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Photosynthesis and the Environment

Photosynthesis and the Environment. Section 3.5 Page 173. Processes a photosynthetic cell undergoes: Photosynthesis Photorespiration Cellular respiration. Photosynthetic rate. 6 CO 2 + 6 H 2 O + light energy  C 6 H 12 O 6 + 6 O 2 How to measure rate experimentally?

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Photosynthesis and the Environment

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  1. Photosynthesis and the Environment Section 3.5 Page 173

  2. Processes a photosynthetic cell undergoes: • Photosynthesis • Photorespiration • Cellular respiration

  3. Photosynthetic rate 6 CO2 + 6 H2O + light energy  C6H12O6 + 6 O2 How to measure rate experimentally? • Rate of CO2usage • Rate of O2evolution

  4. But... It is difficult to measure the rate because:  necessary to measure the net uptake of CO2 or net evolution of O2

  5. Factors affecting photosynthetic rate: • Light intensity • CO2 concentration • Temperature • O2concentration Limiting Factors – can drastically alter the rate

  6. Rate-limiting step • Photosynthesis = Complex series of steps • Overall rate is determined by the slowest step in the process • The “rate-limiting step”.

  7. Limiting factors • Each of the three limiting factors affects a different rate-limiting step.

  8. A. Light intensity • Irradiance = light intensity per unit area of leaf • Light-response curve • Several experiments, at different light intensities • Constant temp • Constant [CO2]

  9. Increase irradiance, increase photosyntheticrate • Zero irradiance: - value for CO2uptake. • Only cell resp. is occurring

  10. increase irradiance, increase photosynthetic rate. • Light reactions produce NADPH • Amount of NADPH dep. on irradiance • Rate-limiting step is in Calvin Cycle, where NADPH reduces 1, 3-BPG • Calvin enzymes are not saturated

  11. Light saturation point: • The Calvin cycle reactions can’t keep up with ATP and NADPH production. • Light is no longer the limiting factor

  12. CO2 availability is the limiting factor. • If [CO2] concentration is low, the rate-limiting step is carbon fixation by rubisco. • If [CO2] is high, the enzymes of the Calvin cycle are saturated; increase in irradiance still does not matter  plateau.

  13. B. CO2 concentration • Higher CO2 concentration, higher rate of photosynthesis (plateaus at a higher rate)

  14. C. Temperature • The reactions of the Calvin Cycle are all enzyme-catalyzed. • The rate of an enzyme-catalyzed reaction is affected by temperature.

  15. Low temperatures: All Calvin enzymes work slowly. NADPH accumulates. • Intermediate temperatures: Another factor is limiting. • High temperatures: enzyme activity is affected

  16. Effect of O2 concentration • High O2 concentrations inhibit photosynthesis by competition for rubisco. • Increase the rate of O2, decrease the photo-synthetic rate (C3 plants)

  17. Photosynthetic efficiency • Def: The net amount of CO2 uptake per unit of light energy absorbed • How much light energy is converted into chemical energy? • Take the initial slope of the light-response curve

  18. As temperature increases, the rate of photorespiration increases more rapidly than the rate of photosynthesis (C3 plants)  slope is less (lower photosynthetic efficiency) at higher temperatures

  19. C4 plants: CO2 uptake rate remains constant over all temperatures • At lower temperatures, C3 plants are more efficient at fixing carbon than C4 plants photosynthetic efficiency

  20. To summarize: Limiting factors to rate of photosynthesis:

  21. O2 concentration also affects photosynthetic rate by competing with CO2 for rubisco

  22. The photosynthetic efficiency is constant for a C4 plant, over all temperatures. • At low temperatures, C3 plants have higher photosynthetic efficiencies than C4 plants.

  23. Homework Photosynthesis & the Environment • Pg. 178 #1-9 Comparing photosynthesis and cellular respiration: • Read pg. 179-181 • Do questions, pg. 182 #1-6 *note: Q5 refers to the energy profile of the respective electron transport chains. Refer to chart on pg. 181

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