# 4.1.3 Interactions of Limiting Factors

Higher Tier Only

### Interactions of Limiting Factors

• 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.

Higher Tier Only

### Light Intensity

• 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:

#### Example question

• Calculate the light intensity when the distance of the plant is 30cm from the lamp
1. Use the equation:
Light intensity =  1/d2
2. Fill in the values you know:
Light intensity = 1/302
3. Calculate the light intensity:
Light intensity = 0.001 au

#### Exam Tip

Remember that ‘au’ stands for arbitrary units.

Higher Tier Only

### Growing in a Greenhouse

• 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

#### Exam Tip

You should be able to use data in the exam to relate limiting factors to the cost-effectiveness of adding heat, light or carbon dioxide to greenhouses.

Remember that spending money on increasing a factor above the point at which it has an effect on the rate of photosynthesis will be a waste of money.

### Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
Close