11.9. Photorespiration

Let us try and understand one more process that creates an important difference between C and C plants – Photorespiration. To understand photorespiration we have to know a little bit more about the first step of the Calvin pathway – the first CO fixation step.

In this chunk, we begin by introducing photorespiration as an important process that differentiates C3 and C4 plants. Photorespiration occurs during the Calvin cycle, which is responsible for fixing carbon dioxide (CO2) into organic compounds. Understanding this process is crucial as it highlights the inefficiency that can arise when RuBisCO, the enzyme responsible for CO2 fixation, binds to oxygen instead of CO2.

Think of photorespiration like a gardener trying to plant seeds but accidentally picking up weeds instead. While they are still doing work in the garden, the effort doesn't yield the intended fruits and vegetables, much like how photorespiration doesn't lead to sugar production.

RuBisCO that is the most abundant enzyme in the world is characterised by the fact that its active site can bind to both CO and O.

RuBisCO, which stands for Ribulose-1,5-bisphosphate carboxylase/oxygenase, is a critical enzyme in plants' photosynthetic processes. It has the unique ability to bind both carbon dioxide (CO2) and oxygen (O2), which can lead to competition between these two gases. Depending on their concentrations, RuBisCO may fix CO2 efficiently when there's more CO2 in the environment or mistakenly fix oxygen, leading to photorespiration.

Imagine a busy restaurant where a waiter has to choose between two types of customers (vegetarians and meat-eaters). If the waiter accidentally serves the wrong meal to a customer, it’s like RuBisCO fixing oxygen instead of carbon dioxide, resulting in no benefit for the restaurant, as the meal won't fulfill the customer’s needs.

In C plants some O does bind to RuBisCO, and hence CO fixation is decreased. Here the RuBP instead of being converted to 2 molecules of PGA binds with O to form one molecule of phosphoglycerate and phosphoglycolate (2 Carbon) in a pathway called photorespiration.

In C3 plants, when oxygen binds to RuBisCO instead of carbon dioxide, the process results in a wasteful reaction where phosphoglycerate (PGA) and phosphoglycolate are produced. This results in lower efficiency since these reactions do not lead to sugar production and instead consume energy and release carbon dioxide.

You can think of this process like a library where instead of checking out books to read, students are accidentally checking out coffee table books that don't help them in their research. They end up wasting their time and resources rather than acquiring the beneficial knowledge from the right books.

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

The photorespiratory pathway does not generate useful energy or sugars for the plant. Instead, it uses ATP and also releases carbon dioxide, which is counterproductive to the goals of photosynthesis. This inefficiency can significantly affect plant growth and productivity in C3 plants under certain environmental conditions.

Consider an athlete wasting energy on a long run but not gaining any strength or muscle because they are simultaneously jogging in a circle rather than going from point A to B. Just as the athlete’s effort does not contribute to their goals, photorespiration impacts the plant's ability to produce food effectively.

In C plants photorespiration does not occur. This is because they have a mechanism that increases the concentration of CO at the enzyme site.

C4 plants, unlike C3 plants, have evolved a mechanism that minimizes the effects of photorespiration. They concentrate carbon dioxide in the bundle sheath cells where the Calvin cycle takes place, ensuring that RuBisCO primarily fixes CO2 rather than oxygen, thereby increasing productivity and growth under conditions that would promote photorespiration in C3 plants.

This is similar to having a special delivery system for your groceries where delivery trucks are redirected straight to homes rather than to the supermarket first. This allows for a more efficient distribution of goods (or carbon dioxide in plants), which helps prevent waste and ensures that the resources are used optimally.

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