H2 Digestion
H2.1 Exocrine Secretions: table of gland secretions into alimentary canal
Secretion (action)
Gland Location
Salivary Amylases (Amylose to Maltose)
Mouth
Pepsin (Protein to Polypeptides)
Stomach
Pancreatic Amylases (Amylose to Maltose)
Trypsin, an endopeptidase (Protein to Polypeptides)
Chymotrypsin, an endopeptidase (Protein to Polypeptides)
carboxypeptidases, exopeptidases (peptides to amino acids)
Pancreas
Carbohydrases: maltase, sucrase, lactase (maltose, sucrose
and lactose to glucose)
Dipeptidases (DIpeptides to Amino Acids)
Enterokinase- (activates the reaction trypsinogen to trypsin)
Small Intestine
H2.2 Structural features of exocrine glands
Exocrine glands release their secretions into a duct. There are three different methods of
secretion:
1. Merocrine in which secretion is via vesicles (the most common) (lactating mammary
glands)
2. Apocrine in which a portion of the secretion cell is lost. (sweat glands)
3. Holocrine in which the whole cell is released.(sebaceous glands)
There is considerable variation in the way the ducts link together to provide tubular or
alveolar shapes
 Secretory cells form a single layer around the
duct.
 The secretory cells are surrounded by a
basement membrane.
 The secretory cells of an acinus will release
the secretion into the lumen of the duct.
 Ducts open onto surfaces such as the skin or
another cavity (Mouth, Alimentary canal).
H2.3 Structural features of exocrine gland cells as seen in electron micrographs
(a) Typical oval nucleus at the base of the
cell
(b) Number of small mitochondria
(c) Golgi body for the modification of post
translational proteins
(d) Rough endoplasmic reticulum in which
the enzymes for secretion are synthesized
(e) Zymogen particles (inactive precursor
particles) of digestive enzymes (store of
Diagram: Exocrine cell of Pancreatic acini
inactive enzymes for secretion)
(f) Small microvilli border
(g) Tight junctions (prevent leakage)
between the apex of adjacent cells
:
An electron micrograph showing some of the
features that might be seen in a pancreatic acinar
cell
(a) Mitochondria (which tend to be fairly small in a
pancreatic acinar cell).
(b) Rough Endoplasmic reticulum (usually at the
base of the cell ) is the site of enzyme synthesis..
(c) Nucleus .
(d) Golgi Apparatus for post translational
modification of the enzymes.
(e) Granular substances often called zymogens and
contain the inactive precursors of the digestive
enzymes. (The enzymes are proteases and the
inactive form prevents auto digestion of cellular
proteins.)
H2.4 State the contents of saliva, gastric juice and pancreatic juice
Substance
Saliva
Gastric Juices
Pancreatic
Juices
Contents



Mucus that serves as a lubricant,
Alpha-amylase, an enzyme that initiates the digestion of starch,
Slightly alkaline electrolyte solution that moistens food.





Hydrochloric Acid
Pepsin (Endopeptidase)
Intrinsic factor ( helps Vit B12 absorption)
Salt and Water
Mucus






Trypsin
Chymotrypsin
Carboxypeptidase
Lipase
Amylase.
Bicarbonate ions; helps to neutralize acidic gastric juice stomach.
H2.5 The control of digestive juice secretion by nerves and hormones
The example shows the secretion of the hormone Gastrin, a polypeptide hormone secreted by the mucous
lining of the stomach; which induces the secretion of gastric juice.
Gastric Juices are secreted by a combination of stimuli and responses:
a) The smell of food leads to a reflex in which
b) gastric juices are released into the stomach.
c) i)The physical presence of food in the lower region of the stomach
stimulates the endocrine cells within then stomach wall to release gastrin.
ii) Gastrin travels through the blood stream to its target tissue which are
the gastric juice cells of the stomach itself.
H2.6 The role of membrane-bound enzymes in the surface cells of the small
intestine
The immobilization of the
enzyme within the epithelial
cell of the villi increases the
efficiency of dipeptide
digestion to amino acids.
The fixing of the enzyme
prevents their removed from
the gut.
Even when rubbed off from
the epithelial surface it can
still continue to function
within the gut.
note: membrane transport
In this second example there
is a membrane bound
disaccharidaze (Maltase)
which provides a method of
completing the digestion of
double sugars like maltose.
Other membrane molecules
complete the absorption of
the monosaccharides.
Note that absorption of
monosaccharides is both an
active process and requires
inorganic ions like sodium
H2.7 Cellulose Digestion
Humans cannot digest cellulose.
Cellulose
Humans do not produce the cellulase enzymes
required to digest this polysaccharide.
Humans do not have bacteria or protozoan in the
gut which produce cellulase (as are found in many
herbivores).
Cellulose is the major constituent of the plant cell
wall.
Undigested within the gut, cellulose is known
within the diet as fiber.
Fiber creates bulk (mass ) which is a stimuli to
maintain peristalsis
H2.8 Activation of pepsin and trypsin
Pepsin and trypsin protease enzymes: Both are endopeptidases and hydrolyze peptide bonds in proteins.
Pepsin and trypsin are synthesized inside endocrine cells of the stomach and pancreas
They are synthesized in an inactive form to prevent auto digestion the proteases.
(a) i) Pepsinogen is the inactive precursor of
Pepsin.
ii) HCl acid is secreted from parietal cells and
activates pepsinogen to pepsin in stomach lumen
(b) i)Trypsinogen is produced by acinar cell of the
pancreas.
ii) Enterokinase is produced by the epithelial
cells of the small intestine
iii) Enterokinase activates the trypsinogen to the
active trysin.
H2.9 Action of endopeptidases and exopeptidases
Endopeptidases are specific for hydrolyzing
internal peptide bond within a peptide. This cuts
the protein to make smaller polypeptides.
This increases the number of terminal (end)
amino acids available for hydrolysis.
Exopeptidases hydrolyze the terminal amino
acid ( carboxy or amino terminal group) at either
end of the small polypeptide chain
Endopeptidases therefore increase the number of
substrate sites for the action of exopeptidases.
Alternatively Endopeptidases could be regarded
as an increasing the surface area for the action of
the exopeptidases
H2.10 Lipid digestion in a hydrophilic medium and the role of bile
Overview of lipid digestion:
Lipid (fats and oils) is insoluble in water
(hydrophobic).
Lipids tend to coalesce into larger droplets
which reduces the surface area for
digestion.
The hydrophobic lipid in the diagram is only
accessible to the water soluble lipases at the
interface between lipid and water.
To increase the access (increased surface
area) and rate of lipid digestion the lipid
droplet must be broken up.
Bile salts secreted from the liver (via
gallbladder) have molecules with a
combination of hydrophobic and (lipophilic)
hydrophilic regions.
Bile salts break up the lipid droplet into
many smaller droplets thereby increasing
the surface area of lipid-water access
Lipid-Water Interface
This diagram illustrates that the increase in
surface area of the lipid-water interface also
increases the presence of substrate for the
lipases.
Note that the bile salts orientate the
triglyceride with the glycerol head into water
and the fatty acid tails into the salt.
The glycerol section of the triglyceride is
hydrophilic.
The fatty acid tails of the triglyceride are
hydrophobic.
The linkage between the two (ester bonds)
is thus presented at the water-lipid interface
which the water soluble lipase (c) can
access.
The hydrolysis of the triglyceride has
produced water soluble glycerol (e) and
fatty acids surrounded by bile salt (d)
.
Absorption of lipids: this system has other
subtle adaptations for absorption
(a) Bile salts and fatty acids. The
phospholipid structure of the salts allows it
to fuse with the cell membrane and the fatty
acid molecules to pass into the epithelial
cells of small intestine villus.
b) The fatty acids and glycerol recombine in
the endoplasmic reticulum to form lipid.
c) Protein is added to the lipid to form
lipoprotein. This is how lipid is transported
around the body.
d) The lipoprotein is formed into vesicles
called chylomicrons.
e) Exocytosis of the vesicles releases the
lipoprotein from the cell
f) The lipoprotein is taken up in the lacteal
vessel a branch of the lymphatic system.
note: glycerol uptake is thought to occur via
carrier mediated transport across the
membrane.
g) The lacteals, lymphatic system and the
lipoproteins eventually enter the general
circulation