H2.1 Gland Secretions into the alimentary canal

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).

digestive glands

 

 

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H2.2 The structure of the exocrine gland

acinusExocrine 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.

There is considerable variation in the way the ducts link together to provide tubular or alveolar shapes

Mc Graw Hill has an excellent account

 

Electron Micrograph Structure of the pancreatic acinus

micrograph acinus

An electron micrograph showing some of the features that might be seen in a pancreatic acinar cell:

Other features: there are tight junctions between the adjacent cells and there is a small microvilli border.

 

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H2.3 A comparison of gland secretions

comparison of digestive secretions

 

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H2.4 The hormonal and nervous control of gland secretions

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:

gastrin

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.

 

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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 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

 

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+

 

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H2.6 Cellulose digestion

 

cellulose

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H2.7 The activation of pepsin and trypsin

 

activation

 

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.

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H2.8 The development of stomach ulcers and stomach cancers.

 

 

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H2.9 The role of bile in lipid digestion.

emulsificationOverview 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.

 

 

bile salts

 

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

 

 


lipid absorption

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

 

 

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Click4Biology: Option H, Digestion

Digestion

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

H2.6 Cellulose digestion

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.