Nutrient Digestion 2 (Fats, Vitamins and Minerals) Flashcards
What form is almost all ingested fat in?
Triacylglycerol
What enzyme is responsible for all fat digestion in the small intestine?
Pancreatic lipase
What are triacylglycerols present as?
Large lipid droplets which are insoluble in water
Why is digestion of fats really slow?
Digestion can only take place at the surface of the droplet because the lipase is water-soluble
What is emulsification?
Dividing of large lipid droplets into smaller droplets
What is the benefit of emulsification?
Increased surface area and increased accessibility to lipase action
What are the 2 main requirements of emulsification?
Mechanical disruption of large lipids into small droplets
Emulsifying agent to prevent small droplets reforming into large droplets
How does mechanical disruption work?
Smooth muscle contraction grinds and mixes lumenal contents
How does emulsifying agent work?
Non-polar portions associate with non-polar interior of lipid droplet leaving polar portions exposed at water surface
Polar portions repel other small lipid droplets prevents them reforming into large droplets
What are emulsifying agents?
Bile salts and phospholipids secreted in bile
Amphiphatic molecules
How is absorption enhanced?
Micelles
What are micelles?
Bile salt + monoglycerides + fatty acids + phospholipids
Similar to emulsion droplets but smaller
What happens when micelles breakdown?
Release of small amounts of free fatty acids and monoglycerides into solution which causes diffusion across plasma membrane of absorbing cells
Dynamic equilibrium between fatty acids and monoglycerides in solution and in micelles - retains most of fat digestion products in solution while constantly replenishing supply of free molecules for absorption
Are micelles absorbed?
No
What happens to fatty acids and monoglycerides after they enter epithelial cells?
They enter smooth endoplasmic reticulum where they form back into triacylglycerols by enzymes
Describe the transport of triacylglycerol through the cell?
Transported in vesicles formed from smooth endoplasmic reticulum
Processes through golgi apparatus
Exocytosed into extracellular fluid at serosal membrane
What are extracellular fat droplets? + content
Chylomicrons
Contain phospholipids, cholesterol and fat-soluble vitamins
What is a lacteal?
A branch of the lymphatic system where the chylomicrons run directly up into the lymph circulation
What are the 2 classes of vitamins? + examples
Fat-soluble: A, D, E, K
Water-soluble: B group, C and folic acid
Describe the transport of fat and water-soluble vitamins?
Fat-soluble: follow some absorptive path as fat
Water-soluble: either absorbed by passive diffusion or carrier-mediated transport
How is vitamin B12 absorbed?
Large charged molecule that binds to intrinsic factor in stomach to form complex which is absorbed via specific transport mechanism in distal ileum
What is B12 deficiency called?
Pernicious anaemia - failure of red blood cell maturation
What percentage of ingested iron is absorbed across intestine into blood?
10%
How is iron transported?
Transported across brush border membrane via DMT1 into duodenal enterocytes
How is iron stored?
Iron ions are incorporated into ferritin (protein-iron complex)
What happens to unbound iron?
Transported across serosal membrane into blood
What does iron bind to in the blood? + why
Transferrin
Keeps iron secured in blood
How is ferritin expression regulated?
Depends on body’s iron status
Iron is the only nutrient regulated by the gut
What happens when hyperaemia occurs?
Increased ferritin levels lead to more iron bound in enterocytes
What happens when anaemia occurs?
Decreased ferritin levels leads to more iron released to blood
