Option H Further human physiology

2. Digestion

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 Salivary amylase (starch to maltose

Stomach Gastric gland – gastric juice

Pepsin (proteins to polypeptides)

Exocrine part of Pancreas(intestine)

Pancreatic juice 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

Intestinal juice Carbohydrases – maltase, sucrase, lactaseDipeptidases – dipeptides to amino acidsEnterokinase – activate trypsinogen to trypsin

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.

H2.2 The structure of the exocrine gland

H2.2 The structure of the exocrine gland

                                                                                                                           Exocrine glands release their secretions into a duct. There are three different methods of secretion:

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•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 secretionsDigestive juice ContentsSaliva Amylase for starch digestion

Lingual lipase to digest triglycerides to fatty acidsWater and electrolytes to moisten Mucus to lubricate food Antibacterial compounds

Gastric juice HCl – activate pepsin, kills bacteriaMucus – protect stomach liningPepsin and rennin – digest proteins

Pancreatic juice Trypsin,chymotrypsin, aminopeptidase and carboxy peptidases – digestion of proteinsPancreatic amylase – Pancreatic lipase – High concentration of bicarbonate ions to neutralise acidic food

H2.4 The hormonal and nervous control of gland secretions

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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 juiceMovement of food to duodenum detected by medulla oblongata and production of gastric juice inhibited.

H2.5 Membrane bound enzymes in gut epitheliumEnzyme 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 membraneDigestion of the disaccarides(maltose)can occur early in the digestive systemEven when the epithelial cell is rubbed off, the enzyme action can still continue.

H2.6 Cellulose digestionHumans 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

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.

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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 intestineEnterokinase 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

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 cellf) 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