Week 2 Digestion and absorption 2 Flashcards
What components of the digestive tract are involved in mechanical digestion?
Teeth - chewing
Stomach - chruning and propulsion
Small intestine - segmentation
Large intestine - churning and propulstion
What are the roles of the oral cavity in chemical digestion?
Salivary amylase and lipase
What are the roles of the stomach in chemical digestion?
Pepsin and gastric lipase
What are the roles of the pancreas in chemical digestion?
Amylase
Trypsin
Chymotrypsin
Carboxypeptidase
Elasate
Lipase-colipase
Phospholipase
Cholesterol esterase
What is the role of the gall bladder in chemical digestion?
Store and concentrate bile
What is the role of the intesinatl brush border in chemical digestion?
Enterokinase
Maltase
Lactase
Trehalse
Isomalatase
Aminooligopeptidase
dipeptidase
What is the role of the large intestine in chemical digestion?
Bacterial fermentaion
What are the four key products of digestion?
Water and ions
Amino acids
Monosaccharides
Fatty acids
What is the role of bile stalts in chemical digestion?
Breaks down fat molecules into smaller molecules for enzyme action
How does absoprtion vary across the GIT?
Very limited in the oral cavity - some glucose gels and drugs
Limited in the stomach (some lipid soluble e,g medication and alcohol)
Majority in the small intestine
What substances are normally absobred in the small intestine?
Water
Electrolytes Na+ K+ cl- Ca2+, Mg2+ Fe 2+
Vitamins
Fatty acids
Glycerol and cholesterol
Amino acids or oligopepetides
Monosaccharides
What is absorbed in the large intestine?
WAter, Na+, K+, Cl-, Vitamin K
What are the different sources of carbohydrates that may be broken down in the GIT?
Startch - potatoes, pasta etc
Trehalose - 2 sugar molecues found in mushrooms, honey and yeast
Lactose - milk and dairy products
Sucrose - table sugar
Describe how staract is broken down by enzymes?
Starch is first broken down by alpha amylase from saliva
Into maltose, alpha dextrins or maltotriose
THese are then broken into glucose by their subsequent enzymes
Maltase, alpha-dextrinases and sucraose
How is trhalose broken down?
By enzyme called trehalase into gluxose
How is lactose borken down?
By an ezyme called lactase
Into glucose and galactose
How is sucrose broken down?
By sucrase
INto glucose and fructose
What are the different types of lipids?
Triglycerides
Cholesterol ester
Phospholipids
How are triglycerides broken down?
By lingual, gastric and pancreatic lipases
Into monoglycerides and 2x fatty acids
How is cholesterol ester droken down?
By cholesterol ester hydrolase
INto cholesterol and a fatty acid
How are phospholipids broken down?
By phospholipase A2 into
Fatty acid and lysolecithin
How is protein broken down?
IN the somtach pepsin breaks down protein into amino acids and oligopeptide
In the small intestine acited on by more luminal and brush border enzymes to by broken down into amino acids, dipietides and tripeptides
What are the different luminal enzymes in the small intestine that can act on protein?
Trypsin
Chymotryspin
Elastase
Carboxypeptidase A and B
What are the different brush borbder enzymes that can act on peptides in the lumen of the small intestine?
Peptidases
How is the flow of substances from the pancreas into the duodenum controlled?
Flow into the ampulla of vater then out through the major duodenal pappila
Exit into the duodenum is controlled by the sphincter of Oddi
What are the two principle cell types contributing to secretions in the pancreas?
Pancreatic acinar cells
Pancreatic duct cells
What is the role of pancreativc acinar cells?
Enzyme secretion - proteases, pnacreatic lipase and pancreatic amylases amylase
Immautre enzymes are stored in zymogen granules
also secrets fluid containing salts
What are zymogen granules?
Found at the apical side of the pancreatic acinar cells
Contain enzyme precuroses in the inactive form
What is the role fo pancreatic duct cells in digestion and absorption?
HCO3- Bicarbonate secretion and water
What molecules timulate the release of enzymes from pancreatic acinar cells?
VIP
Secretin
CCK
Ach
Describe how acetylcholine can activate enzyme ssecretion from pancreatic acinar cells.
Activates a GPCR
Leads to activation of phospholipase C
1. Leads to increase in activate DAG - stimulating Ptorein Kinase C
2. Causes IP3 stimulates release of Calcium ions from the ER - stimulate protein Kinase C or Calcmodulin which then activates Protien Kinase or phosphoprotein phosphatases
Leads to fusion of zymogen granules with the cytoplasm for release
Give the basic activation of pancreatic acinar cells to release enzyme
Stimulates with acetylcholine or cholecystokinin along others
Leads to an increased in intraceullar Ca2+ and stimulates the exocytosis of zymogen granules which contain the secertory enzymes
Describe how VIP and secretin lead to the activation of pancreatic acinar cells?
Stimulate a G protein
Activates adenylate cyclase - leads to conversion of ATP to cAMP
Activates Protein KInase A causing the fusion of symogen granules
How does cholecystokin lead to the activation of pancreatic acinar cells?
By two pathways both involiving a G protein activation
1. Activation by andelyaste cyclase, cAMP and Protein Kinase A pathway
2. Activation of phospholipase C leading to calcium ion release activating PK and PP or DAG activating PKC
Results in fusion of zymogen granules with the cell membrane
What is meant by autodugestion?
When pancreatic enzymes destroy own tissue leading to inflammation
How do we prevent auto-digestion?
Zymogen granules contain inactive precursors to prevent autodigestion
The interior of the symogen granule is very acidic whilst enzymes such as trypsin typically require pH 8 to work
Contain Serine Protease INhibitros e.g SPINK1 gene disrupt the activation site to prevent activation inside the granule
What gene is associated with hereditary pancreatitis?
SPINK1
Describe the process of trypsin release and activation in the pancreas
- Proenzymes contained in acinar cells
- released into pancreatic duct and into duodenum by the Ampulla of Vater, commonly trypsinogen
- Trypsinogen is activated by cleave to trypsin by enteropeptidase in the duodenal brush border
- Trypsin can further cleave trypsiongen to active trypsin form
What is trypsinogen?
An inactive precurose of trypsin released from the pancreas.
Activated to trypsin by cleavage by enteropeptidase in the duodenal brush border
What is the role of trypsin in the pancreas?
Cleaves and activates other proenzymes
Chmotrypisinogen to chymotripsin
Proelastase to elastase
Procarboxypeptidase to carboxypeptidase
How does trypsin result in active pancreatic lipase?
Cleaves procolipase into colipase
Which then binds to pancreatic lipase to form an active pancreatic lipase
Describe how hromone stimulation can cause pancreatic acinar cells to secrete salt rich fluid.
Action of ACh and CCK on receptors
results in a rise in intracellular calcium ions
Stimulates protein kinases fueling the loss of Chloride ions through the apical membrane into the lumen and the loss of potassium ions by the apical membrane