(5) An introduction to what factors affect the rate of photosynthesis?

The rate of photosynthesis is usually limited by three main environmental conditions - factors:

• (i) Shortage of light (usually lack of sunlight) slows photosynthesis - since the greater the light intensity, the greater the rate of photosynthesis in the green chloroplast in plant cells.

• (ii) Low temperature, slows down the rate of photosynthesis - a general rule for all chemical reactions

  • A combination of both (i) and (ii) will cause very different rates between photosynthesis in winter (less sunlight time, less intense light, slower) compared to summer (more sunlight time, more intense light, faster).

  • At night, light is the limiting factor, in winter its usually the temperature in daylight.

  • If the temperature gets too high photosynthesis will slow down due to enzyme damage.

• (iii) A shortage of carbon dioxide will also slow down the rate of photosynthesis but you can artificially increase it by pumping CO₂ into a greenhouse structure.

  • If there is sufficient light and the temperature not too low, the ambient carbon dioxide concentration becomes the limiting factor.

• So, three factors affecting the rate of photosynthesis that can be investigated in the laboratory - see Parts (9) to (11).

• (iv) However, under some circumstances the essential green pigment chlorophyll might be the limiting factor too.

  • Lack of chlorophyll/chloroplasts in the plant cells reduce the plant's capacity to photosynthesise.

    • Stressed or damaged plants may turn pale yellow or develop spots from a fungus, bacteria or virus.

    • Plants maybe affected by disease eg halo blight, tobacco mosaic virus, poor nutrition - lack of vital minerals.

    • Also (v) lack of water denatures cells and plants droop, reducing photosynthesis, and eventually die.

    • Any of these factors can cause damage to chloroplasts or the cell cannot make enough chlorophyll.

    • Therefore the plant cell capacity to absorb sunlight is reduced weakening the plants growth and development.

    • See notes on Plant diseases and defences against pathogens and pests

• (v) Strictly speaking, lack of water is another factor, but that does affect the whole plant.

• Light intensity, temperature and the availability of carbon dioxide interact and in practice any one of them may be the factor that limits the speed (rate) photosynthesis.

  • You can relate the principle of limiting factors to the economics of enhancing the following conditions in greenhouses.

  • You can carry out laboratory experiments to measure the rate of photosynthesis under various conditions i.e. changing any of the three factors and keeping the other two factors constant.

  • These experiments and graphical data analysis are discussed in detail in other Parts of this section.

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

For graph data and a more detailed discussions see parts

(6) Factors controlling the rate of photosynthesis - detailed discussion of typical data graphs

(8) More complex graphs involving more than one limiting factor controlling photosynthesis

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Key points - factors that affect the rate of photosynthesis in plants

Source of information is based on textbooks and syllabus-specifications for students taking the AQA GCSE, Edexcel GCSE and OCR GCSE level biology examinations (~US grades 9-10)

Factors Affecting the Rate of Photosynthesis

Photosynthesis is influenced by several environmental factors.

The rate at which plants photosynthesise can speed up or slow down depending on these conditions.

The main limiting factors are:

1. Light intensity

2. Carbon dioxide concentration

3. Temperature

4. Chlorophyll levels (in some cases)

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1. Light Intensity

• More light = faster photosynthesis (up to a point).

• Light provides the energy needed for the reaction.

• At low light, photosynthesis is limited because there’s not enough energy.

• Once light is no longer the limiting factor, increasing it won’t increase the rate any further.

Graphs show a rapid rise at first, then a plateau when something else becomes limiting.

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2. Carbon Dioxide (CO₂) Concentration

• CO₂ is one of the raw materials in the photosynthesis equation.

• Increasing CO₂ generally increases the rate of photosynthesis.

• Eventually the rate levels off when another factor (like light or temperature) becomes limiting.

CO₂ levels can be raised artificially in greenhouses to boost plant growth.

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3. Temperature

• Photosynthesis is controlled by enzymes—temperature affects enzyme activity.

• As temperature increases, the rate rises until an optimum is reached (often around 25–35°C).

• Above the optimum, enzymes can become denatured, and the rate drops sharply.

Important in agriculture: Temperature is monitored to avoid overheating crops.

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4. Chlorophyll Concentration

• Chlorophyll absorbs light for photosynthesis.

• If a plant is deficient in minerals like magnesium, it makes less chlorophyll.

• Less chlorophyll = less light absorbed = slower photosynthesis.

Common in plants suffering from disease or nutrient deficiency.

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Investigating the Factors Practically

• Use aquatic plants like Elodea to measure bubble production or oxygen output.

• Change light intensity (by moving the lamp), CO₂ levels (adding sodium hydrogencarbonate), or temperature (using a water bath).

Plotting results on a graph helps identify which factor is limiting.

See sections (6), (8), (9) and (10)

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5. Limiting Factor Defined

A limiting factor is the factor in shortest supply that prevents the rate of photosynthesis from increasing further.

For example:

• On a sunny day, CO₂ or temperature may be limiting.

• In the evening, light might be the limiting factor.

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Summary Table

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Exam Tips

• Be ready to explain and interpret graphs showing rate changes.

• Use the term "limiting factor" accurately in your answers.

• Know how plants are grown in controlled environments like greenhouses.

• Revise practical investigations and what the variables represent.

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Keywords, phrases and learning objectives for this part on photosynthesis

Be able to describe and explain factors affecting the rate of photosynthesis including light intensity, carbon dioxide concentration, ambient temperature, formation of chlorophyll in chloroplasts and water limiting environmental conditions.

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