Sign up to access the complete lesson and track your progress!
Unlock This CoursePhotosynthesis is like a recipe - you need all the right ingredients in the right amounts. Just as you can't make more cakes if you run out of flour (even if you have plenty of eggs and butter), plants can't photosynthesise faster if they're short of light, carbon dioxide, or the right temperature. These are called limiting factors and understanding them is crucial for helping plants grow better.
Key Definitions:
Imagine three taps filling a bucket with holes. The rate water stays in the bucket depends on the slowest tap, not the fastest ones. Similarly, photosynthesis rate depends on whichever factor is most limiting - light, CO₂, or temperature.
Plants need three key conditions to photosynthesise effectively. When any one of these is in short supply, it becomes the limiting factor that controls how fast photosynthesis can happen.
Light provides the energy that drives photosynthesis. Without enough light, plants simply can't make food fast enough, no matter how much CO₂ or perfect temperature they have.
High light intensity means fast photosynthesis - up to a point. Eventually, other factors become limiting.
Low light intensity slows photosynthesis dramatically. This is why plants grow poorly in shade.
In darkness, photosynthesis stops completely. Plants must use stored energy instead.
As light intensity increases, the rate of photosynthesis increases proportionally - but only up to a certain point. Once light is no longer the limiting factor, increasing it further won't help. This creates the characteristic curve you'll see in graphs: steep rise, then levelling off.
Carbon dioxide is one of the raw materials for photosynthesis. Plants take in CO₂ from the air through their stomata (tiny pores in leaves). Normal air contains about 0.04% carbon dioxide - quite a small amount!
Commercial greenhouse growers often pump extra CO₂ into their greenhouses to boost photosynthesis rates. They might increase CO₂ levels to 0.1% - more than double the normal amount. This can increase crop yields by 20-30%, making it very profitable for expensive crops like tomatoes and cucumbers.
Just like with light, increasing CO₂ concentration boosts photosynthesis rate - but only until something else becomes limiting. In a graph, you'll see the same pattern: rapid increase, then plateau.
Temperature affects photosynthesis because the process depends on enzymes and enzymes are very sensitive to temperature changes. This makes temperature the trickiest limiting factor to understand.
When it's cold, enzymes work slowly. Photosynthesis rate is low because the chemical reactions happen sluggishly. This is why plants grow poorly in winter.
When it's too hot, enzymes change shape and stop working properly (they denature). Photosynthesis rate drops rapidly. This is why plants wilt on very hot days.
The optimum temperature for most plants is around 25-30°C. Below this, rate increases with temperature. Above this, rate decreases rapidly. This creates a bell-shaped curve on graphs - very different from the light and CO₂ patterns.
Scientists use graphs to show how limiting factors affect photosynthesis. Learning to read these graphs is essential for understanding how plants respond to their environment.
When the line rises steeply, that factor is limiting photosynthesis. More of it means faster photosynthesis.
When the line levels off, that factor is no longer limiting. Something else is now the bottleneck.
For temperature graphs, a steep fall shows enzymes denaturing. The plant is being damaged by heat.
The key skill is identifying which part of the graph shows each factor being limiting. Look for where increasing that factor makes a difference and where it doesn't.
Dutch tomato growers use sophisticated computer systems to control all three limiting factors. They use LED lights for perfect light intensity, CO₂ generators to triple normal CO₂ levels and precise heating/cooling for optimum temperature. Result? They can grow 500 tonnes of tomatoes per hectare - about 10 times more than outdoor farming!
Understanding limiting factors isn't just academic - it's the foundation of modern agriculture and helps explain many plant behaviours you see every day.
Farmers and growers use limiting factor knowledge to maximise crop yields and profits. Different approaches work for different situations and budgets.
Farmers can't control light or temperature easily, but they can increase CO₂ by adding organic matter to soil. Decomposing compost releases CO₂ right where plant roots need it.
Complete control is possible: artificial lights, CO₂ injection, heating and cooling systems. Expensive but very effective for high-value crops.
You can observe limiting factors in action all around you, once you know what to look for.
Forest floors: Plants here are usually light-limited. That's why they have large, thin leaves to catch every photon and why forest floors have few plants.
Deserts: Surprisingly, desert plants are often CO₂-limited, not water-limited for photosynthesis. They close their stomata during hot days to save water, which also keeps CO₂ out.
Winter gardens: Even evergreen plants photosynthesise slowly in winter because low temperatures limit enzyme activity, even when there's bright sunshine.
Several misunderstandings about limiting factors can trip up students. Let's clear these up:
Wrong! Once a factor stops being limiting, adding more won't help. You can't make plants photosynthesise faster by blasting them with intense light if they're already CO₂-limited.
The most efficient approach is to ensure no single factor is severely limiting. Balance gives better results than extremes.
In exam questions, look for clues about conditions. 'Dim light' suggests light is limiting. 'Cold day' suggests temperature is limiting. 'Poorly ventilated greenhouse' suggests CO₂ is limiting. Always explain your reasoning clearly.