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.