(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.

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1. LIGHT INTENSITY - importance and limitations

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• Graph 1. Increasing light intensity increasing rate of photosynthesis, but may be limited by too low a concentration of carbon dioxide or too low a temperature

  • Visible light energy photons are 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)

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• 2. TEMPERATURE - its importance and limitations

• 

• Graph 2. Increasing temperature increasing rate of photosynthesis, but may be limited by degradation of the enzyme at higher temperature

  • 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..

    • Effect of changing the temperature of reactants (GCSE chemistry notes)

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

  • At temperatures over 40^oC enzymes involved in the process are increasingly denatured, so photosynthesis slows down and eventually stops because the photosynthesis enzymes are degraded as bonds in the protein molecule are broken.

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

    • See notes on enzymes and temperature effect

  • 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 e.g. 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 (7) How to successfully operate a commercial  greenhouse!

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• 3. CARBON DIOXIDE CONCENTRATION - its importance and limitations

• 

• Graph 3. Increasing the carbon dioxide concentration increases the rate of photosynthesis, but may be limited by too low a light intensity or too low a temperature

  • 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)..

    • Effect on rate of changing reactant concentration in a solution/gas mixture

  • 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!

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• 4. What about the concentration of chlorophyll in the cells?

  • Theoretically, the concentration of chlorophyll in the leaves could be a limiting factor.

  • A plant derived from essential nutrients including iron (needed to synthesise chlorophyll) and magnesium (part of the chlorophyll molecule), cannot produce sufficient chlorophyll.

  • Therefore there may not be enough chlorophyll for photosynthesis rate to sustain a healthy plant.

  • You could argue a graph of rate of photosynthesis versus chlorophyll concentration would look like graphs 1. or 3.

  • The limiting factor could be too low a concentration of carbon dioxide, or too high a temperature degrading the enzymes or too low a light intensity.

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For more on more complex photosynthesis graph analysis see:

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

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Key points - graphs illustrating limiting factors in photosynthesis

Information sources for Doc Brown's key points: IGCSE-GCSE biology are based on textbooks & syllabus-specifications for students taking the UK AQA, Edexcel, OCR 21st Century Science, OCR Gateway science suite biology, WJEC, CCEA and CIE GCSE physics 9-1 level science examinations.

Rate of Photosynthesis - Biology Revision Notes

1. What is the Rate of Photosynthesis?

The rate of photosynthesis refers to how quickly a plant converts carbon dioxide and water into glucose and oxygen, using light energy.

It’s affected by several limiting factors, meaning the one that’s in shortest supply and prevents the rate from increasing further.

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2. Key Limiting Factors Affecting Rate

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3. Typical Graphs and What They Show

(a) Light Intensity Graph

• Rate increases steadily with more light (more photon energy).

• Then levels off: light is no longer the limiting factor (CO₂ or temperature becomes limiting).

 (b) Carbon Dioxide Concentration Graph

• More CO₂ = faster rate (linear up to a point).

• Plateaus when light or temperature becomes limiting.

(c) Temperature Graph

• As temperature increases, rate increases due to faster enzyme activity.

• Peaks at an optimum (around 25–35°C).

• Then falls sharply as enzymes become denatured (structure is damaged by heat energy).

(d) Chlorophyll Graph?

• Chlorophyll is the important catalyst for photosynthesis.

• A lack of it e.g. from nutrient deficiency, will reduce the rate of photosynthesis.

• Rate of photosynthesis versus chlorophyll concentration might be linear?

• A similar graph to (a) or (b)?, so any of factors (a) to (c) could become the limiting factor.

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4. Understanding the Graphs – Key Features

Tip: Draw axes clearly—Y-axis = rate of photosynthesis, X-axis = varying factor.

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5. Controlling Factors in Greenhouses

In commercial greenhouses, all these factors can be controlled to:

• Maximise crop yield

• Speed up growth using artificial lighting, CO₂ enrichment, and temperature regulation

This links photosynthesis to food security and agricultural biotechnology.

See section (7) How to successfully operate a commercial  greenhouse!

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Exam Tips

• Describe graphs clearly: “Initially, the rate increases because... then it plateaus because...”

• Clearly label the axes.

  • Know how to describe and identify the limiting factors from a graph.

  Revise practical investigations, e.g. using pondweed to measure oxygen bubbles.

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Keywords, phrases and learning objectives for this part on photosynthesis

Be able to interpret graphs in graphical analysis of the rate of photosynthesis for limiting factors intensity of light, optimum temperature, carbon dioxide level concentration in air and generally the effects of changing conditions.

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