Factors Affecting The Rate Of Photosynthesis (AQA GCSE Combined Science: Synergy: Life & Environmental Sciences): Revision Note

Limiting factors

• In order to photosynthesise a plant needs light, water and carbon dioxide

• The availability of light and carbon dioxide can affect how much and how quickly (the rate) photosynthesis occurs

  • Although water is necessary for photosynthesis, it is not considered a limiting factor as the amount needed is relatively small compared to the amount of water transpired from a plant so there is hardly ever a situation where there is not enough water for photosynthesis

• Other environmental factors such as temperature and the amount of chlorophyll in the chloroplasts can also affect the rate of photosynthesis

Temperature

• The temperature of the environment affects how much kinetic energy all particles have – so temperature affects the speed at which carbon dioxide and water move through a plant

• The lower the temperature, the less kinetic energy particles have, resulting in fewer successful collisions occurring over a period of time

• Increasing temperature increases the kinetic energy of particles, increasing the likelihood of collisions between reactants and enzymes which results in the formation of products

• At higher temperatures, however, enzymes that control the processes of photosynthesis can be denatured (where the active site changes shape and is no longer complementary to its substrate) – this reduces the overall rate

Graph showing the effect of temperature on the rate of photosynthesis

Light

• The intensity of the light available to the plant will depend on the amount of energy that it has to carry out photosynthesis

• The more light a plant receives, the faster the rate of photosynthesis

• This trend will continue until some other factor required for photosynthesis prevents the rate from increasing further because it is now in short supply

Graph showing the effect of light intensity on the rate of photosynthesis

At low light intensities, increasing the intensity will initially increase the rate of photosynthesis. At a certain point, increasing the light intensity stops increasing the rate. The rate becomes constant regardless of how much light intensity increases as something else is limiting the rate.

Carbon dioxide concentration

• Carbon dioxide is one of the raw materials required for photosynthesis

• This means the more carbon dioxide that is present, the faster the reaction can occur

• This trend will continue until some other factor required for photosynthesis prevents the rate from increasing further because it is now in short supply

A graph showing the effect of the concentration of carbon dioxide on the rate of photosynthesis

Interactions of Limiting Factors (HT Only)

Higher tier only

• More than one limiting factor can have an effect on the rate of photosynthesis

• Graphs may show the effect of two factors interacting:

 The rate of photosynthesis increases with increasing light intensity, temperature and carbon dioxide

 At the start of the graph, the rate of photosynthesis is limited by the light intensity so both lines are showing the same rate.

As the light intensity increases the rate of photosynthesis at 15℃ is lower than 25℃.

Both lines level off, this shows that light intensity is no longer the limiting factor.

• Graphs may show the interactions between three different factors, the graph below shows the relationship between temperature, carbon dioxide as light intensity is increased:

The rate of photosynthesis increases with increasing light intensity, temperature and carbon dioxide

 All three experiments level off when light intensity is no longer the limiting factor.

In the top line, this has the highest temperature and concentration of carbon dioxide so the rate of photosynthesis is much higher.

In experiment 2, the concentration of carbon dioxide is the limiting factor.

In this graph, the rate of photosynthesis is controlled by carbon dioxide levels.

Light Intensity (HT Only)

Higher tier only

• The inverse square law shows the relationship between light intensity and distance.

• As the distance increases the light intensity decreases. This means that they are inversely proportional to each other

• Light intensity and distance are inversely proportional to each other

• This means that as the distance doubles you decrease the intensity of the light will be four times less

• This is called the inverse square law and shown by the equation below:

Growing in a Greenhouse (HT Only)

Higher tier only

• Commercial horticulturists will grow their plants in a greenhouse

• This means that they are able to control as many of the limiting factors of photosynthesis as possible

• Limiting factors are important in the economics of enhancing the conditions in greenhouses to gain the maximum rate of photosynthesis while still maintaining profit

  • Keeping plants in a greenhouse has associated costs, but the increased yield of the crop and fact that the crop can be harvested more frequently, means that the farmer will make more money

• The levels of heat, light, water, carbon dioxide and nutrients are carefully controlled so only the smallest amounts needed are used so that farmers are not wasting money

  • Eg. spending money on increasing the concentration of carbon dioxide beyond a point when some other factor limits the rate of photosynthesis is a waste

The farmers can increase yield in a greenhouse but this does have an increased cost