C3, C4, and CAM plants (article) | Khan Academy
Relationship between Photosynthesis and Respiration. THE EFFECT OF carbohydrate oxidation and photorespiration are discussed. It is concluded that the The extent to which tricarboxylic acid cycle respiration contin- ues inthe light in. Photorespiration refers to a process in plant metabolism where the enzyme RuBisCO This process reduces the efficiency of photosynthesis, potentially reducing The oxidative photosynthetic carbon cycle reaction is catalyzed by RuBP .. Edit links. This page was last edited on 5 November , at ( UTC). However, the relatively low ratio of CO2 to O2 of mesophyll fluids in contact with air also influences the relative rates of photorespiration and the Calvin cycle. of photosynthesis there was a higher carbon dioxide to oxygen gas ratio in the.
As both substrates combine with the active site of rubisco, they are competitive inhibitors of each other's reactions.
One might recall our earlier discussions about competitive inhibition. The relative concentration of the two substrates and the differential affinity of the enzyme for each substrate will determine which of the reactions Calvin cycle or Photorespiration predominate.
Fortunately for plants and for us indirectly! However, the relatively low ratio of CO2 to O2 of mesophyll fluids in contact with air 0. Temperature also influences the relative rates of photorespiration and the Calvin cycle. Because increased temperature more efficiently removes carbon-dioxide from solution than it does oxygen, high temperatures favor photorespiration.
The photorespiration pathway is an enzymatic one that is not coupled to any electron transfer system. It does not generate ATP. It does use oxygen and it does produce carbon dioxide, and it uses a sugar-phosphate as its primary fuel. The complete pathway is depicted here.
The regulatory interplay between photorespiration and photosynthesis.
It is worthy to note that this diagram, as others of its type, show the organelles tightly appressed to each other. Indeed there are some famous electron micrographs example above that show this, but other micrographs do not show them this way. I say this just to comment that this positioning may be more an efficient design for communication to students than a realistic portrayal of life in a typical cell.
In the chloroplast, rubisco, combines with ribulose-1,5-bisphosphate RuBP and oxygen. The enzymes of this pathway are enumerated in the diagram above.
The 2-phosphoglycolate is converted to glycolate by phosphoglycolate phosphatase in the chloroplast.
The phosphate liberated is returned to the local phosphate pool. The glycolate is transported from the chloroplast into a nearby peroxisome. In the peroxisome, the glycolate is oxidized by oxygen gas to glyoxylate and hydrogen peroxide by glycolate oxidase. The peroxide is converted to water and oxygen gas by catalase. So the consumption of oxygen in the oxidation is replaced by catalase activity in the peroxisome. The glyoxylate is converted to the amino acid glycine in the peroxisome.
The amino group is transferred to the glyoxylate from glutamate another amino acid by glyoxylate: The glycine is transported to the mitochondrion.
Photorespiration - Wikipedia
In the mitochondrion, glycine decarboxylase carves off carbon dioxide gas from the glycine. It also cleaves off the amino group.
- Difference Between Photosynthesis and Photorespiration
- C3, C4, and CAM plants
If you are paying attention to the chemical structures, you realize that the two-carbon amino acid has had both its amino and acid groups removed! Cyanobacteria have three possible pathways through which they can metabolise 2-phosphoglycolate. They are unable to grow if all three pathways are knocked out, despite having a carbon concentrating mechanism that should dramatically reduce the rate of photorespiration see below.
This intermediate is able to react with either CO 2 or O 2. For example, when the stomata are closed to prevent water loss during drought: In algae and plants which photosynthesise underwater ; gases have to diffuse significant distances through water, which results in a decrease in the availability of CO2 relative to O 2.
This is because the enediol intermediate is less stable. Increasing temperatures also reduce the solubility of CO2, thus reducing the concentration of CO2 relative to O 2 in the chloroplast. Biological adaptation to minimize photorespiration[ edit ] Certain species of plants or algae have mechanisms to reduce uptake of molecular oxygen by RuBisCO.
Biochemical carbon concentrating mechanisms[ edit ] Biochemical CCMs concentrate carbon dioxide in one temporal or spatial region, through metabolite exchange. C4[ edit ] Maize uses the C4 pathway, minimizing photorespiration. C4 plants capture carbon dioxide in their mesophyll cells using an enzyme called phosphoenolpyruvate carboxylase which catalyzes the combination of carbon dioxide with a compound called phosphoenolpyruvate PEPforming oxaloacetate.
This oxaloacetate is then converted to malate and is transported into the bundle sheath cells site of carbon dioxide fixation by RuBisCO where oxygen concentration is low to avoid photorespiration. The CO 2 concentrations in the Bundle Sheath are approximately 10—20 fold higher than the concentration in the mesophyll cells. Under these conditions, photorespiration does occur in C4 plants, but at a much reduced level compared with C3 plants in the same conditions. C4 plants include sugar canecorn maizeand sorghum.
Crassulacean acid metabolism allows plants to conduct most of their gas exchange in the cooler night-time air, sequestering carbon in 4-carbon sugars which can be released to the photosynthesizing cells during the day. This allows CAM plants to reduce water loss transpiration by maintaining closed stomata during the day. This process is not fully understood. In many species, biophysical CCMs are only induced under low carbon dioxide concentrations.
There is some debate as to when biophysical CCMs first evolved, but it is likely to have been during a period of low carbon dioxide, after the Great Oxygenation Event 2. Low CO 2 periods occurred around, and — million years ago.