Photorespiration

Let us understand one more process that creates an important difference between C3 and C4 plants — photorespiration. To understand it, we need to know a little more about the first step of the Calvin pathway, the first CO2 fixation step, in which RuBP combines with CO2 to form 2 molecules of 3-PGA, catalysed by RuBisCO:

RuBP + CO₂ → (RuBisCO) 2 × 3-PGA

RuBisCO is the most abundant enzyme in the world. It is characterised by the fact that its active site can bind to both CO2 and O2 — hence its full name, carboxylase-oxygenase. RuBisCO has a much greater affinity for CO2 than for O2, but the 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 RuBP, instead of being converted to 2 molecules of PGA, binds with O2 to form one molecule of phosphoglycerate and phosphoglycolate (a 2-carbon compound) 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. The biological function of photorespiration is not known yet.

In C4 plants:

• Photorespiration does not occur.
• This is because C4 plants have a mechanism that increases the concentration of CO2 at the enzyme site. When the C4 acid from the mesophyll is broken down in the bundle sheath cells, CO2 is released, raising the intracellular concentration of CO2.
• In turn, this ensures that RuBisCO functions as a carboxylase, minimising its oxygenase activity.

Because C4 plants lack photorespiration, they have greater productivity and yields, and, in addition, they show tolerance to higher temperatures. This is one reason C4 plants outperform C3 plants under warm, bright conditions.

Table 11.1 — the NCERT textbook presents this as a fill-in exercise: for each characteristic, choose the correct option(s) from the list given to compare C3 and C4 plants.