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Digestive EnzymesDigestive 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.
Exocrine GlandsExocrine Glands
Enzymes production:
salivary glands
gastric glands in the stomach wall
pancreas
glands in the wall of the small intestine
SalivaSaliva
Saliva is produced by the salivary glands.
It contains mucus and the enzyme salivary amylase.
breaks down starch into maltose (a disaccharide)
starch
amylase
maltose
Gastric JuiceGastric Juice
Gastric juice is produced by the gastric glands in the stomach wall.
It contains:
mucus
hydrochloric acid
the precursor
PepsinPepsin
Pepsinogen is activated by the hydrochloric acid, which converts it into pepsin.
Pepsin converts proteins into peptides.
protein
pepsin
peptides
Pancreatic JuicePancreatic 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 precursor trypsinogen
Pancreatic AmylasePancreatic Amylase
Pancreatic amylase carries out the same reaction as salivary amylase:
starch
amylase
maltose
Pancreatic LipasePancreatic Lipase
Pancreatic lipase breaks down triglycerides into glycerol and fatty acids.
triglycerides
lipase
glycerol
fatty acids
Pancreatic LipasePancreatic Lipase
Lipase reacts too slowly to break apart lipids. To overcome this problem, fat molecules are broken
apart into smaller molecules.
BileBile
Bile 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
TrypsinTrypsin
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
smaller peptidespeptides
MaltaseMaltase
Glands in the wall of the small intestine produce enzymes such as maltase (a disaccharidase).
Maltase breaks down maltose into two glucose molecules.
maltose
maltase
glucose
2
Digestive EnzymesDigestive 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 EnzymesDigestive 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.