The following points highlight the two types of micro-bodies. The types are: 1. Peroxisomes 2. Glyoxysomes.
Micro-Bodies: Type # 1. Peroxisomes:
They are micro bodies which contain enzymes for peroxide biosynthesis. Peroxisomes were discovered by De Duve (1965) with the help of fractionation technique. The term was coined by De Duve in 1969. Peroxisomes are found in both plant and animal cells, generally in close association with endoplasmic reticulum, mitochondria and chloroplasts.
Despite absence of DNA, peroxisomes are believed to be able to replicate like mitochondria and plastids.
They are believed to vestige of an ancient organelle present in protoeucaryotes which performed all oxidation reactions prior to evolution of mitochondria. They contain special docking proteins called peroxins for obtaining materials from cytosol and endoplasmic reticulum. Peroxisomes occur in all eukaryotic cells.
They are quite abundant in liver and kidney cells. A photosynthetic cell may have 70- 100 peroxisomes. Peroxisomes are believed to develop from endoplasmic reticulum.
Their size and shape are variable. Commonly the peroxisomes have a diameter of 0.5-1.0 µm. They are covered over by a single membrane. The interior contains a matrix which may be granular or have fibrils arranged variously. In some cases the matrix has a central dense, crystalline or fibrous core which is called nucleoid.
The peroxisomes contain oxidative enzymes like urate oxidase, D-amino acid oxidase, a- hydroxy acid oxidase and (3-hydroxy acid oxidase. Molecular oxygen is required. The reactions produce hydrogen peroxide which is immediately metabolized by another enzyme called catalase.
(a) In animal cells, peroxisomes metabolize in number of toxic substances like nitrite, phenols, formaldehyde, formic acid, methanol, ethanol etc. 25% of alcohol consumed by a person is detoxified inside liver cells.
(b) Unusual substances or xenobiotic (e.g., D-amino acids, alkanes) which cannot be metabolized by normal enzymes are broken down inside peroxisomes.
(c) Urate produced during catabolism of nucleic acids and some proteins is changed into allontoin inside peroxisomes.
(d) Long chain (e.g., prostaglandins) and branched chain fatty acids are initially broken down by peroxisomes.
(e) In root nodules, they convert fixed nitrogen in ureids for transport.
(f) Plant peroxisomes found in photosynthetic cells, perform photorespiration.
For this, they are associated with chloroplasts and mitochondria. Peroxisomes pick up glycolate from chloroplasts. The same is oxidized with the help of oxygen to produce glyoxylate. Hydrogen peroxide is formed as by-product. Glyoxylate is changed to amino acid glycine. The glycine condenses to produce amino acid serine and carbon dioxide.
Micro-Bodies: Type # 2. Glyoxysomes:
Glyoxysomes are micro bodies which contain enzymes for (5-oxidation of fatty acids and glyoxylate pathway. They are considered to be special peroxisomes. The micro bodies appear transiently in germinating oil seeds and the cells of some fungi till the stored fat is consumed.
Like other micro bodies, glyoxysomes have a single covering membrane and an enzyme rich matrix with a crystalloid core, p-oxidation of fatty acids produces acetyl CoA. The latter is metabolized in glyoxylate cycle to produce carbohydrates.
After completion of their function, glyoxysomes are believed to be changed into peroxisomes. They reappear in senescent plant tissues for degradation of lipids and mobilisation of degradation products.
