Nutrient Digestion and Absorption of Fats and Vitamins

Question 1

What form does almost all ingested fat take?

Answer 1

• triacylglycerol

Question 2

Which enzyme breaks down fats in the small intestine?

Answer 2

• pancreatic lipase

Question 3

What is triacylglycerol initially broken down into?

Answer 3

• monoglyceride

- two fatty acids

Question 4

True or False.

Lipase can only act on the polar end of the triacylglycerol molecule.

Answer 4

True.

- this is because lipase is also a polar molecule.

Question 5

How does the body overcome the very slow process of digestion with lipase?

Answer 5

• the addition of emulsifiers

Question 6

What does action by emulsifiers require?

Answer 6

• mechanical disruption

- emulsifying agents

Question 7

Why is mechanical disruption important?

Answer 7

• breaks down large lipid droplets into smaller droplets

- this increases the surface area available for reaction

Question 8

How is mechanical disruption achieved?

Answer 8

• smooth muscle contracts to grind and mix the luminal contents

Question 9

Why is the addition of emulsifying agents important?

Answer 9

• prevents the smaller droplets from reforming larger droplets
• they are amphipathic, allowing fat and water to mix, and repelling other lipid molecules

Question 10

What is the function of micelles?

Answer 10

• they act as a ‘fat-taxi’, transporting fat

- they are smaller and therefore can get closer to the microvilli

Question 11

How do the micelles come into contact with the microvilli?

Answer 11

• they undergo a randomised introduction process

- they eventually reach the acid microclimate which pulls them closer

Question 12

What happens when the micelles meet the microvilli?

Answer 12

• they break down, releasing small amounts of free fatty acids and monoglycerides into solution
• these then diffuse across the plasma membrane into absorbing cells

Question 13

True or False.

Micelles themselves are absorbed.

Answer 13

False.

Micelles themselves are NOT absorbed.

Question 14

What happens to fatty acids and monoglycerides upon entering the cell?

Answer 14

• they enter the sER

- here, they are reformed into triacylglycerols by enzymes, also located in the sER

Question 15

What happens to triacylglycerols after they are reformed in the sER?

Answer 15

• they are emulsified by coating them with an amphipathic protein
• they are then transported through the cell in vesicles
• they are processed through the Golgi apparatus
• they are then exocytosed into the extracellular fluid at the serosal membrane
• the droplets are now termed chylomicrons

Question 16

What happens to chylomicrons?

Answer 16

• they pass into lacteals between endothelial cells

Question 17

Name some fat-soluble vitamins.

Answer 17

Vitamins A, D, E, K

The follow the same absorptive path as fats.

Question 18

Name some water-soluble vitamins.

Answer 18

Vitamin B group, vitamin C and folic acid

Question 19

How are water-soluble vitamins absorbed?

Answer 19

Either via passive diffusion or carrier-mediated transport.

Question 20

Describe vitamin B12.

Answer 20

It is a large, charged molecule.

Question 21

How is vitamin B12 absorbed?

Answer 21

• it binds to intrinsic factor, which is secreted by the stomach
• together they form a complex
• the complex is then absorbed via specific transport mechanism found in the distal ileum

Question 22

How much B12 does the liver store?

Answer 22

• around three years worth

Question 23

What can a B12 deficiency lead to?

Answer 23

• pernicious anaemia (the failure of red blood cell maturation)

Question 24

True or False.

Around 10% of daily ingested iron is absorbed from the intestines into the blood.

Answer 24

True.

Question 25

Why is it important for the body to store iron effectively?

Answer 25

• it is highly reactive

- it may rust within the body creating free radicals

Question 26

How does the body deal with iron?

Answer 26

• iron is absorbed in the duodenum
• it is transported across the brush border membrane by divalent metal transporter 1 (DMT1)
• iron ions are then lead into cage-type structures called ferritin
• the ferritin will eventually be sent out in the faeces.

Question 27

How often does the entire surface of the small intestine replenish?

Answer 27

• every five days

Question 28

What happens to iron present in the blood?

Answer 28

• it binds to transferrin

Question 29

If someone is anaemic, what happens to their ferritin?

Answer 29

• the body will stop releasing ferritin, therefore more iron will be present in the blood

Question 30

If someone is hyperanaemic, what happens to their ferritin?

Answer 30

• the body will release increased levels of ferritin, therefore there will be less iron present in the blood