Glucose monomers are assembled into starch through enzymatic processes primarily in plant cells, where glucose units are linked by glycosidic bonds to form two main types of glucose polymers: amylose and amylopectin. These polymers form starch granules. The assembly starts with the production of ADP- glucose, which acts as a glucose donor. Starch synthases elongate glucose chains via α-1,4-glycosidic bonds, and branching enzymes introduce α-1,6-glycosidic branches primarily in amylopectin. This provides a semi- crystalline structure to starch granules. The process occurs in plastids such as chloroplasts and amyloplasts and involves a coordinated action of starch synthases, branching enzymes, and debranching enzymes for proper starch biosynthesis. Disassembly of starch into glucose monomers or smaller sugars is carried out by enzymes that hydrolyze the glycosidic bonds. Starch phosphorylation by specific kinases facilitates accessibility for starch- degrading enzymes. Beta-amylase and other enzymes hydrolyze the starch into maltose and glucose, which can then be exported from the plastid for energy use or conversion into other sugars like sucrose. Thus, glucose monomers are linked by α-1,4 and α-1,6 glycosidic bonds into starch polymers through enzymatic synthesis, and starch is broken down back into sugars by enzymatic hydrolysis of these bonds.Glucose monomers are assembled into starch through enzymatic processes in plant cells. The process starts with the production of ADP-glucose, a glucose donor molecule synthesized from glucose-1-phosphate and ATP. Starch synthase enzymes then elongate glucose chains by forming α-1,4-glycosidic bonds, while branching enzymes create α-1,6-glycosidic branch points mainly in amylopectin. This yields two polymers: amylose (mostly linear) and amylopectin (highly branched), which aggregate into semi- crystalline starch granules in plastids such as chloroplasts or amyloplasts.
For disassembly, starch is first phosphorylated to make it accessible to enzymes. Then, enzymes like beta-amylase hydrolyze the α-1,4-glycosidic bonds, releasing maltose and glucose units. Debranching enzymes remove branches by breaking α-1,6 bonds. The breakdown products, mainly maltose and glucose, are exported out of plastids for use in metabolism or converted into other sugars like sucrose. In summary, glucose monomers are linked via enzymatic glycosidic bond formation into starch, and starch is enzymatically hydrolyzed back to glucose and maltose during disassembly.
