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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.
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 C3 and C4 Plants
Around 95 per cent of shrubs, trees, and plants are C3 species. 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 |
C3 plants 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 |
|
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 C3 plants, 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 CO2 and 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.