What form is almost all ingested fat?
Where is all fat digested?
What enzyme digests fat?
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?
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?
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 soluble vitamins