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2.6.1.1. Reserve homopolysaccharides

Homopolysaccharides energy reserve

Starch

The starch is the polysaccharide reserve own vegetables. It accumulates in the form of granules inside the plastids, in the plant cell.

Starch is made up of thousands of glucose molecules. As they are not dissolved in the cytoplasm, they do not influence the internal osmotic pressure and constitute a large energy reserve that occupies little volume.

Starch is found in seeds and tubers, such as potatoes and sweet potatoes. With this energy reserve, plants can obtain energy without the need for light.

But starch is not really a polysaccharide, but a mixture of two, amylose (30%) and amylopectin (70%).

  • Amylose It is made up of a polymer of glucoses  linked by α (1 → 4) bonds in an unbranched chain It has a helical structure with six glucose molecules (three maltose ) per turn. It is made up of between 200 and 300 glucose molecules (α-D-glucopyranose). It is soluble in water, giving colloidal dispersions . With iodine it is stained blackish blue.
  • Amylopectin. It is made up of a polymer of α-D-glucopyranose linked by α bonds (1 → 4), with branches with α bonds (1 → 6). Its structure is also helical, similar to that of amylose, but with a lateral branching caused by an α (1-6) bond every 25-30 glucose molecules. It is less soluble in water than amylose. With iodine it is stained dark red.

Animals can digest starch (not cellulose ) through the action of several enzymes :

  • The amylasesenzymes hydrolases present in the salivary and the pancreatic juice, hydrolyze α (1 → 4) to obtain molecules of glucose  and maltose  as final products.
  • The branches with α (1→6) bonds presented by amylopectin have to be degraded by the debranching enzyme α (1→6) glucosidase, which completes the hydrolysis giving rise to maltose  and glucose.
  • Ultimately, the enzyme maltase will hydrolyze the maltose molecules and glucose will be obtained .

Glycogen

The glycogen is the polysaccharide energy reserve own animals and fungi (and in some bacteria). It is found in the liver and muscles.

Glycogen, like amylopectin, is made up of a polymer of glucoses (α-D-glucopyranose) linked by α bonds (1→4), with branches in position α (1→6). Its structure is similar to that of amylopectin, although with more branches, every eight or ten glucoses .

With iodine, the colloidal dispersion is stained dark red. Enzymes Amylases on glycogen give maltosas and limit dextrin. Then, by means of R-debranching enzymes and maltasesglucose is obtained .

Glucógeno

By Glykogen.svg: NEUROtikerderivative work: Marek M (Glykogen.svg) [Public domain], via Wikimedia Commons

Question of reflection

Glycogen is found in the form of granules dispersed in the cytoplasm of liver cells.

a) After a diet rich in pasta, these granules are very large and abundant in hepatocytes,

b) while after a day of fasting or a cycling race they practically disappear. Please provide a reasoned explanation for these facts [1].

Dextrans

They are the reserve polysaccharides of yeasts. They are composed of branched chains of α-D-glucopyranose, but the links are varied: (1 → 2), (1 → 3), and so on. The dental plaque bacterial dextrans is rich.