W2 Physiology: lipid digestion and absorption of calcium, iron and vitamins Flashcards
what is emulsification and how does it occur
- When solid fats and oils are converted into an emulsion of small oil droplets suspended in water
1. mouth (chewing)
2. Gastric churning with enzyme and squeezing through pylorus
3. segmentation and peristalsis in SI mixing chyme with pancreatic and biliary secretions
how are emulsification droplets stabilised?
by the addition of amphiphilic coat
Contents of amphiphilic coat
Products of lipid digestion eg monoglyceride
cholesterol
biliary phospholipid
bile salts (when droplets are reduced to unilamellar mixed micelles)
Process of digestion of fats
- mouth- lingual lipase
- Gastric phase- gastric lipase
- Duodenum- pancreatic (TAG) Lipase
what triggers production of gastric lipase?
gastric lipase is secreted in response to gastrin release from chief cells
optimum pH of gastric lipase
What is gastric lipase resistant to?
ph4
pepsin
why is gastric lipase inactive in the duodenum
digestion by pancreatic protease and unfavourable pH
where are short and medium chain FA absorbed?
stomach
which cells secrete pancreatic lipase
What is this triggered by
acinar cells of pancreas
in response to CCK which also stimulate bile flow
what substances and conditions does full activity of pancreatic lipase require
colipase co-factor, alkaline pH, Ca2+, bile salts, fatty acids
where is bile salt released from and what into?
bile salt is released from the gall bladder into the duodenum
what is the release of bile salt triggered by
CCK
function of bile salts
- act as DETERGENTS to help emulsify large lipid droplets to small lipid droplets
- increases SA for pancreatic lipase to act
describe the structure of bile salts
AMPHIPATHIC
hydrophilic and hydrophobic parts
result of failure to produce bile salts
- lipid malabsorption, steatorrhea (fatty faeces)
- secondary vitamin deficiency (A,D,E,K)
why may someone have steatorrhea
failure to produce bile salts- lipid malabsorption
problem that bile salts present and how this is solved
block access of lipase enzyme to TAG
colipase from the pancreas binds to bile salts and lipase allowing access to tri- and di-acylglycerols
is colipase directly secreted as itself from the pancreas?
no
colipase is secreted as inactive procolipase then activated by trypsin
what are the final products of lipid digestion stored in
mixed micelles
methods by which mixed micelles can enter enterocyte
passive diffusion
membrane fatty acid translocases
fatty acid binding protein
fatty acid transport protein
- size of short chain and medium chain FA
- how short chain and medium chain FA pass through enterocyte
short chain (<6), medium chain (8-12) diffuse through enterocyte, exit through basolateral membrane and enter villus capillaries
size of long chain FA
what happens to the long chain FA
> 12
long chain FA and monoglycerides are resynthesised into triglycerides in ER and incorporated into chylomicrons
how are chylomicrons formed?
what then happens to chylomicron
- triglycerides are formed in ER by monoglyceride and long FA
- cholesterol added and phospholipid synthesis
- APO-B 48 added to from chylomicron
- chylomicron exits enterocyte by exocytosis and enters the central lacteal then carried in lymph vessels to systemic circulation via thoracic duct
transport of cholesterol absorption
endocytosis in clatherin coated pits by NPC1L1
