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Gastrointestinal > Physiology of lipid digestion and absorption of calcium, iron and vitamins > Flashcards

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

1
Q

What are the different types of lipid?

A

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

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2
Q

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

A

Small intestine

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3
Q

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

A

Lipases

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4
Q

What happens during the gastric phase of lipid digestion?

A

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

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5
Q

Why does the rate of hydrolysis increase as it proceeds?

A

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

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6
Q

What is the main lipid digestive enzyme?

A

Pancreatic lipase

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7
Q

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

A

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

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8
Q

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

A

Bicarbonate (HCO3-) in pancreatic juice

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9
Q

Name the peptide hormone released by the duodenum and its function

A

Cholecystokinin (CCK)

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

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10
Q

What is the role of bile salts?

A

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

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11
Q

What type of molecule are bile salts?

A

Amphipathic - hydrophilic head and hydrophobic tail.

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12
Q

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

A

Lipid malabsorption resulting in steatorrhoea

Secondary vitamin deficiency due to failure to absorb lipid vitamins

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13
Q

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

A

Co-factor colipase

amphipathic polypeptide secreted with lipase by the pancrease

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14
Q

What are the products of triglyceride breakdown by pancreatic lipase?

A

2-monoglyceride and two fatty acid molecules

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15
Q

Where does the pancreatic lipase cut the triglyceride?

A

The first and third backbone

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16
Q

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

A

Mixed micelles

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17
Q

What do mixed micelles contain?

A

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

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18
Q

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

A

Passive diffusion

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19
Q

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

A

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

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20
Q

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

A

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

21
Q

How are chylomicrons formed?

A

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

22
Q

How do chylomicrons leaves the enterocyte?

A

Exocytosis

23
Q

How do the chylomicrons get the the systemic circulation?

A

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

24
Q

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

A

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

25
Q

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

A

Albumen

binds to fatty acid and glycerol released from the chylomicron metabolism

26
Q

What is the name for the remainder of the chylomicron?

A

Chylomicron remnant

27
Q

What happens to the chylomicron remnant?

A

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

28
Q

How is cholesterol absorbed?

A

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

29
Q

Where is the cholesterol bound protein deposited?

A

In an endocytic compartment

30
Q

What does the ezetimibe do?

A

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

31
Q

How is Ca2+ absorbed?

A

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

32
Q

What is the difference between paracellular and transcellular transport?

A

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.

33
Q

When to calcium absorption in a low calcium diet?

A

Calcium uptake becomes a much more regulated process involving transporters

34
Q

What regulates the expression of calcium channels?

A

Vitamins

35
Q

What is rickets?

A

A disease of children caused by vitamin D deficiency

Results in a lack of calcium channels resulting in low calcium absorption

36
Q

What regulates active Ca2+ absorption?

A

1,25-dihydroxyvitamin D3 and parathyroid hormone

37
Q

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

A

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

38
Q

What is iron used for in the body?

A

Important constituent of haemoglobin, myoglobin and many enzymes

39
Q

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

A

It is reduced

Fe3+ –> Fe2+

40
Q

What enzyme breaks down haem?

A

Haem oxidase

41
Q

What happens to iron in the cell?

A

It can be stored as ferratin or transported across the membrane

42
Q

What is iron transported through to get into the blood?

A

Ferroportin

43
Q

How does vitamin B12 get into the body?

A

Vitamin B12 is ingested in food

44
Q

What is the process of vitamin B12 absorption?

A

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

45
Q

How is the Vitmain B12 intrinsic factor complex is absorbed?

A

Endocytosis

46
Q

What are the fat soluble vitamins?

A

A, D, E and K

47
Q

How are fat soluble vitamins absorbed?

A

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

48
Q

What are the water soluble vitamins?

A

B (not B12), C and H

49
Q

How are water soluble vitamins transported across the apical membrane?

A

Na transporters

Gastrointestinal (11 decks)

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