Digestion of Fats, minerals and vitamins

Question 2

What are the parts of a triacylglycerol molecule?

Answer 2

1 Glycerol

3 stearic acids

Question 3

Where does all fat digestion take place and what enzyme is responsible?

Answer 3

Digested in small intestine by pancreatic lipase

Question 4

Why would digestion of lipid droplets be very slow if only lipase was involved?

Answer 4

Triacylglycerol molecules stick together and form large lipid droplets:

• because it is not water soluble

Lipase is water soluble so can not penetrate/enter the lipid droplets - so could only act on the outer molecules

Question 5

When lipase acts on triacylglycerol molecules, what is produced?

Answer 5

1 monoglyceride & 2 fatty acids

Question 6

What process is needed to allow faster digestion of lipid droplets?

Answer 6

Emulsification

Then formation of Micelles

Question 7

Why does emulsifacation/micelle formation make lipid digestion faster?

Answer 7

Increases the total surface area of lipid droplets and thus increases the area accessible to lipase

Question 8

What is a micelle?

Answer 8

Very small droplet made up of:

• Bile salt
• Monoglycerides
• fatty acids
• phospholipids

They are similar to emulsion droplets but much smaller, so to further increase the exposed area of lipids

Question 9

Describe the structure of micelles and how this keeps them very small

Answer 9

Phospholipids and bile salts form the surface of the micelle. They are anphipathic which means the have a polar and non polar end.

The polar (hydrophilic) end sticks outwards and the non-polar (hydrophobic) end sticks in.

The polar end interfaces with the water of the lumen and repels other droplets so they dont coagulate together

Question 10

The digested products of fats are contained within micelles.

How are these absorbed out of the small intestine?

Answer 10

1) Micelles must be broken down at the epithelium, releasing their contents into solution
2) The acid microclimate around villi donates the H+ to the fat products making them uncharged/unpolarised.
3) This makes them lipid soluble so they can simply diffuse across the plasma membranes, out of the lumen

Question 11

Explain how the equilbirum between fatty acids and monoglycerides works between the solution and inside micelles

Answer 11

Dynamic equilibrium between fatty acids and monoglycerides in solution and in micelles - retains most of fat digestion products in solution while constantly replenishing supply of free molecules for absorption

(odd question but just learn that)

Question 12

What happens to micelles after they are broken down?

Answer 12

They are not absorbed so their non-digestive components just float about and then form new micelles

Question 13

What happens to fat digestion products after leaving the intestinal lumen?

(answer up to the point they leave the sER

Answer 13

After entering the epithelial cells:

1. Fatty acids and monoglycerides enter Smooth endoplasmic reticulum (sER)
2. Enzymes within the sER reform the FAs & monoGs back into triacylglycerols (TAGs)
3. TAGs coated in anphiphatic protein => emulsification
4. They then leave sER in vesicles (more on this in next few slides)

Question 14

Explain what happens to TAG droplets in the epithelial cells when they leave the sER

Answer 14

1. TAG droplets are carried through the cell in vesicles formed from sER membrane
2. Vesicles travel to Golgi apparatus - where they processed (stuffs added and theyre folded and stuff) into Chylomicrons
3. Chylomicrons are exocytosed into extracellular fluid at serosal membrane

Question 15

What are chylomicrons?

Answer 15

Extracellular fat droplets

They also contain phospholipids, cholesterol & fat-soluble vitamins

In many ways they are similar to micelles - however - they are much smaller (~1 µm diameter)

Question 16

What is the fate of chylomicrons after leaving enterocytes?

Answer 16

Chylomicrons pass into lacteals between endothelial cells (cannot pass through capillary basement membrane)

Question 17

What are the two classes of vitamins?

Answer 17

Fat soluble (aka lipid derived)

Water soluble

Question 18

What are the fat soluble vitamins?

How are they absorbed?

Answer 18

A, D, E, K

To remember, think of asking a random abdn person:

Ah Dont Even Know

They follow the same absorptive path as fats (why theyre found in chylomicrons)

Question 19

What are the water soluble vitamins and how are they absorbed?

Answer 19

B group, C and folic acid

Either absorbed by passive diffusion or carrier-mediated transport. (kinda similar to Glucose etc) with exceptions

Question 20

How is vitamin B₁₂ absorbed?

Answer 20

B12 is a large, charged molecule so cant follow the routes of others

Instead, it binds to ‘Intrinsic factor’ - which is secreted in the stomach

This forms a complex which is absrobed by a specific transport mechanism in distal ileum

Question 21

What does vitamin B12 deficiency cause?

Answer 21

Pernicious anaemia

(failure of RBC maturation)

Question 22

What role do the intestines play in iron absorption?

Answer 22

10% of daily ingested iron absorbed across intestine into blood

Question 23

How is iron absorbed?

Answer 23

Transported across membrane via DMT1 into duodenal enterocytes

Here, some iron ions are joined to a specific protein to form ferritin for storage in the cell

Other iron ions pass through the serosal membrane into the blood and bind to Transferrin

Question 24

What regulates the expression of ferritin in the body?

Answer 24

Ferritin expression regulated depending on body’s iron status

Hyperaemia => increased ferritin levels => more iron is then stored in enterocytes

Anaemia => decreased ferrtin levels => more iron released from enterocytes into the blood