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H2: Digestion

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Rachel Lau

on 16 December 2014

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Transcript of H2: Digestion

H2: Digestion
Digestive juices are secreted into the alimentary canal by glands.

salivary glands
gastric glands in the stomach wall
the pancreas
the wall of the small intestine

The control of digestive juice secretion by nerves and hormones
The role of membrane-bound enzymes on the surface of epithelial cells in the small intestine in digestion
Some digestive enzymes are immobilised on the plasma membrane of the epithelial cells of the small intestine

1. The enzyme is fixed in place, meaning it can be reused

2. Can be linked to secondary functions (e.g. membrane transport)


Maltase is immobilised on the epithelial lining with its active site facing towards the intestinal lumen
Maltase digests the disaccharide maltose into two glucose monomers

The roles of gastric acid & Helicobacter pylori in the development of stomach ulcers and stomach cancers

H. pylori is a bacterium that can survive the acid conditions of the stomach
It secretes urease which neutralises the gastric acid to lower the acidity of the stomach
secretes proteases to degrade the mucosal lining of the stomach wall, allowing it to burrow into this lining
The degradation of lining by H. pylori allows for damage to the stomach wall by gastric acids. ulcers.
The prolonged presence of stomach ulcers may lead to the formation of stomach cancers
Structural features of exocrine gland cells
Have ducts which secretes products
Formed by a cluster of cells called acinus.
Acini are lined by a single layer of secretory cells.
Exocrine products is released into the lumen by secretory vesicles
Secretory cells contain rough ER and Glogi apparatus for material secretion.
Compostition of saliva, gastric juice and pancreatic juice
Contains water, mucus, salts and ions
The reason for cellulose not being digested in the alimentary canal
Glucose can exist in one of two isomeric forms: a-glucose or ß-glucose

Humans can digest polymers of a-glucose (e.g. starch),
Cannot digest the polymer of ß-glucose (cellulose)

Do not produce the enzyme (cellulase)
lack bacteria capable of digesting cellulose

Pepsin and trypsin are initially synthesised as inactive precursors and are subsequently activated
Secretory cells
Duct cells
Lumen/ duct




Starch to maltose
Gastric juice




Protein to polypeptides
Pancreatic juice

Small intestine


Tripsin(ogen), Lipase
& Amylase
Starch to maltose
Proteins to polypeptides
Triglycerides to Glycerol + Fatty acid



Example: secretion of gastric juice

Sight and smell of food triggers a reflex response
Gastric juice is secreted from gastric pits in the stomach wall
Ensures that gastric juice is in the stomach by the time the food is consumed


Food entering the stomach causes distension
Detected by stretch receptors in the stomach lining
Impulses sent to the brain, triggers the secretion of gastrin from the pits lining the stomach wall
Gastrin causes the sustained release of gastric juice
When the pH drops too low, gastrin secretion is inhibited by hormones (secretin and somatostatin)
Pepsin and trypsin are protease enzymes
Hydrolyse peptide bonds to digest proteins
Proteins are essential components of cells, these enzymes could digest the cells that secrete them
They are synthesised as inactive forms (zymogens) and subsequently activated in the digestive tract (lined with mucus to protect cells)
The problem of lipid digestion in a hydrophilic medium and the role of bile in overcoming this
Lipids are hydrophobic, insoluble within the aqueous environment of the body
group together to form fat globules
The enzyme lipase is water soluble and can only bind the lipids to its active site at the lipid-water interface
the interior of the fat globule is inaccessible to lipase, making lipid digestion very slow
Bile is a fluid that contains bile salts and pigments, produced by liver cells and stored in the gall bladder
Bile salt molecules have both a hydrophobic and hydrophilic end
The hydrophobic end attaches to the lipid while the hydrophilic end interacts with water
This divides the fat globule into smaller droplets (emulsification), increasing the total surface area available for enzyme activity
Full transcript