The Digestive System … Notes II
The Digestive Process
…see “Notes” handout
When food is ingested, it is put into the mouth where it is chewed.
Chewing is a physical digestion process that increases the
surface area so that chemical digestion can take place faster.
Connected to the mouth are a series of salivary glands, which
release saliva through ducts (tiny tubes) that lead into the mouth.
Saliva contains water and an enzyme. The water acts both as a
lubricant to aid in swallowing and as a reactant in the hydrolytic
reactions of digestion. The enzyme, salivary amylase
(ptyalin), breaks the ester bonds between the sugar molecules of
starch and begins the chemical conversion of cooked starch into
The tongue is a muscle that rolls the food into swallow able sized
balls, each one called a bolus, and pushes them to the back of the
mouth from where swallowing takes place.
The chamber at the back of the mouth is called the pharynx. It is
a common chamber for both food and air. The trachea and the
esophagus split off from the base of the pharynx and transport
their contents to their respective destinations.
In order to prevent the food materials from going down the
trachea, the epiglottis, a ventral flap of tissue, covers it. The
muscle contractions moving the bolus are called peristalsis. At
the base of the esophagus, the bolus encounters a constriction, the
cardiac sphincter, that must relax and open before the bolus can
enter the stomach.
The stomach is a large "J-shaped" organ.
Its walls have three layers of muscle that churn the food materials
over and over. The presence of food in the stomach causes the
release of gastrin, a hormone, which travels from the walls of the
stomach into the blood stream.
As gastrin circulates the body, it affects the stomach and causes
the release of gastric juice. Gastric juice contains water as well as
HCI and pepsinogen.
Under the influence of these, the bolus becomes acid chyme
(chyme, literally, means "runny").
The HCI has two functions. It creates an environment with a low
pH (about 2.5) that will kill any bacterial growth that may be on
the food material. HCI also reacts with pepsinogen and converts it
into pepsin, a protease. Pepsin is secreted as the precursor
pepsinogen rather than in its final active form. Being a protein
digesting enzyme, a cell would not survive the production of
active pepsin. The stomach walls produce mucous to protect
themselves from the pepsin. Deterioration of the mucous lining of
the stomach results in an ulcer.
The pyloric sphincter is similar to the cardiae sphincter, except it is
located at the base of the stomach where it controls the passage of
the liquid acid chyme, a small amount at a time, into the
duodenum (first part of the small intestine). The duodenum is
specialized by the presence of chemoreceptors, chemical sensitive
nerve endings that are able to detect the different biochemicals in
the food material. In this way, the digestive system can regulate
which secretions are released.
The presence of the acid chyme triggers the release of secretin
from the duodenum. Secretin is a hormone that travels through the
blood to the pancreas where it causes the release of pancreatic
juice. Sodium bicarbonate is a component of pancreatic juice.
Bicarbonate ions (HCO31-) over neutralize the acid chyme and it
becomes alkaline with a pH of about 8.5. The other components of
pancreatic juice are enzymes that are active at this alkaline pH.
The presence of lipids in acid chyme causes the release of a
hormone called CCK (cholecystokinin). CCK affects both the
gall bladder and the pancreas. It causes the release of bile and
pancreatic juice containing the lipase enzymes that will digest
The enzymes in pancreatic juice are:
a. lipase - converts lipids into fatty acids and glycerol
b. trypsin - a protease that breaks many peptide bonds to convert
protein into peptides of varying lengths
c. pancreatic amylase which breaks remaining starches into
d. nucleases - which partially convert nucleic acids into
These enzymes are active in the duodenum.
The small intestine also produces and secretes its own digestive
enzymes. The hydrolysis of disaccharides by enzymes like
maltase completes the digestion of carbohydrates.
Peptidases break any remaining peptide bonds in the proteins to
release amino acids, the unit molecules.
The last part of the small intestine is called the ileum. The ileum
is specialized to absorb the products of digestion. It has a huge
surface area, which maximizes the potential for absorption. It
is also lined with specialized structures for absorption. These
structures are called villi
The products of fat digestion enter the lacteals, part of the
lymphatic system. The rest of the products of digestion enter
the blood stream. The cells lining the villi are equipped with
lots of mitochondria to produce the A TP energy required for
the active transport of these products of digestion.
Once the nutrient value from the food is absorbed, the remains
are indigestible materials (cellulose), water, and other
components of the food that the body didn't require. These
things pass from the ileum through the ileo-caecal valve
(another sphincter) into the large intestine ( colon). The first
segment of the colon is called the caecum. Extending down
from the caecum is the appendix.
The major roles of the colon are to absorb a great amount of
the water that was added to the food material all the way
along the digestive system, and to house E. coli.
E. coli is a bacterium that metabolizes some of what our bodies
were unable to. It lives in symbiosis with humans because it is
able to obtain its nutrients from the waste material. Part of
what it does is release minerals, manufacture vitamins and
amino acids, which get absorbed along with the water into the
circulatory system. The bacteria begin the decomposition of
the waste materials and convert them to feces with a changed
color, smell, and texture. The last part of the colon is the
rectum that stores feces until defecation. It ends with the
anal sphincter, which controls defecation.