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Limiting factors

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The rate of photosynthesis can be monitored as a function of either CO2 uptake or O2 production.


Since organisms that photosynthesise (such as plants) also undergo respiration (to have usable energy from the food they just made through photosynthesis), the relationship between the amount of photosynthesis and the amount of respiration that takes place at any time can be analysed.


Photosynthesis taken alone is the gross photosynthesis taking place. After accounting for any respiration that is using up the products of photosynthesis, extra products of photosynthesis amount to the net photosynthesis.


net photosynthesis = gross photosynthesis – respiration


The point at which photosynthesis and respiration are taking place at the same rate is called the compensation point.



If photosynthesis had no limiting factors, what would glasshouse growers have to exploit?



Precisely. Photosynthesis, just like all other physiological processes in living things as well as chemicals and beyond, is subject to external influences. The main factors that weigh in on the efficiency and speed of photosynthesis are :


1. Temperature
2. CO2 Concentration
3. Light intensity


Both the concentration of carbon dioxide and the intensity of light are similar in that they are both direct ingredients in the overall photosynthesis reaction. But since temperature insists on having the first say, let it be that way…


1. Temperature


The optimum for a lot of plants is 25 degrees Celsius – regardless, the rate of photosynthesis forms a bell curve in response to increasing temperature:



The rate of photosynthesis is sluggish at lower temperatures, while at higher temperatures it drops sharply. What’s happening? It’s all in the enzymes. Enzymes are subject to the same laws of thermodynamics as everything else. Put simply, temperature influences the random movement and collisions between molecules; at low temperatures the movement decreases, so the activity of enzymes involved in photosynthesis, among others, also decreases.


Turn up the heat a few notches, and hey presto, photosynthesis speeds up! Turn it up beyond 30 degrees or so, and you kill the party. Just what’s happened now? It’s a very general property of most enzymes that at high temperatures they denature. Photosynthesis enzymes, such as RuBP, are no exception. Denatured enzymes have a misfolded 3D structure.



In this state they cannot bind their substrates and carry out their catalytic activity. Hence, no photosynthesis!


2. CO2 concentration


Needless to repeat, carbon dioxide is one of the prerequisites for photosynthesis. After all, it supplies the carbon (C) atoms without which glucose and other organic molecules wouldn’t exist. This is how plants react to changing CO2 concentration:



As the CO2 concentration increases, so does the rate of photosynthesis, as the much-loved carbon dioxide is becoming more and more plentiful! So why does it have to end so tragically and abruptly? It seems as if the plant has enough CO2 but it’s just not good enough. Why?


Well, it’s simple: CO2 isn’t the only thing needed in photosynthesis. In other words, there are other limiting factors. Perhaps they are the same as the others listed here! When the plant has more CO2 than it can use it’s because it doesn’t have enough light, or heat, or has too much heat (denatured enzymes), or…


3. Light intensity


Temperature has degrees, CO2 concentration has pressure/volume, so what does light have? Would you believe it, there’s a special unit of measurement for light called the lux. Pretty awesome. Around 100,000 lux are available in an average day to a photosynthesising plant. Unsurprisingly, light is very much welcomed.



Just like CO2 concentration, increasing light intensity will only result in so much increased photosynthesis rate before another limiting factor comes in.


Plant growers must take into account all these different factors affecting photosynthesis and know which one becomes limiting when. The environment within a glasshouse, for example, must be optimised by adding extra CO2, increasing the temperature especially during winter, and maximising light exposure including adding artificial light.






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