Starch is made up of two forms of glucose polymers. These are the linear sequenced amylose
and the branched out tree-like amylopectin. Amylose is a linear chain of glucose linked together
by alpha 1-4 glycosidic bonds. In amylopectin, the glucose molecules are also linked by alpha 1-
4 glycosidic bonds, but the branched points are linked by alpha 1-6 glycosidic bonds. The mouth
will break down the starch physically by the jaws teeth and tongue and chemically by the
salivary glands. The job of the salivary glands is not only to secrete saliva but also the enzyme
within it called salivary-alpha amylase. Alpha-amylase hydrolyzes (breaks down) alpha 1-4
glycosidic bonds. Amylase only breaks down starch partially. From the mouth the starch travels
to the pharynx and by means of a segment of contractions known as peristalsis, the esophagus
delivers the consumed starch into the stomach. Starch is only hydrolyzed partially 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
amylase becomes inactivated. This is because the acidic environment of the stomach actually
inactivates the salivary amylase. So starch digestion does not occur within the stomach. The
stomach will only mix the content around and allow the starch to slowly reach the small
intestine. It is within the small intestine where most of the digestion and absorption of starch
takes place. Within the lumen of the small intestine we can find the cells of the intestine known
as enterocytes. The enterocytes are also called the absorptive cells hence they absorb the
nutrients. The enterocytes also contain brush border enzymes that play a role in the digestion of
starch. Below the enterocytes we have the bloodstream. So when starch reaches the small
intestine, it is already in a partially hydrolyzed form. When the starch reaches the small intestine,
the pancreas will begin to secrete alpha-amylase. The pancreatic alpha-amylase will be secreted
into the small intestine where it will break down the alpha 1-4 glycosidic bonds (Just as what the
salivary amylase did) breaking down the starch further. The enterocytes have brush border
enzymes that participate in the digestion of starch. One enzyme known as maltase hydrolyzes
two glucose molecules linked together also known as maltose. You also have another brush
border enzyme called sucrase-isomoltase. Isomoltase will hydrolyze both the alpha 1-4
glycosidic bonds and alpha 1-6 glycosidic bonds. Because of this, we are going to end up with
many glucose molecules conceived from starch digestion in the small intestine. 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. Sodium-glucose linked transporters are found in the apical
surface of the enterocytes. These transporters function as cotransporters for 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 GLUT 2 transporter. The GLUT 2 transporter is found
on the basal surface of the enterocyte. 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. It is also good to note that insulin helps control blood glucose levels by signaling
the liver and muscle and fat cells to take in glucose from the blood. Insulin help cells take in
glucose. Insulin can also signal the liver to take up glucose and store it as glycogen. A muscle
contraction requires the muscle cells to have energy. In our bodies this energy is stored in a
specific molecule called Adenosine Triphosphate. ATP is a large molecule, not to mention it is
unstable in water, which is bad news. Since we are made mostly of water. ATP makes up for
these drawbacks though, through the fact that if the third phosphate chain is released. It provides
the power for a muscular contraction. Since ATP is a large and unstable molecule, our muscles
can only store enough to power around 10 seconds worth of a contraction before they run out.
Since only about 10 seconds of ATP is stored, the body has three-generation systems
(Glycolysis, Krebs/Citric acid Cycle, Electron transport Chain) which work in real time to keep
ATP levels topped off. Powering these generation systems and stored in your muscles is glucose.
It is this glucose, which the first generation system (Glycolysis) turns into ATP. Glycolysis
(which occurs in the cytoplasm of our cells) is basically the lysing of glucose’s 6-carbon ring
into two 3-carbon molecules called pyruvic acids or pyruvates. Using two ATPs as a source of
fuel in our investment stage, what we generate out of Glycolysis is a net pay off of two ATPs net
and 2 NADHs net. In the absence of oxygen (anaerobic), the pyruvates formed through
glycolysis get rerouted into a process called fermentation. Unlike Glycolysis, The Krebs cycle
and Electron Transport chain are both aerobic processes; they require oxygen to function. The
Krebs cycle takes place in the matrix of the mitochondria. One of the pyruvates is oxidized, one
of the carbons of the three carbon chain bonds with an oxygen molecule and leaves the cell as
CO2. What is left is a two-carbon compound called Acetyl-coenzyme A. Another NAD+ comes
along, picks up a hydrogen and becomes NADH. The two pyruvates create another 2 molecules
of NADH to be used later (Pyruvate oxidation). Enzymes bring together a phosphate with ADP
to create another ATP molecule for each pyruvate. Enzymes also help join the two-carbon acetyl
CoA and a 4-carbon molecule called oxaloacetic acid in which they form a 6-carbon molecule
known as citric acid. Each pyruvate yields 3 NADHs and 1 FADH2 per citric acid cycle for a
total of 6 NADHs and 2FADH2s for both pyruvates that were once a glucose molecule. After
Glycolysis and the citric acid cycle we end up with a total of 4 ATPs (2ATPs from Glycolysis
and 2ATPs from the Krebs Cycle) 10 NADHs (2NADHs from Glycolysis 2NADHs from both
pyruvate oxidation and 6NADHs from both pyruvates oxidized into Acetyl-CoA merging with
oxaloacetic acid and undergoing the Citric Acid Cycle.) and 2 FADH2s (one per each citric acid
cycle, 2 pyruvates generate a total of 2FADH2s). During the Electron Transport Chain, each
NADH is going to be responsible for the production of three ATPs and each FADH2 will be
responsible for the production of two ATPs. The electrons of the NADHs and FADH2s we made
in the Krebs cycle are going to provide the energy that will work as a pump along a chain of
channel proteins across the inner membrane of the mitochondria where the Krebs cycle occurred.
These proteins will swap these electrons to send hydrogen protons from inside the very center of
the mitochondria, across its inner membrane to the outer compartment of the mitochondria and
once they are out, the protons will want to get back to the other side of the inner membrane. This
is because there is many other protons out there. What we want in the end is equilibrium on both
sides of the membrane. These protons are allowed back in through the protein ATP synthase.
The energy of this proton flow drives this spinning mechanism that squeezes some ADP and
some phosphates together to form ATP. The electrons from the 10 NADHs that come out of the
Krebs cycle have just enough energy to produce three ATPs each and also do not forget that the
2 FADH2s we have will make 2 ATPs each for a total of the average result of 38 ATP. Insoluble
fiber appears to speed the passage of foods through the stomach and intestines and adds bulk to
the stool. The portion of starch that resists 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
resistant starch when it reaches 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 resistant starch will actually undergo fermentation by the gut microbiota. Through
bacterial fermentation, the bacteria will produce a byproduct such as short-chain fatty acids
which will be subsequently used by the human body. Starch that is not fermented, absorbed or
digested will be waste and excreted by the human body.
