Lecture 26 GI Physio Digestion Flashcards
Chemical Digestion
The chemical hydrolysis of food is caused by?
Enzymes secreted
Enzymes attached to enterocytes of small intestine (brush border enzymes)
what are enzymes secreted by?
salivary glands
chief cells of stomach
acinar cells of the pancreas
what are the Main nutrients that undergo chemical digestion?
Carbohydrates (sugars)
Proteins
Lipids (fats)
whats carbohydrates?
Important source of energy
Consist of chains of monosaccharides (eg Glucose)
whats the Average amount of carbs ingested?
250 - 800 g/day in typical western diet
whats the composition of ingested carbs?
starch and glycogen
Cellulose
disaccharides
monosaccharides
whats starch and glycogen?
Main carbs in diet
Storage polysaccharides
Long chains of glucose - a1-4 glycosidic bonds
whats cellulose?
large amount in diet
cant digest
Structural polysaccharide in plants
b1-4 glycosidic bonds
whats ingest disaccharides?
Sucrose
Lactose
Maltose
whats ingest monosaccharide?
limited amount
Glucose
how much proteins do we ingest?
70-100 g/day in our food
what does the intestine also have to digest an equivalent amount of?
endogenous proteins such as enzymes, immunoglobulins
whats proteins required for?
amino acids
20 amino acids
Approximately 10 essential in diet
whats Ingested proteins?
Long chains of amino acids
Linked by peptide bond
how much lipids do we consume?
100 - 150 g/day.
what do lipids consist mainly of?
triglycerides
Contains fat soluble vitamins
what are Triglycerides?
Glycerol backbone with 3 fatty acids attached by ester bonds.
whats fatty acids variable chain length
short chain
medium chain
long chain
< 6 C
6-12 C
12-24 C
what size are ingest nutrients (same size even after digestion) and what size are absorb nutrients (to get them across epithelium into body)?
Large complex molecules (carbs, proteins, lipids)
Small molecules
what does chemical digestion do?
reduce nutrients to size that allows them to cross the epithelial lining of GI tract
what does chemical digestion need?
digestive enzymes
whats digestive enzymes?
Are extracellular
Are organic catalysts
Function optimally at a specific pH
Are specific
what does it mean that digestive enzymes are specific?
Will only act on a specific substrate
Require different enzymes for different substrates
E.g. proteases, lipase, amylase
what 2 processes occurs in chemical digestion?
Luminal digestion
Contact digestion
whats luminal digestion?
Initial digestion involving enzymes secreted into lumen
what enzymes are involved in luminal digestion?
Salivary glands - amylase
Stomach - pepsin
Small intestine - trypsin, chymotrypsin, lipase and
amylase (from pancreas)
whats contact digestion (in SI)?
Digestion completed by enzymes produced by enterocytes (absorptive cell)
These enzymes attached to brush border of enterocytes
what is the site of contact digestion?
Brush border microvilli (mucus granules, microvilli form brush border, enterocyte-absorptive cell)
Chemical digestion of CARBOHYDRATES
whats the LUMINAL digestion of carbs?
Use Salivary and pancreatic amylase
Polysaccharides converted to disaccharides
Chemical digestion of CARBOHYDRATES
whats the CONTACT digestion of carbs?
Disaccharidases
Sucrase, lactase and maltase
Bound to brush border of SI
to Convert disaccharides to monosaccharides
Chemical digestion of PROTEINS
whats does the LUMINAL digestion of proteins involve?
Pepsin in stomach
Trypsin, chymotrypsin
- Secreted by pancreas
- Operate in small intestine
Chemical digestion of PROTEINS
what is LUMINAL digestion of proteins?
Convert proteins into polypeptides and proteoses
Lumen of stomach and SI
use: Pepsin in stomach Trypsin, chymotrypsin - Secreted by pancreas - Operate in small intestine
Chemical digestion of PROTEINS
what is CONTACT digestion of proteins?
Involves peptidases
- Attached to brush border of SI
Many types
Convert polypeptides to individual amino acids
Chemical digestion of lipids
where does digestion occur?
intestinal lumen
NO contact digestion
Digestive enzymes dissolved in luminal fluid
Protein and sugars (soluble in water)
Fats (insoluble in water) - requires more complex process
what are the stages of Chemical digestion of LIPIDS?
Emulsification
Stabilisation
Hydrolysis (digestion)
Formation of micelles
whats Emulsification of LIPIDS?
Motility separates large fat droplets down into smaller droplets (0.5 - 1.0 μm)
- forms an EMULSION
increase SA for digestion
where does Emulsification of LIPIDS occur?
Stomach - retropulsion
SI - segmentation
whats Stabilisation of LIPIDS?
use lecithin and bile salts
- secrete in bile
- stabilise emulsion droplets
- allow formation of smaller droplets
Increase SA for digestion even further
where does Stabilisation of LIPIDS occur?
SI
what does Hydrolysis of LIPIDS involve?
lipase and colipase (cofactor) - Both secreted by the pancreas
where does Hydrolysis of LIPIDS occur?
lumen of SI at the surface of emulsion droplets
NO contact digestion at microvilli
whats Hydrolysis of LIPIDS?
Co-lipase anchors lipase to surface of emulsion droplet
Lipase converts triglycerides to: Monoglyceride and Free fatty acids
whats the formation of micelles?
Products of fat digestion are insoluble in water. Monoglyceride and Long chain fatty acids.
Kept in solution through the formation of micelles
whats micelles?
Small droplets (4 - 6 nm diameter)
Consist of 20 - 30 molecules
- Bile salts/lecithin
- Amphipathic compounds
- Fatty acids
- Monoglycerides
what do 20-30 molecules do micelles consist of?
Bile salts/lecithin
- Amphipathic compounds
Fatty acids
Monoglycerides
what do Carbohydrates, Proteins, Lipids get digested to (smallest size)?
Monosaccharides
Amino acids
Micelles
