UK GCSE level age ~14-16, ~US grades 9-10 Biology revision notes re-edit 21/05/2023 [SEARCH]

Photosynthesis: 6. The rate of photosynthesis and graphs illustrating the limiting factors of light, temperature and carbon dioxide

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(6) Factors controlling the rate of photosynthesis - detailed discussion of typical data graphs for light intensity, ambient temperature and carbon dioxide level in the air

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

1. LIGHT INTENSITY

  • graph of rate of photosynthesis speed versus light intensity shape of photosynthesis graph explained Graph 1. Light intensity limitation

    • Light energy is needed for photosynthesis, so as the light intensity increases, the rate of photosynthesis chemical reactions steadily increases in a linear manner - 1st part of the graph is 'light limiting'.

      • More light, more molecules 'energised' to react.

      • BUT, at the point where the graph becomes horizontal, light is no longer the limiting factor.

    • However, eventually the rate levels off to become constant due to limitation of the carbon dioxide concentration (too low) or the temperature (too low) and any increase in light intensity has no further effect on the rate of photosynthesis for plant growth.

      • Two points to bear in mind when studying any of the graphs dealing with photosynthesis.

      • Since the graph line has become horizontal (flattened out, constant rate), this also means that light intensity is no longer the limiting factor - you must increase carbon dioxide concentration or temperature to increase the rate of photosynthesis - in other words you need increase some other factor.

      • Remember: Whenever the graph line on a photosynthesis graph becomes horizontal, a limiting factor is coming into play.

    • Light intensity falls to ~zero at night and there is much less light in winter, so these place limits on photosynthesis.

      • Plants have adapted to live in shaded areas by having larger and thinner leaves to increase the number of chlorophyll molecules to absorb light (see Part 8. graph 8).

    • Greenhouse design/operation and light intensity.

      • Lots of glass window panes to let light in.

      • Site the greenhouse in a non-shaded area.

      • At night artificial light can be supplied.

      • However, the light level with have its limit (either sunlight or artificial light at night), so for maximum effect you may still need a warm temperature and a fresh supply of carbon dioxide.

      • For more on this read the section

      • How to successfully operate a commercial  greenhouse!

    • Light initiated reactions - effect of changing intensity (GCSE chemistry notes)

 

  • 2. TEMPERATURE

  • graph of rate of photosynthesis speed versus temperature of solution gas shape of photosynthesis graph explained Graph 2. Temperature limitation

    • Photosynthesis chemical reactions cannot happen without the help of enzymes.

    • Raising the temperature gives the molecules more kinetic energy so more of them react on collision, and initially, you get the expected (exponential) increase in the speed of the photosynthesis reaction - initially an accelerating curve upwards (non-linear) with increase in temperature increasing plant growth..

    • However, too high a temperature is just as bad as too a low temperature (which would be too slow).

    • At temperatures over 40oC enzymes involved in the process are increasingly destroyed, so photosynthesis slows down and eventually stops because the photosynthesis enzymes are destroyed.

    • The denaturing of the protein structure caused by the higher temperatures affects the active sites on enzymes (x-reference key and lock mechanism) and they can no longer catalyse the photosynthesis reactions.

    • A graph of rate of photosynthesis versus temperature rises at first (usual rate of chemical reaction factor), goes through a maximum (optimum temperature) and then falls as the enzymes are becoming increasingly denatured and eventually cease to function.

      • The final shape of the graph is due to the combination of the two graph trends from increasing rate of reaction versus increase denaturing, both coincident with increase in temperature.

    • Greenhouse design/operation and temperature

      • Ideally in greenhouses you would want the optimum temperature, a constant adequate supply of carbon dioxide and plenty of light - hence the use of transparent glass!

      • A greenhouse warms up by trapping the heat radiation from the sun - the 'greenhouse effect'.

      • BUT take care that the greenhouse does not get too hot eg by opening ventilation systems or putting up shades.

      • In cold weather, heaters might be employed in a greenhouse because the temperature may be too low for efficient photosynthesis for plant growth - but heaters increase cost of production.

      • If the heaters are not electric and burn a fuel like paraffin, then lots of carbon dioxide is produced - quite handy, two factors catered for at the same time!

      • For more on this read section How to successfully operate a commercial  greenhouse!

 

  • 3. CARBON DIOXIDE CONCENTRATION

  • graph of rate of photosynthesis versus level of carbon dioxide concentration shape of photosynthesis graph explained Graph 3. Carbon dioxide limitation

    • Carbon dioxide is needed for photosynthesis, so as the carbon dioxide concentration increases, the rate of photosynthesis chemical reactions steadily increases in a linear manner - initially the reaction rate of photosynthesis is directly proportional to CO2 concentration (can be in air or water)..

    • However, eventually the rate levels off due to limitation of the light intensity (too low) or the temperature (can be too low or too high) no matter what the increase in the CO2 concentration.

      • Since the graph line has become horizontal (flattened out), this also means that carbon dioxide concentration is no longer the limiting factor - you must increase light intensity or temperature to increase the rate of photosynthesis.

      • You should note that the concentration of carbon dioxide in air is only ~0.04%, and is often the limiting factor, especially on warm bright sunny days  ..

      • BUT, short dull winter days (low light intensity) and low temperature (slows chemical reactions) can also be the limiting factors.

    • Greenhouse design/operation and carbon dioxide concentration

      • If the ambient temperature is warm and the plants/greenhouse in bright sunshine, the limiting factor might be the concentration of carbon dioxide in air.

      • You do need some ventilation or the level of carbon dioxide gas will fall if the air is not replenished as the carbon dioxide is used up by the plants.

      • BUT, for maximum effect you need a warm temperature, plenty of light and extra CO2 if you can supply it!

      • For more on this read the section How to successfully operate a commercial  greenhouse!

For more on photosynthesis graphs see:

More complex graphs involving more than one limiting factor controlling rate of photosynthesis


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