Figure 1. The structure of the starch polymers and the starch granule. Top left: representations of the structures of amylose and amylopectin. The chains in the amylose molecule are 1000 or more glucosyl residues in length. The short chains within the clusters of the amylopectin molecule are typically 12–20 glucosyl residues in length. Top right: adjacent chains within the clusters of the amylopectin molecule form double helices, and these associate together to form crystalline lamellae. The regions between the clusters that contain the branch points do not crystallize, giving rise to alternating crystalline and amorphous lamellae with a periodicity of 9 nm. The layers of the sandwich are parallel with the surface of the granule; in other words, the lamellae form concentric shells within the granule matrix. Bottom: scanning electron micrographs of the inner face of a starch granule from a potato tuber, cracked open and treated with a starch-degrading enzyme to reveal the growth rings. Each ring consists of tens of the 9 nm repeats shown above. The bar represents 5 m; the picture on the right is a closer image of part of the picture on the left.

Figure 2. The actions of ADPglucose pyrophosphorylase, starch synthase and starch-branching enzyme. Starch synthase catalyses the addition of the glucosyl moiety of ADPglucose on to the nonreducing end of a chain via an 1,4 linkage. Starch-branching enzyme cleaves sections of chains from the nonreducing end and adds them to the side of the same or an adjacent chain via an 1,6 linkage.

Figure 3. The pathway of starch degradation in the endosperm of a germinating cereal seed. The starch granule is attacked by the endoamylase -amylase, which releases soluble linear and branched glucans. These are acted on by the debranching enzyme limit dextrinase and the exoamylase -amylase to produce maltose. Maltose is then hydrolysed to glucose by -glucosidase. The glucose is taken up into the growing embryo.

Figure 4. The pathway of starch degradation in an Arabidopsis leaf at night. The enzyme that attacks the starch granule is not known, but the rate at which the process occurs is controlled in part by the level of phosphorylation of amylopectin via glucan, water dikinase. Debranching of the starch polymers is mainly via isoamylase3, and linear glucans are metabolized via -amylase to yield maltose as the main product, with maltotriose as a more minor product. Maltose is exported from the chloroplast to the cytosol via the maltose transporter MEX1, then metabolized via the transglucosidase DPE2. DPE2 releases one of the glucosyl moieties of maltose as free glucose and transfers the other to an unknown acceptor molecule, from which it is presumed to be released via an unknown enzyme and converted to a hexose phosphate. Currently, the best candidates for these roles are a cytosolic heteroglycan and the enzyme glucan phosphorylase. The maltotriose product of -amylase is converted via a disproportionating- or D-enzyme to maltopentaose and free glucose. The maltopentaose is a substrate for the further action of -amylase, and the glucose is assumed to be transported to the cytosol via a glucose transporter. Hexose phosphates produced in the cytosol from free glucose and the deglucosylation of the DPE2 acceptor molecule are converted to sucrose for export to the nonphotosynthetic parts of the plant.