Physiology of Lipid Digestion Flashcards
What 3 areas does emulsion occur in?
Mouth (chewing), stomach (gastric churning and squirting through narrow pylorus), small intestine (segmentation and peristalsis).
What are the 3 types of emulsion droplets and describe them?
- multilamellar vesicle (two double layers). 2. Unilamellar vesicle (one double layer). 3. Mixed micelle (one single layer).
How are emulsion droplets stabilised?
By addition of a coat of amphipathic molecules that form a surface layer on the droplets.
What make up the amphipathic coat?
Fatty acids, monoacylglycerols, biliary phospholipids, cholesterol, bile salts (when droplets have progressively been reduced to unilammelar and mixed micelles).
When is gastric lipase released and where from?
In response to gastrin, released from chief cells.
Describe gastric lipase.
Has a pH optimum of 4 and is resistant to pepsin. Is inactive in the duodenum due to digestion by pancreatic protease and unfavourable pH, preferentially hydrolyses TAGs at the 3 position.
What type of fatty acids are absorbed by the stomach?
Short and medium chain fatty acids.
What do triglycerides get broken down into when digested by gastric lipase?
Diacylglycerol and a free fatty acid.
What does the full activity of pancreatic lipase require?
Colipase co-factor, alkaline pH, calcium, bile salts and fatty acids.
What position does pancreatic lipase usually hydrolyse triglycerides at?
The 1 and 3 positions.
What other lipases are there?
Carboxyl ester hydrolase, phospholipase A2, lingual lipase.
What pushes the vesicles apart when bile salts bind?
The negative charges on the surface of the vesicles.
Describe the function of colipase.
Binds to the bile salts and lipase, allowing lipase to access tri and di-acylglycerols (as bile salts block access of lipase to TAGs).
How do fatty acids enter the enterocytes from mixed micelles?
Passive diffusion and/or membrane fatty-acid translocases, fatty-acid binding protein and fatty-acid transport proteins.
What happens to short or medium chain fatty acids in the enterocytes?
They exit through the basolateral membrane and enter the villus capillaries.
What happens to long chain (>12 carbons) fatty acids and monoglycerides in the enterocytes?
They are resynthesised into triglycerides in the ER and are subsequently incorporated into chylomicrons.
What protein allows the absorption of cholesterol?
Niemann-Pick-C1-like 1 protein (NPC1L1).
What are the places where NPC1L1 is located?
Clatherin coated pits.
What drug prevents cholesterol absorption and how?
Ezetimibe - binds to NPC1L1.
Where does passive (paracellular) and active (transcellular) absorption of calcium occur?
Passive is whole length of duodenum. Active is mainly duodenum and upper jejunum.
When calcium concentration in chyme is less than 5mM, what type of absorption predominates?
Active.
What is active calcium absorption regulated by?
1,25-dihydroxyvitamin D3 (calcitriol), parathyroid hormone (increases 1,25-dihydroxyvitamin D3 synthesis).
How does increased 1,25-dihydroxyvitamin D3 (calcitriol) lead to increased calcium transport?
It causes more expression of calcium channels on the apical membrane and Ca2+ATPase on the basolateral membrane.
What does Fe2+ bind to in the stomach?
Gastroferrin.
What reduces Fe3+ to Fe2+?
HCl, vitamin C, brush border cytochrome B ferric reductase.
What transports iron into the enterocyte?
Divalent metal transporter 1 (DMT1) - coupled to H+ transport.
What causes increased and decreased expression of DMT1?
Expression increased by blood loss, reduced by human haemochromatotis protein (HFE).
What converts haem into Fe3+?
Haem oxidase.
Where is ferroportin 1 located and what hormone negatively regulates it and when?
The basolateral membrane. Hepcidin is released from the liver when body iron levels are high.
What causes vitamin B12 to be released from proteins when it is ingested?
Stomach acid.
What binds to B12 in the stomach?
Haptocorin (secreted to saliva).
What cells in the stomach release intrinsic factor?
Parietal cells.
What digest haptocorin and where?
Pancreatic proteases in the small intestine.
Where does B12 bind to intrinsic factor?
In the small intestine.
Where and how is B12-intrinsic factor complex absorbed?
In terminal ileum by endocytosis.
Why are vegans susceptible to B12 deficiency?
B12 is not present in vegetables.
Describe the absorption of fat soluble vitamins?
Incorporated into mixed micelles -> usually passively transported into enterocytes -> incorporated into chylomicrons or VLDLs.
What are the water soluble vitamins?
B complex vitamins (not B12), C and H.
What are transport processes for water soluble vitamins similar to?
Those of monosaccharides, amino acids and small peptides. May be either Na+-dependent or independent.
