(8) More complex graphical analysis involving more than one limiting factor controlling the rate of photosynthesis

Reminder: The limiting factor is one that controls the maximum possible rate of the photosynthesis reactions for given set of conditions.

• In experiments using eg Canadian pondweed, you can immerse the green weed in sodium hydrogencarbonate solution to supply the carbon dioxide (one variable) needed for photosynthesis.

• The other two photosynthesis rate variables are temperature and light intensity.

• Remember, one of the three variables must be kept constant for a given set of experiments involving the changing of the other two variables.

• Graph 4.

• Graph 4. Combined limiting factors of light intensity and temperature

  • Graph 4. Rate of photosynthesis versus light intensity at different temperatures (2 factors)

  • Initially the graph lines are linear as the rate of photosynthesis is proportional to the light intensity (see also Graph 1), so light intensity is the limiting factor.

  • However, just prior to point X on the graph, the increase in rate slows down, and finally at point X on the graph becomes horizontal, and the rate of photosynthesis reaches a maximum irrespective of the light intensity - the graph becomes horizontal.

  • This is shown by the graph line becoming horizontal, and the maximum rate is now dependent on the temperature i.e. the higher the temperature the greater the maximum rate of photosynthesis possible - the temperature has become the limiting factor.

  • For these experiments a suitable concentration of CO₂/NaHCO₃ must be chosen and kept constant!

  • -

• Graph 5.

• Graph 5. Carbon dioxide concentration and light intensity as combined limiting factors

  • Graph 5. Rate of photosynthesis versus light intensity with different carbon dioxide (CO₂ from NaHCO₃ solution) concentrations (2 factors)

  • Initially the graph lines are linear as the rate of photosynthesis is proportional to the light intensity (see also Graph 1) - so the light intensity is the limiting factor.

  • However, just prior to point X on the graph, the increase in rate slows down, and finally at point X the graph has becomes horizontal, and the rate of photosynthesis reaches a maximum irrespective of the light intensity.

  • This is shown by the graph line becoming horizontal, and the maximum rate is now dependent on the concentration of the carbon dioxide (from the 1%-3% NaHCO₃ solution) i.e. the higher concentration of carbonate/carbon dioxide, the greater the maximum rate of photosynthesis possible.

  • For these experiments a suitable temperature must be chosen and kept constant! (eg lab. temp. of ~20-25^oC)

  • -

• Graph 6.

• Graph 6. Carbon dioxide concentration and temperature combined as limiting factors

  • Graph 6. Rate of photosynthesis versus CO₂/NaHCO₃ at different temperatures (2 factors)

  • Initially the graph lines are linear as the rate of photosynthesis is proportional to the carbon dioxide (or sodium hydrogencarbonate) concentration (see also Graph 3).

  • However, just prior to point X on the graph, the increase in rate slows down, and finally at point X the graph becomes, and the rate of photosynthesis reaches a maximum irrespective of the carbon dioxide concentration.

  • This is shown by the graph line becoming horizontal, and the maximum rate is now dependent on the temperature i.e. the higher the temperature the greater the maximum rate possible.

  • For these experiments a suitable light intensity must be chosen and kept constant!

  • -

• Graph 7.

• Graph 7. Light intensity and carbon dioxide concentration combined as limiting factors

  • Graph 7. Rate of photosynthesis versus CO₂/NaHCO₃ concentrations at light intensities (2 factors)

  • Initially the graph lines are linear as the rate of photosynthesis is proportional to the carbon dioxide (or sodium hydrogencarbonate) concentration (see also Graph 3).

  • However, just prior to point X on the graph, the increase in rate slows down, and finally at point X the graph becomes horizontal, and the rate of photosynthesis reaches a maximum irrespective of the carbon dioxide concentration.

  • This is shown by the graph line becoming horizontal, and the maximum rate is now dependent on the light intensity i.e. the higher the light intensity the greater the maximum rate possible.

  • For these experiments a suitable temperature (eg 20-25^oC) must be chosen and kept constant!

  • -

• Graph 8.

• Graph 8 Chlorophyll combined with light intensity combined as limiting factors

  • Graph 8 shows the rate of photosynthesis for two plants A and B.

  • We have looked at the way in which light, temperature or carbon dioxide can be limiting factor.

  • A shortage of chlorophyll can also be the 4th limiting factor.

  • Assume the graph for plant A is typical of most plants which are not adapted to live in shaded areas and receive an sample of sunlight i.e. do not live in a very shaded area.

    • In this case the rate of photosynthesis is limited by temperature or carbon dioxide concentration in the air.

  • Some plants, like plant B, live in continuous shade i.e. a low level of light intensity.

    • These plants have adapted to these conditions by evolving to grow a higher ratio of leaves to roots compared to other plants.

    • The leaves are larger and thinner with a greater surface area so more chlorophyll in chloroplasts is available to absorb light, so increasing the plant's photosynthesis efficiency.

    • The graph for B show a faster initial rate of photosynthesis because of the higher concentration of chlorophyll, but the rate of photosynthesis levels off before that of plant A as a limiting factor comes into play.

      • The limiting factor might a low temperature in a shaded area,

      • or carbon dioxide level if there is no air movement.

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Keywords, phrases and learning objectives for this part on photosynthesis

In the context of photosynthesis, be able to analyse, understand and explain more complex graphical analysis involving more than one limiting factor controlling the rate of photosynthesis.

This may involve combined factors of light intensity, temperature, carbon dioxide level and chlorophyll concentration in the chloroplasts.

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