W2 Physiology: lipid digestion and absorption of calcium, iron and vitamins

Question 1

what is emulsification and how does it occur

Answer 1

• When solid fats and oils are converted into an emulsion of small oil droplets suspended in water
  1. mouth (chewing)
  2. Gastric churning with enzyme and squeezing through pylorus
  3. segmentation and peristalsis in SI mixing chyme with pancreatic and biliary secretions

Question 2

how are emulsification droplets stabilised?

Answer 2

by the addition of amphiphilic coat

Question 3

Contents of amphiphilic coat

Answer 3

Products of lipid digestion eg monoglyceride
cholesterol
biliary phospholipid
bile salts (when droplets are reduced to unilamellar mixed micelles)

Question 4

Process of digestion of fats

Answer 4

1. mouth- lingual lipase
2. Gastric phase- gastric lipase
3. Duodenum- pancreatic (TAG) Lipase

Question 5

what triggers production of gastric lipase?

Answer 5

gastric lipase is secreted in response to gastrin release from chief cells

Question 6

optimum pH of gastric lipase

What is gastric lipase resistant to?

Answer 6

ph4

pepsin

Question 7

why is gastric lipase inactive in the duodenum

Answer 7

digestion by pancreatic protease and unfavourable pH

Question 8

where are short and medium chain FA absorbed?

Answer 8

stomach

Question 9

which cells secrete pancreatic lipase

What is this triggered by

Answer 9

acinar cells of pancreas

in response to CCK which also stimulate bile flow

Question 10

what substances and conditions does full activity of pancreatic lipase require

Answer 10

colipase co-factor, alkaline pH, Ca2+, bile salts, fatty acids

Question 11

where is bile salt released from and what into?

Answer 11

bile salt is released from the gall bladder into the duodenum

Question 12

what is the release of bile salt triggered by

Answer 12

CCK

Question 13

function of bile salts

Answer 13

• act as DETERGENTS to help emulsify large lipid droplets to small lipid droplets
• increases SA for pancreatic lipase to act

Question 14

describe the structure of bile salts

Answer 14

AMPHIPATHIC

hydrophilic and hydrophobic parts

Question 15

result of failure to produce bile salts

Answer 15

• lipid malabsorption, steatorrhea (fatty faeces)

- secondary vitamin deficiency (A,D,E,K)

Question 16

why may someone have steatorrhea

Answer 16

failure to produce bile salts- lipid malabsorption

Question 17

problem that bile salts present and how this is solved

Answer 17

block access of lipase enzyme to TAG

colipase from the pancreas binds to bile salts and lipase allowing access to tri- and di-acylglycerols

Question 18

is colipase directly secreted as itself from the pancreas?

Answer 18

no

colipase is secreted as inactive procolipase then activated by trypsin

Question 19

what are the final products of lipid digestion stored in

Answer 19

mixed micelles

Question 20

methods by which mixed micelles can enter enterocyte

Answer 20

passive diffusion
membrane fatty acid translocases
fatty acid binding protein
fatty acid transport protein

Question 21

• size of short chain and medium chain FA

- how short chain and medium chain FA pass through enterocyte

Answer 21

short chain (<6), medium chain (8-12)
diffuse through enterocyte, exit through basolateral membrane and enter villus capillaries

Question 22

size of long chain FA

what happens to the long chain FA

Answer 22

> 12

long chain FA and monoglycerides are resynthesised into triglycerides in ER and incorporated into chylomicrons

Question 23

how are chylomicrons formed?

what then happens to chylomicron

Answer 23

• triglycerides are formed in ER by monoglyceride and long FA
• cholesterol added and phospholipid synthesis
• APO-B 48 added to from chylomicron
• chylomicron exits enterocyte by exocytosis and enters the central lacteal then carried in lymph vessels to systemic circulation via thoracic duct

Question 24

transport of cholesterol absorption

Answer 24

endocytosis in clatherin coated pits by NPC1L1

Question 25

action of ezetimibe and what is it used for

Answer 25

• binds to NPC1L1 preventing internalisation and cholesterol absorption
• used in conjunction with statins in hypercholesterolemia

Question 26

transport in Ca2+ absorption

which is the main one?

Answer 26

passive and active (mainly active)

Question 27

what is action calcium absorption regulated by?

Answer 27

1,25-dihydroxyvitamin D3 and parathyroid hormone

Question 28

what does long term iron balance in the body depend on

Answer 28

Absorption of iron across the duodenum

Question 29

Form of Dietary iron and daily requirement

Answer 29

Fe3+

12-15mg (5-10% absorbed)

Question 30

what can iron deficiency cause

Answer 30

microcytic anaemia

Question 31

effect of excess of dietary iron

Answer 31

toxic to pancreas, heart, liver

Question 32

can Fe3+ be transported across enterocyte

Answer 32

no

most iron in diet is Fe3+, but only Fe2+ can be absorbed across duodenal enterocytes

Question 33

• how is Fe3+ reduced to Fe2+

- what promotes this

Answer 33

• Fe3+ accepts an electron

- HCl in stomach, Vitamin C, ferric reductase (duodenal cytochrome B), Gastroferrin

Question 34

what is Gastroferrin secreted by and its action on

Answer 34

secreted by gastric parietal cells

reversibly binds to Fe2+, preventing formation of insoluble anion salts

Question 35

what is ferratin

Answer 35

storage form of iron

Question 36

daily requirement of Vitamin B12

Answer 36

5-15ug

Question 37

another name for vitamin B12

Answer 37

Cobalamin

Question 38

describe vitamin b12 absorption

Answer 38

stomach acids removed vitamin B12 from protein
Haptocorin (from salivary enzymes) binds to VB12
Stomach parietal cells release intrinsic factor
Pancreatic protease digest Haptocorin, releasing VB12
with then binds to intrinsic factor in SI
Vitamin B12-Intrinsic factor complex absorbed in terminal ileum by endocytosis

Question 39

what are the fat soluble vitamins?

Answer 39

A, D, E, K

Question 40

what are the water soluble vitamins

Answer 40

B, C, H

not B12