The process of respiration is responsible for oxygen metabolism and carbon dioxide production.
It is an important process in cell respiration, and a positive term. Photorespiration, however, is a totally negative term. It indicates a loss in the ability of photosynthetic organisms using light energy to fix carbon for subsequent carbohydrates.
Photospiration can cause the photosynthesis to stop working by causing a loss of as much as half of the carbon that was fixed at the price of light energy.
RuBisCO is the most abundant enzyme worldwide. It can bind to both CO2 and O2. RuBisCO’s affinity for CO2 is much greater than O2. It is determined by the relative concentrations of O2 or CO2 to which enzyme it will bind.
Definition: This is a trend that can be seen in nearly all C3 plants, where an increase of carbon dioxide results in a decreased photosynthesis rate.
Photorespiration in C3 and C4 Pathways
Photorespiration in C3 plants
Any O2 that binds to RuBisCO is C3 plants, and therefore CO2 fixation is decreased.
The RuBP is able to bind with O2 rather than being converted into two PGA molecules. This allows for one phosphoglycerate molecule and one phosphoglycolate molecule in a process called photorespiration.
The photorespiratory pathway does not produce ATP or sugars. It aids in the release of CO2 through the use of ATP.
The photorespiratory pathway does not produce ATP or NADPH. Photo-Respiration, therefore, is a costly operation.
Photorespiration within C4
plants
Photorespiration is not possible in C4 plants. They have a process that increases the CO2 concentration at the enzyme site.
This occurs when the mesophyll acid C4 is broken down by the bundle cells (sheath) to release CO2. This results in an increase in intracellular CO2.
This reduces oxygenase activity by ensuring that RuBisCO acts like a carboxylase.
It is not surprising that C4 plants don’t have photorespiration. These plants also have a higher tolerance for temperature.
Photorespiration in cam plants
In the pathway carbon dioxide reserves are created ( temporal isolation) to increase Rubisco CO2 binding.
- CAM plants are well-suited to dry environments, where water loss is high. Stomata must also be kept closed during daylight hours
- The CO2 becomes a 4C compounds at night, when the stomata are open. CO2 then diffuses into the leaf
- This allows CO2 reserves to become available for use during the day. Stomata are closed, and O2 cannot been released