Week 11 · Lesson 1 Photosynthesis (B): light-independent reactions and the equation, starch, factors that affect the rate, vertical farming · 3 lessons
3
Light, carbon dioxide and temperature: what limits photosynthesis
Science Prep 2 Term 1, Unit 3: pp. 70–74
GoalRead graphs of the rate of photosynthesis and name the factors that limit it, and describe vertical farming.
New words
limiting factor · the factor in shortest supply, which sets the ratelight intensity · how bright the light isvertical farming · growing plants in stacked layers indoors
Explanation
The rate of photosynthesis depends on several factors. As light intensity rises, the rate first rises and then levels off, when another factor becomes the limit. The same is true for the concentration of carbon dioxide: the rate rises, then levels off. Temperature is different: the rate rises up to a best temperature, then falls, because enzymes stop working well in heat, and it stops near 50 °C. The best temperature depends on the plant; for our invented plant it is 30 °C. Water and minerals also matter. A limiting factor is the one in shortest supply: improving anything else does not help. Vertical farming grows food in stacked layers indoors, using artificial light and water with dissolved nutrients (hydroponics). It saves land and water but uses energy. Whether it is a good idea depends on the place and the cost, so people have different views.
Worked examples
Read the light graph above: the rate goes from 0 to 56 as light intensity rises from 0 to 6, then it stays near 60. Beyond intensity 8, more light does not help, so something else, such as carbon dioxide or temperature, is limiting.
This invented temperature graph rises to a peak at 30 °C and then falls to nothing at 50 °C. At 40 °C the rate is lower than at 30 °C, because the enzymes are working less well.
A farmer in a closed building grows lettuce on stacked trays with lamps, using water with nutrients. The plants get the light they need all day, no matter the weather, but the lamps use electricity.
Class activity
Graph practice, on paper only. Use these invented data: light intensity 0, 2, 4, 6, 8, 10 and rate 0, 15, 30, 40, 44, 44. Draw the graph and join the points. Mark the place where the line levels off and name another factor that could be the limit there, such as carbon dioxide or temperature.
Practice
1
What is a limiting factor?
The factor in shortest supply, which sets the rate.
2
Describe the shape of the rate against temperature graph.
It rises to a peak (best temperature), then falls to nothing.
3
Why does the rate stop rising when light intensity is raised a lot?
Another factor, such as carbon dioxide or temperature, has become the limit.
4
Give one advantage and one disadvantage of vertical farming.
It saves land and water; it uses a lot of energy for lighting.
