Starch (Carbohydrate) Digestion and Absorption

Summary

This video delves into the digestion of starch, highlighting its structure composed of amylose and amylopectin. It explains the partial breakdown by salivary amylase in the mouth, the inactivation of this enzyme in the stomach, and the complete digestion in the small intestine by pancreatic amylase and brush border enzymes. The process results in glucose absorption via sodium-glucose linked transporters and its use as energy or storage as glycogen. Undigested starch, known as resistant starch, ferments in the colon, producing short-chain fatty acids, while undigested portions are excreted, illustrating the complex journey of starch in the human body.

Takeaways

• 🍞 Starch is a key component of many foods, such as bread, and is made up of two types of glucose polymers: amylose and amylopectin.
• 🔗 Amylose is a linear chain of glucose molecules linked by α-1,4 glycosidic bonds, while amylopectin has a tree-like structure with branches linked by α-1,6 glycosidic bonds.
• 🦷 The digestion of starch begins in the mouth, where physical actions like chewing and chemical actions from salivary amylase start breaking down starch into smaller units.
• 🌊 Salivary amylase specifically targets α-1,4 glycosidic bonds, but its activity is limited as it becomes inactivated in the acidic environment of the stomach.
• 🔄 Starch reaches the small intestine in a partially hydrolyzed form, where most of the digestion occurs with the help of pancreatic amylase and brush border enzymes.
• 🧬 Pancreatic amylase continues the breakdown of α-1,4 glycosidic bonds, further hydrolyzing starch into smaller components.
• 🛠 Brush border enzymes, such as maltase and isomaltase, play crucial roles in breaking down the final bonds of starch, including the branch points.
• 🚰 The small intestine is equipped with sodium-glucose linked transporters (SGLTs) that facilitate the absorption of glucose into the body.
• 🔄 Glucose absorption is coupled with the transport of sodium ions, and once inside the cells, glucose is reabsorbed into the bloodstream through GLUT2 transporters.
• 🚀 The absorbed glucose can be used by the body for energy or stored in the liver as glycogen for future use.
• 🌱 Some starch, known as resistant starch, escapes digestion in the small intestine and reaches the colon, where it is fermented by gut microbiota, producing short-chain fatty acids.
• 🚮 Undigested and unabsorbed starch is ultimately excreted from the body as waste.

Q & A

What is starch made up of?

Starch is made up of two forms of glucose polymers: the linear amylose and the branched amylopectin.

How are the glucose molecules in amylose linked together?

In amylose, glucose molecules are linked together by alpha 1-4 glycosidic bonds to form a linear chain.

What is unique about the structure of amylopectin compared to amylose?

Amylopectin has a tree-like structure with linear chains of glucose and branch points, which are created by alpha 1-6 glycosidic bonds.

What is the role of salivary amylase in starch digestion?

Salivary amylase, secreted by the salivary glands, begins the digestion of starch by breaking down alpha 1-4 glycosidic bonds in the mouth.

Why does starch digestion not occur within the stomach?

Starch digestion does not occur in the stomach because the acidic environment inactivates the salivary amylase.

Where does most of the starch digestion and absorption take place?

Most of the starch digestion and absorption takes place in the small intestine.

What is the role of pancreatic amylase in starch digestion?

Pancreatic amylase, secreted by the pancreas, further breaks down the alpha 1-4 glycosidic bonds in the small intestine, continuing the digestion of starch.

What are brush border enzymes and how do they participate in starch digestion?

Brush border enzymes are enzymes found on the surface of the intestinal cells (enterocytes) that participate in the digestion of starch, including enzymes like maltase and isomaltase that hydrolyze different types of glycosidic bonds.

How is glucose absorbed into the body from the small intestine?

Glucose is absorbed into the body through sodium-glucose linked transporters (SGLTs) on the apical surface of enterocytes, which function as co-transporters for both sodium and glucose.

What happens to the portion of starch that is resistant to digestion in the small intestine?

The portion of starch that is resistant to digestion in the small intestine, known as resistant starch, reaches the colon where it undergoes fermentation by the gut microbiota.

What is the final fate of the starch that is not fermented or absorbed in the colon?

The starch that is not fermented or absorbed in the colon is excreted as waste by the human body.

Outlines

00:00

🍞 Starch Digestion Overview

This paragraph introduces the topic of starch digestion, explaining the structure of starch which consists of two glucose polymers: linear amylose and branched amylopectin. It describes the initial stages of digestion that occur in the mouth, where the physical action of chewing and the chemical action of saliva, containing the enzyme salivary amylase, begin to break down starch into smaller units. The paragraph also touches on the inactivation of salivary amylase in the stomach due to its acidic environment, and the partial hydrolysis of starch into oligosaccharides and shorter polysaccharides.

05:01

🌡 Small Intestine: The Main Stage for Starch Digestion

This paragraph delves into the process of starch digestion in the small intestine, where most of the digestion and absorption of starch takes place. It explains the role of pancreatic amylase in further breaking down starch by hydrolyzing alpha 1,4 glycosidic bonds. The paragraph also highlights the function of brush border enzymes, such as maltase and isomaltase, which complete the digestion of starch into glucose molecules. The absorption of glucose into the bloodstream via sodium-glucose linked transporters (SGLTs) is described, along with the subsequent use of glucose for energy or storage as glycogen in the liver. The paragraph concludes by discussing the fate of resistant starch, which reaches the colon and undergoes fermentation by gut microbiota, producing short-chain fatty acids that are utilized by the body.

Keywords

💡Starch

Starch is a polysaccharide, a large carbohydrate molecule composed of glucose units joined by glycosidic bonds. In the context of the video, starch is the primary focus as it is a major component of the human diet, particularly in foods like bread. The video discusses the structure and digestion of starch, highlighting its importance in human nutrition.

💡Amylose

Amylose is a form of starch that consists of linear chains of glucose molecules linked by α-1,4 glycosidic bonds. It is one of the two forms of glucose polymers that make up starch. The video explains that during digestion, enzymes like amylase target these bonds to break down amylose into simpler sugars.

💡Amylopectin

Amylopectin is the branched form of starch, characterized by its tree-like structure due to the presence of branch points created by α-1,6 glycosidic bonds. The video script mentions that amylopectin, along with amylose, constitutes starch and is subject to enzymatic breakdown during digestion.

💡Salivary amylase

Salivary amylase is an enzyme secreted by the salivary glands and is the first enzyme to interact with starch during digestion. The video describes how salivary amylase begins the process of starch breakdown in the mouth by hydrolyzing α-1,4 glycosidic bonds, initiating the conversion of starch into simpler sugars.

💡Oligosaccharides

Oligosaccharides are short chains of sugars, typically resulting from the partial hydrolysis of starch. The video script refers to the partial hydrolysis of starch by salivary amylase in the mouth, resulting in the formation of oligosaccharides, which are further broken down in the small intestine.

💡Small intestine

The small intestine is the primary site for the digestion and absorption of nutrients in the human body. The video explains that after partial digestion in the mouth and inactivation of salivary amylase in the stomach, starch reaches the small intestine where most of its digestion occurs, facilitated by enzymes like pancreatic amylase and brush border enzymes.

💡Pancreatic amylase

Pancreatic amylase is an enzyme secreted by the pancreas that continues the breakdown of starch in the small intestine. The video emphasizes its role in further hydrolyzing α-1,4 glycosidic bonds, aiding in the complete digestion of starch into glucose molecules.

💡Brush border enzymes

Brush border enzymes are a group of enzymes located on the surface of the intestinal cells (enterocytes) that participate in the final stages of starch digestion. The video mentions that these enzymes, such as maltase and isomaltase, break down the remaining glycosidic bonds in starch, including the branch points, into glucose.

💡SGLTs (Sodium-glucose linked Transporters)

SGLTs are transport proteins found on the apical surface of enterocytes that facilitate the absorption of glucose into the cells. The video describes how SGLTs work by co-transporting sodium and glucose into the cells, where glucose can then be absorbed into the bloodstream via GLUT2 transporters.

💡Resistant starch

Resistant starch refers to the portion of starch that is not digested in the small intestine and reaches the colon. The video explains that resistant starch undergoes fermentation by gut microbiota in the colon, producing short-chain fatty acids that can be used by the body for energy.

💡Fermentation

Fermentation is a metabolic process carried out by bacteria, including those in the human gut, to break down complex molecules like resistant starch. The video describes how, in the colon, resistant starch is fermented by gut microbiota, leading to the production of beneficial byproducts such as short-chain fatty acids.

Highlights

Starch is composed of two forms of glucose polymers: linear amylose and branched amylopectin.

Amylose consists of a linear chain of glucose molecules linked by alpha 1-4 glycosidic bonds.

Amylopectin features a tree-like structure with branch points created by alpha 1-6 glycosidic bonds.

Salivary glands secrete saliva and the enzyme salivary amylase, which begins starch digestion in the mouth.

Salivary amylase breaks down alpha 1-4 glycosidic bonds, partially hydrolyzing starch into oligosaccharides and shorter polysaccharides.

The acidic environment of the stomach inactivates salivary amylase, halting starch digestion temporarily.

Most starch digestion occurs in the small intestine, facilitated by enzymes from the pancreas and the intestinal cells.

Pancreatic amylase continues the breakdown of alpha 1-4 glycosidic bonds, further hydrolyzing starch.

Brush border enzymes, such as maltase and isomaltase, play a role in starch digestion by hydrolyzing specific glycosidic bonds.

Isomaltase is crucial for hydrolyzing the branch points of starch, allowing complete digestion.

Starch digestion results in the production of many glucose molecules, which are absorbed into the bloodstream.

Sodium-glucose linked transporters (SGLTs) on the intestinal cells facilitate the absorption of glucose into the body.

Glucose is either used as energy by tissues or stored in the liver as glycogen after absorption.

Resistant starch, which is not digested in the small intestine, reaches the colon and undergoes fermentation by gut microbiota.

Fermentation of resistant starch by colon bacteria produces short-chain fatty acids, which are beneficial for the human body.

Undigested starch is excreted as waste, highlighting the end of the starch digestion process.

Transcripts

in this video we will 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 the bread contains

starch starch is made up of two forms of

glucose polymers these are the linear

amalo and the branched amalo pectin

amose is a linear chain of glucose

linked together by Alpha 124 glycosidic

bonds Amal optin are linear chains of

glucose with Branch points creating a

tree likee

figure in Amal optin the glucose

molecules are also linked by alpha1 124

glycosidic bonds but the branch points

are linked by alpha1 126 glycosidic

bonds so now now let's see what happens

when starch is

ingested what happens first is that the

mouth will break down the starch both

physically and chemically physically by

the Jaws teeth and tongue chemically by

the salivary

glands the salivary gland secret secret

saliva but also the enzyme within it

called salivary Alpha

amas what Alpha amalay essentially does

is that it will break these bonds here

the alpha 124 glycosidic

bonds so alpha amas hydes alpha 124

glycosidic Bonds hydrolyzing in this

respect means breaking

down Amala will only break down the

starch partially and then from the mouth

the starch will travel towards the

stomach

here starch is only partially hydrolized

into oligosaccharides and shorter

polysaccharides

once it reaches the stomach starch is

only hydrolyzed partially because once

the starch comes down to the esophagus

into the stomach the amas becomes

inactivated this is because the acidic

environment of the stomach actually

inactivates the salivary

amalay 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 and it is within the small

intestine where most of the digestion

and absorption of starch takes

place now let's zoom into the small

intestine and see what happens with the

starch so we're zooming into the Lumen

of the small

intestine within the Lum of the small

intestine we can find the

cells the cells of the intestine known

as inyes

the interos sites are also called the

absorptive cells because they absorb the

nutrients but the ocytes also

contain these enzymes called brush B

enzymes that play a role in the

digestion of

starch below the interos sites we have

the

bloodstream so when starch reaches the

small intestine it is already in a

partially hydrolized form remember

now when starch reaches the small

intestine another organ known as the

pancreas which is situated here will

begin to secrete Alpha amas so there was

Alpha amase secreted from the salivary

glands and the

pancreas the pancreatic Alpha amas will

be secreted into the small intestine

where it will break down the alpha 124

glycosidic bonds

just as what the salivary amalay

did so pancreatic Alpha amas hydrolized

the alpha 124 glycosidic bonds breaking

down the starch

further the interos sites as I mentioned

also have enzymes called brush water

enzymes that participate in the

digestion of

starch these enzymes include

molas which which will hydr moltos now

molos is essentially two glucose

molecules linked together so molese will

hydrolize

these then you have another brush butter

enzyme called sucra Isom

molas isomaltase will hydroly both the

alpha1 124 glycosidic bonds and alpha1

126 glycosidic bonds

so what is important to know is that

Isom molas will hydroly the branch

points of starch

here therefore starch will encounter all

these enzymes in the small intestine and

will be digested it 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 interos

sites we find these

Transporters uh called sodium glucose

linked Transporters or

sglts these sodium glucose link

Transporters are found on the appical

surface of the anas sites so on the

top and what they do is that they

function as a co-transporter for both

sodium and glucose two sodium ions will

enter for one glucose

molecule once glucose is within the cell

it can be reabsorbed by the bloodstream

through a glute 2

transporter the glute 2 transporter is

found on the basal surface of the

interos

side when glucose is in the bloodstream

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 so that was the fate of starch

one it once it is broken down in the

small intestine it is absorbed by the

body and can be used as

energy however not all all portions of

starch is digested in the small

intestine the portion of starch that

resist digestion in the small intestine

are known as resistant

starch and this fraction of starch will

essentially reach the

colon so what happens to this resistance

stch when it reaches the

colon so here we're just zooming into

the colon

the colon is also known as a large

intestine the resistant starch will

reach the colon after escaping digestion

in the small

intestine now within the colon the

resistance starch will actually undergo

fermentation by the gut

microbiota so zooming into the colon

here we have the colon

cells and the

mucus the resistance starch here will

encounter all these bacteria within the

colon and through bacterial

fermentation the bacteria will produce a

byproduct such as short chain fatty

acids which will be subsequently used by

the human

body and of course starch that is not

fermented or absorbed or digested will

be waste and will be excreted by the

human body and that is where I end this

video hope you enjoyed it thank you for

watching

[Music]