The reactions involved in the digestion of starch are exothermic but occur slowly if there is no catalyst.
Two types of molecule in starch: amylose and amylopectin
Amylose: Unbranched chains of alpha-glucose linked by 1,4 bonds
Amylopectin: Chains of alpha-glucose linked by 1,4 bonds with some 1,6 bonds which make the molecule branched.
Amylase is the enzyme that begins the digestion of both forms of starch
Most starch digestion occurs in small intestine, catalysed by pancreatic amylase.
Pancreatic amylase and break down any 1,4 bond, as long as there is a chain of at least four glucose monomers.
Amylose is digested into a mixture of two- and three-glucose fragments (maltose and maltotriose)
Amylase cannot break 1,6 bonds in amylopectin (specificity of active site) and fragments that cannot digest are dextrins.
Three enzymes in the membranes of microvilli on villus epithelium cells complete the digestion of starch.
Maltase, glucosidase and dextrinase digest maltose, maltriose and dextrins into glucose.
Glucose is absorbed into villus epithelium cells by co-transport with sodium ions.
The glucose then moves by facilitated diffusion into the fluid in interstitial spaces in the villus.
The dense network of capillaries close to the epithelium ensures glucose only has to travel a short distance to enter the blood system.
Capillary walls: consist of a single layer of thin cells with larger pores than usual between adjacent cells, aiding the entry of glucose.
Blood carrying glucose and other products of digestion flows through villus capillaries to venules in the sub-mucosa of the wall of the small intenstine.
Blood in these venules is carried via the hepatic portal vein to the liver, where excess glucose can be absorbed by liver cells and converted to glycogen for storage.
Glycogen structure is similar to amylopectin but with more 1,6 bonds (extensive branching)
