GI physiology - Digestion and absorption Flashcards
What is digestion?
How ingested nutrients are broken down.
What is absorption?
How ingested nutrients are taken up in the GI tract.
What initiates digestion?
The salivary glands.
What are the glands that secrete saliva?
The sublingual salivary gland the submandibular salivary gland.
What cells are present in the salivary glands?
Serous cells.
What do the cells in the salivary glands secrete?
Alpha amylase, lingual lipase, mucin, lgA and lysozyme.
What is the function of alpha amylase in the saliva?
It breaks down complex carbohydrates (starch) in the mouth. However, it is inactivated by the low pH in the stomach.
What is the function of lingual lipase in the saliva?
Breakdown of triglycerides. Its optimal pH is 405 and its activity is persistant throughout the GI tract.
What is the function of mucin in the saliva?
A lubricant.
What is the function of lysozyme?
It is an anti-microbial.
What is the function of lgA in the saliva?
It is an immunoglobin (antibody).
How does the concentration and composition of saliva differ to that of plasma?
It is hypotonic and alkaline.
What is the purpose of bicarbonate in the saliva?
It is a buffering function to minimise acid decay and damage from acid reflux.
How much saliva is produced each day?
Between 1000 and 1500ml.
Why is taste produced in the mouth?
Saliva moistens and lubricates the food and also solubilises a small amount of it which releases molecules that activate taste receptors.
What percent of total lipid digestion does lipase account for?
10-30%.
How is protein broken down in the stomach?
Chief cells secrete pepsinogen that is a precursor for pepsin that partially breaks down the proteins.
What cells secrete HCl?
Parietal cells.
What do mucous cells do in the stomach and what is their function?
They secrete mucous that is essential for protection and lubrication.
What is the purpose of G cells in the stomach?
They produce gastrin which controls motility and production of HCl, pepsinogen and mucous.
How does pepsin work?
It is an exopeptidase that clips amino acids from the end of polypeptide chains.
How much HCl is secreted by the stomach per day?
Around 2 litres.
What is the origin of hydrogen ions in the stomach?
CO2 in the parietal cells.
How is CO2 converted to hydrogen ions?
Carbonic anhydrase causes the CO2 to react with water to produce carbonic acid which dissociates into HCO3- and H+.
How do hydrogen ions reach the lumen of the stomach?
Primary H+/K+ - ATPases in the luminal membrane of the parietal cells pump the ions into the lumen actively, against a steep electrochemical gradient.
What happens to the K+ in this process?
The K+ leaks back into the lumen through K+ channels.
uptake of K+ ionis into the parietal cell, which are passed on into the blood
What happens to the HCO3-?
It is secreted on the opposite side of the cell into the blood in exchange for Cl-, at the baselateral membrane.
What chemical messengers regulate acid secretion?
Gastrin, acetylcholine, histamine and somatostatin.
What is the difference between protein digestion by chief cells and parietal cells?
Pepsin from chief cells acts as an exopeptidase whereas acid from parietal cells acts an endopeptidase.
What is the difference between exopeptidases and endopeptidases?
Exopeptidases clip amino acids from the end of polypeptides whereas endopeptidases catalyse the cleavage of internal peptide bonds.
How is pepsin produced from pepsinogen?
Pepsinogen is only activated by the low pH in the stomach.
Which function of the pancreas is utilised in digestion?
The exocrine function.
What is the significance of the bicarbonate in the pancreatic fluid?
It acts as a buffer to ensure chyme is not acidic and the enzymes can remain active.
Where is bile stored?
The gall bladder.
How is bile injected into the lumen of the small intestine during feeding?
The gall bladder muscle contracts and the bile enters via the common bile duct.
What is the function of bile?
It is alkaline and neutralises gastric acid entering the duodenum. It is also important for digestion and absorption of fats along with acting as a surfactant to reduce surface agent.
What are zymogens?
Enzymes precursors that require some change to become active.
What do the enzymes secreted from the pancreas digest?
Fat, polysaccharides, proteins, nucleic acids, sugars, amino acid and nucleotides.
What is the role of enterokinase?
It is a proteolytic enzyme that splits off a peptide from pancreatic trypsinogen to form the active trypsin.
What is the function of trypsin?
It activates other pancreatic zymogens and also digests proteins.
How is pancreatic sections controlled?
By the hormones secretin and CCK (cholecystokinin).
What is the role of secretin?
It is the primary stimulant for HCO3- secretion.
What is the role of CCK?
It stimulates enzyme secretion.
What is the brush border of an epithelial cell?
It contains a variety of protein carriers that facilitate the uptake of free amino acids.
What do peptidases do on the brush border?
Break large peptides (too big to get through the brush border) into small peptides or free amino acids that can then enter the cell via carriers.
What happens when small peptide enter the epithelial cell?
They are hydrolysed into small amino acids by intracellular peptidases.
What happens when amino acids are inside the epithelial cell?
They converge on the basal lateral membrane to be transported into the bloodstream.
How are carbohydrates digested?
Salivary and pancreatic amylase to reduce the complex carbohydrates into smaller sections.
How are maltase, sucrose and lactose digested?
On the brush border, maltase, sucrase and lactase are present to digest the carbohydrates into fructose or glucose and galactose.
How do glucose and galactose enter the cell?
There are transporters that are specialised for their uptake into the cell.
How does sodium affect the transport of glucose and galactose?
Sodium travels down an electrochemical gradient (from high to low) that drives the uptake by cotransporters of glucose and galactose.
Why is ATPase present intracellularly?
It is needed to maintain the intracellular concentrations of sodium low - it removes Na+ to maintain the gradient.
How do oral rehydration solutions work?
They need to be sweet so that sugar is present so sodium can be transported across the membrane,
Why can sucrose, lactose or maltose not enter the epithelial cell?
They are too large and the correct transporters are not present.
What happens when cholesterol is broken down?
It forms cholic acid and chenodeoxycholic acid which are primary bile acids.
What happens to primary bile acids?
They are stored in the gall bladder.
What happens to cholic acid?
It undergoes bacterial hydroxylation and deoxycholic acid is formed (secondary bile acid)
What happens to chenodeoxycholic acid?
It undergoes bacterial hydroxylation to form lithocholic acid (secondary bile acid)
What happens to these secondary bile acids?
They are released into the duodenum.
please look at bile salts properly
this is very unprofessional
What are triglycerides broken down into?
Monoglycerides and two molecules of FFA.
What are phospholipids broken down into?
Lysophosphatides and FFA.
What is colipase?
A protein co-enzyme required for optimal pancreatic triglyceride lipase activity.
How is colipase activated?
It is secreted by the pancreas in an inactive form which is then activated in the intestinal lumen by trypsin.
What happens after colipase is activated?
It binds to the bile-salt covered triacylglycerol interface of micelles, allowing the access of the PTGL to its substrates at the water-lipid interface.
pepsin function
initiates protein digestion, breaks them up into peptides and amino acids
what converts pepsinogen into pepsin
hcl
where are zymogens produced
pancreatic bulb
How are zymogens released into lumen of pancreatic duct
exocytosis
By what process do amino acids enter epithelial cells
active transport or by Na+ dependent active transport
transport of sugars into blood overview
facilitated diffusion transports fructose into epi cells
secondary active transport moves glucose and galactose in, using Na+ ions
How do bile slats digest lipids
Cling to mono, di and triglycerides of fat molecules and break them up into triglyceride emulsion droplets
lipases action
cleave off fatty acids from glycerol and generate monoglycerides
FFAs bind to phosphates
Why emuslify fats with bie
forms lipid droplets which increase the surface area for lipases
how does co-lipase act as a co enzyme
lodges on lipid droplet surface, binds lipase
why can bile salts and and lipids form micelles
amphipathic
