1. Oral cavity: Mechanical digestion, Lingual lipase secreted but no digestion
2. Stomach: Continues in small intestine, Peristaltic movement, heat of stomach, and acid juice (gastric and lingual lipases) aid in lipid digestion (not >20-30%)
3. Small intestine: >80-90% of lipid digestion, Emulsification of lipids, Enzymes secreted by pancreas: Pancreatic lipase, Phospholipase, Cholesterol ester hydrolase
Free fatty acids (FFA)
Monoacylglycerol, Cholesterol
Steatorrhea
Impaired digestion and absorption of dietary lipids, Excretion of excessive fat in faeces, Causes: Bile salts deficiency, Pancreatic enzyme deficiency, Disorders of small intestine
Absorption of lipids by intestinal mucosal cells (enterocytes)
Re-synthesis of TAG, PL and CE, Assemble with phospholipids, apolipoprotein B-48 to form chylomicrons, Secreted into lymphatic system, Transport to blood, Transport to peripheral tissues
Chylomicrons
Newly synthesized TAG and CE are very hydrophobic and aggregate in aqueous environment, Chylomicrons released by exocytosis from enterocytes into the lacteals (lymphatic vessels originating from vili of small intestine), Chylomicrons follow lymphatic system to thoracic duct, then left subclavian vein, then enter blood circulation
Metabolism of fatty acids
Fatty acyl-CoA synthetase catalyzes the formation of fatty acyl-CoA, Carnitine shuttle, Beta-oxidation, Acetyl-CoA enters TCA and ETC
Fatty acid oxidation
Major source of ATP in tissues like liver, skeletal muscle, heart, especially in fasting conditions, Catabolic process by which fatty acid molecules are broken down, Cleaving two carbons every cycle to form 1 acetyl-CoA, 1 NADH and 1 FADH2
Fatty acid synthesis
Transfer of mitochondrial acetyl-CoA to the cytosol via Citrate-Malate-Pyruvate shuttle, Condensation of acetyl-CoA and oxaloacetate to produce citrate, Cleaved by citrate lyase to produce cytosolic acetyl-CoA and oxaloacetate, Carboxylation of acetyl-CoA to form malonyl-CoA, Fatty acid synthase catalyses the remaining series of reactions
Triacylglycerol (TAG) synthesis and metabolism
TAG synthesized from acyl-CoA and glycerol 3-phosphate, Glycerol phosphate produced from glucose via glycolysis, TAG undergoes hydrolysis by hormone-sensitive lipase to form free-fatty acids and glycerol, Glycerol enters blood and transported to liver and kidney, FFA reconverted to acyl-CoA and re-esterified with glycerol 3-phosphate to form TAG
Ketone bodies
Formed when acetyl-CoA cannot enter TCA cycle due to lack of oxaloacetate, Occurs during starvation, prolonged fasting, diabetes mellitus
Carbohydrates are essential for metabolism of fat
Acetyl-CoA formed from fatty acid can enter and oxidized in TCA cycle when carbohydrates are available
During starvation, prolonged periods of fasting and diabetes mellitus, oxaloacetate is depleted owing to its preferentially utilization in the process of gluconeogenesis
Acetyl-CoA take the alternative route of formation of ketone bodies in liver mitochondria
gradual deposition of cholesterol in the tissues, particularly in the endothelial linings of blood vessels leading to life-threatening events.
-stroke
-brain blockage
-effects on memory
-chest pain (angina)
-numbness in legs
components of cholymicrons
A) cholesterol
B) phospholipid
C) cholseterol ester
D) triglyceride
Cholesterol undergo degradative reactions in humans with conversion of cholesterol to physiologically important products like:
-bile acids and bile salts
-steroid hormones
-vitamin D
what is bile
-a watery mixture of organic and inorganic compounds.
-Phosphatidylcholine and bile salts are quantitatively the most important organic components of bile
-stored in gallbladder
§ The continuous process of secretion of bile salts into bile, their passage to the duodenum where some are converted to bile acids, and their subsequent return to liver as a mixture of acids and salts is known as enterohepatic circulation.