1. Chapter 10: Photosynthesis AP Biology Mrs. Ramon 2010

2. Overview of Photosynthesis Nutritional modes Nutritional modes Autotrophs Chemoautotrophs Heterotrophs

3. Plant Structure and Function Fig. 10.2

5. Where does the oxygen come from? O 2 given off by plants comes from H 2 O, not CO 2. O 2 given off by plants comes from H 2 O, not CO 2. Before the 1930s: Before the 1930s: Step 1: CO 2  C + O 2 Step 1: CO 2  C + O 2 Step 2: C + H 2 O  CH 2 O Step 2: C + H 2 O  CH 2 O C.B. van Niel C.B. van Niel In a certain bacteria hydrogen sulfide (H 2 S) is used in photosynthesis In a certain bacteria hydrogen sulfide (H 2 S) is used in photosynthesis Yellow globules of sulfur are produced as a waste. Yellow globules of sulfur are produced as a waste. Van Niel proposed this reaction: Van Niel proposed this reaction: CO 2 + 2H 2 S  CH 2 O + H 2 O + 2S CO 2 + 2H 2 S  CH 2 O + H 2 O + 2S

6. From water?!? Applied to plants: Applied to plants: CO 2 + 2H 2 O  CH 2 O + H 2 O + O 2 CO 2 + 2H 2 O  CH 2 O + H 2 O + O 2 Confirmation of van Niel’s hypothesis: Confirmation of van Niel’s hypothesis: Using 18 O, a heavy isotope, as a tracer. Using 18 O, a heavy isotope, as a tracer. Label either CO 2 or H 2 O. Label either CO 2 or H 2 O. The 18 O label only appeared if water was the source of the tracer. The 18 O label only appeared if water was the source of the tracer. Essentially, hydrogen extracted from water is incorporated into sugar and the oxygen released to the atmosphere (respiration). Essentially, hydrogen extracted from water is incorporated into sugar and the oxygen released to the atmosphere (respiration).

7. Chapter 10: Photosynthesis Photosynthesis is a redox reaction. Photosynthesis is a redox reaction. Reverses the direction of electron flow in respiration. Reverses the direction of electron flow in respiration. Water is split and electrons transferred with H + from water to CO 2, reducing it to sugar. Water is split and electrons transferred with H + from water to CO 2, reducing it to sugar. Polar covalent bonds (unequal sharing) are converted to nonpolar covalent bonds (equal sharing). Polar covalent bonds (unequal sharing) are converted to nonpolar covalent bonds (equal sharing). Light boosts the potential energy of electrons as they move from water to sugar. Light boosts the potential energy of electrons as they move from water to sugar.

10. Excited electrons are unstable. Excited electrons are unstable. Generally, they drop to their ground state in a billionth of a second, releasing heat energy. Generally, they drop to their ground state in a billionth of a second, releasing heat energy. Some pigments, including chlorophyll, release a photon of light, in a process called fluorescence, as well as heat. Some pigments, including chlorophyll, release a photon of light, in a process called fluorescence, as well as heat.

11. Photosynthetic Pigments Thomas Engelmann (1883) Thomas Engelmann (1883) Different segments of a filamentous alga were exposed to different wavelengths of light. Different segments of a filamentous alga were exposed to different wavelengths of light. Areas receiving wavelengths favorable to photosynthesis should produce excess O 2. Areas receiving wavelengths favorable to photosynthesis should produce excess O 2. Engelmann used the abundance of aerobic bacteria clustered along the alga as a measure of O 2 production. Engelmann used the abundance of aerobic bacteria clustered along the alga as a measure of O 2 production.

12. Absorption Spectrum Pigments differ in their absorption spectrum. Pigments differ in their absorption spectrum. Chlorophyll a Chlorophyll a Best: red and blue Best: red and blue Least: green. Least: green. Action spectrum Action spectrum Measures changes in some measure of photosynthetic activity (for example, O 2 release) as the wavelength is varied. Measures changes in some measure of photosynthetic activity (for example, O 2 release) as the wavelength is varied.

13. Two types: Two types: Photosystem I Photosystem I 700nm 700nm Photosystem II Photosystem II 680nm 680nm The differences between these reaction centers (and their absorption spectra) lie in the proteins associated with each reaction center. The differences between these reaction centers (and their absorption spectra) lie in the proteins associated with each reaction center. These two photosystems work together to use light energy to generate ATP and NADPH. These two photosystems work together to use light energy to generate ATP and NADPH. Photosystems

14. Chapter 10: Photosynthesis

15. Two routes for electron flow: Two routes for electron flow: Noncyclic electron flow produces both ATP and NADPH. Noncyclic electron flow produces both ATP and NADPH. Cyclic Electron Flow Cyclic Electron Flow Noncyclic electron flow produces ATP and NADPH in roughly equal quantities. Noncyclic electron flow produces ATP and NADPH in roughly equal quantities. Calvin cycle consumes more ATP than NADPH. Calvin cycle consumes more ATP than NADPH. Cyclic electron flow generates enough surplus ATP to satisfy the higher demand for ATP in the Calvin cycle Cyclic electron flow generates enough surplus ATP to satisfy the higher demand for ATP in the Calvin cycle Electron Flow

19. Three phases: Three phases: 1. Carbon fixation 2. Reduction 3. Regeneration Calvin Cycle – Where sugar is FINALLY made

20. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 10.17.1

21. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 10.17.2

22. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 10.17.3

23. Alternative C Fixation Plan B Plan B Regulation of stomata can cause problems for photosynthesis Regulation of stomata can cause problems for photosynthesis CO 2 levels drop as CO 2 is consumed in the Calvin cycle. CO 2 levels drop as CO 2 is consumed in the Calvin cycle. O 2 levels rise as the light reaction converts light to chemical energy. O 2 levels rise as the light reaction converts light to chemical energy. While rubisco normally accepts CO 2, when the O 2 / CO 2 ratio increases (on a hot, dry day with closed stomata), rubisco can add O 2 to RuBP. While rubisco normally accepts CO 2, when the O 2 / CO 2 ratio increases (on a hot, dry day with closed stomata), rubisco can add O 2 to RuBP.

24. When rubisco adds O 2 to RuBP, RuBP splits into a three-carbon piece and a two-carbon piece in a process called photorespiration. When rubisco adds O 2 to RuBP, RuBP splits into a three-carbon piece and a two-carbon piece in a process called photorespiration. The two-carbon fragment, glycolate, is exported from the chloroplast and degraded/oxidized to CO 2 by peroxisomes. The two-carbon fragment, glycolate, is exported from the chloroplast and degraded/oxidized to CO 2 by peroxisomes. Unlike normal respiration, this process produces no ATP, nor additional organic molecules. Unlike normal respiration, this process produces no ATP, nor additional organic molecules. Photorespiration decreases photosynthetic output by siphoning organic material from the Calvin cycle. Photorespiration decreases photosynthetic output by siphoning organic material from the Calvin cycle. Reversing percentage of carbon fixation Reversing percentage of carbon fixation Photorespiration can drain away as much as 50% of the carbon fixed by the Calvin cycle on a hot, dry day. Photorespiration can drain away as much as 50% of the carbon fixed by the Calvin cycle on a hot, dry day. Why Rubisco isn’t the smartest enzyme

25. C 4 Photosynthesis C 4 Photosynthesis C 4 plants bypass photorespiration to increase the efficiency of photosynthesis C 4 plants bypass photorespiration to increase the efficiency of photosynthesis The C 4 plants use a different enzyme for C fixation The C 4 plants use a different enzyme for C fixation The key enzyme, phosphoenolpyruvate (PEP) carboxylase, adds CO 2 to phosphoenolpyruvate (PEP) to form oxaloacetetate (4-C molecule). The key enzyme, phosphoenolpyruvate (PEP) carboxylase, adds CO 2 to phosphoenolpyruvate (PEP) to form oxaloacetetate (4-C molecule). PEP carboxylase has a very high affinity for CO 2 (does not react with O 2 )and can fix CO 2 efficiently when rubisco cannot, i.e. on hot, dry days when the stomata are closed. PEP carboxylase has a very high affinity for CO 2 (does not react with O 2 )and can fix CO 2 efficiently when rubisco cannot, i.e. on hot, dry days when the stomata are closed. It’s all about contigencies

26. In C 4 plants, mesophyll cells on the surface of leaves pump CO 2 into the bundle sheath cells, keeping CO 2 levels high enough for rubisco to accept CO 2 and not O 2. In C 4 plants, mesophyll cells on the surface of leaves pump CO 2 into the bundle sheath cells, keeping CO 2 levels high enough for rubisco to accept CO 2 and not O 2.

27. CAM CAM A second strategy to minimize photorespiration is found in succulent plants, cacti, pineapples, and several other plant families. A second strategy to minimize photorespiration is found in succulent plants, cacti, pineapples, and several other plant families. These plants, known as CAM plants for crassulacean acid metabolism (CAM), open stomata during the night and close them during the day. These plants, known as CAM plants for crassulacean acid metabolism (CAM), open stomata during the night and close them during the day. Temperatures are typically lower at night and humidity is higher. Temperatures are typically lower at night and humidity is higher. During the night, these plants fix CO 2 into a variety of organic acids in mesophyll cells. During the night, these plants fix CO 2 into a variety of organic acids in mesophyll cells. During the day, the light reactions supply ATP and NADPH to the Calvin cycle and CO 2 is released from the organic acids. During the day, the light reactions supply ATP and NADPH to the Calvin cycle and CO 2 is released from the organic acids. Alternatives