Hormonal Control of Digestion
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Transcript Hormonal Control of Digestion
Digestive Enzymes
• Throughout the digestive system, enzymes break down the
food into useful substances.
Recall that enzymes are protein catalysts – they speed
up chemical reactions.
The substance that an enzyme acts on is called its
substrate.
Digestive Enzymes
• Enzymes are contained within the digestive juices that are
secreted into the digestive tract by exocrine glands.
An exocrine gland is a gland that secretes substances
through ducts or tubes onto a body surface or into a cavity.
This is in contrast to endocrine glands, which secrete
substances directly into the blood stream.
Exocrine Glands
• The exocrine glands that play an important role in the digestive
system are the:
salivary glands
gastric glands in the stomach wall
pancreas
glands in the wall of the small intestine
Saliva
• Saliva is produced by the salivary glands.
• It contains mucus and the enzyme salivary amylase.
breaks down starch into maltose (a disaccharide)
amylase
maltose
starch
Gastric Juice
• Gastric juice is produced by the gastric glands in the stomach wall.
• It contains:
mucus
hydrochloric acid
the inactive enzyme precursor pepsinogen
Some digestive enzymes are secreted as inactive precursors because, in
their active state, they would be potentially very harmful to the
exocrine gland cells that produce and secrete them.
Pepsin
• Pepsinogen is activated by the hydrochloric acid, which
converts it into pepsin.
• Pepsin converts proteins into peptides.
pepsin
protein
peptides
Pancreatic Juice
• Pancreatic juice is produced by the exocrine glands in the
pancreas
• It contains:
bicarbonate ions (alkaline)
many enzymes, including pancreatic amylase and
pancreatic lipase
the inactive enzyme precursor trypsinogen
Pancreatic Amylase
• Pancreatic amylase carries out the same reaction as salivary
amylase:
amylase
maltose
starch
Pancreatic Lipase
• Pancreatic lipase breaks down triglycerides into glycerol and
fatty acids.
glycerol
lipase
triglycerides
fatty acids
Pancreatic Lipase
• The breakdown of lipids by pancreatic lipase poses special
problems, because lipids are insoluble in the aqueous
environment of the digestive tract.
As food travels through the digestive tract, the lipids within
them melt and coalesce into large droplets.
Lipase is water-soluble, so is unable to enter the lipid droplets
and would only be able to breakdown the lipids on the surface
of the droplets.
Food does not remain in the digestive tract long
enough for lipase to be able to completely digest the
lipids in this manner.
Bile
• This problem is overcome by the action of bile.
• Bile molecules have a hydrophobic end and a hydrophillic end,
so are able to interact with both the lipids and the water,
causing the lipids to break up into smaller droplets. This
process is called emulsification.
• This speeds up the digestion of the lipids in the small intestine.
http://www.biologyinmotion.com/bile/index.html
Trypsin
• Trypsinogen is activated by an enzyme called enteropeptidase,
which is secreted by the lining of the small intestine.
Enteropeptidase converts trypsinogen to trypsin.
• Trypsin continues the breakdown of proteins.
trypsin
peptides
smaller peptides
Maltase
• Glands in the wall of the small intestine produce enzymes such
as maltase (a disaccharidase).
• Maltase breaks down maltose into two glucose molecules.
maltase
maltose
2
glucose
Digestive Enzymes
• Enzymes produced by the wall of the small intestine are not
secreted like the other enzymes of the digestive tract.
• Instead, they remain attached to the plasma membrane of the
cells lining the intestine, with their active sites exposed to the
food in the intestine.
• With this arrangement, the substrates can be digested and then
the products of digestion can immediately be absorbed into the
body.
Digestive Enzymes
• Some macromolecules cannot be digested by humans, for
example, cellulose.
Humans cannot digest cellulose because they do not have
the gene that produces the enzyme cellulase.
Control of Digestive Juice Secretion
• The secretion of digestive enzymes is regulated by both nerves
and hormones.
• Nerves carry signals to and from the brain and cause specific
responses throughout the body.
• Hormones are chemical regulators that are secreted in one part
of the body (by endocrine glands) and transported by the
bloodstream to another part, where they cause a response.
Control of Digestive Juice
Secretion
• The sight or smell of food causes the brain to send nerve impulses to the
stomach to start secreting gastric juices.
• Much more gastric juice is secreted once food enters the stomach:
Receptors in the stomach lining send impulses to the brain, which
then sends more impulses to the exocrine gland cells
Impulses are also send to endocrine glands in the stomach wall, which
then release the hormone gastrin into the bloodstream.
Gastrin travels to the upper part of the stomach where it
stimulates increased secretion of hydrochloric acid.