Ms.TRS
H2.1 Gland Secretions into the
alimentary canal
Gland
location
Gland and digestive juice Main enzymes in the secretion
Mouth
Salivary gland - Saliva
Stomach
Gastric gland – gastric juice Pepsin (proteins to polypeptides)
Salivary amylase (starch to maltose
Exocrine part Pancreatic juice
of Pancreas
(intestine)
Pancreatic amylase (amylose to maltose)
Trypsin ( proteins to poly peptides –
endopeptidase)
Chymotrypsin (proteins to polypeptides –
endopeptidase)
Amino/carboxy peptidases(remove
individual amino acids from polypeptides)–
exopeptidases
Goblet cells
of Small
intestine
Carbohydrases – maltase, sucrase, lactase
Dipeptidases – dipeptides to amino acids
Enterokinase – activate trypsinogen to
Intestinal juice
Features of exocrine cell
(a)Rough Endoplasmic reticulum is usually at the base of
the cell and is the site of enzyme synthesis.
(b) Golgi Apparatus for post translation modification of
the enzymes.
(c) Granular substances often called zymogens contain the
inactive precursors of the digestive enzymes. The enzymes
are proteases and the inactive form prevents auto digestion
of cellular proteins.
Other features: there are tight junctions between the
adjacent cells and there is a small microvilli border.
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H2.2 The structure of the exocrine
gland
Exocrine glands release their secretions into a duct. There are three different methods of secretion:
•Exocrine glands release their secretions into a
duct.
•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 A comparison of gland
secretions
Digestive juice
Contents
Saliva
Amylase for starch digestion
Lingual lipase to digest triglycerides to fatty acids
Water and electrolytes to moisten
Mucus to lubricate food
Antibacterial compounds
Gastric juice
HCl – activate pepsin, kills bacteria
Mucus – protect stomach lining
Pepsin and rennin – digest proteins
Pancreatic juice
Trypsin,chymotrypsin, aminopeptidase and carboxy
peptidases – digestion of proteins
Pancreatic amylase –
Pancreatic lipase –
High concentration of bicarbonate ions to neutralise
acidic food
.
H2.4 The hormonal and nervous
control of gland secretions
Gastric Juices are secreted by a combination of
nervous and hormonal stimuli and responses:
Nervous:
a) The smell of food leads to a reflex in which
b) gastric juices are released into the
stomach.
Hormonal:
a)The physical presence of food in the lower
region of the stomach stimulates the endocrine
cells within then stomach wall to release
gastrin.
b) Gastrin travels through the blood stream to
its target tissue which are the gastric juice cells
of the stomach itself and stimulate production
of gastric juice
Movement of food to duodenum detected by
medulla oblongata and production of gastric
juice inhibited.
H2.5 Membrane bound enzymes in
gut
epithelium
Enzyme immobilisation is when the protein molecule is
attached to a fixed surface.
Being fixed to the membrane of the epithelium of the
duodenum is more efficient since the enzyme is not
removed (reused) and can be linked to secondary
functions including membrane transport.
Products are released close to the membrane for rapid
transport to the membrane
Digestion of the disaccarides(maltose)can occur early in the
digestive system
Even when the epithelial cell is rubbed off, the enzyme
action can still continue.
H2.6
Cellulose
digestion
 Humans cannot digest cellulose.
 Humans do not produce the cellulase enzymes
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required to digest this polysaccharide.
Humans do not have bacteria or protozoan's 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 fibre.
Fibre creates bulk (mass ) which is a stimuli to
maintain peristalsis
H2.7 The activation of pepsin and
trypsin
 Both pepsin and trypsin are protease enzymes.
Specifically that are both endopeptidases which
hydrolyse peptide bonds in proteins to produce smaller
polypeptides.
 Pepsin and trypsin are synthesised inside exocrine
cells of the stomach and pancreas respectively.
 If they are synthesised in an active form they will
autodigest the internal protein structures of the
exocrine cells.
 To prevent autodigestion the proteases are synthesised
in an inactive form.
•
Pepsinogen is the inactive precursor of Pepsin.
Pepsinogen in secreted by the Chief cells in the gastric
pits of the stomach.
HCl acid is secreted from the parietal cells and activates
the pepsinogen to pepsin in the lumen of the stomach.
Trypsinogen is produced by acinar cell of the pancreas.
Enterokinase is produced by the epithelial cells of the
small intestine
Enterokinase activates the trypsinogen to the active form
in the small intestine.
H2.8 The development of stomach
ulcers and stomach cancers.
 To protect the stomach wall from protein digesting enzymes and acid a
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mucus lining covers the surface.
Stomach ulcers are sore in the walls of the stomach which have become
inflamed or infected.
The development of stomach ulcers is associated with the presence of
the bacterium Helicobacter pylori.
H.pylori is able to survive the acid conditions of the stomach through
the secretion of ureases (enzyme) this leads to the local
neutralisation of the stomach acid
This process along with the production of proteases lead to damaged
epithelial cells (stomach lining) and to the mucus lining.
The damaged lining may be attacked by stomach acid and lead to the
formation of the ulcer.
It is further suggested that ulcers may lead in time to the formation of
tumours.
Stomach cancers may have a range of causes including those associated
with the presence of the bacterium.
H2.9 The role of bile in 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 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.
Absorption of lipids
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.
g) The lacteals, lymphatic system and the
lipoproteins eventually enter the general
circulation