Gastrointestinal system 7 Flashcards
What are the main nutrients that undergo chemical digestion
Carbohydrates (sugars)
Proteins
Lipids (fats)
Why are carbohydrates an important source of energy
250-800 mg per day
Storage polysaccharides
Describe structure of carbohydrates
Large complex chains of monosaccharides - (like glucose)
Describe the composition of starch and glycogen
Long chains of glucose joined by alpha 1-4 glycosidic bonds
What are three disaccharides
Sucrose (glucose and fructose)
Lactose (glucose and galactose)
Maltose (2 glucose)
How much protein do we ingest a day
70-100g per day
Are proteins a source of energy
No
Why are proteins important
Required for amino acids
How many amino acids can we synthesise
12
What essential amino acids need to be ingested
Histidine
Leucine
Lysine
Where do our proteins come from
50% diet
50% endogenous proteins - secreted into intestine (enzymes and immunoglobins)
How much lipids do we ingest
100-150 per day
Why do we need lipids
Source of energy - not essential -
Fat soluble vitamins (A,D, E & K),
Slow gastric emptying
What is the main lipid
Gylcerol (backbone with 3 fatty acids attached)
What are the viable lengths of fatty acids
Short chain - <6 carbons
Medium chain - 6-12 carbons
Long chain - 12-24 carbons
Why do we need chemical digestion
Large complex molecules only absorbed as small molecules - reduces size of nutrients to allow absorption
How is chemical and mechanical digestion related
Chemical digestion breaks up food particles to increase surface area for chemical digestion
What are general properties of digestive enzymes
Extracellular
Organic catalysts
Very specific
Have optimal pH
What is the composition of cellulose
Long chains of beta 1-4 glycosidic bonds
What are the optimal pH’s of different enzymes
Salivary enzymes - alkaline
Gastric enzymes - Acidic
Small intestine - Alkaline
What are the two stages of chemical digestion called
Luminal digestion
Contact digestion
Describe luminal digestion
Enzymes secreted in lumen
Salivary - amylase
Stomach - pepsin
SI - Pancreatic enzymes
Describe Contact digestion
In SI - completes digestion before absorption
Involves enzymes produced by enterocytes - attached to brush boarder
Describe luminal digestion of carbohydrates
Salivary and pancreatic amylase converts polysaccharides to disaccharides
Describe contact digestion of carbohydrates
Disaccharides converted to monosaccharides - Sucrase, lactase, maltase
Describe luminal digestion of proteins
Pepsin in stomach
trypsin, Chymotrypsin and carbopeptidase in SI convert proteins into polypeptides
Describe contact digestion of proteins
Peptidase attaches to brush boarder and converts polypeptides into amino acids
Describe chemical digestion of lipids
Pancreatic lipase - lingual lipase and gastric lipase minor
No contact digestion
What is the problem with lipid digestion
Digestive enzymes dissolved in luminal fluid - as lipids are fat soluble requires more complex process
Describe the steps in chemical digestion of fats
Emulsification
Stabilisation
Digestion (Hydrolysis)
Formation of micelles
What does emulsification do
Breaks up lipid droplets into small droplets (0.5 - 1 um)
In stomach - simple
In SI - complex - bile salts stabilise
Describe Stabilization
In SI
Bile salts - released with arrival of food - stabilize emulsion in small intestine - reduces size further
Describe bile salts
Secreted by liver and concentrated in gall bladder
Have a hydrophobic and hydrophilic side
Describe hydrolysis
Surface area of emulsion droplets in SI
Lipase converts triglycerides to monoglycerides and free fatty acids
What is involved in hydrolysis
Involves Lipase and cofactor colipase - secreted by pancreas
Colipase anchors lipase to surface of droplet
Why do we need micelles
Products of fat digestion insoluble in water
Describe formation of micelles
Small droplets consisting of 20-30 molecules - Bile salts (amphipathic) surround fatty acids and monoglycerides
