3 - GI physiology three

what form is fat digested in?

triclyglycerol

what is fat digested by

pancreatic lipase in small intestine

what is lipase

water soluble enzyme

why is fat digestion slow?

digestion can only take place at surface of triclyglycerol droplet

what is emulsification

breaking down of large fat droplets into smaller droplets

why is emulsification important?

increases surface area and accessibility to lipase speeding up fat digestion

what does emulsification require

mechanical disruption

emulsifying agent

what is mechanical disruption

large droplets to small droplets - smooth muscle contractions to grind and mix - lumen contraction

emulsifying agents purpose

prevention of small fat droplets reforming into large droplets

what are emulsifying agents made of?

bile salts and phospholipids

prevent small droplets reforming into large droplets

what type of molecule are emulsifying agents

amphipathic - non polar part and polar part

how is fat absorption enhanced

formation of micelles

what are micelles

bile salt + monoglycerides + fatty acids + glycerol

fatty acid micelle structure

HYDROPHOBIC

- fatty acids and monoacylglycerols in interior

- bile salts on exterior

micelle breakdown 2 steps

release small amount of free fatty acids and monoglycerides into solution

diffusion across plasma membrane of absorbing cells

micelles are not.... during fat absorption

ABSORBED

why is dynamic equilibrium important between fatty acids and monoglycerides in solution and in micelles

retains most of fat products in solution while constantly replenishing supply of free molecules for absorption

after entering epithelial cells where do fatty acids and monoglycerides go?

enter smooth endoplasmic reticulum

what happens to fatty acids and monoglycerides within SER

reformed into triacylglycerols by enzymes

what happens to triacylglycerols in SER

coated with amphipathic coating for emulsification

where do triacylglycerols go after SER modification

droplets transported through cell in vesicles to be processed by golgi and exocytosed into ECF at serial or basolateral membrane

what are extracellular fat droplets called

chylomicrons (smallest one)

what do chylomicrons contain

phospholipids

cholesterol

fat soluble vitamins - ADEK

what to chylomicrons pass to and why?

pass into lacteals between endothelial cells as they cannot pass through capillary basement membrane

2 classes of vitamins

fat soluble

water soluble

how are fat soluble vitamins absorbed

Same way as fats with them

how are water soluble vitamins absorbed (B,C and folic acid)

diffusion (passive) or by carrier mediated transport

what is vitamin B12

large charged molecule

How is vitamin B12 absorbed?

binds to intrinsic factor in stomach

froms B12 complex

absorbed via specific transport mechanism in distal ileum

B12 deficiency

pernicious anemia - failure of RBC maturation

how is iron absorbed?

iron transported across brush border membrane via DMT1 into duodenal enterocytes

incorporated into ferritin to create protein-iron complex as an intracellular iron store

unbound iron transported across serosal membrane into blood

hyperaemia

increased ferritin levels - more iron bound in enterocytes - haemochromatosis

anaemia

lack of iron due to decreased ferritin

what percent of daily ingested iron absorbed crosses the intestine into the blood

10%

Which Vitamin enhances iron absorption

Vitamin C

reduces ferric (3+) to become ferrous (2+) which is absorbed

Heme iron

from meat & fish

10% food iron, 25% absorbed

non heme iron

from veg, milk, egg

90% food iron

10% absorbed