13.9 Photorespiration

Let us try and understand one more process that creates an important difference between C3 and C4 plants – Photorespiration.

To understand photorespiration we have to know a little bit more about the first step of the Calvin pathway – the first CO2 fixation step.

This is the reaction where RuBP combines with CO2 to form 2 molecules of 3PGA, that is catalysed by RuBisCO.

     RuBP +CO2 –(Rubesco)-> 2 X 3PGA

RuBisCO that is the most abundant enzyme in the world (Do you wonder why?) is characterised by the fact that its active site can bind to both CO2 and O2 – hence the name.

Can you think how this could be possible? RuBisCO has a much greater affinity for CO2 than for O2. Imagine what would happen if this were not so! This binding is competitive. It is the relative concentration of O2 and CO2 that determines which of the two will bind to the enzyme.

In C3 plants some O2 does bind to RuBisCO, and hence CO2 fixation is decreased. Here the RuBP instead of being converted to 2 molecules of PGA binds with O2 to form one molecule and phosphoglycolate in a pathway called photorespiration.

In the photorespiratory pathway, there is neither synthesis of sugars, nor of ATP. Rather it results in the release of CO2 with the utilisation of ATP. In the photorespiratory pathway there is no synthesis of ATP or NADPH.

Therefore, photorespiration is a wasteful process. In C4 plants photorespiration does not occur. This is because they have a mechanism that increases the concentration of CO2 at the enzyme site.

This takes place when the C4 acid from the mesophyll is broken down in the bundle cells to release CO2 – this results in increasing the intracellular concentration of CO2. In turn, this ensures that the RuBisCO functions as a carboxylase minimising the oxygenase activity.

Now that you know that the C4 plants lack photorespiration, you probably can understand why productivity and yields are better in these plants.

In addition these plants show tolerance to higher temperatures.

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