When acetate serves both as a source of energy and as a source of various intermediates required to synthesise the carbon skeletons of all the major cellular components (which may occur in microorganisms such as E.coli, Pseudomonas and algae, as well as in higher plants), the tricarboxylic acid cycle is modified to form another cycle called the glyoxylate cycle. The glyoxylate cycle provides both energy and four-carbon intermediates for the biosynthetic pathways of the cell. This pathway does not occur in higher animals, which do not possess the enzyme isocitrase and maltose synthase. It is prominent, however, in the seeds of higher plants, which can convert acetyl residues derived from the fatty acids of storage triacylglycerols into carbohydrate via succinic acid. The enzymes of the glyoxylate cycle, particularly isocitrase and maltose synthase are localised
in glyoxysomes. They are found only in plant cells capable of converting fatty acids to sugar.