Respiration leads to oxygen metabolism, and carbon dioxide production. In cellular respiration it is a positive term, a critical process for survival. Yet Photorespiration is a completely negative term because it indicates a serious loss to the method of using light energy in photosynthetic organisms to fix carbon for subsequent carbohydrates.

By causing the loss of up to half the carbon fixed at the cost of light energy, photospiration undoes the photosynthesis work.

RuBisCO is the globally most abundant enzyme. Its active location can bind both to CO2 and to O2. But RuBisCO 's affinity to CO2 is far greater than O2. The relative concentration of O2 and CO2 determines which enzyme will bind.

Definition: It is a trend seen in almost all C3 plants where an increase in carbon dioxide concentration results in a decrease in photosynthesis rate.

Photorespiration in C3 Plants

Any O2 binds to RuBisCO in C3 plants and hence CO2 fixation is reduced.

Here the RuBP binds with O2 instead of being converted into 2 PGA molecules to form one phosphoglycerate and phosphoglycolate molecule in a pathway called Photorespiration.

There is no synthesis of sugars or ATP in the photorespiratory pathway. Instead it helps in CO2 release with the use of ATP.

There is no synthesis of either ATP or NADPH in the photorespiratory pathway. Photo-Respiration is therefore a costly operation.

Photorespiration cycle is explained in detail through the Photorespiration diagram below.

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Photorespiration in C4 plants

Photorespiration does not occur in C4 plants. This is because they have a mechanism which increases the CO2 concentration at the site of the enzyme.

This happens when the mesophyll C4 acid is broken down in the bundle sheath cells to release CO2 this results in an increase in the intercellular CO2 concentration.

This in turn ensures that the RuBisCO acts as a carboxylase which minimizes oxygenase activity.

Now it’s understandable that C₄ plants lack Photorespiration. Additionally, these plants show higher temperature tolerance.

Detailed Difference between C₃and C₄ Plants

Around 95 per cent of shrubs, trees, and plants are C₃species. C₄ plants, on the other hand, are those that employ the C₄ pathway during the dark response. These plants' chloroplasts are dimorphic, and unlike C₃ plants, C₄ plants' leaves have kranz anatomy. C₄ plants make up around 5% of all plants on the planet. Here's how to tell the difference between C₃ and C₄ plants.

Character	C₃ plants	C₄ plants
Definition	For the dark, C₃ plants use the C₃ pathway, often known as the Calvin cycle reaction of photosynthesis.	For the dark response of photosynthesis, C₄ plants employ the C₄ pathway, also known as the Hatch-Slack Pathway.
Season	These are cold-season plants that thrive in chilly, moist environments.	These are warm-season plants that thrive in arid environments.
Product	The result of the C₃ cycle is phosphoglyceric acid, a three-carbon molecule.	The result of the C₄ cycle is Oxaloacetic acid, a four-carbon molecule.
Presence	C₃plants account for 95% of all green plants on the planet.	C₄ plants make up around 5% of all plants on the planet..
Conditions	In temperate climates, these plants are plentiful.	In tropical climates, these plants are plentiful.
Kranz anatomy	Kranz anatomy does not exist in leaves.	Kranz anatomy may be found in leaves.
Chloroplast	The bundle sheath cells in this case lack chloroplasts.	Chloroplasts are found in the bundle sheath cells.
CO₂ acceptors	Only one CO₂ acceptor exists in C₃ plants.	Two CO₂ acceptors are found in C₄ plants.
Secondary acceptor	Secondary CO₂ acceptors are absent in C₃ plants.	Secondary CO₂ acceptors are found in C₄ plants.
Photosynthesis	Only when the stomata are open can it accomplish photosynthesis.	Even when the stomata are close together, it accomplishes photosynthesis.
Peripheral reticulum	The peripheral reticulum does not make up the chloroplasts.	The peripheral reticulum is made up of chloroplasts.
Temperature	Photosynthesis occurs at a relatively low temperature.	Photosynthesis occurs at a high temperature.
Efficiency	Photosynthesis is less efficient in C₃ plants.	Photosynthesis is more efficient in C₄ plants.
Photorespiration	The rate of Photorespiration is really high.	There is no Photorespiration.
CO₂ fixation	In C₃plants, it takes a long time.	In C₃ plants, it occurs at a quicker rate.
Mesophyll Cell	The dark response occurs solely in the mesophyll cells in this case.	Mesophyll cells will only conduct the first phases of the C₄ cycle in this case. The majority of the work is done in bundle sheath cells.
CO₂ Composition	These plants have a high carbon dioxide composition point.	These plants have a low carbon dioxide composition point.
Growth	When the soil temperature is between 4 and 7 degrees, growth begins.	When the soil temperature is between 16 and 21, growth begins.
Example	Wheat, oats, rice, sunflower, and cotton are some of the most common crops.	Amaranthus, maize, and sugarcane

Conclusion

RuBisCO is the globally most abundant enzyme. Its active location can bind both to CO₂and to O₂. The relative concentration of O₂ and CO₂ determines which enzyme will bind to the enzyme. Photorespiration is a trend where an increase in carbon dioxide concentration results in a decrease in photosynthesis rate.

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FAQs on Photorespiration in C3 and C4 Plants: Detailed Comparison

1. What is photorespiration, and why is it often described as a wasteful process?

Photorespiration is a metabolic pathway that occurs in photosynthetic organisms when the enzyme RuBisCO acts on oxygen rather than carbon dioxide. It is considered wasteful because, unlike photosynthesis, it does not produce any energy in the form of ATP or reducing power as NADPH. Instead, it consumes energy and releases previously fixed carbon as CO₂, effectively reducing the overall efficiency of photosynthesis by up to 25% in C3 plants.

2. What is the primary difference in how C3 and C4 plants handle photorespiration?

The primary difference lies in their leaf anatomy and initial carbon fixation step.

C3 plants (e.g., rice, wheat) perform the Calvin cycle directly in mesophyll cells where RuBisCO is exposed to atmospheric oxygen, making them highly susceptible to photorespiration.
C4 plants (e.g., maize, sugarcane) have a specialised Kranz anatomy. They use the enzyme PEP carboxylase to first fix CO₂ in mesophyll cells, then transport this fixed carbon to bundle-sheath cells. This process actively pumps CO₂ around RuBisCO, creating a high-CO₂ environment that effectively outcompetes oxygen and prevents photorespiration.

3. Why is photorespiration almost non-existent in C4 plants?

Photorespiration is negligible in C4 plants due to a highly efficient CO₂ concentrating mechanism. They possess a unique leaf structure called Kranz anatomy and a two-step photosynthetic process. Initially, the enzyme PEP carboxylase, which has no affinity for oxygen, fixes CO₂ in the mesophyll cells. The resulting 4-carbon compound is transported to the deeper bundle-sheath cells, where it is decarboxylated. This floods the bundle-sheath cells with a high concentration of CO₂, ensuring that RuBisCO primarily binds with CO₂ and not O₂, thus bypassing the photorespiratory pathway.

4. What are the key steps and organelles involved in the photorespiration pathway in C3 plants?

The photorespiration pathway in C3 plants is a complex process that involves three different cell organelles: the chloroplast, peroxisome, and mitochondrion.

Chloroplast: RuBisCO fixes O₂ to RuBP, forming one molecule of PGA and one molecule of phosphoglycolate. The phosphoglycolate is converted to glycolate.
Peroxisome: Glycolate is converted to glyoxylate and then to the amino acid glycine.
Mitochondrion: Two molecules of glycine are converted into one molecule of serine, releasing a molecule of CO₂ in the process. This step is where the fixed carbon is lost.

The serine then travels back to the peroxisome and then the chloroplast to be recycled into the Calvin cycle, consuming ATP along the way.

5. How do environmental conditions like temperature and CO₂ levels affect the rate of photorespiration?

Environmental conditions significantly influence the rate of photorespiration, especially in C3 plants. High temperatures and low CO₂ concentrations favour photorespiration. This happens because as temperatures rise, the specificity of the enzyme RuBisCO for CO₂ decreases, and its affinity for O₂ increases. Furthermore, in hot and dry conditions, plants close their stomata to conserve water, which reduces the intake of CO₂ and leads to an accumulation of O₂ produced during photosynthesis within the leaf, further promoting photorespiration.

6. Why does the enzyme RuBisCO bind with oxygen, initiating photorespiration?

The enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) has an active site that can bind to both carbon dioxide (for carboxylation) and oxygen (for oxygenation). This dual functionality is an evolutionary relic from a time when the Earth's atmosphere had very low oxygen and high carbon dioxide levels. The binding is competitive, meaning the outcome depends on the relative concentrations of O₂ and CO₂ at the enzyme's active site. When O₂ levels are high relative to CO₂, the oxygenase activity is triggered, initiating the photorespiratory pathway.

7. If photorespiration is so wasteful, why might it still exist in C3 plants from an evolutionary perspective?

While seemingly wasteful, scientists hypothesise that photorespiration may have some protective roles, which could explain its persistence. One major theory is that it acts as a photoprotective mechanism. Under high light and low CO₂ conditions (e.g., when stomata are closed), the photosynthetic machinery can become over-energised, leading to cellular damage. Photorespiration helps dissipate this excess energy and reducing power, thus protecting the plant. It is also linked to other vital metabolic processes like nitrogen assimilation and plant defence.