30 Questions
What is the primary challenge in lipid digestion and absorption?
Solubility of lipolytic enzymes in an aqueous medium
What percentage of dietary triglycerides may be digested in the stomach?
30 per cent
Which of the following is a vegetable source of lipids?
Groundnut oil
What is the optimal pH range for lingual lipase?
pH 4.0 to 4.5
In which phase of lipid digestion and absorption do events take place inside intestinal epithelial cells?
Absorption phase II
What type of fatty acid chains are more susceptible to digestion in the stomach?
Short-chain fatty acids
What is the primary site of fat digestion?
Small intestine
What is the optimum pH for pancreatic lipase?
6
What is the role of Ca++ in fat digestion?
It facilitates the action of pancreatic lipase
What is the function of secretin?
Regulation of electrolyte and fluid components
What is the role of bile salts in fat digestion?
They help in emulsification of fats and enhance lipase activity
What is the enzyme that breaks down phospholipids?
Phospholipase A2
What is the primary component of chylomicrons?
Triglycerides
What is the function of apo-B48 in chylomicrons?
To facilitate the transportation of triglycerides
Where are chylomicrons synthesized?
Intestinal wall
What is the approximate diameter of chylomicrons?
0.075-1 mμ
Why can't resynthesised TG pass into the lymphatics or portal blood?
Because it is insoluble in water
What is the difference between nascent and circulating chylomicrons?
Circulating chylomicrons have apo-C, while nascent chylomicrons do not
What is the characteristic of fatty acids that are more readily absorbed?
Short and medium chain FA (6 to 10 C) and unsaturated FA
Where are the products of digestion taken up and resynthesized into TG?
Smooth endoplasmic reticulum
What is the role of the enzyme glycerokinase?
To convert glycerol to α-Glycerol-(P)
What happens to short-chain FA and medium-chain FA (less than 8 to 10 C) and unsaturated FA?
They are absorbed into the portal blood directly
What is the function of the enzyme thiokinase?
To convert FA to Acyl-CoA
What is the ultimate fate of all FFA present in the intestinal wall?
They are reincorporated into TG after activation
What is the final product of fat digestion in the small intestine?
α-monoglycerides
What is the process by which bile salts are returned to the liver?
Enterohepatic circulation of bile salts
What is the purpose of micelles?
Collection of bile salts and fatty acids for absorption
What is the site of fat absorption?
Duodenum and jejunum
What happens to less than 1/4 of the ingested fat?
It is broken down to glycerol and FA
What is the first step of pancreatic lipase action?
Removal of a terminal FA to produce an α, β-diglyceride
Study Notes
Dietary Sources of Lipids
- Animal sources: dairy products, meat, fish, and eggs
- Vegetable sources: sunflower oil, groundnut oil, cotton seed oil, and mustard oil
Challenges of Lipid Digestion and Absorption
- Insolubility of fats in water
- Solubility of lipolytic enzymes in an aqueous medium
Phases of Lipid Digestion and Absorption
- Preparatory phase: digestion of lipids in the intestine
- Absorption phase I: transport of digested fats across the membrane of intestinal villous layer into intestinal epithelial cells
- Absorption phase II: events that take place inside intestinal epithelial cells and its passage through lacteals to lymph or portal blood
Preparatory Phase
- Little or no fat digestion takes place in the mouth
- Lingual lipase (Ebner's gland): pH 4.0 to 4.5
- Stomach: about 30% of dietary triglycerides may be digested; more active on triglycerides with shorter or medium fatty acid chains; more specific for ester linkage at 3-position rather than position-1
Digestion in Small Intestine
- Major site of fat digestion
- Lipase (steapsin) - pancreatic juice
- Sequence of events for intestinal fat digestion:
- Secretion of pancreatic juice stimulated by acid gastric contents, GI hormones (secretin and CCKPZ), and bile salts
- Role of bile salts: combination with lipase, protection against inhibitory effects of bile salts, and emulsification of fats
- Role of Ca++: facilitates lipase action and precipitates free fatty acids as 'soaps'
Absorption and Formation of Chylomicrons
- Short- and medium-chain fatty acids and unsaturated fatty acids are more readily absorbed
- Products of digestion taken up by smooth endoplasmic reticulum and resynthesized into triglycerides
- Formation of chylomicrons: composed of triglycerides, cholesterol, phospholipids, and apolipoprotein B-48
- Average composition of chylomicron molecule: triglycerides (87-88%), phospholipids (8%), free and esterified cholesterol (3%), and apolipoprotein B-48 (0.05-2%)
Absorption of Resynthesized Triglycerides
- α-Monoglycerides are further hydrolyzed by intestinal lipase to produce free fatty acids and glycerol
- Fatty acids absorbed and formed from hydrolysis of α-monoglycerides are activated to acyl-CoA
- Glycerol is converted to α-glycerol-phosphate by glycerokinase in the presence of ATP
- α-Glycerol-phosphate combines with acyl-CoA to form triglyceride molecule
Mode of Action of Pancreatic Lipase
- Complete hydrolysis of fats produces glycerol and fatty acids
- Digestion steps: removal of terminal fatty acid to produce α, β-diglyceride, removal of terminal fatty acid to produce β-monoglyceride, isomerization of β-monoglyceride to α-monoglyceride
- α-Monoglycerides are the major endproducts of fat digestion
Test your knowledge of lipid metabolism, including dietary sources of lipids, challenges, and phases of digestion and absorption. This quiz is designed for 200-level MBBS students in the Department of Medical Biochemistry at Baze University of Abuja.
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