ul><ul><li>i.e. rubisco catalyzes RuBP + CO 2 </li></ul>2005-2006
  20. 20. CAM Plants (crassulacean acid metabolism) <ul><li>CAM plants are adapted to life in arid conditions by conserving water. </li></ul><ul><li>During the night, the CAM plant's stomata are open, allowing CO 2 to enter and be fixated as organic acids (CAM) that are stored in vacuoles . During the day the stomata are closed (thus preventing water loss), and the carbon is released to the Calvin cycle so that photosynthesis may take place. </li></ul>2005-2006
  21. 21. <ul><li>The carbon dioxide is fixed in the mesophyll cell's cytoplasm by a PEP reaction similar to that of C4 plants . But, unlike C4 plants, the resulting organic acids are stored in vacuoles for later use; that is, they are not immediately passed on to the Calvin cycle. </li></ul><ul><li>Of course, the latter cannot operate during night because the light reactions that provide it with ATP and NADPH cannot take place without light. </li></ul>2005-2006
  22. 22. <ul><li>During the day </li></ul><ul><li>The carbon in the organic acids is freed from the mesophyll cell's vacuoles and enters the chloroplast's stroma and, thus, into the Calvin cycle. </li></ul>2005-2006
  23. 23. The benefits of CAM <ul><li>The most important benefit to the plant is the ability to leave most leaf stomata closed during the day. </li></ul><ul><li>Being able to keep stomata closed during the hottest and driest part of the day reduces the loss of water through evapotranspiration , allowing CAM plants to grow in environments that would otherwise be far too dry. </li></ul>2005-2006
  24. 24. <ul><li>C3 plants , for example, lose 97% of the water they uptake through the roots to transpiration - a high cost avoided by CAM plants. </li></ul>2005-2006
  25. 25. Any Questions?? 2005-2006