Limiting Factors - Photosynthesis Ep 3

The environmental conditions must be just right for photosynthesis to take place at the fastest possible rate. Today we’re going to take a look at how light intensity, temperature and carbon dioxide (CO₂) concentration can affect and limit the rate of photosynthesis. They will also affect plant growth, because photosynthesis produces glucose. Plants require glucose for respiration and to build other molecules such as starch, cellulose, lipids and amino acids.

Light intensity and wavelength

If you remember from the article about the light-dependent reaction, light was absorbed by the photosynthetic pigments (e.g. chlorophyll) in the photosystems. However different photosynthetic pigments absorb different wavelengths of light. Plants appear green because the photosynthetic pigments reflect green light but absorb red and blue light. Therefore it is important to remember that the light not only needs to be of the right intensity, but also the right wavelength.

As light intensity increases, so does the rate of photosynthesis because more light energy is available for the light-dependent reaction. However this is only true up to a saturation point when there is no further increase in rate. At the saturation point, something else (probably temperature or carbon dioxide concentration) has become the limiting factor. If the light intensity is too low, the light-dependent stage will be limited. Remember that the light-dependent reaction produces ATP and reduced NADP which are essential for the light-independent reaction. Specifically, they are needed for the conversion of GP to triose phosphate in the Calvin cycle. So low light intensity would mean an increase in GP but a decrease in TP and RuBP (see diagram further down the page).

Temperature

Remember that temperature affects the rate of enzyme-controlled reactions, and that photosynthesis requires enzymes such as Rubisco. The optimum temperature for photosynthetic enzymes is around 25°C, so that is the temperature at which photosynthesis is at its highest rate. Similar to many other enzymes, if the temperature is too high (about 45°C) the photosynthetic enzymes will denature. But even before 45°C is reached, the rate of photosynthesis slows down. This is due to closing of the stomata – if the temperature is high, the plant tries to reduce water loss through transpiration by closing the stomata. Consequently, less CO₂ can enter the leaves.

At lower temperatures there is not enough heat energy to be transferred to kinetic energy so less successful collisions occur between enzyme and substrate. Although you only need to know about Rubisco specifically for A-Level biology, there are enzymes involved at all stages of the Calvin cycle. So at low temperatures (and at high temperatures if the enzymes are denatured), the concentrations of all the Calvin cycle compounds will drop because none of the reactions can be catalysed.

Carbon dioxide concentration

CO₂ only makes up 0.04% of the atmospheric gases, but if increased artificially the optimum concentration for photosynthesis is about 0.4%. Beyond that point the stomata will close. If you really want to know why that is, try reading this research paper. If carbon dioxide concentration is low, conversion of RuBP to GP in the Calvin cycle is limited. Consequently, GP and triose phosphate are reduced, but RuBP levels increase because it can still be synthesised while there is still some GP and triose phosphate available.

This diagram shows how different limiting factors can affect the Calvin cycle.

Other factors affecting rate of photosynthesis

Although light, temperature, and CO₂ are the main limiting factors to learn about, don’t forget that water availability is also important. For example, water is needed for photolysis in the light-dependent reaction. However, if you water a plant too much the rate of photosynthesis may decrease due to waterlogging the soil. Absorption of minerals is hindered by waterlogged soil because the roots cannot absorb enough oxygen for aerobic respiration, and magnesium ions are an important part of the structure of chlorophyll. Less magnesium ion absorption means less chlorophyll, which means less light energy is absorbed for the light-dependent reaction. In addition, a plant that cannot access enough water will close its stomata to reduce water loss through transpiration. This will reduce the amount of CO₂ entering the leaves for photosynthesis.

Artificial conditions to optimise rate of photosynthesis

A careful balance of all these environmental conditions allows an optimum rate of photosynthesis and therefore an optimum rate of plant growth. So in agriculture, it is in the interest of farmers to modify the environment their crops are grown in. Glasshouses and polytunnels are good examples of how this can be achieved. In sunny weather, they allow plenty of light in and trap heat energy. If there is a danger of the temperature becoming too high, cooling systems can be installed. At night, artificial lights can be used to keep photosynthesis going. Even the CO₂ concentration can be increased slightly be using a heater that burns a fuel such as propane. All these things will help farmers to increase their yield.

Summary

With this topic it’s good to try and think about the whole picture of photosynthesis and think about how the reactions will be affected if something changes, such as reducing activity of the enzymes. Here are a few summary points:

The three main environmental factors that can limit photosynthesis are light intensity, temperature, and carbon dioxide concentration.
Light must not only be intense, but also the right wavelength to be absorbed by the photosynthetic pigments.
Temperature affects the activity of enzymes involved in photosynthesis (e.g. Rubsico).
Carbon dioxide is needed to react with RuBP to produce GP in the Calvin cycle.