Nutrient Digestion II (Fats, Vitamins, Minerals)

Question 1

What is the digestive process for conversion of fats to triacylglycerols and monoglycerides?

Answer 1

• All fat digestion occurs in small intestine by pancreatic lipases
• Pancreatic lipases convert TAG’s to Monoglycerides & 2 FA’s

Question 2

Why is emulsification of ingested fats required?

Answer 2

• To help digestion, lipid droplet surface area is increased by emulsification to allow lipases to work over a greater area.
• Emulisification is aided by smooth muscle contraction of the stomach and small intestine

Question 3

What role do bile salts play in the production of emulsification droplets and micelles?

Answer 3

• Emulsifying agents (bile salts & phospholipids) prevent small lipid droplets from reforming back into large lipid molecules by polarising ends of the lipids so they repel each other
• Emulsifying agents (bile salts) also aid the formation of Micelles by keeping the fat molecules away from water
• Micelles are like ‘taxis’ for fat droplets
• Micelles = bile salts + phospholipids + monoglycerides + FA’s

Question 4

How are fat soluble and water soluble vitamins absorbed?

Answer 4

• Fat soluble vitamins (A, D, E and K) are absorbed in the small intestine along with fats.
• Water soluble vitamins (C and B) are absorbed by mediated transport

Question 5

How are important dietary minerals absorbed?

Answer 5

• Iron and Calcium are absorbed by active transport.
• The intestinal absorption of these minerals is actively regulated within the intestine, e.g. if body concentrations of these minerals drop then there is an enhanced uptake at the intestine.
• Heme Iron is absorbed by an apical transporter on the enterocyte.
• Ionised Iron is actively absorbed by apical cotransport with H+ on a protein called the divalent metal transporter (DMT1).
• Once inside the cell, enzymes convert heme iron to Fe2+, and both pools of ionized iron leave the cell on a transporter called ferroportin.
• Iron uptake is regulated by hormone hepcidin
• When levels of iron in the body are high, the liver secretes hepcidin, which binds to ferroportin.
• The hepcidin-bound transporter is targeted for destruction by the enterocyte, which results in decreased iron uptake across the intestine.
• Most Ca2+ absorption in the gut occurs by passive, unregulated movement through paracellular pathways.
• Ca2+ Transcellular transport is regulated by vitamin D3. Ca2+ is actively transported across the basolateral membrane by a Ca2+ ATPase or by Na+-Ca2+ antiporter.