Apart from the well-known compartments just discussed occur a variety of so-called microbodies in plant cells. On average are they of 0.3 - 1.2 µ diameter. They are distinguished into peroxysomes and glyoxysomes.
The two types differ in their functions: peroxysomes are the sites of photorespiration, while glyoxysomes are in charge of the mobilization of storage compounds (fats). Both peroxysomes and glyoxysomes contain crystalline inclusions.
The two compartment types are not distributed evenly. Peroxysomes are usually found in the vicinity of chloroplasts and mitochondria, glyoxysomes occur only near mitochondria.
Photorespiration: transport of metabolites (according to H. W. HELDT und U. I. FLÜGGE, 1986
Photorespiration is a process at least partially opposing photosynthesis. In mitochondria develops carbon dioxide as a product of the serine synthesis. Serine is synthesized under light exposure and oxygen uptake from two glycerines. During the synthesis of glyoxalate from glycolate is the heavy cytotoxin hydrogen peroxide produced that is immediately afterwards broken down by the enzyme catalase. Catalase is an enzyme specific for peroxysomes. In addition is the fraction-1-protein involved, the enzyme that catalyzes carbon dioxide fixation during photosynthesis. It has two activities:
ribulose 1,5 bisphosphate carboxylase (= Rubisco) (photosynthesis)
ribulose bisphosphate oxygenase (photorespiration).
Photorespiration is only indirectly dependent on light. The light is necessary to produce 3-P-glycerate via photosynthesis. 3-P-glycerate is the starting compound of photosynthesis. It has to be supplied constantly since it is quite short-lived.
What is the use of photorespiration anyway? The main product of photorespiration is the amino acid glycine and this again is the starting compound for the synthesis of the amino acid serine.
Glyoxysomes are the site of the glyoxalate cycle that is tightly linked to the breakdown of fatty acids.
During plant germination are glyoxysomes in a key-position. They control and catalyze the degradation of storage fat and they channel the degradation products towards the synthesis of numerous carbon compounds (mainly carbohydrates). Especially during growth are a lot of carbohydrates required for the synthesis of new cell walls. The activities in peroxysomes and glyoxysomes show how important compartmentalization is for the regulation of metabolic pathways. Opposing processes are distributed onto different compartments. They demonstrate, too, how the different compartments interact and that membranes as selective filters regulate the throughput of metabolites.
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