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PHOTOSYNTHESIS

PHOTOSYNTHESIS. Chapter 10. Energy Intake. Heterotrophs – ingest energy-rich compounds (sugars) from other organisms Autotrophs – synthesize energy-rich compounds (sugars) from an energy source (light, chemical reactions) Photoautotrophs – synthesize sugars from sunlight (plants).

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PHOTOSYNTHESIS

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  1. PHOTOSYNTHESIS Chapter 10

  2. Energy Intake • Heterotrophs – ingest energy-rich compounds (sugars) from other organisms • Autotrophs – synthesize energy-rich compounds (sugars) from an energy source (light, chemical reactions) • Photoautotrophs – synthesize sugars from sunlight (plants)

  3. Pigments • Absorb light energy to “excite” electrons • Chlorophyll a and b→ green pigment • Chlorophyll a : P680 & P700 • Cartenoids → pink, orange, yellow pigments • Anthocyanins → red, purple, blue pigments • Xanthophylls → yellow pigment

  4. Photosynthesis 6CO2 + 6H2O + light → C6H12O6 + 6O2

  5. Chloroplast • Light-dependent reactions in thylakoid membranes • Calvin cycle in stroma

  6. Light-dependent Reactions • Overall goal: • Light from the sun energizes electrons • High energy carriers bring energy to Calvin cycle

  7. Photosystems I & II • PS I = P700 • PS II = P680 • Energized electrons transfer energy between antenna pigment molecules • Energy reaches primary electron acceptor in reaction center

  8. Noncyclic Photophosphorylation • Water splits: electron, oxygen, hydrogen • Oxygen leaves (product – we breathe in!) • Electron charged, accepted by NADP+ to create NADPH • Hydrogen builds up in thylakoid, chemiosmosis to phosphorylate ADP • NADPH and ATP carry energy to the Calvin cycle

  9. Cyclic Photophosphorylation • Electrons recycle to phosphorylate ADP (do not get accepted by NADP+) • Occurs simultaneously with Noncyclic to produce additional ATP

  10. Calvin cycle • Overall goal: • Take energy from NADPH and ATP • Store energy in chemical bonds of sugar

  11. Step 1: Carbon fixation • CO2 (1C) combines with RuBP (5C) (enzyme = rubisco) • Makes PGA (3C) • Called C3 Photosynthesis

  12. Step 2: Reduction • Reduction = accepting high-energy electrons • ATP and NADPH “drop off” high-energy electrons • Converts PGA (3C) to G3P (3C) – higher energy molecule • ADP and NADP+ return to light dependent reactions

  13. Step 3: Regeneration of RuBP • 1 G3P is used for carbohydrate synthesis (2 G3P → 1 glucose) • The remaining G3P (3C) are rearranged (using ATP) to regenerate RuBP (5C) to accept more CO2

  14. Also Known As… • Calvin-Benson cycle • Carbon reduction cycle • Light-independent reactions • Dark reactions >:( Remember that the Calvin cycle can NOT occur in the absence of light! It is dependent on ATP and NADPH from photophosphorylation (created in light-dependent reactions)

  15. Environmental Factors • Photosynthetic rate can vary, based on environmental conditions: • Light Intensity • Temperature • Oxygen concentration

  16. Light Intensity Photosynthetic rate increases as light intensity increases, due to increased excitation of electrons in the photosystems Photosystems will eventually become saturated, limiting further increase in photosynthetic rate

  17. Temperature Linked to action of enzymes Photosynthetic rate increases as temperature increases, since molecules move faster and collide with enzymes more frequently At high temperatures, however, photosynthetic rate decreases sharply as enzymes are denatured.

  18. Oxygen Concentration Photosynthetic rate decreases as oxygen increases, due to a phenomenon called PHOTORESPIRATION

  19. Photorespiration • Rubisco fixes CO2 in Calvin cycle, but it can ALSO fix O2 (photorespiration) • This is a PROBLEM! • Reduces CO2-fixing efficiency (competitive inhibition) • Products are not useful (must be processed by peroxisomes) • No glucose is produced 

  20. Photorespiration • Primarily problem for plants under water stress (exceptionally hot environments) • Stomata close to reduce water loss • Oxygen concentration builds (produced through light reactions) • Carbon dioxide is not entering the leaf and is being used up (in the Calvin cycle)

  21. Prevention of Photorespiration • Plants in hot, dry climates have developed mechanisms to prevent photorespiration: • C4 plants • CAM plants

  22. C4 Photosynthesis Calvin cycle moved to bundle sheath cells, which have lower O2 exposure than mesophyll cells In the mesophyll cell, CO2 is fixed by PEP carboxylase through the C4 cycle to create a compound that can be transported via plasmodesmata to the bundle sheath cell Higher efficiency means stomata are open less, reducing water loss Converting CO2 in this way has an energetic cost to the plant, but is worth the higher efficiency

  23. CAM Photosynthesis • Plants take in CO2 at night, allowing stomata to remain closed during the day (reduce water loss) • CO2 taken in at night, converted, and stored in vacuole • During the day, convert back to CO2 and then used in Calvin cycle • Converting CO2 in this way has an energetic cost to the plant, but is worth the higher efficiency

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