Chemical Digestion in the GI Tract Flashcards
describe the macronutrient carbohydrate (CHO)
- important source of energy
- dietary carbohydrate 250-800g per day
- monosaccharides (a little of dietary CHO) eg. glucose, fructose
- disaccharides (some dietary CHO) eg. sucrose (which sugar) = glucose + fructose
- polysaccharides (majority of dietary CHO) - large storage forms of carbohydrate (starch - amylose and amylopectin [digestable]: plants, fibre [indigestable]: plants)
describe the macronutrient protein
source of amino acids to make body proteins, not a major source o energy under normal conditions
macronutrient with the most complex structure:
- long chains of amino acids linked by peptide bonds
- peptide chains are folded into secondary and tertiary structures
- may join with other peptide chains to form quaternary structure
12 of the 20 amino acids can be made in te body by adults (essential amino acids can no the made in the body, 8 in adults)
sources of protein:
- 50% diet: plant and animal protein
- 50% endogenous (body) proteins secreted into the intestine (enzymes and immunoglobulins)
describe the macronutrient lipids
- important source of energy
- most lipids are not essential BUT: omega -3 and -6 fatty acids and fat soluble vitamins A, D, E and K
- mostly triglycarides (TAGs): glycerol backbone and three fatty acids
fatty acids have different chain lengths:
- short chain fatty acids (<6 carbons)
- medium chain fatty acids (6-12 carbons)
- long chain fatty acids (13-24 carbons)
describe chemical digestion in the GIT relative to all the macronutrients
the food we eat contains nutrients mostly in their storage (large) forms
- carbohydrates: starch, disaccharides
- proteins: large peptides to tertiary/quaternary structures
- lipids/fat: triglycerides (TAGs)
BUT the GIT can only absorb nutrients as small molecules
- carbohydrates: monosaccharides
- proteins: amino acids, small paper ides (2-3 AAs)
- lipids/fat: free fatty acids, monoglycerides (glycerin + 1 fatty acid)
describe the process of chemical digestion overall
chemical digestion is the process of breaking macronutrients down from their large/storage forms into small molecules which can be absorbed using chemicals: acid, digestive enzymes (-ase at the end of the name)
chemical digestion occurs at the surface of food particles
- mechanical digestion breaks up food particles which increases the surface area for chemical digestion
digestive enzymes are extracellular, organic catalysts
Enzyme + substrate <–> enzyme-substrate complex <–> enzyme + product
describe the digestive enzymes that do chemical digestion
they are very specific!
- need different enzymes for different substrates/macronutrients
- need different enzymes for different types of bonds for carbohydrates and protein (eg. carbohydrates: amylase for starch, disaccharidase for disaccharides)
- have an optimum pH they work best at: salivary and pancreatic - slightly alkaline pH, stomach - acidic pH
starch: can be digested by the body, amylase can break alpha-bonds between glucose molecules
cellulose: can’t be digested by the body, amylase can’t break beta-bonds between glucose molecules
sucrose: disaccharides are digested by different enzymes
- they have different sugars
- and therefore different bonds
describe the two stages of chemical digestion
- luminal digestion: starts chemical digestion
- chemical digestion by acid and enzymes which have been secreted into the lumen of the GIT and mixed through the food/chyme: carbohydrate, protein and lipids/fat
- mouth: salivary amylase
- stomach: HCl acid, pepsin, lingual and gastric lipase
- small intestine: pancreatic digestive enzymes - contact digestion: finishes chemical digestion
- chemical digestion by enzymes made by and attached to the brush boarder of enterocytes (small intestine epithelial cells): carbohydrate and protein
- small intestine: brush boarder enzymes - brush boarder disaccharidases and brush boarder peptidases
describe the process of chemical digestion for carbohydrates
- luminal digestion
- initiated in the mouth by salivary amylase
- continued in the small intestine by pancreatic amylase
- starch goes to oligo- and disaccharides - contact digestion
- in the small intestine disaccharidases are bound to the brush boarder
- different enzymes for different disaccharides eg. sucrase for sucrose
- end up with monosaccharides
describe the process of chemical digestion for protein
- luminal digestion
- initiated in the stomach by HCl acid (denatures protein) and pepsin (protease) (cuts up large polypeptide chains)
- continues in the small intestine by pancreatic proteases (trypsin chymotrypsin, carboxypeptidase) and end up with small polypeptides - contact digestion
- in the small intestine peptidases are bound to the brush boarder
- different enzymes for different peptide bonds
- small polypeptides go to amino acids (1, 2 or 3 joined together)
describe how the chemical digestion of lipids/fat is different
- lipids/fats are insoluble in water
BUT - digestive enzymes are water soluble and dissolved in the fluid within the lumen of the GIT
- the body needs a mechanism to allow lipids to dissolve in fluid so that they can be digested - so chemical digestion of fats is a much more complex process
- fats undergo luminal digestion (no contact digestion), which occurs in the stomach of the small intestine
- lingual and gastric lipase digest roughly 30% of TAGs in the stomach
- pancreatic lipase is the main digestive enzyme and digests TAGs in the small intestine
what are the four stages of the chemical digestion of lipids/fat?
- emulsification: mixing
- motility - emulsification: stabilisation
- bile salts and others - digestion: hydrolysis
- enzymes - formation of micelles
- bile salts
describe the first phase of chemical digestion of lipids/fats, emulsification
emulsification: process by which large lipid droplets are broken down into smaller, stabilised droplets increasing the surface area for digestion
- mixing causes mechanical disruption of fat droplets
- motility breaks up large lipid droplet into smaller emulsification droplets- stomach: retropulsion
- small intestine: segmentation
- results in: droplets 0.5-1 micrometers, and increasing surface area for digestion
describe the second phase of chemical digestion of lipids/fats, emulsification
- bile salts (and other molecules) stabilise the emulsion droplets
- bile salts (and other molecules) coat the emulsion droplets- stomach: simple emulsion (other molecules)
- small intestine: more complex emulsion (bile salts)
- results in: stabilisation of emulsion droplets which prevents them from reforming into larger droplets
describe the structure, function and how the bile salts are made that are used in phase two of chemical digestion of lipids/fats
Bile salts
- made and secreted by liver and concentrated in the gallbladder
- released into small intestine with arrival of chyme
amphipathic: have hydrophobic and negatively charged hydrophilic side chain
- stabilise the emulsion droplets in small intestine
- also reduce the size of the emulsion droplets
- which further increases the surface area for digestion
describe the third phase of chemical digestion of lipids/fats, hydrolysis
lipase converts triglycerides to:
- 2x free fatty acids
- 1x monoglyceride
hydrolysis of TAGs requires both lipase and is cofactor colipase, both secreted by the pancreas
- colipase anchors lipase to the surface of the droplet so it can do its chemical digestion
hydrolysis of fats (break down of TAGs) occurs:
- in the small intestine
- at surface of emulsion droplets
