Photorespiration C3 and C4 Pathways

Introduction

Respiration results in the metabolism of oxygen and the generation of carbon dioxide. It is a positive expression in cellular respiration, and it is a necessary activity for a living. On the other hand, photorespiration is a wholly negative expression that denotes a significant loss in utilizing light energy in photosynthetic organisms to fix carbon fixation for later carbohydrate fixation

Photorespiration

Photorespiration undoes the action of photosynthesis by causing the loss of up to half of the carbon fixed at the cost of light energy. RuBisCO is the most common enzyme on the planet. Its active site is capable of binding both CO2 and O2. However, RuBisCO has a far stronger affinity for CO2 than it does for O2. Which will bind to the enzyme is determined by the relative concentrations of O2 and CO2.

An exponential increase in carbon dioxide content decreases the photosynthetic rate, which is a pattern found in virtually all C3 plants.

Photorespiration is sometimes called oxidative photosynthetic photosynthesis, C2 photosynthesis.

To better understand photorespiration, consider the Calvin cycle, the first step in the biosynthetic phase in C3 plants (plants that exclusively employ the Calvin cycle to fix carbon dioxide).

Carbon fixation refers to the chemical process when carbon dioxide and water combine to form carbohydrates. The Calvin cycle is the initial phase in carbon fixation, in which CO2 reacts with Ribulose-1 and 5-bisphosphate (RuBP) to generate two molecules of 3-phosphoglyceric acid, a 3-carbon acid (PGA). The most prevalent enzyme in the world, RuBisCO, catalyzes the reaction (RuBP carboxylase-oxygenase). RuBisCO is an enzyme that binds to both CO2 and O2 but prefers CO2. As a result, CO2 and O2 binding are competitive with the concentration of the molecules in the atmosphere determining who wins.

Photorespiration in plants occurs through:

When RuBisCO attaches to oxygen molecules in C3 plants, the process deviates from the regular metabolic route. When RuBP and oxygen molecules combine, one molecule of phosphoglycerate and one molecule of phosphoglycolate is formed. This process is known as photorespiration. During photorespiration, neither sugar nor ATP molecules are created; instead, CO2 is emitted at the expense of ATP, rendering the entire process ineffective.

C4 plants, on the other hand, do not have photorespiration due to a unique method for increasing CO2 levels for enzyme binding. The C4 acid, oxaloacetic acid (OAA), degrades via the Hatch and Slack Pathway, releasing CO2. This guarantees a high intercellular CO2 concentration. As a result, in C4 plants, RuBisCO acts as a carboxylase enzyme rather than an oxygenase. This is why C4 plants are more productive.

Process of Photorespiration in C3 Plants

Any O2 binds to RuBisCO in C3 plants, reducing CO2 fixation. 

Instead of being transformed into two PGA molecules, RuBP interacts with O2 to generate one phosphoglycerate and one phosphoglycolate molecule in a process known as photorespiration. In the photorespiratory route, neither sugars nor ATP are synthesized. 

Instead, it further aids CO2 release through the usage of ATP. In the photorespiratory pathway, neither ATP nor NADPH is synthesized. 

As a result, photo-respiration is a cost ridden procedure.

Process of Photorespiration in C4 Plants

C4 plants do not perform photorespiration. They contain a mechanism that elevates the CO2 concentration at the enzyme’s site. This occurs when mesophyll C4 acid is broken down in bundle sheath cells to release CO2, increasing intracellular CO2 concentration. 

This guarantees that the RuBisCO functions as a carboxylase, reducing oxygenase activity. It’s easy to see why C4 plants don’t have photorespiration. Furthermore, these plants have a better temperature tolerance.

Photorespiration connects C3 and C4 Pathways

C4 plants evolved separately from C3 predecessors more than 60 times. C4 photosynthesis is a complicated feature, and its development from the original C3 photosynthetic pathway required modifications in leaf morphology and physiology and changes in the expression of hundreds of genes. The C4 route is more efficient than the C3 pathway under high temperature, high light, and the present CO2 content in the environment because it raises the CO2 concentration surrounding the key CO2 fixing enzyme Rubisco. The oxygenase reaction and, as a result, photorespiration are significantly reduced.

Crassulacean Acid Metabolism (CAM)

This branch of flora uses a method similar to the C4 division, except that they consume carbon dioxide at night and convert it to malic or aspartic acid. They are stored in the vacuoles of their photosynthetic cells. As soon as the sun shines, these plants close their stomata and break down the malic acid to maintain a high enough carbon dioxide ratio to prevent photorespiration. This allows the stomata of the leaves to be complete to avoid wilting. Kranz anatomy is not depicted in this section of flora.

Conclusion

C4 plant evolution occurred in stages distinguished by the selective pressures that drive the changes. The dense venation pattern is initially chosen for high-light, high-temperature environments where soil water availability prevents stomatal closure if water conductance is high enough. Carbon limitation drives the second phase of evolution, which can occur whenever stomatal aperture is limited, such as in salt stress or drought stress conditions or in niches that are exceptionally rich in other nutrients. 

Adopting the C4 cycle to replace nitrogen following the emergence of the photorespiratory pump instantly places the species on a slippery slope towards C4, and species are projected to slide as long as the selection pressure is present. In theory, species can regress if selection pressure is reduced. This is only viable until more optimisations occur, such as the loss of photorespiratory enzyme mesophyll activity.

In this sense, C4 is an evolutionary dead end, although a very productive one.

Suppose the plants continue to fix carbon dioxide when their stomata are closed. In that case, all of the carbon dioxide stored will be eaten, and the oxygen proportions will soar compared to carbon dioxide levels.

As we all know, Photosynthesis is a biological process that uses light energy to produce carbohydrates. The entire procedure is divided into two stages.

• Photochemical phase – ATP and NADPH are created in the photochemical step

• Biosynthetic Phase- The ultimate product glucose is generated during the biosynthetic phase

Plants are classed as C3 or C4 plants based on how they progress through the biosynthetic phase. Photorespiration is another feature that distinguishes C4 plants from C3 plants.