Nutrient Digestion II (Fats, Vitamins and Minerals)

Question 1

What form is ingested fat in?

Answer 1

Triacylgerol (glycerol and 2 stearic acids).

Question 2

What enzyme digests fat in the small intestine?

Answer 2

Pancreatic lipase.

Question 3

Describe the breakdown of triacylglycerol by lipase.

Answer 3

Triacylglycerol –> monoglyceride + 2 fatty acids

• Triacylglycerols present as large hydrophobic lipid droplets.
• Lipase is hydrophilic and cannot enter the lipid droplet.
• Thus, digestion can only take place at the surface, very slow (6-8 hours).

Question 4

Describe emulsification.

Answer 4

Dividing large lipid droplets into smaller droplets, creating increased surface area and accessibility to lipase action.

Question 5

Describe the requirements of emulsification.

Answer 5

Mechanical disruption;
- Of large lipid droplets into small droplets.
- Smooth muscle contraction grinds and mixes lumen contents.
Emulsifying agent;
- Prevents small droplets reforming into large droplets.
- Bile salts and phospholipids secreted in bile.

Question 6

Describe micelles.

Answer 6

• Similar to emulsion droplets but much smaller (still too large to go into a cell.
• Made of bile salt, monoglycerides, fatty acids and phospholipids.
• Polar portion at micelle surface; non-polar portions form micelle core.

Question 7

Describe micelle breakdown and recycling process.

Answer 7

Breakdown;
Release of small amounts of free fatty acids (FFAs) and monoglycerides into solution –> diffusion across plasma membrane of absorbing cells.

Recycling;
Any leftover FFAs or monoglycerides reform into micelles and process repeats.
Micelles move down gut before being absorbed by transporter, moved into liver and reloaded with bile salts and phospholipids.

Question 8

Describe what happens to FFAs and monoglycerides after they pass through the epithelium of the small intestine.

Answer 8

• FFAs and monoglycerides enter smooth endoplasmic reticulum (sER) where there are reformed into triacylglycerols (by enzymes).
• Triacylglycerol droplets transported through cell in vesicles (of sER membrane), processed through Golgi apparatus and exocytosed into extracellular fluid at serial membrane.
• Extracellular flat droplets = chylomicrons (contain phospholipids, cholesterol and fat-soluble vitamins)
• Chylomicrons pass into lacteals between endothelial cells (cannot pass through capillary basement membrane).

Question 9

Describe what happens to FFAs and monoglycerides after they pass through the epithelium of the small intestine.

Answer 9

• FFAs and monoglycerides enter smooth endoplasmic reticulum (sER) where there are reformed into triacylglycerols (by enzymes).

Question 10

Describe fat-soluble vitamins and their absorption.

Answer 10

A, D, E, K;

- Follow same absorptive path as fat

Question 11

Describe water-soluble vitamins and their absorption.

Answer 11

B group, C and folic acid;

- Either absorbed by passive diffusion or carrier-mediated transport

Question 12

Describe the absorption of iron.

Answer 12

• 10% of daily ingested iron absorbed across intestine into blood.
• Iron transported across across brush border membrane (via DMT1) into duodenal enterocytes.
• Iron ions incorporated into ferritin (protein-iron complex —> intracellular iron store)
• Unbound iron transported across serosal membrane –> blood
• Iron in blood binds to transferrin
• Ferritin expression regulated depending on body’s iron status
• Hyperaemia –> increased ferritin levels –> more iron bound in enterocytes
• Anaemia –> decreased ferritin levels –> more iron released to blood