Download Digestion notes D.2 File

Topic H.2 – Digestion
H.2.1 - State that digestive juices are secreted into the alimentary canal by glands,
including salivary glands, gastric glands in the stomach wall, the pancreas and
the wall of the small intestine.
 The alimentary canal of the digestive system includes the mouth, esophagus,
stomach, small intestine, and large intestine. Food travels in one direction
through the alimentary canal and is digested along the way, nutrients are
absorbed and waste products are formed.
 The digestive system also contains accessory organs which contribute secretions
that aid in the chemical digestion of food. These accessory glands include the
salivary glands, gastric glands, pancreas, liver and glandular cells in the intestinal
wall.
 Remember the primary role of digestion is to convert the large macromolecules we
eat into their much smaller subunits that can then be absorbed into our cells.
These accessory glands contribute digestive secretions or “juice” that aid in the
breakdown of macromolecules.
 The following chart summarizes the digestive glands and their secretions.
Digestive gland
Secretion
Saliva containing salivary
Salivary gland
amylase
Mucus, hydrochloric acid
Gastric glands
and pepsinogen
Pancreatic juice
containing a protease,
more amylase and lipase.
Pancreas
Also contains a form of
hydrogen carbonate to
neutralize stomach acids
Liver
Bile
Intestinal glandular Variety of digestive
cells
enzymes
Action
Begins digestion of
polysaccharides
Begins digestion of proteins
in the stomach
Continues digestion of
carbohydrates and proteins
that began in the mouth and
stomach. Lipase digests fats
once they have been
emulsified by bile.
Emulsifies lipids
Completes digestion of the
macromolecules
H.2.2 - Explain the structural features of exocrine gland cells.
 An exocrine gland is a collection of cells that produce a product and release it into
a very specific location. An exocrine gland cell contains ducts which transfer a
secretion from the gland to a specified location.
 More often than not the secretion is a protein either in the form of a digestive
enzyme or a hormone. Due to the fact that the secretions from an exocrine gland
are often proteins these glands tend to contain high numbers of organelles that
are involved in the synthesis and processing of proteins. This would include high
numbers of ribosomes, endoplasmic reticulum, Golgi bodies, vesicles, and
mitochondria.
 The pancreas is an example of an exocrine gland that secretes enzymes into a
duct. The exocrine cells of the pancreas are grouped around the end of a very
small branch of the much larger pancreatic duct known as a ductule. The
ductules empty into larger and larger ducts until the secretions reach the
pancreatic duct and can then be released into the small intestine. The grouping
or arrangement of the exocrine glands around the ductule is called an acinus.
H.2.3 - Compare the composition of saliva, gastric juice and pancreatic juice.
Saliva
 Solvent is water
Gastric Juice
 Solvent is water
Pancreatic Juice
 Solvent is water
 Amylase
 Mucus
 Amylase
 Mucus
 Hydrochloric acid
 Bicarbonate
 Pepsin in the form of
pepsinogen
 Trypsin in the form of
trypsinogen
 Lipase
Source: Heinemann Baccalaureate Biology Higher. New York : Heinemann International
Inc., 2007.
H.2.4 - Outline the control of digestive juice secretion by nerves and hormones,
using the example of secretion of gastric juice.
 This concept had its beginnings when Ivan Pavlov performed his classic
experiment with salivating dogs and how the body prepares itself for digestion of
food. You may recall that the dogs would begin to salivate at the occurrence of a
stimulus that signaled food was on its way.
 Digestive juices are not continuously secreted. They are only released when they
are needed to hydrolyze molecules. For example, digestive enzymes in the
stomach are released once a stimulus is received. As we saw earlier, the sight
and smell of food can stimulate the release of gastric juices. Presence of food in
the stomach stimulates receptors in the stomach wall to send a message to the
brain which in turn signals the secretion of more gastric juices.
 There is also a hormone called gastrin involved in the control of the secretion of
gastric juices. When the stomach is distended (widened due to presence of food)
it promotes the production of gastrin. Gastrin is a hormone that causes a
sustained release of gastric fluid, most of all hydrochloric acid.
H.2.5 - Outline the role of membrane-bound enzymes on the surface of epithelial
cells in the small intestine in digestion.
 Most enzymes which enter the alimentary canal catalyse their specific hydrolytic
reaction by mixing with the substrates that have been consumed. They often
have a limited molecular lifespan and are normally digested themselves or
eliminated as wastes.
 There are some exceptions which are membrane-bound digestive enzymes which
are produced by and remain in the membranes of the cells (epithelial) lining the
small intestine.
 An example of such an enzyme would be maltase. Maltase breaks down the
disaccharide maltose into two glucose molecules. Maltase remains embedded
within the membranes of the inner epithelial cells of the villi and microvilli. As
maltose comes in contact with the active site of maltase the enzymes catalyses
the hydrolysis reaction.
 The advantage of the membrane bound enzymes is that once the reaction has
been catalysed the end products are exactly where they need to be in order to be
absorbed.
H.2.6 - Outline the reasons for cellulose not being digested in the alimentary
canal.
 Despite the fact that many mammals are herbivores, no species of mammal
including humans produces an enzyme that can digest cellulose, a
polysaccharide composed of thousands of glucose molecules and a major
structural component of plant cell walls.
 Mammals known as grazers (i.e. cows) contain a huge number of mutualistic
bacteria in their intestines which produce an enzyme known as cellulase that can
hydrolyze cellulose into glucose. Despite the number of bacteria present these
grazers still cannot get a high yield of energy from plant material so they must
continually ingest large amounts of plant matter.
 Due to the fact that we do not contain cellulase producing bacteria and we cannot
produce cellulase on our own, most plant material we consume exits the body in
our feces.
H.2.7 - Explain why pepsin and trypsin are initially synthesized as inactive
precursors and how they are subsequently activated.
 Pepsin and trypsin are known as proteases; enzymes which catalyze the
hydrolysis of peptide bonds in proteins. The difficulty is that proteases cannot
distinguish between a protein that has been ingested and a structural protein that
is part of the human body.
 In order to control the hydrolysis of essential proteins these proteases are initially
released in an inactive molecular form. Pepsin is released as pepsinogen and
trypsin is released as trypsinogen.
 The inactive molecular form of pepsin has 44 additional amino acids attached to
the primary structure of the enzyme. When pepsinogen is released into the
stomach it is exposed to hydrochloric acid which removes the extra 44 amino
acids converting pepsinogen into pepsin, the active protease. There is a lining of
mucus that protects the lining of the stomach from being digested by pepsin and
hydrochloric acid.
Pepsinogen
Pepsin
 Trypsinogen is released from the pancreas via the pancreatic duct and enters the
small intestine at the duodenum. When the partially digested food from the
stomach enters the small intestine it stimulates the release of an enzyme known
as enterokinase. Enterokinase converts trypsinogen into its active form trypsin.
H.2.8 - Discuss the roles of gastric acid and Helicobacter pylori in the
development of stomach ulcers and stomach cancers.
 It was believed for a long time that stomach ulcers were caused by too much
hydrochloric acid in the stomach perhaps brought on by stress. Until very
recently scientists believed that the acidic level in the human stomach was too
high for any organism to survive there.
 Dr. Barry J. Marshall and Dr. J. Robin Warren isolated bacterial cells from the
intestines of patients with stomach ulcers in 1982-83. This has lead us to what
we now know and understand about this strain of bacteria known as Helicobacter
pylori.
 These bacteria can survive in the human stomach by burrowing beneath the
mucus layer and infecting the cells of the lining of the stomach. They use the
enzyme urease to create ammonia which neutralizes the stomach acids. The
infection in the cells leads to gastritis and stomach ulcers.
 Patients can be treated with antibiotics but those who have had gastritis for 20-30
years have a significant increase in their risk of stomach cancer as compared to
the general population.
H.2.9 - Explain the problem of lipid digestion in a hydrophilic medium and the role
of bile in overcoming this.
 Lipids serve many important functions within the human body (think
phospholipids!) however they are difficult to digest due to their insolubility in
water. As we saw earlier the solute for digestive enzymes is water and lipids are
not water soluble. This means we have hydrophobic molecules in a hydrophilic
medium.
 When lipids are exposed to an aqueous environment they tend to stick together or
coalesce. When the molecules clump together it decreases the surface area in
comparison to the volume of molecules. This means that the lipid molecules on
the outside of this cluster of lipids can be digested but the enzymes cannot reach
the lipid molecules in the interior.
 This problem is overcome by the addition of bile to the small intestine. Bile is
produced by the liver and stored in the gall bladder. Bile molecules have both a
hydrophobic and hydrophilic end which allows them to be partially soluble in both
water and lipids. This means that bile molecules can wedge themselves inside
that large lipid globule, break them apart and prevent them from coalescing again
into the large lipid molecule. This process is known as the emulsification of lipids.
The now much smaller lipid molecules can be digested by lipase.