Photorespiration: C3 and C4 pathways, CAM Plants

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