(Narrator) This is Armando.
This video was created as part of the Hungry Microbiome Project,
which I made at CSIRO.
In this video we’ll explore the carbohydrate, starch.
Starch is the major source of carbohydrate in the human diet.
We have it every day.
Examples of starchy food include potatoes, bread, pasta and beans.
Starch is stored in plants and seeds, and is used as an energy source.
In this video we will look at where and how starch is stored.
Then we will learn about the structure of starch
and how it is digested in the human body.
Finally we’ll focus on resistant starch.
So let’s begin this journey by looking at a regular plant.
Here we have a regular plant with roots,
and it’s receiving some sunlight
so that the plant can participate in what’s called photosynthesis.
If we zoom into the leaf of this plant we can find plant cells.
So here is a cell, with a cell wall.
The cell wall of the plant cell is made up of
the carbohydrate known as cellulose.
Within the plant cell we can also find the nucleus,
the rough endoplasmic reticulum, the smooth endoplasmic reticulum,
golgi apparatus, ribosomes, vacuole.
And here is what’s known as a chloroplast,
the organelle that carries out photosynthesis.
And actually within the chloroplast we can find starch granules,
where starch is stored.
So let’s have a closer look at the chloroplast.
So here we are looking at one chloroplast.
A chloroplast is a pigmented organelle
because it participates in photosynthesis,
and therefore is pigmented and contains colour.
Chloroplasts are about five micrometres in diameter,
but the size can vary again depending on the plant.
Chloroplasts contain thylakoids
and all the structures related to photosynthesis,
but within the chloroplast you can find starch granules.
The starch granules are where starch is located.
However starch is not only stored within chloroplasts,
it is also stored within another organelle known as amyloplasts.
So let’s have a look at where amyloplasts come from.
A good example is by looking at wheat.
Now wheat contains grains.
If we look inside a wheat grain there is a region in it called an endosperm.
Endosperm, the region endosperm are also found within seeds.
Anyways, endosperm is the region that contains starch granules,
stored in amyloplasts.
So essentially amyloplasts are found within endosperms.
So if we were to look into this endosperm area
of the grain and the seed we can find amyloplasts.
Amyloplasts are non-pigmented organelles.
It is non-pigmented because it contains no colour.
As you can see amyloplasts are made up mostly of these starch granules.
Now seeds contain many amyloplasts
because they need all this stored energy as they grow,
and therefore it’s no surprise that roots of plants
also contain many amyloplasts for energy reserves.
So if we were to differentiate chloroplasts to amyloplasts,
it would be that chloroplasts contain transitory starch granules,
and amyloplasts have reserve starch granules.
Let us zoom at a cross section of a starch granule and focus more on it.
The starch granule can vary in size,
from one micrometre to a hundred micrometres,
depending on the plant.
Starch granules contain granule rings.
Now running through these granule rings we find glucose polymers.
Starch is made up of two specific glucose polymers.
Let’s zoom into this section
of the granule to learn a bit more about them.
Now the granule as I mentioned have granule rings.
The granule actually have alternating crystalline and amorphous regions.
Here are the alternating amorphous and crystalline regions.
Now running through these regions we have the two glucose polymers.
As you can see the crystalline region,
they have the defined branched glucose polymer running through them.
Within the amorphous region
we can actually find an unbranched glucose polymer.
The unbranched and branched glucose polymers
are the two glucose polymers that make up starch.
These two glucose polymers are known as amylopectin and amylose.
So the unbranched glucose polymer is amylose,
and here the branched glucose polymer is known as amylopectin.
So amylose is a chain,
linear chain of glucose molecules
connected by alpha 1-4 glycosidic bonds.
Amylopectin on the other hand are made up of
many linear chains of glucose molecules, but with branch points.
The linear glucose are connected by alpha 1-4 bonds,
and the branch points are connected by alpha 1-6 bonds.
Interestingly amylopectin, the branched glucose polymer,
make up 70 to 80% of the starch granule,
so typically there would be a great...
there would be a greater amylopectin to amylose ratio.
In reality the amylose and amylopectin are not flat;
rather they form coils, like so.
Here is an amylopectin in a coiled form.
Humans eat starch
and the amylose and amylopectin coils can be digested by humans to glucose,
which will then be subsequently used for energy.
Now that we know how starch is stored within the starch granules,
let us see what happens when we humans eat it.
So here we have bread, which contains starch,
it contains amylose and amylopectin.
And here we have a hungry human and his digestive system.
The amylose and amylopectin, the starch,
is digested and absorbed predominately by the small intestine.
So within the small intestine here
the starch will encounter digestive enzymes
which will break it down into glucose molecules.
However in reality starch is digested at different rates.
A portion of starch can be digested quickly, another portion slowly,
and another one can resist digestion in the small intestine altogether.
So for example, here we have starch represented as amylopectin.
Starch can be divided into three types,
depending on the rate it is digested in the human body.
Starch can be rapidly digested,
which is the portion of starch digested
within 20 minutes of ingestion.
Starch can be slowly digested,
which is the portion digested between 20 minutes to 120 minutes.
And finally a portion of starch can be resistant to digestion altogether,
with the remaining portion after 120 minutes.
Let us now focus on resistant starch.
A better way to define resistant starch
is the fraction of starch that is not digested in the small intestine
but reach the colon is called resistant starch.
Examples of foods that are high in resistant starch
include unripe bananas, cooked and cooled potatoes, beans and legumes.
The extent to which starch resist digestion can
influence the development or progression of certain diseases,
such as diabetes and colon cancer.
This quality of resistant starch
have many health benefits for the human body.
One of the main reasons why this is so
is because resistant starch undergo fermentation by bacteria in the colon
to produce substances such as short chain fatty-acids.
Therefore it is important
that we see and understand how resistant starch
actually resist digestion in the human body.
There are actually four types of resistant starches.
The types are the physically inaccessible starch,
resistant granules, retrograded starch and chemically modified starch.
Now let’s look at each one and see how they differ.
Remember that each one is a type of resistant starch,
and that a resistant starch
is starch which has resisted digestion in the small intestine.
So type one resistant starch is the physically inaccessible starch,
because it is physically inaccessible.
A good example of this is because of the
presence of the cell wall that protects the starch granules.
So here is a plant cell with its tough cell wall.
Here are the starch granules within the plasts.
And so with the cell wall the digestive enzymes
secreted by the humans cannot access the starch.
Foods that are high in type one resistant starch
are legumes and partially milled grains.
Resistant starch type two, called resistant granules,
is resistant to digestion
because of the characteristic of the starch granules itself.
Each plant contains starch granules with amylose and amylopectin.
The starch granules differ in size, shape,
amylose and amylopectin ratio, amongst other things.
It is thought that a combination of these factors
make some granules have a more compact structure,
and so are more resistant to attack by digestive enzymes.
Good examples of food containing resistant starch type two
are raw potatoes and unripe bananas.
Resistant starch type three,
also known as retrograded starch,
is the most abundant of the resistant starches.
This is what we eat most of the time, such as bread and cereals.
Now resistant starch type three is formed through food processing,
it is formed by first cooking the starch and then cooling it down.
So let me draw it out and explain.
Resistant starch type three is formed when the starchy granules
containing amylose and amylopectin are cooked in water,
they are hydrolysed.
This causes the starch granules to become completely
hydrated and gelatinised.
The starch granules will swell up
and the amylose will start to leak out.
The starch is in a gelatinised form.
Then when the food is cooled down
the amylose and amylopectin will recrystallise
to a completely new compact structure.
This process is known as retrogradation or recrystallisation.
And here we see a rearrangement of
amylose and amylopectin to a more crystalline structure.
We also see amylose cross-linking occurring through hydrogen bonds.
This new structure formed by cooking and cooling
decreases access to digestive enzymes,
and so this type of starch will resist digestion.
Examples of food containing resistant starch type three
are cooked and cooled potatoes.
Now finally some authors have proposed a type four resistant starch,
which is the chemically modified starch.
This type of starch is said to be possibly attributed
to the different bonds formed through chemical modification,
and so with these new bonds
this type of starch will resist digestion by digestive enzymes.
Examples of food with chemically modified starches
are certain cakes and breads.
And so that concludes the video on starch and resistant starch.
I hope you enjoyed it. Thank you for watching.
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