Question

What is photorespiration?

Short answer

Photorespiration is a process where RuBisCO oxygenates RuBP, leading to a loss of energy and carbon in plants.

Step-by-step solution

- Understand the Concept of Photorespiration

Photorespiration is a process in plant metabolism where the enzyme RuBisCO oxygenates RuBP, leading to a loss of energy and carbon that could otherwise be used for photosynthesis.

- The Role of RuBisCO

RuBisCO is an enzyme that typically helps in the fixation of carbon dioxide to RuBP during photosynthesis. However, it can also bind to oxygen, which initiates photorespiration.

- The Process

When RuBisCO binds to oxygen, it forms one molecule of 3-phosphoglycerate (3-PGA) and one molecule of phosphoglycolate. The phosphoglycolate must be recycled through a series of reactions that consume ATP and release CO₂, reducing the efficiency of photosynthesis.

- Conditions Favoring Photorespiration

Photorespiration is more likely to occur under conditions of high light intensity, high oxygen concentration, and low carbon dioxide concentration.

- Summary

In summary, photorespiration is a process that competes with photosynthesis, decreasing the efficiency of energy production and carbon fixation in plants.

Key concepts

RuBisCO

RuBisCO, short for Ribulose-1,5-bisphosphate carboxylase/oxygenase, is a crucial enzyme in the process of photosynthesis. This enzyme is highly abundant in plant chloroplasts because it plays a pivotal role in capturing carbon dioxide from the atmosphere.
Typically, it catalyzes a reaction where carbon dioxide combines with RuBP to form two molecules of 3-phosphoglycerate. However, RuBisCO can also bind to oxygen instead of carbon dioxide, leading to photorespiration. This alternative reaction is wasteful for the plant because it consumes energy and releases carbon dioxide, which is contrary to the goals of photosynthesis.
Despite its inefficiency, RuBisCO is essential for life on Earth as it facilitates the transformation of inorganic carbon into organic forms that can be used by almost all living organisms.

RuBP

Ribulose-1,5-bisphosphate (RuBP) is a five-carbon sugar that acts as the substrate for the enzyme RuBisCO in the Calvin cycle of photosynthesis. Its role is to combine with carbon dioxide to form two molecules of 3-phosphoglycerate.
This reaction is the first major step in the carbon fixation process, where inorganic carbon is converted into organic compounds.
When RuBisCO binds with oxygen instead of carbon dioxide, RuBP gets oxygenated, resulting in the production of one molecule of 3-phosphoglycerate and one molecule of phosphoglycolate. The latter must go through a series of energy-consuming reactions to be converted back into a usable form, making the process less efficient.
Thus, while RuBP is critical for photosynthesis, its unintended reactions contribute to the inefficiencies observed during photorespiration.

3-phosphoglycerate

3-phosphoglycerate (3-PGA) is a three-carbon molecule formed during the Calvin cycle of photosynthesis. Specifically, it's created when RuBisCO catalyzes the reaction between RuBP and carbon dioxide.
This molecule serves as an essential intermediate that eventually contributes to the formation of glucose and other carbohydrates, which are vital for the plant's energy storage and growth.
In the context of photorespiration, one molecule of 3-PGA is formed alongside one molecule of phosphoglycolate when RuBisCO binds to oxygen.
Although 3-PGA can be readily used in the Calvin cycle, the accompanying phosphoglycolate cannot, leading to inefficiencies and energy loss for the plant.
Understanding the balance between producing 3-PGA and other, less useful molecules helps explain why photorespiration is considered a wasteful process.

phosphoglycolate

Phosphoglycolate is a by-product of the reaction between RuBP and oxygen during photorespiration. Unlike 3-phosphoglycerate, it is not directly useful for the Calvin cycle.
The presence of phosphoglycolate necessitates a salvage pathway, which involves multiple steps occurring in different parts of the cell, including the chloroplast, peroxisome, and mitochondrion.
This recycling process consumes ATP and releases carbon dioxide, reducing the overall efficiency of photosynthesis.
Essentially, phosphoglycolate production represents a loss for the plant because it leads to wasted energy and fewer carbon molecules available for synthesizing sugars.
Therefore, minimizing the conditions that favor the formation of phosphoglycolate is crucial for maintaining optimal photosynthetic efficiency.

Photosynthesis Efficiency

Photosynthesis efficiency refers to how effectively a plant converts light energy and carbon dioxide into chemical energy and oxygen. Ideally, this process should result in minimal waste and maximum production of glucose and other carbohydrates.
However, photorespiration is a competing reaction that hampers this efficiency. When RuBisCO binds to oxygen instead of carbon dioxide, it initiates a sequence of reactions that consume energy and release carbon dioxide. This diminishes the plant’s ability to produce and store energy.
Factors that increase the likelihood of photorespiration include high light intensity, high oxygen concentrations, and low carbon dioxide levels.
By understanding and addressing these factors, scientists and agriculturalists can develop strategies to improve photosynthetic efficiency, ensuring better crop yields and more sustainable plant growth.