Structure & function
Microbodies are roughly spherical in shape, bound by a single membrane, and are usually 0.5 to 1 micrometre in diameter. There are
several types, by far the most common of which is the peroxisome. Peroxisomes derive their name from hydrogen peroxide, a reactive intermediate in the process of molecular breakdown that occurs in the microbody. Peroxisomes contain type II oxidases, which are enzymes that use molecular oxygen in reactions to oxidize organic molecules. New microbodies are formed by incorporation of required proteins and lipids from the cytoplasm and subsequent splitting when they reach a certain size. Although structurally similar, their roles, and thus their contents, are different.
several types, by far the most common of which is the peroxisome. Peroxisomes derive their name from hydrogen peroxide, a reactive intermediate in the process of molecular breakdown that occurs in the microbody. Peroxisomes contain type II oxidases, which are enzymes that use molecular oxygen in reactions to oxidize organic molecules. New microbodies are formed by incorporation of required proteins and lipids from the cytoplasm and subsequent splitting when they reach a certain size. Although structurally similar, their roles, and thus their contents, are different.
PeroxisomesPeroxisomes are present in leaves, and their oxidative enzymes are involved in the breakdown of hydrogen peroxide and,more important, in photorespiration. Photorespiration occurs when carbon dioxide levels in the leaves drop and oxygen levels increase, a typical phenomenon on hot, sunny days when a plant is experiencing some level of water stress. Peroxisomal oxidase converts glycolate to glyoxylate, and hydrogen peroxide is produced as a by-product. Because hydrogen peroxide is toxic, it is quickly converted by a catalase to water and oxygen. Glyoxylate goes through several more steps which involve reactions in the mitochondria and then back again in the peroxisomes. Eventually, glycerate is formed in the peroxisomes. Glycerate is then transported out of the peroxisomes and into chloroplast, where it is converted to PGA, which can reenter the Calvin cycle.
|
GlyoxysomesGlyoxysomes are found in the cells of fat-rich seeds. Fats are synthesized and stored as oil bodies, sometimes called spherosomes. Spherosomes are surrounded by a single layer of lipids instead of a lipid bilayer and are therefore not organelles in the strict sense. Glyoxysomes are responsible for converting fats and fatty acids into sucrose. The fat used by glyoxysomes comes from spherosomes. Glyoxysomes are considered to be a type of peroxisome. In some plants, small glyoxysomes are found in the cotyledons of developing seeds. During germination and seedling development, they mature into fully functional glyoxysomes. They function until the fats are completely digested into sucrose. The energy from sucrose is required to drive early seedling development before photosynthesis begins. Large fat molecules are difficult to transport into the plant embryo; they must be converted to the more mobile sucrose molecules.Breakdown of fats is a collaborative effort between enzyme-containing glyoxysomes and fat containing spherosomes. Direct contact between spherosomes and glyoxysomes must occur. Fats from spherosomes leak out close to the membrane of glyoxysomes.
|