Nutrient Digestion 2 Flashcards
What form is almost all ingested fat?
Tricylglycerol
Where is all fat digested?
Small intestine
What enzyme digests fat?
Pancreatic lipase
What is a lipase?
Water-soluble enzyme that digests fat
Why is digestion of triacylglycerols without modifications very slowly?
They present as large lipid droplets and digestion can only take place at the surface of the droplet
What makes triacylglycerol?
Glycerol molecule and 3 stearic acids
What does lipase break triacylglycerol down into?
Monoglyceride and 2 fatty acids
What is emulsification?
Dividing large lipid droplets into smaller droplets (about 1mm in diameter) to increase surface area and accessibility to lipase action
Why does emulsification occur?
Dividing large droplets into smaller ones increases surface area so can be broken down by lipase faster
What does emulsification require?
Mechanical disruption of large lipid droplet into small droplets
Emulsifying agent (prevents small droplets reforming into large droplets)
What provides mechanical disruption of large lipid droplets into small droplets?
Smooth muscle contraction grinds and mixes luminal content
What is the function of an emulsifying agent?
Prevents small droplets reforming into large droplets
What is an example of an emulsifying agent?
Bile salts and phospholipids in bile
What can be said about the charge of emulsifying agents?
Amphipathic molecules (polar charged and non-polar charged portions
What does the polar and non-polar portion of emulsifying agents bind to?
Non-polar associated to non-polar lipid interior of fat, leaving polar portion exposed at water surface
How do emulsifying agents prevent droplets reforming into large ones?
Polar portion of emulsifying agent surrounds the outside of the fat which prevents them reforming into large droplets
Why are micelles required to be formed after emulsification?
Absorption is still slow after emulsification, micelles speed this up
How is absorption of fat enhanced?
By forming micelles
What are micelles formed from?
Bile salt
Monoglycerides
Fatty acids
Phospholipids
What are the charges of micelles?
Polar surface, non-polar core
What size are micelles?
4-7 um (much smaller than emulsion droplets)
What do micelles release when they are broken down?
Free fatty acids and monoglycerides which diffuse across plasma membrane of absorbing cells
Explain the equilbrium between fatty acids and monoglycerides in solution and in micelles?
Dynamic equilbrium exists, so micelles retains most of fat digestion products in solution while constantly replenishing supply of free molecules for absorption
What is not absorbed once micceles reach the epithelial surface of lumen cells?
The micelles themselves are not absorbed, whilst their breakdown products are
Summarise the process of fat absorption prior to the fat being inside of the epithelial cells?
1) Emulsification
2) Formation of micelles
3) Travel to surface of cells and release fatty acids
4) Fatty acids enter epithelial cells
Summarise the process of fat absorption after entering epithelial cells?
1) Fatty acids and monoglycerides enter smooth endoplasmic reticulum (SER) where they are reformed into triaclyglycerols
2) Coated with amphiphatic protein (emulsification)
3) Transported though cell in vesicles formed from SER membrane, processed though golgi apparatus
4) Exocytosed into ECF at serosal membrane
5) Chylomicrones pass into lacteals between endothelial cells (cannot pass through capillary basement membrane)
Where are fatty acids and monoglycerides reformed into triaglyerols once they enter epithelial cells after the gut lumen?
Smooth endoplasmic reticulum (SER)
Where do chylomicrons enter to be transported around the body?
Lacteals between endothelial cells (cannot pass through capillary basement membrane)
What are chylomicrones?
Extracellular faty droplets
What do chylomicrons contain?
Fat droplets
Phospholipids
Cholesterol
Fat soluble vitamins
Summarise the entire process of fat absorption?
1) Emulsification in gut lumen
2) Formation of micelles
3) Micelles release fatty acids and monoglycerol at cell surface which pass through cell membrane
4) Enter smooth endoplasmic reticulum (SER) and reformed into triacylglycerols
5) Coated with amphiphatic protein (emulsification)
6) Transported through cell in vesicles formed from SER membrane
7) Processed though golgi apparatus and exocytosed into ECF at serosal membrane
8) Chylomicrones pass into lacteals between endothelial cells and are transported around the body in lymphatic system
What are the 2 classes of vitamins?
Fat soluble vitamins
Water soluble vitamins
What are examples of fat soluble vitamins?
Vitamin A
Vitamin D
Vitamin E
Vitamin K
What are examples of water soluble vitamins?
Vitamin B group
Vitamin C
Folic acid
What absorption mechanism do fat soluble vitamins use?
Same absorptive path as fat
How are water soluble vitamins absorbed?
Passive diffusion or carrier mediated transport
What is absorbed differently from other vitamins and why?
Vitamin B12 because it is a large charged molecule
What is the absorption mechanism of vitamin B12 summarised?
1) Binds to intrinsic factor in stomach to form complex
2) Absorbed via specific transport mechanism in distal ileum
Where does vitamin B12 join intrinsic factor?
Stomach
What is the disease of vitamin B12 deficiency called?
Pernicious anaemia
What does pernicious anaemia cause?
Failure of red blood cell maturation
Where do bile salts get absorbed?
Distal end of ileum
What is vitamin B12 required for?
Red blood cells to mature to correct shape
How many years of vitamin B12 are stored in the liver?
About 3 years
What percentage of ingested iron is transported into the circulatory system, and what happens to the rest?
10% is transported into the circulatory system, the rest is held in the gut
What is the only nutrient regulated by the gut?
Iron
What is meant by iron is the only nutrient regulated by the gut?
Every nutrient continues to be absorbed to be ‘saved for later’
Only a certain amount of iron is absorbed
Summarise the absorptive process of iron?
1) Transported across brush border membrane of intestine (via DMT1) into duodenal enterocytes
2) Iron ions incorporated into ferritin (protein-iron complex) which acts as intracellular iron store
3) Unbound iron transported across serosal membrane into the blood
4) Iron in blood binds to transferrin
What transporter is used to get iron from the brush border of intestine into duodenal enterocytes?
DMT1 transporter protein
What is ferritin?
Protein-iron complex
What is the function of ferritin?
Act as intracellular iron store
Why does iron in blood bind to transferrin?
All metals generate oxygen free radicals which are toxic so iron is stored in a way it cannot do any damage
Where does the pancreas deliver all of its enzymes to?
Duodenum
How is ferritin expression regulated?
By bodies iron status:
- hyperaemia causes increased ferritin levels so more iron bound in enterocytes
- anaemia causes decreased ferritin levels so more iron released to blood
What is hyperaemia?
Excess of blood in vessels
When is each gut surface cell shed?
Roughly every 3 days
What happens to ferritin when the cell it is in dies?
Leaves with the faeces
