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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Limiting Factors in Photosynthesis
Plants need the right conditions to photosynthesize efficiently. When one of these conditions isn't at its optimum level, it can slow down the whole process. These conditions are called limiting factors because they limit how quickly photosynthesis can happen.
Key Definitions:
Light provides the energy for photosynthesis. Without enough light, the light-dependent reactions cannot proceed at their maximum rate.
Effect on photosynthesis: As light intensity increases, the rate of photosynthesis increases proportionally until another factor becomes limiting.
Carbon dioxide is a key reactant in photosynthesis. It combines with water to produce glucose in the light-independent reactions.
Effect on photosynthesis: Higher CO₂ concentrations lead to faster rates of photosynthesis until another factor becomes limiting.
Photosynthesis involves enzyme-controlled reactions. Like all enzymes, those involved in photosynthesis work best at their optimum temperature.
Effect on photosynthesis: As temperature increases, the rate of photosynthesis increases until an optimum is reached. Above this optimum, the rate decreases as enzymes begin to denature.
Although water is a reactant in photosynthesis, it's rarely a limiting factor in most environments. However, severe water shortage can indirectly limit photosynthesis by causing stomata to close, reducing CO₂ intake.
At any given time, the rate of photosynthesis is controlled by the factor that is furthest from its optimum level. This is known as Blackman's Law of Limiting Factors.
"When a process depends on several factors, its rate is limited by the factor in shortest supply."
This means that increasing a non-limiting factor will have no effect on the overall rate of photosynthesis until the current limiting factor is addressed.
Scientists use graphs to show how different factors affect photosynthesis. These graphs are crucial for understanding the concept of limiting factors.
When light intensity is plotted against the rate of photosynthesis, the graph typically shows:
If we increase CO₂ concentration or temperature (when they are limiting), the plateau occurs at a higher rate of photosynthesis.
When temperature is plotted against the rate of photosynthesis, the graph typically shows:
Understanding limiting factors is crucial for commercial growers who want to maximize crop yields in greenhouses and polytunnels.
Growers use artificial lighting to extend growing seasons and supplement natural light during darker months. LED lights are increasingly popular as they can provide specific wavelengths that plants use most efficiently.
CO₂ levels in greenhouses can be artificially increased to 2-3 times atmospheric levels (from ~400ppm to ~1000ppm). This is often done by burning natural gas or using CO₂ cylinders.
Heating systems maintain optimal temperatures for photosynthesis. Ventilation and cooling systems prevent overheating in summer. Computerized systems can precisely control conditions.
The Netherlands is a world leader in greenhouse technology despite its small size and cloudy climate. Dutch greenhouses use:
As a result, Dutch tomato yields can exceed 80 kg per square meter per year - about 4 times higher than open field growing.
Scientists and students can investigate limiting factors through controlled experiments. Here's how you might set up an investigation:
A common experiment uses pondweed (Elodea) to measure oxygen production as an indicator of photosynthesis rate:
Variables to control: Temperature, CO₂ concentration, same piece of pondweed, same volume of water
In real-world situations, limiting factors interact with each other in complex ways:
At higher temperatures, carbon dioxide becomes less soluble in the cell cytoplasm. This means that even though enzyme activity increases with temperature, CO₂ availability might decrease, potentially making CO₂ the limiting factor at higher temperatures.
Plants in high light environments often have higher optimum temperatures for photosynthesis. This is because they have more energy available to support the faster chemical reactions that occur at higher temperatures.
When answering questions about limiting factors:
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