Lecture 26: Digestion Flashcards
What are the main nutrients that undergo chemical digestion?
- carbohydrates (sugars)
- proteins
- lipids
what are carbohydrates good for?
- important source of energy
- storage polysaccharides
what is the structure of carbohydrates?
large complex chains of monosaccharides
e.g. glucose is a monosaccharide
what are most common of sources of carbohydrates?
- starch
- glycogen
what is the composition of starch and glycogen?
long chains of glucose joined by alpha 1-4 glycosidic bonds
what are common forms of disaccharides? what are the composed of?
Sucrose
- glucose and fructose
Lactose
- glucose and galactose
Maltose
- Glucose and glucose
what do we ingest a limited amount of?
monosaccharides - glucose
what is protein like in food?
- ingest 70-100g per day of proteins in food
- not a major source of energy
what are proteins needed for?
amino acids
- there are 20 amino acids
- 12 can be synthesised
- others essential, cannot be synthesised (e.g. histidine, leucine, lysine)
what are the sources of protein?
- 50% diet
- 50% endogenous proteins, enzymes and immunoglobulins secreted into small intestine
what is the structure of ingested proteins?
long chains of amino acids linked by peptide bonds
what are lipids like in food?
- 100-150g/day
- not essential
- important source of energy
- has fat soluble vitamins A,D,E,K
- slows gastric emptying
what is the main structure of lipids?
Mainly triglycerides
- glycerol backbone with 3 fatty acids attached
Fatty acids variable chain length
- short chain (less than 6 carbons)
- medium chain (6-12 carbons)
- long chain (12-24 carbons)
why do we need chemical digestion?
- we ingest nutrients in the form of large complex molecules, but can only absorb nutrients as small molecules.
- chemical digestion reduces the size of nutrients to allow them to be absorbed
where does chemical digestion occur?
at the surface of food particles
- mechanical digestions breaks up food to increase surface area available for chemical digestion
what does chemical digestion utilise to function?
digestive enzymes
what are features of digestive enzymes?
- extracellular
- they are organic catalysts
- they are very specific so different enzymes are needed for different substrates (e.g. amylase, protease, lipase)
- have optimal pH
what is the optimal pH of salivary, gastric and small intestinal enzymes?
salivary - alkaline
gastric - acidic’
small intestinal - alkaline
what is cellulose and its structure?
- structural polysaccharide of plants
- large amount of cellulose in diet
- long chains of beta 1-4 glycosidic bonds
what are the 2 stages of chemical digestion?
- luminal digestion
- contact digestion
what does luminal digestion involve?
initial digestion involving enzymes secreted into the lumen:
- salivary glands secrete salivary amylase
- stomach secretes pepsin
- small intestine has pancreatic enzymes (pancreatic amylase, trypsin, chymotrypsin, carboxypeptidase, lipase)
what does contact digestion involve?
- occurs in small intestine
- completes digestion before absorption
- involves enzymes produces by enterocytes and attached to brush border of enterocytes
what happens in the luminal digestion of carbohydrates?
- uses salivary and pancreatic amylase
- polysaccharides converted into oligosaccharides and disaccharides
what happens in the contact digestion of carbohydrates?
- contact digestion of brush border disaccharidases
- disaccharides are converted into monosaccharides
- involved sucrase, lactase, maltase and others bound to brush border
carbohydrate digestion summary
what happens in the luminal digestion of proteins?
- involves pepsin in stomach
- involves trypsin and chymotrypsin in small intestine
- involves carboxypeptidase in small intestine
- convert proteins into polypeptides
what happens into contact digestion of proteins?
- involved peptidases
- there are many different types attached to brush border
- converts polypeptides into individual amino acids
chemical digestion of proteins summary
where does chemical digestion of lipids occur? what does it involve?
- occurs in lumen of small intestine
- No contact digestion!
- pancreatic lipase is main digestive enzyme
- lingual lipase and gastric lipase have minor role
what is the problem with lipid digestion?
- digestive enzymes are dissolved in aqueous luminal fluid
- this is no problem for carbohydrates and proteins as they are soluble in water
- lipids are insoluble in water so they need a more complex process
what are the stages of lipid digestion?
- emulsification
- stabilisation
- digestion (hydrolysis)
- formation of micelles
what happens in emulsification? where does it occur?
1st stage of fat digestion:
Motility breaks up lipid droplets into small droplets
- forms an emulsion that increases surface area for digestion
- simple emulsion occurs in the stomach (retropulsion)
- more complex emulsion occurs in small intestine (segmentation) and bile salts stabilise droplets
what happens in stabilisation? where does it occur?
2nd stage of fat digestion:
- occurs in the small intestine
- involves bile salts which are secreted by the liver and concentrated in the gallbladder
- it is released into the small intestine with the arrival of food
- has a hydrophobic and hydrophilic side
- they stabilise the emulsion in the small intestine and reduce the size of the emulsion droplets to further increase surface area
what happens in hydrolysis? where does it occur?
3rd stage of lipid digestion
- occurs in small intestine at surface of emulsion droplets
- involves lipase and cofactor colipase
- both are secreted by the pancreas
- colipase anchors lipase to the surface droplet
- lipase converts triglycerides to monoglycerides and free fatty acids
what happens in formation of micelles? what are micelles?
4th stage of fat digestion
- products of fat digestion are insoluble in water, especially monoglycerides and long chain fatty acids
- so they are kept in solution through the formation of micelles
- micelles are small droplets consisting of 20-30 molecules of bile salts, fatty acids and monoglycerides
steps of fat digestion
