Digestion of Fat, Vitamins and Minerals Flashcards
What form is almost all ingested fat in?
Triacylglycerol
What is responsible for all fat digestion in the small intestine?
Pancreatic lipase
How does triacylglycerol present?
Large lipid droplets which are insoluble in water
What is lipase?
A water soluble enzyme
What does lipase break triacylglycerol into?
Monogylceride and 2 fatty acids
Why is the digestion of fat very slow?
Digestion can only take place at the surface of triacylglycerol droplets
Emulsification
Dividing large lipid droplets into smaller droplets
What is the purpose of emulsification?
To increase the surface area and accessibility to lipase action
What does emulsification require?
- Mechanical disruption
- Emulsifying agent
How does mechanical disruption take place in emulsification?
Large lipid droplets mechanically disrupted into small droplets as smooth muscle contraction grinds and mixes luminal contents
What does the emulsifying agent do?
Prevents small droplets reforming into large droplets
What forms emulsifying agents?
Bile salts and phospholipids secreted in bile
What are emulsifying agents?
Amphiatic molecules (polar and non-polar portions)
How do the amphiatic molecules work as emulsifying agents?
- The non-polar portions associate with non-polar interior of liquid droplet leaving polar portions exposed at water surface
- Polar portions repel other small lipid droplets to prevent reforming into large droplets
How is absorption lipase digestion products enhanced?
By the formation of micelles
What are micelles?
Made up of:
- Bile salt
- Monoglycerides
- Fatty acids
- Phospholipids
How do micelles compare in size to emulsion droplets?
Much smaller
How are the components of micelles arranged?
- Polar portions of molecules a micelle surface
- Non-polar portions form micelle core
What does micelle breakdown result in?
Release of small amounts of free fatty acids and monoglycerides into solution leading to diffusion across plasma membrane of absorbing cells
What does the dynamic equilibrium between fatty acids and monoglycerides in solution and in micelles result in?
Retains most of fat digestion products in solution while constantly replenishing supply of free molecules for absorption
What does not get absorbed with the products of lipase digestion?
Micelles
What happens to fatty acids and monoglycerides after entering epithelial cells?
They enter smooth endoplasmic reticulum where they are reformed into triacylglycerols
What is responsible for the reforming of triacylglycerols?
Enzymes located in the smooth endoplasmic reticulum
What happens to triacylglycerol droplets after they are reformed?
Emulsified by coating of amphiphatic proteins
How are triacylglycerol droplets transported through the cell?
In vesicles formed from the smooth endoplasmic reticulum membrane
What happens to the triacylglycerol droplets transported in vesicles?
They are processed through the Golgi apparatus and exocytosed into extracellular fluid at serosal membrane
What are chylomicrons
Extracellular fat droplets
Other than fat, what do chylomicrons contain?
- Phospholipids
- Cholesterol
- Fat soluble vitamins
What happens to chylomicrons?
They pass into lacteals between endothelial cells
What can chylomicrons not pass through?
Capillary basement membrane
What are the 2 classes of vitamins?
- Fat soluble vitamins
- Water soluble vitamins
What are the fat soluble vitamins?
- A
- D
- E
- K
What happens to fat soluble vitamins?
Follow the same absorptive path as fat
What are the water soluble vitamins?
- B group
- C
- Folic acid
What happens to water soluble vitamins?
Either absorbed by passive diffusion or carrier-mediated transporter
What is vitamin B12?
A large charged molecule
What happens to B12?
It binds to intrinsic factor in the stomach to form complex which is absorbed via specific transport mechanism is distal ileum
What does B12 deficiency result in?
Pernicious anaemia
What is pernicious anaemia?
Failure of red blood cells maturation
What happen to 10% of our daily ingested iron?
Absorbed across intestine into blood
How is iron transported across the brush border membrane?
Via DMT1
Where does DMT1 transport iron to?
Duodenal enterocytes
What are iron ions incorporated into?
Ferritin
What is ferritin?
Protein-iron complex which acts as an intracellular iron store
What happens to unbound iron?
Transported across serosal membrane to the blood
What happens to iron in the blood?
Binds to transferrin
How is ferritin expression regulated?
Depending on body’s iron status
Hyperaemia
Increased ferritin levels which leads to more iron bound to enterocytes
Anaemia
Decreased ferritin levels which leads to more iron released to blood
