Digestion and Absorption Flashcards
What is the definition of digestion?
- The mechanical, chemical and microbial breakdown of large, insoluble food molecules into simple absorbable compounds
What is the definition of absorption?
- The process by which these simple compounds are taken across the intestinal membranes into the blood
Where does most digestion and absorption occurs in an omnivore/carnivore?
- 90% occurs in the small intestine
Where does most digestion and absorption occur in herbivores? (in both hindgut fermenters and ruminants)
- hind gut fermenters = more in the large intestine
- ruminants = significant uptake in the rumen too
Where do digestive secretions come from and where do they secrete into?
- salivary glands - secrete into the mouth and tongue
- stomach
- exocrine pancreas - secretions into the small intestine
- liver - secretes bile into small intestine
- small intestinal glands and brush border
What does alpha amylase do?
- catalyses the hydrolysis of starch into sugars
What is the optimum pH of salivary alpha amylase and where is it inactivated?
- optimum pH = 6.6-6.8
- inactivated at stomach pH
What is the function of salivary alpha amylase?
- to start carbohydrate digestion and act on starch
Where is pancreatic alpha amylase secreted from?
- secreted by the exocrine pancreas
What is the function of pancreatic alpha amylase?
- to end carbohydrate digestion and act on complex carbohydrates
Where does most carbohydrate digestion occur?
- occurs in the small intestine by the action of pancreatic alpha amylase
How do alpha amylases work?
- attack the alpha 1,4 glycosidic links only
- attack only in the middle of a CHO chain = endoglycosidases
What are the two phases of carbohydrate digestion?
- luminal phase
- membranous phase
Describe the luminal phase of carbohydrate digestion:
- starch and glycogen are degraded into compounds containing 2-9 glucose units
- products of luminal degradation cannot be absorbed by the epithelial cells
Describe the membranous phase of carbohydrate digestion?
- di, tri and oligosaccharides are broken down to monosaccharides by enzymes bound to the apical membrane of the epithelial cells
Where does the luminal phase of carbohydrate digestion occur?
- Luminal phase happens mostly in the duodenum as this is where the ducts of the pancreas empty
What are enterocytes?
- columnar epithelial cells found at the surface of villi which act as absorptive functional units of the digestive tract
What 3 types of vessel can be found within each of the individual villi?
- artery
- vein
- lymphatic vessel
What is the lifespan for an enterocyte and what happens when they die?
- 2-3 days
- when they die they will either be passed out with faeces/degraded and reabsorbed
What are the 4 different mechanisms for absorption?
- passive diffusion
- facilitated diffusion
- active transport
- endocytosis
What happens once nutrients are within cells?
- they cross over the baso-lateral membrane and enter circulation, directed to the hepatic portal vein to liver or indirectly via lymphatic system
What are the products of carbohydrate digestion?
- mainly glucose
- smaller amounts of fructose and galactose
How is fructose absorbed?
- absorbed by carrier-mediated diffusion down its concentration gradients into the cell and the out of the cell into the blood
What the transport of glucose and galactose into intestinal cells require?
- energy
Glucose transport has a number of characteristics which provide information about the mechanisms involved - what are these characteristics?
- saturable
- competitively inhibited by galactose
- inhibited by glucose analogues
- sodium dependent
- inhibited by the sodium/potassium ATPase inhibitor
- requires energy
How is glucose transported from the lumen of the GI tract into the blood?
- glucose and sodium are co-transported together from the lumen into the intestinal cell
- the concentration of sodium and glucose is higher inside the cell than in the blood
- glucose is moved into the blood via facilitated diffusion
- sodium pumped into the blood and potassium pumped out of the blood - needs energy
What is the general function of proteases?
- break down proteins and peptides
How do proteases work?
- They cause cleavage of the peptide bonds by hydrolysis
What are zymogens?
- zymogens are inactive precursors of proteases
What are the two groups of gastrointestinal proteases?
- endopeptidases
- exopeptidases
What are endopeptidases?
- attack and cleave in the middle of the amino acid chain
- secreted by the stomach and pancreas
What are exopeptidases?
- split off amino acids from the end of the chain
- secreted by pancreas and small intestine glands
What are pepsins?
- gastric proteases
where does protein digestion start?
- starts in the stomach via endopeptidase - pepsin
What pH does pepsin require, what does it do and what is it secreted by
- active at low pH <3
- attacks all proteins (except keratin and mucin)
- secreted by pepsinogen by chief cells
What is secreted simultaneously with pepsinogen? what is it secreted by?
- H+
- secreted by parietal cells
Where are both pepsin and H+ produced?
- gastric pits
How is HCl secreted by parietal cells?
- by carrier mediated active transport from the parietal cell into the lumen
Why is HCL needed?
- HCL activates pepsinogen to make activated pepsinogen
- activated pepsinogen loses 44 amino acids from its chain to make pepsin
How does pepsin make a positive feedback loop?
- when pepsin is made it stimulates more activated pepsinogen to become pepsin
Where do peptides from pepsin digestion in the stomach pass?
- into the small intestine
What are the 3 endopeptidases produced by the pancreas
- trypsinogen (trypsin)
- chymotrypsinogen (chymotrypsin)
- pro-elastase (elastase)
Why do pancreatic endopeptidases have an alkaline optimum pH?
- because carbonate HCO3- ions are also secreted from the pancreas
What happens to peptides after pancreatic endopeptidases digestion?
- peptides are attacked by exopeptidases
What are the two types of pancreatic exopeptidase?
- Carboxypeptidases
- Aminopeptidases
What do carboxypeptidases do and where are they secreted from?
- split amino acids from the carboxy terminal
- secreted by the pancreas as zymogens
What are aminopeptidases and where are they made and found?
- split amino acids from the N- terminal
- made in the small intestine glands
- found on intestinal cell membrane
Complete the sentence:
Proteases are both stored and secreted as …
- zymogens
What do lipases do?
- digest fats, removing fatty acids from triacylglycerol’s
Where is lingual lipase active?
- active in the mouth and stomach
Where is pancreatic lipase active?
- active in the small intestine
- requires phospholipids and bile acids for activation
When is bile released?
in response to CCK
What is bile, where is it stored, where is it synthesised?
- bile is a greeny-brown fluid which is synthesized in the liver and stored in the gall bladder
What does bile excrete?
- excess cholesterol and toxic breakdown products of haemoglobin (bile pigment)
How is bile returned to the liver?
- enterohepatic circulation
How is fat emulsified?
- Bile salt is sprayed through the pyloric sphincter
- lipid droplets are coated with bile salts, preventing them from coalescing
How is fat digested?
- gastric lipids act o the fat droplets and starting to break them down
- once in the duodenum pancreatic lipase binds to the droplets and with the aid of colipase start to breakdown the fats into fatty acids and monoglycerides
- FA and monoglycerides diffuse into micelles which merge with the brush border and are absorbed into enterocytes
How are chylomicrons formed?
- TAG is conjugated with proteins and phospholipids
How are fatty acids released into the tissues?
- some of the TAG is broken down
Where vitamins and minerals absorbed?
- mainly in the small intestine
What mechanisms are used to absorb vitamins and minerals?
- passive diffusion
- carrier mediated transport
- active transport
How is water absorbed?
- the absorption of water follows electrolytes and is mainly linked to sodium movement, occurs in the small and large intestines
