Name: ハングリーマイクロバイオーム澱粉の消化 (Hungry Microbiome: Starch Digestion)
Uploaded: 2021-01-14T06:35:07.000Z
Duration: 8 min 53 s
Description: VoiceTubeの動画で発音を聞きながら英語表現を覚えよう！学べる英語：

This video was created as part of the Hungry Microbiome Project,

In this video we’ll focus on starch digestion.

We will first review the structure of starch,

and finally we will look at the steps involved in starch digestion.

So we begin here with a human eating a sandwich.

Starch is made up of two forms of glucose polymers.

These are the linear amylose, and the branched amylopectin.

linked together by alpha 1-4 glycosidic bonds.

Amylopectin are linear chains of glucose with branch points,

also linked by alpha 1-4 glycosidic bonds,

but the branch points are linked by alpha 1-6 glycosidic bonds.

So now let’s see what happens when starch is ingested.

What happens first is that the mouth will break down the starch,

physically by the jaws, teeth and tongue,

but also the enzyme within it called salivary alpha amylase.

so alpha amylase hydrolysis alpha 1-4 glycosidic bonds.

Hydrolysing in this respect means breaking down.

Amylase will only break down the starch partially,

and then from the mouth the starch will travel towards the stomach here.

Starch is only partially hydrolysed into oligosaccharides

and shorter polysaccharides once it reaches the stomach.

because once the starch comes down to the oesophagus,

into the stomach, the amylase becomes inactivated.

This is because the acidic environment of the stomach

actually inactivates the salivary amylase,

and so starch digestion does not occur within the stomach.

The stomach will only mix the content around,

and then allow the starch to reach the small intestine.

where most of the digestion and absorption of starch takes place.

Now let’s zoom into the small intestine

So we’re zooming into the lumen of the small intestine.

Within the lumen of the small intestine we can find the cells,

the cells of the intestine known as enterocytes.

The enterocytes are also called the absorptive cells,

But the enterocytes also contain these enzymes

called brush border enzymes that play a role in the digestion of starch.

Below the enterocytes we have the blood stream,

so when starch reaches the small intestine

it is already in a partially hydrolysed form, remember.

Now when starch reaches the small intestine

another organ known as the pancreas, which is situated here,

from the salivary glands and the pancreas.

The pancreatic alpha amylase will be secreted into the small intestine

where it will break down the alpha 1-4 glycosidic bonds,

So pancreatic alpha amylase hydrolysis the alpha 1-4 glycosidic bonds,

also have enzymes called brush border enzymes

that participate in the digestion of starch.

These enzymes include maltase, which will hydrolyse maltose.

Now maltose is essentially two glucose molecules linked together,

Then you have another brush border enzyme called sucrase/isomaltase.

Isomaltase will hydrolyse both the alpha 1-4 glycosidic bonds

So what is important to know is that isomaltase

will hydrolyse the branch points of starch here.

Therefore starch will encounter all these enzymes in the small intestine

will be broken down into their building blocks, which is glucose.

So through starch digestion we will end up with many glucose molecules.

Now within the lumen of the small intestine

we also have many sodium ions that actually

play a critical role in the absorption of glucose into the body.

What happens is that on enterocytes we find these transporters

called sodium glucose linked transporters, or SGLTs.

The sodium glucose linked transporters are

found on the apical surface of the enterocytes, so on the top,

and what they do is that they function as a co-

Two sodium ions will enter for one glucose molecule.

it can be reabsorbed by the blood stream through a glut two transporter.

The glut two transporter is found on the basal surface of the enterocyte.

When glucose is in the blood stream it will increase blood glucose levels.

The glucose can be used as energy by tissues,

or it can be stored away in the liver as glycogen.

once it is broken down in the small intestine,

it is absorbed by the body and can be used as energy.

However, not all portions of starch is digested in the small intestine.