Chapter 26.1 Digestive System Processes

This textbook is designed specifically for Kansas State's Biology 198 Class. The course is taught using the studio approach and based on active learning. The studio manual contains all of the learning objectives for each class period and is the record of all student activities. Hence, this textbook is more of a reference tool while the studio manual is the learning tool.
Authors: Robert Bear, David Rintoul, Bruce Snyder, Martha Smith-Caldas, Christopher Herren, and Eva Horne
Kansas State University Libraries
New Prairie Press
Bear, Robert; Rintoul, David; Snyder, Bruce; Smith-Caldas, Martha; Herren, Christopher; and Horne, Eva, "Principles of Biology" (2016). Open Access Textbooks. 1. https://newprairiepress.org/textbooks/1
The textbook was originally published and is also available to download at http://cnx.org/contents/[e-mail] is licensed under a Creative Commons Attribution License 4.0 license.

Episode Transcript

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Welcome to Principles of Biology. Thisbook was written by the Open Alternative Textbook
Initiative at Kansas State University and isbeing released as a podcast and distributed under
the terms of the Creative Commons AttributionLicense. Today's episode is Chapter twenty six
point one Digestive system processes. Allhyperlinks, images and sources can be found

(00:22):
at the link to the book.In the description, every creature has its
own food and an appropriate alchemist withthe task of dividing it. The alchemist
takes the food and changes it intoa tincture, which he sends through the
body to become blood and flesh.This alchemist dwells in the stomach, where
he cooks and works. The maneats a piece of meat in which is

(00:44):
both bad and good. When themeat reaches the stomach, there is the
alchemist who divides it. What doesnot belong to health. He casts away
to a special place and sends thegood wherever it is needed. Philippus or
Realists Paracelsus in voluming thes Sinny Paramyromecirc Of fifteen twenty. Obtaining nutrition and
energy from food is a multi stepprocess that, contrary to the thinking of

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Paracelsis does not involve alchemy or analchemist. Many physical and biochemical processes are
involved in digestion of food, andit is also a highly regulated process for
true animals. The first step isingestion, the act of taking in food.
This is followed by digestion, absorption, and elimination. In the following

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sections, Each of these steps willbe discussed in detail. Ingestion, the
large polymeric molecules found in intact foodcannot pass through plasma membranes, so these
polymers need to be broken into smallermonomers so that animal cells can absorb and
metabolize them to produce energy. Thefirst step in this process is ingestion.

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Ingestion is the process of taking infood through the mouth. Invertebrates, the
teeth, saliva, and tongue playimportant roles in mastication, preparing the food
into a bolus. While the foodis being mechanically broken down, the enzymes
in saliva begin to chemically process thefood as well. The combined action of
these processes modifies the food from largeparticles to a soft mass that can be

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swallowed and can travel the length ofthe esophagus. Digestion and absorption Digestion is
the mechanical and chemical breakdown of foodinto small organic molecules. It is important
to break down macromolecules into smaller monomersthat are of suitable size for absorption across
the digestive epithelium. Large complex molecules, example, proteins, polysaccharides, nucleic

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acids, and lipids must be hydrolyzedinto monomers before they can be absorbed by
the digestive epithelial cells. If thisterminology seems a bit crazy, you may
want to review hydrolysis reactions link beforeproceeding. Different organs play specific roles in
the digestive process. The animal itneeds carbohydrates, protein, nucleic acids,

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and fat, as well as vitaminsand inorganic components for nutritional balance. We
will briefly discuss digestion and absorption ofsome of these in the sections below.
Carbohydrates. The digestion of carbohydrates beginsin the mouth. The salivary enzyme amalase
begins the breakdown of food starches intomaltose a disaccharide. No significant further digestion

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of carbohydrates takes place in the stomach. The esophagus produces no digestive enzymes,
but does produce mucus for lubrication.The acidic environment in the stomach inhibits the
action of the salivary amalase enzyme.The next step of carbohydrate digestion takes place
in the duodenum. The chime fromthe stomach enters the duodenum and mixes with

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the digestive secretions from the pancreas,liver, and gall bladder. Pancreatic juices
also contain an amalase enzyme, whichcontinues the breakdown of arch and glycogen into
maltose, a disaccharide. The disaccharidesare broken down into monosaccharides by enzymes called
maltases, sucrases, and lactases,which are also present in cells lining the

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small intestine. Maltase breaks down maltoseinto glucose. Other disaccharides, such as
sucrose and lactose, are broken downby sucrase and lactase, respectively. Sucrase
breaks down sucrose or table sugar intoglucose and fructose, and lactase breaks down
lactose or milk sugar into glucose andgalactose. The monosaccharides, example, glucose

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and fructose, thus produced, areabsorbed by the intestinal cells and transported into
the bloodstream. The steps in carbohydratedigestion are summarized in figure and table.
Pathways for the breakdown of starch andglycogen sucrose and lactose are shown. Starch
and glycogen, which are both polysaccharides, are broken down into the disaccharide maltose.

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Maltose is then broken down into themonosaccharide glucose. Sucrose. A disaccharide
is broken down by sucrose into themonosaccharides glucose and fructose. Lactose, also
a disaccharide, is broken down bylactase into glucose and galactose. Digestion of
carbohydrates is performed by several enzymes.Starch and glycogen are broken down into glucose

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by amylase and maltase. Sucrose,table sugar and lactose. Milk sugar are
broken down by sucrase and lactase,respectively. Digestion of carbohydrates protein. A
large part of protein digestion, takesplace on the stomach. The enzyme pepsin
plays an important role in the digestionof proteins by breaking down the intact protein

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to peptides, which are short chainsof four to nine amino acids. In
the duodenum, other enzymes trypsin,elastase and chymotrypsin act on the peptides,
reducing them to smaller peptides. Trypsinelastase, carboxypeptidase, and chymotrypsin are produced
by the pancreas and released into theduodenum, where they act on the chyme.

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Further breakdown of peptides to single aminoacids is aided by enzymes called peptidases.
Those that break down peptides, specifically, carboxypeptidase, dipeptidase, and aminopeptidase
play important roles in reducing the peptidesto free amino acids. The amino acids
are absorbed into the blood stream throughthe small intestines. The steps in protein

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digestion are summarized in figure and table. Protein digestion begins in the stomach,
where pepsin breaks proteins down into fragmentscalled peptides. Further digestion occurs in the
small intestine, where a variety ofenzymes break peptides down into smaller peptides and
then into individual amino acids. Severalof the protein digesting enzymes found in the

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small intestine are secreted from the pancreasamino acids are absorbed from the small intestine
into the blood stream. The liverregulates the distribution of amino acids to the
rest of the body. A smallamount of dietary protein is lost in the
feces. Protein digestion is a multistep process that begins in the stomach and

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continues through the intestines. Digestion ofprotein. The bulk of lipid digestion occurs
in the small intestine via the actionof pancreatic lipase. When kime enters the
duodenum, it triggers a hormonal response, resulting in the release of bile,
which is produced in the liver andstored in the gall bladder. Biolates in
the digestion of lipids, primarily triglyceridesby emulsification. Emulsification is a physical process

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in which large lipid globules are dispersedinto several small lipid globules. Lipids are
hydrophobic substances in the presence of water, they will aggregate to form large globules
to minimize exposure to water. Thesesmall globules have a larger surface tovolume ratio
and less an increased surface area forthe lipases to interact with. Bile contains

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bile salts, which are amphipathic,meaning they contain hydrophobic and hydrophilic parts.
Thus, the bile salts hydrophilic sidecan interface with water on one side and
the hydrophobic side interfaces with lipids onthe other. By doing so, bile
salts emulsify large lipid globules into smalllipid globules. By forming an emulsion,

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bile salts increase the available surface areaof the lipid particles significantly. The pancreatic
lipases can then act on the lipidsmore efficiently and digest them, as detailed
in figure. Lipases break down thedietary triglycerides into fatty acids and monoglycerides one
fatty acid attached to a glycerol molecule. These molecules can pass through the plasma

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membrane of the cell and enter theepithelial cells of the intestinal lining. Lipase
products fatty acids and monoglycerides pass throughthe intestinal cells, where they are reassembled
into triglycerides and then are combined withproteins to form large fatty complexes called kilo
microns. Kilo microns contain triglycerides,cholesterol, and other lipids, and have

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proteins on their surface. The surfaceis also composed of the hydrophilic phosphate heads
of phospholipids. Together, they enablethe kylo micron to move in an aqueous
environment without exposing the lipids to water. Kilo Microns leave the absorptive cells via
exocytosis. Kilo Microns enter the lymphaticvessels and then enter the blood via the

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thoracic duct on their way to theliver. Illustration shows a row of absorptive
epithelial cells that line the intestinal lumin. Hair like microvillide project into the lumin.
On the other side of the epithelialcells are capillaries and lymphatic vessels.
In the intestinal lumin, lipids areemulsified by the bile. Lipases break down

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fats, also known as triglycerides,into fatty acids and monoglycerides. Fats are
made up of three fatty acids attachedto a three carbon glycerol backbone. In
monoglycerides, two of the fatty acidsare removed. The emulsified lipids form small
spherical particles called my cells that areabsorbed by the epithelial cells. Inside the

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epithelial cells, the fatty acids andmonoglerides are reassembled into triglycerides. The triglycerides
aggregate with cholesterol, proteins, andphospholipids to form spherical chylomicrons. The chylomicrons
are moved into a lymph capillary,which transports them to the rest of the
body. Lipids are digested and absorbedin the small intestine. Summary of digestion

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steps in mechanical and chemical digestion areshown. Digestion begins in the mouth,
where chewing and swallowing mechanically breaks downfood into smaller particles and enzymes. Chemically
digest carbohydrates stomach. Mechanical digestion includesperistaltic mixing and propulsion. Chemical digestion of
proteins occurs, and lipid soluble substancessuch as aspirin are absorbed in the small

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intestine. Mechanical digestion occurs through mixingand propulsion, primarily by segmentation. Chemical
digestion of carbohydrates, lipids, proteins, and nucleic acid occurs. Peptides,
amino acids, glucose, fructose,lipids, water, vitamins, and minerals
are absorbed into the blood stream.In the large intestine, mechanical digestion occurs

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through segmental mixing and mass movement.No chemical digestion occurs except for digestion by
bacteria. Water, ions, vitamins, minerals, and small organic molecules produced
by bacteria are absorbed into the bloodstream. Mechanical and chemical digestion of food
takes place in many steps, beginningin the mouth and ending in the rectum

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elimination. The final step in digestionis the elimination of undigested food content and
waste products. The undigested food materialenters the colon, where most of the
water is reabsorbed. Recall that thecolon is also home to the microflora called
intestinal flora that aid in the digestionprocess. The semisolid waste is moved through

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the colon by peristaltic movements of themuscle and is stored in the rectum.
As the rectum expands in response tostorage of fecal matter. It triggers the
neural signals required to set up theurge to eliminate the solid waste is eliminated
through the anus using peristaltic movements ofthe rectum. This podcast will be released

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episodically and follow the sections of thetext book in the description For a deeper
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