POWER

Photorespiration

 

When the O2 concentration in the leaf's air space is higher than the CO2 concentration, rubisco accepts O2 and transfers it to RuBP. This pathway produces no ATP molecules and reduces the number of organic molecules that can be reduced by the Calvin Cycle. This metabolic pathway is called photorespiration and it reduces photosynthetic output. The initial CO2 fixation reaction involves the enzyme ribulose-1, 5-bisphosphate carboxylase/oxygenase (RuBisCO), which can react with either oxygen (leading to a process named photorespiration and not resulting in carbon fixation) or with CO2. The probability with which RuBisCO reacts with oxygen vs. CO2 depends on the relative concentrations of the two compounds. Photorespiration consumes O2 and organic fuel and releases CO2 whithout producing ATP or sugar. 

 

Photorespiration in plants is a metabolic pathway that consumes oxygen, evolves carbon dioxide, produces no ATP and decreases photosynthetic output. This occurs because the active site of rubisco can accept O2 as well as CO2. The "respiration" part of photorespiration refers to the fact that this process uses O2 and releases CO2 without producing ATP or sugar.  

 

Some scientists believe that photorespiration is a metabolic relic from earlier times when the atmosphere contained less oxygen and more carbon dioxide than is present today. Under these conditions, when rubisco evolved, the inability of the enzyme's active site to distinguish carbon dioxide from oxygen would have made little difference. This affinity for oxygen has been retained by rubisco and some photorespiration is bound to occur. 

 

It is known that some crop plants, such as soybeans, lose as much as 50% of the carbon fixed by the Calvin cycle to photorespiration. Photorespiration is fostered by hot, dry, bright days. Under these conditions plants close their stomata to prevent dehydration by reducing water loss from the leaf. Photosynthesis then depletes available carbon dioxide and increases oxygen within the leaf air spaces. This condition favors photorespiration.

 

Certain species of plants, which live in hot arid climates, have evolved alternate modes of carbon fixation that minimize photorespiration. C4 and CAM are the two most important of these photosynthetic adaptations.  

 

 

C3 Pathway

Certain plant species have evolved alternate modes of carbon fixation to minimize  The Calvin cycle occurs in most plants and produces 3-phosphoglycerate, a three-carbon compound, as the first stable intermediate. These plants are called C3 plants because the first stable intermediate has three carbons. Agriculturally important C3 plants include rice, wheat, and soybeans.  Plants with a C3 pathway also have their stomata open during the day. RuBisCO, the enzyme used in photosynthesis, is also the enzyme involved in the uptake of CO2. Photosynthesis takes place throughout the leaf, and most plants have a C3 pathway due to it being more efficient than C4 and CAM. 

 

 

C4 Pathway

 Many plant species precede the Calvin cycle with reactions that incorporate carbon dioxide into four-carbon compounds. These plants are called C4 plants.  The C4 pathway is used by several thousand species in at least 19 families including corn and sugarcane, important agricultural grasses.  

This pathway is adaptive, because it enhances carbon fixation under conditions that favor photorespiration, such as hot, arid environments. Leaf anatomy of C4 plants spatially segregates the Calvin cycle from the initial incorporation of CO2 into organic compounds. 

In C4 plants there are two distinct types of photosynthetic cells:

1. Bundle-sheath cells: (Primarily used in CO2 fixation) Are arranged into tightly packed sheaths around the veins of the leaf. The thylakoids in the chloroplasts of bundle-sheath cells  are not stacked into grana. The Calvin cycle is confined to the chloroplasts of the bundle-sheath. 

2. Mesophyll Cells: (Specialize light reactions) Are more loosely arranged in the area between the bundle-sheath cells and the leaf surface. The Calvin cycle of C4 plants is preceded by incorporation of CO2 into organic compounds in the mesophyll.

 

ATP, CO2 and reduced NADP in mesophyll cells is used for synthesis of 4-carbon organic acids (such as malate), which are transported to bundle sheath cells. Here the organic acids are converted releasing CO2 and reduced NADP, which are used for carbon fixation. The resulting 3-carbon acid is returned to the mesophyll cells. 

 

Steps of C4 Carbon Fixation

#1. CO2 is added to phosphoenolpyruvate (PEP) to form oxaloacetate, a four-carbon product. PEP carboxylase is the enzyme that adds CO2 to PEP. Compared to rubisco, it has a much greater affinity for CO2 and has no affinity for O2. Thus, PEP carboxylase can fix CO2 efficiently when rubisco cannot under hot, dry conditions that cause stomata to close, CO2 concentrations to drop and O2 concentrations to rise. 

 

#2. After CO2 has been fixed by mesophyll cells, they convert oxaloacetate to another four-carbon compound (usually malate). 

 

#3. Mesophyll cells then export the four-carbon products (i.e. malate) to the bundle-sheath cells.

In the bundle-sheath cells, the four carbon compounds release CO2, which is then fixed by rubisco in the Calvin cycle. Mesophyll cells thus pump CO2 into bundle-sheath cells, minimizing photorespiration and enhancing sugar production by maintaining a CO2 concentration sufficient for rubisco to accept CO2 rather than oxygen.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CAM Pathway

(Crassulaean Acid Metabolism)

 

Some plants living in desert climates, such as cacti, keep their stomata closed during the day to minimize evaporation. These plants take up CO2 during the night when the stomatas are open, and temporarily bind the CO2 to organic acids in the leaf. The organic acids made at night are stored in vacuoles of mesophyll cells until morning, when the stomata close. During the day the CO2 is released from the acids and used for photosynthesis. Plants using this mechanism of CO2 fixation are called CAM. During daytime, light reactions supply ATP and NADPH for the Calvin cycle. At this time, CO2 is released from the organic acids made the previous night and is incorporated into sugar in the chloroplasts.