Enzymes that breakdown the large polymer food molecules into smaller soluble forms are called 'digestive juices'.
These digestive enzyme juices operate in the same way as enzymes discussed in other sections of the core materials.
Digestive juices are however secreted outside of the epithelial cells of the Alimentary canal into the lumen of the gut (for digestion).
Exocrine glands release their secretions into a duct. There are three different methods of secretion:
1. Merocrine in which secretion is via vesicles ( most common)
2. Apocrine in which a portion of the secreting cell is lost.
3. Holocrine
in which the whole cell is released.
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).
There is considerable variation in the way the ducts link together to provide tubular or alveolar shapes
Mc Graw Hill has an excellent account
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 is usually at the base of the cell and is the site of enzyme synthesis.
(c) Nucleus found at the base, sometimes oval.
(d) Golgi Apparatus for post translation modification of the enzymes.
(e) Granular substances often called zymogens contain the inactive precursors of the digestive enzymes. The enzymes are proteases and the inactive form prevents autodigestion of cellular proteins.
Other features: there are tight junctions between the adjacent cells and there is a small microvilli border.
The example here is the secretion of the hormone gastrin. Gastrin is a polypeptide hormone secreted by the mucous lining of the stomach; induces the secretion of gastric juice.
Gastric Juices are secreted by a combination of 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.
Enzyme immobilisation is when the protein molecule is attached to a fixed surface.
Being fixed to the membrane of the gut epithelium is more efficient since the enzyme is not removed (reused) and can be linked to secondary functions including membrane transport
Even when the epithelial cell is rubbed off, the enzyme action can still continue.
Maltase action in the gut:
Maltase in incorporated as an immobilised enzyme into the epithelial cells of the small intestine.
The disaccharide maltose in received into the active site of the cell membrane enzyme.
The glycosidic bond is hydrolysed into glucose molecules.
Monosaccharides are absorbed along with ions such as Na+
Humans cannot digest cellulose.
Humans do not produce the cellulase enzymes 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
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 endocrine cells of the stomach and pancreas respectively.
If they are synthesised in an active form they will autodigest the internal protein structures of the endocrine 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.
The stomach has an acid environment caused by the secretion of HCl from the Parietal cells.
This acid environment is a barrier to infection from microorganisms ingested with food.
To protect the stomach wall from protein digesting enzymes and acid a mucus lining covers the surface.
Stomach ulcers occur with an infection of the bacterium Helicobacter pylori.
Stomach cancers may have a range of causes including those associated with the presence of the bacterium.
Robin Warren and Barry Marshall (2005) received the Noble Prize for Medicine for this work that replaced the previous hypothesis of causation.
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 can access.
The hydrolysis of the triglyceride has produced water soluble glycerol and fatty acids surrounded by bile salts
Absorption: the 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.
g) The lacteals, lymphatic system and the lipoproteins eventually enter the general circulation
H2.1. Gland secretions into the alimentary canal
H2.2 Exocrine gland secretion.
H2.3 A comparison of gland secretions
H2.4 Nervous and hormonal control of digestive gland secretions
H2.5 Membrane bound enzymes in gut epithelium
H2.7 Activation of Pepsin and Trypsin enzymes.
H2.8 The development of stomach ulcers and stomach cancers
H2.9 The role of bile in lipid digestion.