11.9 Photorespiration

Let us try and understand one more process that creates an important difference between \(C _3\) and \(C _4\) plants – Photorespiration. To understand photorespiration we have to know a little bit more about the first step of the Calvin pathway – the first \(CO _2\) fixation step. This is the reaction where RuBP combines with \(CO _2\) to form 2 molecules of 3PGA, that is catalysed by RuBisCO.
\(
RuBP+CO_2 \xrightarrow{RuBisCo} 2 \times 3 PGA
\)
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 \(CO _2\) and \(O _2\) – hence the name. Can you think how this could be possible? RuBisCO has a much greater affinity for \(CO _2\) when the \(CO _2: O _2\) is nearly equal. Imagine what would happen if this were not so! This binding is competitive. It is the relative concentration of \(O _2\) and \(CO _2\) that determines which of the two will bind to the enzyme.

In \(C _3\) plants some \(O _2\) does bind to RuBisCO, and hence \(CO _2\) fixation is decreased. Here the RuBP instead of being converted to 2 molecules of PGA binds with \(O _2\) to form one molecule of phosphoglycerate and phosphoglycolate ( 2 Carbon) 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 \(CO _2\) with the utilisation of ATP. In the photorespiratory pathway there is no synthesis of ATP or NADPH. The biological function of photorespiration is not known yet.

In \(C _4\) plants photorespiration does not occur. This is because they have a mechanism that increases the concentration of \(CO _2\) at the enzyme site. This takes place when the \(C _4\) acid from the mesophyll is broken down in the bundle sheath cells to release \(CO _2\) – this results in increasing the intracellular concentration of \(CO _2[latex]. In turn, this ensures that the RuBisCO functions as a carboxylase minimising the oxygenase activity.

Now that you know that the [latex]C _4\) 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.

Based on the above discussion can you compare plants showing the \(C_3\) and the \(C_4\) pathway? Use the table format given in table 11.1 and fill in the information.

Table 11.1 Fill in the Columns 2 and 3 in this table to highlight the differences between \(C _3\) and \(C _4\) Plants