Photorespiration C3 and C4 Pathways

Introduction

Photorespiration is the process in which a plant consumes oxygen and releases carbon dioxide during photosynthesis in the presence of light. This reduces photosynthetic output because no ATP is produced, and carbon is lost invariably. Photorespiration is a challenge that plants, particularly C3 plants (plants that have no special feature to combat photorespiration), experience. Some plants, such as CAM (Crassulacean Acid Metabolism: plants that minimise photorespiration and save water by separating these steps in time between night and day) and C4 (plants that minimises photorespiration by separating CO2 fixation and the Calvin cycle in space and performing them in different cell types) have evolved photorespiration-avoidance mechanisms commonly called Photorespiration C and C pathways to solve the problem of carbon loss. 

The Calvin cycle is a series of chemical events in which plants fix carbon from CO2 by converting it into three-carbon sugars. Most new organic matter is formed through this “carbon fixation” process. 

Plants utilise the sugars generated in the Calvin cycle for long-term energy storage, unlike ATP, which is quickly depleted after it is created. However, sometimes instead of utilising carbon, some plants at high temperatures show affinity towards oxygen. 

This side reaction sets in motion a process known as photorespiration, which, rather than fixing carbon, results in the loss of already-fixed carbon in the form of CO2. As a result, photorespiration costs energy and reduces sugar synthesis. Certain plants overcome this by utilising Photorespiration C and C pathways.

Calvin cycle: C3 Pathway

Plants and algae use the Calvin cycle to convert carbon dioxide from the air into sugar, which autotrophs need to grow. All living creatures in the world rely on the Calvin cycle. Plants get their energy and nutrition from the Calvin cycle. 

Herbivores, for example, are indirectly dependent on it because they eat plants. Even species that eat other organisms, such as carnivores, require the Calvin cycle. Without it, they wouldn’t be able to survive since they wouldn’t have enough food, energy, or nutrients.

The Calvin cycle contains four key phases

• Carbon fixation
• Reduction
• Carbohydrate synthesis
• Regeneration

Chemical molecules such as ATP and NADPH, which store the energy that plants have collected from sunlight, provide energy to chemical activities in the sugar-making process. Carbon atoms from CO2 are used in the Calvin cycle and are utilised to make three-carbon sugars.

The Calvin cycle reactions take place in the stroma(the inner space of chloroplasts), as opposed to the thylakoid membrane, where light reactions occur. In addition to CO2, the stroma contains two other molecules that initiate the Calvin cycle: RuBisCO, an enzyme and ribulose bisphosphate (RuBP). 

On each end of RuBP, there are five carbon atoms and a phosphate group. The interaction of CO2 with RuBP is catalysed by RuBisCO, resulting in a six-carbon molecule that is swiftly converted into two three-carbon molecules (3-PGA). 

Carbon fixation is a process in which CO2 is “fixed” from its inorganic state into organic molecules. The stored energy of ATP and NADPH is used to transform the three-carbon molecule 3-PGA into another three-carbon complex, G3P. Because it involves the gain of electrons, this reaction is called a reduction reaction. 

One of the G3P molecules exits the Calvin cycle to help produce the carbohydrate molecule, which is most typically glucose (C6H12O6). RuBP is regenerated by the remaining G3P molecules, allowing the system to prepare for the carbon-fixation step. The side reaction linked with the Calvin cycle is photorespiration and a related combat mechanism called Photorespiration C and C pathways, which we will study in detail.

Photorespiration 

Photorespiration occurs when a plant consumes oxygen and releases carbon dioxide during photosynthesis in the presence of light. This decreases photosynthetic output as no ATP is produced, and as a result, carbon is lost invariably. It is a counterproductive metabolic route that produces glyceraldehyde 3-phosphate by fixing oxygen molecules rather than carbon dioxide (G3P).

RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) enzyme present in stroma catalyses two processes. The first is to increase the carboxylase activity of ribulose-1,5-bisphosphate (RuBP) by adding CO2. The second step is to increase RuBP’s oxygenase activity by introducing oxygen.

RuBisCO’s oxygenase activity produces the three-carbon molecule 3-phosphoglycerate (3-PGA) and the two-carbon molecule glycolate, just like in light-independent processes. The glycolate enters peroxisomes, consuming O2 to generate intermediates that are then broken down to CO2 in mitochondria. As with aerobic cellular respiration, this mechanism utilises O2 and releases CO2, hence the name photorespiration. It undoes photosynthesis’s work, which is to generate sugars.

The carboxylase action is favoured by high CO2 and low O2 concentrations. The oxygenase action is favoured by high O2 and low CO2 concentrations. The light reactions of photosynthesis liberate oxygen, and at higher temperatures, more oxygen dissolves in the cell’s cytosol.  High light intensities and high temperatures (over 30°C) favour the second process, which leads to photorespiration. To combat photorespiration, plants have evolved a mechanism called Photorespiration C and C pathways.

Conclusion

The Calvin cycle is used by all C3, C4, and CAM plants to produce carbohydrates from CO2. These CO2 fixation pathways each have their own set of benefits and drawbacks, making plants suitable for a variety of environments. Plants with the C3 mechanism thrive in temperate climates, while those with the C4 and CAM mechanisms thrive in hot, dry climates.

Both C4 and CAM pathways have evolved independently over time, implying that they may provide a considerable evolutionary benefit to plant species living in hot climes.