Download File

H2-1 State that digestive juices are secreted into the
alimentary canal by glands including salivary
glands, gastric glands in the stomach wall, pancreas,
and wall of the small intestine
• In humans, 3 pairs of salivary glands secrete
saliva into the mouth through ducts in the mouth’s
mucosal lining
• Saliva moistens and lubricates food to make it
easier to swallow and keep it from scratching the
esophageal tissue as it moves toward the stomach
• Saliva also contains the enzyme, salivary amylase,
which starts the breakdown of carbohydrates (starch) in the
mouth into maltose
• Salivary gland secretion is controlled by the nervous
system, maintaining a constant level of secretion to keep the
mouth moist
• Food in the mouth triggers increased secretion via
taste-sensitive neurons which send impulses to the brain to
stimulate the salivary glands
• In the stomach, there is an extra layer of smooth
muscle for churning food and mixing it with gastric juice
• Gastric juice is an acidic secretion of the tubular
gastric glands of the mucosa
• These exocrine glands contain parietal cells, which
secrete hydrochloric acid, and chief cells which secrete
pepsinogen, a protein-digesting enzyme that is activated by
the low pH of the stomach which converts pepsinogen to
pepsin, the active form of the enzyme
• Having an inactive form of the enzyme (pepsinogen)
which gets converted to the active form (pepsin) outside of
the chief cells keeps the chief cells from digesting
themselves
• In the stomach, only proteins are partially digested
• The parietal cells also secrete intrinsic factor which is
a polypeptide necessary for the absorption of Vitamin B12
which is required for the production of rbcs
• Individuals who lack sufficient amounts of intrinsic
factor develop a type of anemia called pernicious anemia
• The pancreas is an accessory organ that contributes
secretions to the digestive tract
• Pancreatic fluid is released into the duodenum via the
pancreatic duct, therefore, the pancreas is an exocrine gland
• Pancreatic fluid contains trypsin and chymotrypsin
which digest proteins, pancreatic amylase which digests
starch, and lipase which digests fat; pancreatic fluid is
released by secretory cells called acini
• These enzymes are inactive as they are released into
the duodenum and are called zymogens
• Zymogens are activated by the brush border enzymes
of the small intestine
• The microvilli of the small intestine participate in
digestion because a number of digestive enzymes are
embedded within the epithelial cells’ plasma membrane,
with their active sites exposed to the chyme
• These brush border enzymes include those that
hydrolyze the disaccharides lactose and sucrose, as well as
others
• The brush border enzymes complete the digestive
process that started with the pancreatic enzymes released
into the duodenum
H2.2 Explain the structural features of exocrine
glands
• Multicellular exocrine glands are structurally
complex with 2 parts: an epithelium-derived duct and a
secretory unit consisting of secretory cells called acini
• Supportive connective tissue surrounds the
secretory unit and supplies it with blood vessels and
nerve fibers
• The connective tissue often forms a fibrous
capsule that extends into the gland proper and divides
the gland into lobes
• The pancreas, most sweat glands, and salivary glands
are merocrine exocrine glands and release their products by
exocytosis as they are produced, with the secretory cells not
being altered in any way
• The secretory cells of a gland produce the product
and secrete it (into acini).
• The acini collect the product and pass it into the
duct which leads into a lumen or out of the body
H2.3 Compare the composition of saliva, gastric
juice and pancreatic juice
Saliva
• Saliva is 99.5% water
• Inorganic constituents include salts (chlorides,
carbonates, phosphates, sulfates) and dissolved gases
• Organic constituents include the enzymes
amylase and lysozyme, proteins (mucin, albumin, and
globulins), small amounts of urea, and unusual waste
products such as acetone
• Epithelial cells and leukocytes are also present
Gastric juice
• The hormone gastrin increases the secretion of gastric
juice
• Pepsin digests proteins to peptones (secondary proteins; a
water soluble compound formed by partial hydrolysis of a protein
by an acid or enzyme during digestion)
• Hydrochloric acid converts pepsinogen to active pepsin
• Intrinsic factor in gastric juice combines with vitamin B12
(extrinsic factor) to prevent its digestion and promote its
absorption in the small intestine
• Gastric juice also contains mucus and the enzyme rennin
(chymosin)
Pancreatic juice
• Contains sodium bicarbonate which neutralizes the
acidity of chyme
• The enzyme trypsinogen is converted to the active
trypsin by enterokinase, and trypsin in turn converts
chymotrypsinogen to the active chymotrypsin (pancreatic
proteases)
• Amylase hydrolyzes starch to maltose and lipase
digests emulsified fats to fatty acids and glycerol
H2.4 Outline the control of digestive juice secretion
by nerves and hormones using the example of
secretion of gastric juice
• The presence of food in the mouth triggers the production
of the hormone gastrin by cells in the wall of the duodenum as
well as cells in the part of the stomach nearest the duodenum
• Secretions of the stomach are increased by
parasympathetic impulses (vagus nerve) after sight or smell of
food, and there is sustained release as stomach distention is
detected by stretch receptors
• Gastrin will stimulate acid production, and as pH falls too
low in the stomach, gastrin release is inhibited by the hormones
secretin and somastostatin
H2.5 Outline the role of membrane-bound enzymes
on the surface of epithelial cells in the small intestine
in digestion
• The villi and microvilli of the small intestine
greatly increase the surface area of the small intestine
• It is over this surface area that products of
digestion are absorbed
• The microvilli participate in digestion because a
number of digestive enzymes are embedded within the
epithelial cells’ plasma membranes, with their active
sites exposed to the chyme
• These brush border enzymes include those that
hydrolyze the disaccharides lactose and sucrose as well as
others and complete the digestive process that started with
the action of pancreatic enzymes released into the duodenum
• Some digestive enzymes, such as maltase, are
immobilized in the surface membrane of cells on the surface
of intestinal villi.
• These enzymes continue working even if the cell is
rubbed off the villus and mixed into the intestinal contents
H2.6 Explain the reasons for cellulose not being
digested in the alimentary canal
• Humans lack the enzyme necessary to digest
cellulose (cellulase), the carbohydrate that functions as
the chief structural component of plants
• Much of the food value of plants is tied up in
cellulose and the digestive tract of many animals
harbor colonies of cellulose-digesting microorganisms
• The relationships between these microorganisms
and their animal hosts are mutually beneficial and
provide an excellent example of symbiosis
• Animals that exhibit these symbiotic relationships
include cows and sheep with cellulose-digesting
bacteria and termites with an intestinal protozoan
• Intestinal microorganisms also produce molecules
such as vitamin K that are important to the well-being
of their vertebrate hosts
H2.7 Explain why pepsin and trypsin are initially
synthesized as inactive precursors, and how they are
subsequently activated
• Chief cells which are found in exocrine glands of the
stomach mucosa secrete pepsinogen, a weak protease that
requires a very low pH to be active
• The low pH is provided by the HCl which is also present
in the stomach
• Activated pepsinogen molecules then cleave one another
at specific sites, producing a much more active protease, pepsin
• The process of secreting an inactive enzyme which is
then converted into one that is more active outside the cell
prevents the chief cells from digesting themselves
• In humans, only proteins are partially digested in the
stomach
• Pancreatic fluid contains trypsin which along with
chymotrypsin, digest proteins in the small intestine
• Released primarily as the inactive enzymes,
trypsinogen and chymotrypsinogen, they are activated by
brush border enzymes of the intestine.
• Trypsinogen is activated by enterokinase (a brush
border enzyme) to trypsin, which in turn activates
chymotrypsinogen
• This prevents the pancreas from self-digestion
H2.8 Discuss the roles of gastric acid and
Heliobacter pylori in the development of stomach
ulcers and stomach cancers
• A stomach ulcer is an open sore in the stomach
wall where digestive juices (acid and the enzyme
pepsin) have begun to eat away at the stomach lining
• Twenty years ago, it was believed that stress
contributed to stomach ulcers and that cigarette
smoking, alcohol abuse, and eating spicy foods were
contributing factors
•
It is now recognized that about 80% of ulcers are
caused by infection from the bacterium called Heliobacter
pylori
•
The remaining 20% are caused by overuse of antiinflammatory drugs such as aspirin and ibuprofen
•
H. pylori survives in the stomach by producing the
enzyme urease which neutralizes stomach acid and allows
the bacterium to colonize the stomach’s mucous lining
•
This opens up the stomach wall to attack from
digestive fluids
•
Infection with H. pylori is now considered as a
primary cause of peptic ulcers and their recurrence
• The changes caused in the stomach lining by the
bacterium link it as a strong risk factor for stomach cancer.
• When an H. pylori infection is identified, it can be
treated with antibiotics.
• A blood test to check for H. pylori antibodies is
available and can be easily done.
H2.9 Explain the problem of lipid digestion in a
hydrophilic medium and the role of bile in
overcoming this problem
• The main exocrine secretion of the liver is bile
which is a fluid mixture consisting of bile pigments and
bile salts
• Bile salts play a very important role in the
digestion of fats
• Fats are insoluble in water making them difficult
to digest and enter the intestine as large drops within
the watery chyme
• Fat molecules are only accessible to the enzyme
lipase at the lipid-water interface and can only work on the
surface of the lipid sphere making digestion very slow
• Lipase has an active site to which the hydrophobic
lipid molecule can bind
• The bile salts are lipid-soluble and partly watersoluble
• They work like detergents and disperse the large fat
drops into a fine suspension of smaller droplets
• This process, called emulsification, produces a
greater surface area of fat upon which lipase can act, and
allows the digestion of fats to occur more rapidly
• Fats are pretreated with bile salts
• Lipase attaches at the water-lipid
interface
• The polar parts of the bile salt face
the aqueous phase with the non-polar part
clinging to the fat globule
• The polar parts repel each other
• They interact with water causing
fatty globule to be broken up into smaller fat
droplets forming an emulsion
Chemical digestion
Using a range of specialised enzymes that work in different
conditions of the different compartments of the organsystem.