Lipid Digestion Flashcards
What are the different lipids?
Fats/oils
Phospholipids
Cholesterol
Fatty acids
What is saturated?
Single bonds only
What is monounsaturated?
One double bond
What is polyunsaturated?
2 or more double bonds
Describe general process of lipid digestion
Emulsification
Enzymatically digested to yield monoglycerides + FAs
What does lingual lipases do (mouth)?
Hydrolysis of triglycerides at sn3
Cleaves short chain FAs
What do gastric lipases do?
Highest activity against short + medium chain triglycerides
Hydrolysis gets faster as goes on = FAs produce act as surfactants
What does pancreatic lipase do?
Hydrolysis of dietary triglycerides
Hydrolyses FA at 1 + 3 position
Produces 2-monoacylglycerol + 2 FAs
What do chyme do?
Stimulate secretin, cholecystokinin (CCK) + pancreatic juice secretion
What is secretin?
Increase secretion of electrolyte + fluids of pancreatic juice
What does CCK do?
Cause contraction of gall bladder + discharge of bile into intestines + stimulates secretion of enzymes
Describe pancreatic lipase
Main digestive enzyme
*Aided by bile salts from gall bladder
HCO3- in pancreatic juice neutralises gastric acid: pH for optimal enzyme action
Briefly describe what micelles do
Carry monoglycerides and FFA to brush border and diffuse into enterocytes
Why can triglycerides not diffuse across brush border?
Too big
What are the role of bile salts in lipid digestion?
Surround fat and break it into smaller droplets, preventing fat from reclumping
Increases the surface area for pancreatic lipase to act upon fat droplets
What does failure to secrete bile salts result in?
Lipid malabsorption
Secondary vitamin deficiency
What do bile salts do to the lipid droplet?
Expose hydrophobic core
= change surface charge
What are the primary products of dietary lipid digestion?
Free FAs
Free cholesterol
Monoglycerides
Lysophospholipids
What happens to fat soluble vitamins?
Fat soluble vitamins packaged + absorbed from mixed micelles
Describe lipid digestion
Emulsification of fat droplets by bile salts
Hydrolysis of triglycerides in emulsified fat droplets into FAs + monoglycerides
Dissolving of FAs + monoglycerides into micelles to produce “mixed micelles”
What happens to mixed micelles?
Trapped in brush border of intestinal mucosa
Lipid components absorbed by passive diffusion
What happens to short + medium triglycerides?
Triglycerides absorbed + hydrolysed by intestinal lipases
What do short + medium FAs NOT require?
Bile salts
Describe absorption + transport in the blood
FA + monoglyceride enter enterocyte by simple diffusion
Transported to endoplasmic reticulum + converted to triglycerides
Golgi = packaged with cholesterol
Chylomicrons into vesicles
Vesicles transported to basolateral surface of enterocyte
Vesicles fuse with plasma membrane + undergo exocytosis
= transfer chylomicrons outside
Transported to lymphatic vessel
Lymph drains into lymphatic system then into blood
Excreted into lacteals + circulate into blood
What does lipoprotein lipase (LPL) do?
Hydrolyses triglycerides held within chylomicron
What do free FAs bind to?
Albumin + circulate in plasma
What does lipoprotein core contain?
Cholesterol ester + TG
What does outer layer of lipoprotein particle contain?
Free cholesterol, phospholipid + specific apolipoproteins
What do apolipoproteins do?
Coat lipoprotein particles
= important in recognition
Describe VLDL
Bind to capillary cells
Become IDLs
How are IDL converted to LDL?
By hepatic lipase
Describe LDL
Product of VLDL losing triglycerides
Carry majority of cholesterol in blood
In peripheral cells
How do peripheral cells utilise LDL cholesterol?
For cell membrane structure + production of hormones
Describe HDL
Produce in peripheral tissue
Bind to receptors in vessel walls
What does lecithin-cholesterol acyltransferase (LCAT) do?
Convert free cholesterol in cholesterol ester, which moves into centre of HDL particle
How does the absorption of cholesterol work?
Transport by endocytosis by NPC1L1 protein
What are higher levels of LDL associated with?
Increased cardiovascular disease risk
What are increased HDL levels associated with?
Decreased risk for coronary heart disease
Low levels = RISK
What is the cardioprotective role of HDL?
Facilitate transfer of cholesterol from lipoproteins to liver
What happens to fat soluble vitamins?
A, D, E + K
Incorporated into mixed micelles
Passively transported
Incorporated into chylomicrons
Distributed by intestinal lymphatics
What happens to H2O-soluble vitamins?
Energy dependent process
Require Na+ in/dependent transporters
How is vitamin B12 (cobalamin) absorbed?
Ingested in food
Salivary glands secrete haptocorin
Stomach acid released B12
Haptocorin binds B12
Stomach parietal cells release intrinsic factor
Pancreatic protease digest haptocorin in small intestine, B12 released
B12 binds to intrinsic factor
Absorbed in terminal ileum by endocytosis
