Physiology of lipid digestion and absorption of calcium, iron and vitamins

Question 1

What are the different types of lipid?

Answer 1

Fats/oils - triacylglycerols (TAG)
Phospholipids
Cholesterol and cholesterol esters
Fatty acids

Question 2

What is the most important stage of digestion for ingested lipids?

Answer 2

Small intestine

Question 3

What is the name of the type of enzyme involved in lipid digestion?

Answer 3

Lipases

Question 4

What happens during the gastric phase of lipid digestion?

Answer 4

Heat and movement in the stomach mix the food with gastric lipase which begins digestion and forms an emulsion

Question 5

Why does the rate of hydrolysis increase as it proceeds?

Answer 5

The fatty acids produced act as surfactants breaking down lipid globules (aids emulsification)

Question 6

What is the main lipid digestive enzyme?

Answer 6

Pancreatic lipase

Question 7

What happens to the emulsified fats once that have been ejected from the stomach into the duodenum?

Answer 7

Further broken down by pancreatic lipase added by bile salts from the gall bladder.

Question 8

What neutralises the stomach acid to create a suitable pH for enzyme action in the intestine?

Answer 8

Bicarbonate (HCO3-) in pancreatic juice

Question 9

Name the peptide hormone released by the duodenum and its function

Answer 9

Cholecystokinin (CCK)

Stimulates the release of bile into the intestine and the secretion of enzymes by the pancreas

Question 10

What is the role of bile salts?

Answer 10

Act as detergents to emulsify large lipid droplets to small droplets
Increase the surface area for the action of lipase

Question 11

What type of molecule are bile salts?

Answer 11

Amphipathic - hydrophilic head and hydrophobic tail.

Question 12

What happens if there is a failure to secrete bile salts?

Answer 12

Lipid malabsorption resulting in steatorrhoea

Secondary vitamin deficiency due to failure to absorb lipid vitamins

Question 13

How do the enzymes gain access to the triglycerides that are blocked by the hydrophobic core?

Answer 13

Co-factor colipase

amphipathic polypeptide secreted with lipase by the pancrease

Question 14

What are the products of triglyceride breakdown by pancreatic lipase?

Answer 14

2-monoglyceride and two fatty acid molecules

Question 15

Where does the pancreatic lipase cut the triglyceride?

Answer 15

The first and third backbone

Question 16

Where are the final products of lipid digestion stored and released from?

Answer 16

Mixed micelles

Question 17

What do mixed micelles contain?

Answer 17

Fatty acid, monoglyceride, cholesterol and phospholipid contained int the bile salt

Question 18

How do fatty acids and monoglycerides transfer between mixed micelles and the apical membrane of enterocytes?

Answer 18

Passive diffusion

Question 19

What happens to short chain (<6 carbon) and medium (8-12 carbon) fatty acids after they have entered the enterocyte?

Answer 19

Diffuse through the enterocyte, exit through the basolateral membrane and enter the villus capillaries

Question 20

What happens to long chain fatty acids (>12 carbons) and monoglycerides?

Answer 20

They are resynthesised to triglycerides in the endoplasmic reticulum and are subsequently incorporated into chylomicrons

Question 21

How are chylomicrons formed?

Answer 21

Monoglyceride and fatty acids are resynthesised in the ER
Cholesterol esters are added and phospholipids are synthesised to form a nascent chylomicron
Apolipoprotein then coats the chylomicron

Question 22

How do chylomicrons leaves the enterocyte?

Answer 22

Exocytosis

Question 23

How do the chylomicrons get the the systemic circulation?

Answer 23

Carried in lymphatic vessels to the systemic circulation (subclavian vein) via the thoracic duct

Question 24

Where and by what enzyme, is the chylomicron triglyceride metabolised?

Answer 24

In the capillaries (particularly muscle and adipose tissue) by lipoprotein lipase present on endothelial cells

Question 25

Name the major blood protein constituent that transports fatty acids and glycerol around the body

Answer 25

Albumen | binds to fatty acid and glycerol released from the chylomicron metabolism

Question 26

What is the name for the remainder of the chylomicron?

Answer 26

Chylomicron remnant

Question 27

What happens to the chylomicron remnant?

Answer 27

Undergoes endocytosis by hepatocytes. Cholesterol is released to be stored, secreted unaltered in bile or oxidised to bile salts

Question 28

How is cholesterol absorbed?

Answer 28

Niemann-Pick C1 like 1 protein binds to cholesterol and stimulated endocytosis. The protein also stimulates the production of a clathrin scaffold that stabilized the pit and is used in transport

Question 29

Where is the cholesterol bound protein deposited?

Answer 29

In an endocytic compartment

Question 30

What does the ezetimibe do?

Answer 30

Binds to NPC1L1 and prevents the internalization of cholesterol inside the cell Prevents the absopion of cholesterol

Question 31

How is Ca2+ absorbed?

Answer 31

Passive transport (paracellular) the whole length of the small intestine and active transport (transcellular) mainly in the duodenum and upper jejunum

Question 32

What is the difference between paracellular and transcellular transport?

Answer 32

Paracellular transport is the transfer of substances across the epithelium by passing through the intracellular space between the cells. Transcellular transport is where the substances travel through both the apical and basolateral membrane.

Question 33

When to calcium absorption in a low calcium diet?

Answer 33

Calcium uptake becomes a much more regulated process involving transporters

Question 34

What regulates the expression of calcium channels?

Answer 34

Vitamins

Question 35

What is rickets?

Answer 35

A disease of children caused by vitamin D deficiency | Results in a lack of calcium channels resulting in low calcium absorption

Question 36

What regulates active Ca2+ absorption?

Answer 36

1,25-dihydroxyvitamin D3 and parathyroid hormone

Question 37

How does calcium leave the cell and get into the vasculature?

Answer 37

Calcium pump that is driven by sodium moving down its concentration gradient into the cell

Question 38

What is iron used for in the body?

Answer 38

Important constituent of haemoglobin, myoglobin and many enzymes

Question 39

What happens to iron in order for it to be transported into the cell?

Answer 39

It is reduced | Fe3+ --> Fe2+

Question 40

What enzyme breaks down haem?

Answer 40

Haem oxidase

Question 41

What happens to iron in the cell?

Answer 41

It can be stored as ferratin or transported across the membrane

Question 42

What is iron transported through to get into the blood?

Answer 42

Ferroportin

Question 43

How does vitamin B12 get into the body?

Answer 43

Vitamin B12 is ingested in food

Question 44

What is the process of vitamin B12 absorption?

Answer 44

Salivary glands secrete haptocorin Stomach acid releases B12 from food Haptocorin binds vitamin B12 released in the stomach Stomach parietal cells release intrinsic factor Pancreatic proteases digest haptocorin in the small intestine and vitamin B12 is released B12 binds to intrinsic factor Vitmain B12 intrinsic factor complex is absorbed by terminal ileum

Question 45

How is the Vitmain B12 intrinsic factor complex is absorbed?

Answer 45

Endocytosis

Question 46

What are the fat soluble vitamins?

Answer 46

A, D, E and K

Question 47

How are fat soluble vitamins absorbed?

Answer 47

Incorporated into mixed micelles Usually passively transported into enterocytes Incorporated into chylomicrons or VLDLs Distributed by intestinal lymphatics

Question 48

What are the water soluble vitamins?

Answer 48

B (not B12), C and H

Question 49

How are water soluble vitamins transported across the apical membrane?

Answer 49

Na transporters