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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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Processes a photosynthetic cell undergoes:

  • Photosynthesis
  • Photorespiration
  • Cellular respiration
photosynthetic rate
Photosynthetic rate

6 CO2 + 6 H2O + light energy  C6H12O6 + 6 O2

How to measure rate experimentally?

  • Rate of CO2usage
  • Rate of O2evolution

It is difficult to measure the rate because:

 necessary to measure the net uptake of CO2 or net evolution of O2

factors affecting photosynthetic rate
Factors affecting photosynthetic rate:
  • Light intensity
  • CO2 concentration
  • Temperature
  • O2concentration


Factors – can drastically alter the rate

rate limiting step
Rate-limiting step
  • Photosynthesis = Complex series of steps
  • Overall rate is determined by the slowest step in the process
    • The “rate-limiting step”.
limiting factors
Limiting factors
  • Each of the three limiting factors affects a different rate-limiting step.
a light intensity
A. Light intensity
  • Irradiance = light intensity per unit area of leaf
  • Light-response curve
    • Several experiments, at different light intensities
    • Constant temp
    • Constant [CO2]

Increase irradiance, increase photosyntheticrate

  • Zero irradiance: - value for CO2uptake.
  • Only cell resp. is occurring

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

Light saturation point:

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

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.
b co 2 concentration
B. CO2 concentration
  • Higher CO2 concentration, higher rate of photosynthesis (plateaus at a higher rate)
c temperature
C. Temperature
  • The reactions of the Calvin Cycle are all enzyme-catalyzed.
  • The rate of an enzyme-catalyzed reaction is affected by temperature.

Low temperatures: All Calvin enzymes work slowly. NADPH accumulates.

  • Intermediate temperatures: Another factor is limiting.
  • High temperatures: enzyme activity is affected
effect of o 2 concentration
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)
photosynthetic efficiency
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

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


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

to summarize
To summarize:

Limiting factors to rate of photosynthesis:


The photosynthetic efficiency is constant for a C4 plant, over all temperatures.

  • At low temperatures, C3 plants have higher photosynthetic efficiencies than C4 plants.

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