Lecture 7 - Absorption of Fats, Vitamins and Minerals Flashcards
Almost all ingested fat is in the form of what?
Triacylglycerol
What enzyme does ALL the fat digestion in the small intestine?
Pancreatic lipase
What is the issue with lipase?
Water soluble - digestion only at surface of a droplet of fat + so digestion very slow + in normal transit time fat would go undigested –> steatorrhoea
What are triacylglycerols composed of?
3 stearic acids + glycerol
What does lipase break triacylglycerols down into?
A monoglyceride + 2 fatty acids
What does the process of emulsifying fats involve?
Dividing the large lipid droplets into smaller ones to increase SA for lipase action
What are the 2 things that emulsification requires?
- Mechanical disruption of large lipid droplets into small ones
- Emulsifying agents (prevent small droplets reforming into large ones)
How does the gut mechanically disrupt large lipid molecules?
Muscularis externa (smooth muscle) contraction grinds + mixes luminal contents
What are the emulsifying agents in the gut?
Bile salts + phospholipids in bile
What kind of molecules are bile salts/phospholipids?
Amphiphatic molecules = have polar and non-polar portions
How do the amphiphatic molecules work in preventing lipid molecules reforming large droplets?
Non-polar portions associate with non-polar interior of lipid problem, with polar portion exposed at water
Polar portions repel other small lipid molecules which are also coated with amphiphatic molecules
Even with the emulsifying agents + mechanical disruption of lipid droplets, digestion via lipase is still very slow, what is the other component lipid digestion that overcomes this?
Formation of micelles (‘fat taxis’)
Describe a micelle
Like an emulsion droplet but much smaller
Composed of bile salt, monoglycerides, fatty acids, phospholipids
Are micelles extracellular or intracellular
Extracellular - never go inside cells
What molecules form the micelle surface?
Polar
What molecules form the micelle core?
Non-polar
What happens once the micelle has moved over to the brush border?
It breaks down and releases small amounts of free fatty acids and monoglycerides which diffuse across the plasma membrane of absorbing cells
Why does micelle breakdown occur at exactly the right spot?
Acid microclimate - fatty acids take on hydrogen ions and become uncharged + can diffuse across the lipid membrane
After the fatty acids/monoglycerides enter the epithelium cells where are they transported?
Into the smooth endoplasmic reticulum
What happens to FFA/monoglycerides in the smooth endoplasmic reticulum?
They are immediately reformed into triacyclglycerols
What occurs after the triacylgylcerols are formed in the sER?
Triacylgylcerol droplets are coated with ampiphatic protein and then transported through cells in vesicles, processed through Golgi apparatus and exocytosed into etracellular fluid at the serosal membrane
What are the extracellular fat droplets produced and leaving at the serosal membrane called?
Chylomicrons
Apart from triacylglycerols what do the chylomicrons contain?
Phospholipids, cholesterol, fat soluble vitamins
What structure do the chylomicrons pass into?
Lacteals
What system is fat carried in from the gut?
Lymphatic
Eventually the fat in the lymphatic system from the gut will enter the where? At this point what will it be coupled up with?
Vena cava system
Coupled up with carrier proteins
What are the two groups of vitamins?
Fat soluble
Water soluble
What are the fat soluble vitamins?
ADEK
What are the water soluble vitamins?
B, C, folic acid
How are fat soluble vitamins absorbed from the gut?
Same route as fat
How are water soluble vitamins absorbed from the gut?
Passive diffuse or carrier mediated transport
Why can’t B12 undergo passive diffuse?
As it is very large
How is B12 absorbed from the gut?
Binds to intrinsic factor in stomach to form a complex which is absorbed via a transport mechanism in the distal ileum
Where are 95% of nutrients from the gut absorbed?
Jejunum
Where is B12 stored? How much is stored there?
Liver
3 years worth
What is the most common cause of B12 deficiency?
Pernicious anaemia
What is the function of B12?
Allows RBCs to mature properly + form their biconcave shape which allows them to transport O2 properly
Who is B12 deficiency most common in?
Elderly
Those who have had surgery to remove part of the stomach or small intestine
What % of daily ingested iron is absorbed across the intestine into blood?
10%
Into which cells of the GIT is iron absorbed?
Duodenal enterocytes
What transporter is used to absorb iron?
Divalent metal transporter 1 (DMT1)
What are the two forms iron comes as?
3+
2+
What form of iron can the transporter take up?
2+ only
How is iron stored in the duodenal cells?
As ferritin (12 iron ions)
Why is iron stored as ferritin in the enterocyte?
As it is a very reactive molecule and can cause a lot of free radicals
How does iron get from the duodenal enterocyte into the blood?
Unbound iron transported across serosal membrane via specific transporter
Iron in blood binds to transferrin which is then taken to the liver and can be made into Hb etc.
How is ferritin expression regulated?
Depends on body’s iron status
If anaemia - signal to small intestines to stop them making ferritin so less ferritin made and more iron released into blood
What happens to the ferritin in the duodenal cells?
As cells go through cell cycle and migrate up + differentiate on the villi they die and are sloughed off along with the ferritin inside
What is the only nutrient the gut regulates?
Iron
How does hyperaemia affect ferritin levels?
Increases ferritin levels (so more bound in enterocytes)
How does anaemia affect ferritin levels?
Decreases (so more iron released into blood)
