Download Biology I—Digestion Lab Resource Guide

Biology I—Digestion Lab Resource Guide
Video:
Start by viewing the “Crash Course” video on digestion available at YouTube:
https://www.youtube.com/watch?v=s06XzaKqELk&index=28&list=PL3EED4C1D684D3ADF
Background reading on digestion of carbohydrates:
The digestion of starches begins in the mouth
with the action of the enzyme salivary
amylase. As you chew, your salivary glands
release amylase which begins to break bonds in
the giant starch molecules. Your teeth and
saliva together start the processes of
mechanical and enzymatic digestion of food in
your mouth.
After you swallow your chewed-up food bits, the bolus, or food-bit, is swallowed down the
esophagus into the stomach where it is broken down into smaller pieces by hydrochloric acid (HCl) and
the physical churning and turning—much like a blender full of acid. This process is called physical
digestion; very little chemical digestion occurs in the stomach. The mucous lining of the stomach
protects it from the HCL so you don’t digest your own cells. If the mucous lining breaks down, ulcers can
occur in places where the acid touches the stomach lining tissue.
When the acidic food liquid, now called chime, passes
into the small intestine, the disaccharide sugars are
hydrolyzed (broken down into simple sugars using water)
into monosaccharides by enzymes like maltase,
isomaltase, sucrase and lactase present in the border of
the small intestine. Chemicals like pancreatic juice and
bile are added through the bile duct into the small
intestine. Pancreatic juice helps your cells to uptake
simple sugars like glucose and the bile emulsifies or
breaks down fats and lipids in your food. This process is
known as chemical digestion.
As the food particles are simplified into monosaccharides,
amino acids and fatty acids, they are absorbed through the
villi, tiny hair-like cells that line the small intestine, into the
blood stream. In a typical Western diet, digestion and
absorption of carbohydrates is fast and takes place usually
in the duodenum, or upper third of the small intestine.
However, when the diet contains carbohydrates not easily
digestible, digestion and absorption take place mainly in the jejunum and ileum, or middle and lower
portion of the small intestine. All remaining nutrients are absorbed by the small intestine before passing
on the waste products to the large intestine where it will be packed tightly and water will be removed.
Resistant foods such as potato, bean, oat, wheat flour, and several starches, are digested by bacteria
which live in the large intestine through the process of biological digestion. These leftover starches are
broken down anaerobically in the absence of oxygen which produces gases (hydrogen, carbon dioxide
and stinky old methane). The gases are either absorbed into the bloodstream or passed out of the body
through the end of the digestive tract.
Background reading on digestion of proteins:
Most proteins are decomposed from polypeptides into
single amino acids through digestion in the stomach and
intestines. To break a polypeptide, specific enzymes add
water to the chain causing it to break apart through
hydrolysis. As the bonds of the polypeptide are broken
apart, energy is released to your cells. The single amino
acids in the proteins are then absorbed into your
bloodstream and re-used as the building blocks of body
parts like muscles and other tissues. Proteins are also
important for cell signaling, chemical reactions and
building immunity from disease.
The digestion of proteins typically begins with digestive enzymes.
Digestive enzymes are special proteins that help chemical reactions
occur inside the body. A substrate, such as a protein or a
carbohydrate, binds with the enzyme on its active site and is them
broken apart into the products, which are usually smaller
monomers of the polymer.
In the stomach with the enzyme pepsin which
starts to break apart the polypeptide chains.
Pepsin is most functional in the acidic hydrochloric
acid (HCl) slurry of the stomach which is normally
at a pH of 2-3. After the chyme, or partially
digested food goo, moves to the small intestine,
digestion is continued by the enzymes trypsin and
chymotrypsin. Before the absorption in the small
intestine, most proteins are already reduced to
single amino acids or short peptide chains. If the
peptides are longer than four amino acids, they are
usually not able to be absorbed by the villi of the
small intestine and will pass on to the large
intestine to be packed and excreted as waste.
The absorption rates of individual amino
acids are highly dependent on the protein source.
For milk proteins, about 50% of the ingested
protein is absorbed between the stomach and the
jejunum (middle section of the small intestine) and
90% is absorbed by the time the digested food
reaches the ileum (lower section of the small
intestine).
All undigested proteins will be excreted
through the end of the digestive tract as solid
waste after it has been compacted by the large
intestine and all the water has been absorbed back
into the cells of the digestive tract.