Nutrient Digestion & Absorption 2

Question 1

What form does most ingested fat take? And what are hte products of its digestion?

Answer 1

Triacylglcyerol, A union of glycerol and 3 fatty acids

Its broken down to Monoglyceride and 2 FA

Question 2

What enzymes digests TAG? what is the major issue with this enzyme?

Answer 2

Pancreatic Lipase
Its water soluble vs TAG which is fat soluble and so forms lipid droplets.
This decreases the surface area for lipase to act making it very slow

Question 3

How do we increase hte surface area for fat digestion?

Answer 3

By emulsification
(produces droplets around ~1mm)

Question 4

Explain the process of fat emulsification

Answer 4

1) Smooth muscle contraction grinds and mixes the luminal contents, breaking up the fatty droplets
2) emulsifing agents bile salt and phospholipids prevent the droplets reforming into big ones

Question 5

How do emulsifying agents work?

Answer 5

Phospholipids and bile salts are anphipathic (i.e. they have a polar and non-polar end).
The polar end faces outward interfacing with the water and repelling other droplets

Question 6

How do we speed up fat absorption in the gut?

Answer 6

By forming the products into micelles, same idea as emulsified droplets but smaller (~4-7um).

Question 7

What are the constituents of a micelle?

Answer 7

• Products of fat digestion (monoglyceride + FA)
• Bile salt
• Phospholipids

Question 8

How do the products of Fat digestion get from micelles to the intestinal epithelium?

Answer 8

1) They reach the brush border by being squelched around (brownian motion)
2) In the acid microclimate aorund the villi the fat products take on H+ ions, becoming uncharged
3) The products can then diffuse across the plasma membrane

The micelle itself is not absorbed

Question 9

Explain the equilibrium of Fatty Acids & monoglyceride in solution vs in micelles

Answer 9

A dynamic equibilibrium where products are reduced by H+ ions and pass from micelles into solution in the brush border whilst more are taken up by micelles in the gut lumen.

Question 10

What happens to the products of fat digestion once theyre in the enterocytes?

Answer 10

1) Taken up by Smooth ER (because not water soluble so cant sit in cytoplasm)

2) Reformed into TAG

Question 11

How are the products of fat digestion passes form the enterocyte into the blood?

Answer 11

1) TAG formed into vesicles from sER membrane

2) Processed through golgi apparatus into chylomicron

3) Exocytosed through basolateral membrane into ECM

4) Chylomicrons are still too big to pass through capillary basement membrane so enters lacteals -> Intestinal duct -> Thoracic Duct -> Blood

Question 12

What is a chylomicron?

Answer 12

A lipoprotein
Made of Phospholipids, cholesterol, TAG & proteins/vitamins

Question 13

What is a lacteal?

Answer 13

A lymphatic vessel of the small intestine whose function is to absorb fats

Question 14

What are the important vitamins we absorb from our gut and what are their classes?

Answer 14

Fat Soluble:
- A
- D
- E
- K

Water Soluble:
- B Group
- C
- Folic Acid

Question 15

How do vitamins get absorbed through the gut wall?

Answer 15

Fat soluble ones follow the same path as fat.
= Micelle -> solution -> diffusion -> chylomicron -> Lacteal

Water soluble ones are absorbed by passive diffusion or carrier mediated transport

Question 16

How does vitamin B12 get absorbed?

Answer 16

Its large and charged so passive diffusion ain’t an option.
Binds to intrinsic factor (glycoprotein) forming a complex which is absorbed by a specific transport mechanism in the ileum.

Question 17

Where is vitamin B12 stored?

Answer 17

In the liver, around 3 years worth at any time

Question 18

What happens if you have a B12 deficiency?

Answer 18

Red blood cells fail to mature and you get pernicious anaemia

Question 19

How much iron reaches our blood?

Answer 19

~10% of what we ingest daily

Question 20

Define enterocyte?

Answer 20

A cell of intestinal lining

Question 21

How is iron transported from the gut lumen into enterocytes?

Answer 21

By DMT1 (divalent metal transporter 1) in the duodenum. This is H+ coupled transport just like that of peptides

Question 22

What happens to the bulk of iron in enterocytes?

Answer 22

• Its incorporated into a protein-iron complex called ferritin (an intracellular iron store)
• Its stored there until the enterocyte dies (5 day life) and is shat out
• Hence more iron in your diet -> More ferritin -> Darker faeces. Isnt science awesome

Question 23

How does iron get from the enterocyte to your blood? and what happens to it then

Answer 23

Any that doesn’t form ferritin is transported across the serosal membrane,
From there its bound to the protein transferrin and taken to the liver for use.

Question 24

How is iron intake regulated?

Answer 24

Ferritin expression is regulated based on your body iron level.
Too little iron (anaemia) -> Less ferritin -> More iron absorbed
and vice versa for hyperaemia