Starch (Carbohydrate) Digestion and Absorption
Summary
TLDRThis 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
🍞 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.
🌡 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.
Mindmap
Keywords
💡Starch
💡Amylose
💡Amylopectin
💡Salivary amylase
💡Oligosaccharides
💡Small intestine
💡Pancreatic amylase
💡Brush border enzymes
💡SGLTs (Sodium-glucose linked Transporters)
💡Resistant starch
💡Fermentation
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]
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