digestion and absorption Flashcards
fats
saturated: meat fat, butter, cheese, cream
unsaturated: olive, rape seed, nuts
cholesterol: plants and animals, dairy products, palm oil
Too much…..
Atherosclerosis (MI, stroke)
Obesity
protein
Polymers of 20 amino acids
Essential (9): phenylalanine, valine,
tryptophan, threonine, isoleucine,
methionine, histidine, leucine,
& lysine
digestion
Digestionis the process by which food is broken down into components simple enough to be absorbed in the intestine.
Carried out by enzymes secreted by
-glandular cells in the mouth
-chief cells in the stomach
-exocrine cells of the pancreas
-enzymes bound to the apical membrane
of enterocytes.
salivary secretion
Production: parotid, submandibular and sublingual salivary glands
Moistens food
Starts the digestion of
carbohydrates a-amylase
Starts the digestion of lipids (minor: lingual lipase)
stomach
Start of protein digestion
Chief cells: secrete pepsinogen
Parietal cells: hydrochloric acid secretion
denatures proteins activates pepsin
= endopeptidase
zymogens
Many enzymes are synthesised as inactive precursors that are subsequently activated by cleavage of one/a few specific peptide bonds. The inactive precursor is called azymogen(orproenzyme).
Pepsinogen inactive ——> pepsin active
proteolytic enzymes, PANCREATIC SECRETIONS
Trypsin- proteins
Chymotrypsin- proteins
Carboxypeptidase- proteins
Elastase- proteins (elastin)
- released as zymogens
non-proteolytic enzyme, pancreatic secretions
Phospholipase - phospholipids
Amylase- starch
Lipase- triglycerides
Ribonuclease- RNA
Deoxyribonuclease- DNA
- released in active forms
liver
Production and secretion of bile
Stored in gallbladder and released into
the duodenum after a meal
Emulsification of fat particles
Aids absorption of fats by forming
complexes called micelles
bile
bile salts:
synthesised from cholesterol——> colic acid, chenodeoxycholic acid
Conjugated to amino acid (glycine or taurine)
Amphipathic
Functions:
1.Emulsification of lipid aggregates
2.Solubilisation & transport of lipids in aqueous environment
small intestine
Duodenum:
Mixing secretions from pancreas, liver and duodenum with food
Neutralisation of acid
Further digestion
Absorption
Jejunum:
Completing breakdown
Nutrient absorption
Ileum:
Nutrient absorption
small intestine
Brush border enzymes:
Integral part of the
membrane
(not free in lumen)
Peptidases
Lactase
Sucrase
Maltase
mouth
Mastication
Bolus
Carbohydrates:
Starch broken down by
a-amylase to maltotriose,
maltose and a-limit dextrin
Proteins: nothing
Lipids: lingual lipase present
but minor contribution
stomach
Churning
Chyme
Carbohydrates: nothing
Proteins: HCl denatures proteins & activates pepsin
(endopeptidase that cleaves proteins to
smaller peptides)
Lipids: gastric lipase present
but minor contribution
duodenum, chyme and carbs
Chyme mixes with pancreatic juice and bile
Slightly alkaline
Carbohydrates: a-amylase further digests starch
Brush border disaccharidases (maltase, sucrase, lactase)
Result in monosaccharides
duodenum, proteins and lipids
Proteins: cleaved by trypsin, chymotrypsin, elastase,
carboxypeptidases to produce increasingly
smaller peptides/dipeptides
Brush border peptidases produce dipeptides/amino acids
Lipids: pancreatic lipase digests lipids to
monoglycerides and fatty acids. Phospholipase
digests phospholipids to lysolecithin & fatty acids
absorption
Passive absorption – diffusion
slow
needs concentration gradient or charge gradient
Facilitated transport
slightly faster
involves membrane carrier
controllable
Active transport
uses energy
fast
uses membrane carrier
controllable
absorption of lipids- formation of micelles
polar head group
non polar hydrocarbon chain
hydrophobic lipid core (insoluble long chain fatty acids)
chylomicron formation
If fatty acids are less than
12 carbons long they go
straight to the portal blood
If larger,
triglyceride reformed into
protein/lipid structure called
a chylomicron.
Chylomicrons 76-100nm
diameter secreted into
lymphatics via lacteals.
