Lipid, Iron, Calcium and Vitamins

Question 1

state the problems posed by digestion of fats

Answer 1

lipids are either insoluble or poorly soluble - causing special problems for digestion and absorption

Question 2

how are the problems of digestion of fats overcome

Answer 2

emulsified into small oil droplets suspended in water;
via bile salts and pancreatic secretions
gastric churning through narrow pylorus

Question 3

stabilisation of emulsion droplets

Answer 3

by addition of a coat of amphiphilic molecules that form a surface layer on the droplets

Question 4

amphiphillic molecules on the surface of droplets

Answer 4

products of lipid digestion (fatty acids)
biliary phospholipids
cholesterol
bile salts

Question 5

describe TAG lipase

Answer 5

pancreatic lipase is a main lipid digestive enzyme in adults (hydrolyses TAGs)
released in response to CCK, also stimulating bile flow

Question 6

where is TAG secreted from

Answer 6

acinar cells of pancreas (exocrine)

Question 7

how does pancreatic lipase hydrolyse TAGs

Answer 7

at 1 and 3 positions

Question 8

release of bile salts

Answer 8

released into duodenum in bile from gallbladder in response to CCK
act as detergents to emulsify large lipid droplets to small droplets

Question 9

describe bile salts

Answer 9

amphipathic;
hydrophillic - projects from surface of droplet
hydrophobic - adsorbs onto droplets

increased surface area for action of lipase but block access of enzyme to TAGs

Question 10

failure of bile salt secretion

Answer 10

lipid malabsorption

secondary vitamin deficiency due to failure to absorb fat soluble vitamins

Question 11

colipase

Answer 11

amphipathic polypeptide secreted with lipase by the pancreas

Question 12

role of colipase

Answer 12

binds to bile salts and lipase allowing access by rrhe latter to tri- and di-acylglycerols

Question 13

activation of colipase

Answer 13

secreted as inactive procolipase

activated by trypsin

Question 14

formation of micelle

Answer 14

As TAGs towards the surface of the emulsion droplets are hydrolysed, they are replaced by TAGs within the core, decreasing droplet size until a mixed micelle results

Question 15

absorption of free fatty acids and monoacylglycerols

Answer 15

Transfer between mixed micelles and the apical membrane of enterocytes entering by the cell by passive diffusion and/or membrane fatty-acid translocases, fatty-acid binding protein and fatty-acid transport proteins

Question 16

absorption of short/medium chain fatty acids

Answer 16

diffuse through enterocyte

exit through basolateral membrane and enter villus capillaries

Question 17

absorption of long chain fatty acids and monoacylglycerols

Answer 17

resynthesized to triglycerides in the endoplasmic reticulum and are incorporated into chylomicrons (which are then carried in lymph vessels to systemic circulation via thoracic duct)

Question 18

absorption of cholesterol

Answer 18

transport by endocytosis in clatherin coated pits by NPC1L1 protein
Ezetimibe binds to NPC1L1, prevents internalization, and thus cholesterol absorption. Used in conjunction with statins in hypercholesterolaemia

Question 19

absorption of calcium

Answer 19

passive transport mechanisms - paracellular (whole small intestine)
active transport mechanisms - transcellular (duodenum and upper jejunum)

Question 20

active transport of calcium absorption;

regulation

Answer 20

[Ca2+] <5mM –> mainly active absorption

regulated by 1,25 - dihydroxyvitamin (calcitriol) and parathyroid hormone (increase 1,25-dihydroxyvitamin D3 synthesis)

Question 21

daily loss of ion form the body

Answer 21

unregulated process:
iron balance within the body, long-term, depends upon tightly regulated absorption of iron across the duodenum which matches losses

Question 22

how is iron lost from the body on the daily

Answer 22

urine
sweat
desquamated enterocytes

Question 23

what is dietary ion

Answer 23

mainly in oxidised form
present in meat or vegetbales as;
inorganic iron
haem 
ferratin

Question 24

what does iron deficiency cause

Answer 24

microcytic anaemia

Question 25

effects of iron excess cause

Answer 25

toxic - accumulation in liver, pancreas and heart and molecularly the production of hydroxyl radicals and hydroxide ions

Question 26

absorption of iron

Answer 26

PP
reduced iron absorbed across apical membrane (across enterocyte) to basolateral membrane via molecular chaperone
iron oxidised and transported to tissues
haem imported across apical membrane followed by cytoplasmic metabolism to release iron

Question 27

how is oxidation of iron promoted

Answer 27

HCl within the stomach
vitamin C
ferric reductase, duodenal cytochrome B (Dctyb) - present on the brush border membrane of enterocytes
gasteroferrin (secreted by gastric parietal cells) reversibly binds Fe2+ -preventing the formation of insoluble anion salts

Question 28

absorption of vitamin B12

Answer 28

stomach releases vitamin B12 from protein
haptocorin secreted in saliva binds vitamin B12 released in stomach
stomach parietal cells release intrinsic factor
pancreatic proteases digest haptocorin in small intestine, vitamin B12 released
vitamin B12 binds to intrinsic factor in small intestine
vitamin B12-intrinsic factor complex absorbed in terminal ileum by endocytosis

Question 29

where is vitamin B12 found

Answer 29

not in vegetables - vegans deficiency

Question 30

absorption of fat soluble vitamins

Answer 30

requires adequate bile secretion and an intact intestinal mucosa
Incorporated into mixed micelles
passively transported into enterocytes
Incorporated into chylomicrons, or VLDLs
Distributed by intestinal lymphatics

Question 31

examples of fat soluble vitmains

Answer 31

vit A, D, E, K

Question 32

absorption of water vitamins

Answer 32

similar to those described for monosaccharides, amino acids and di- and tri-peptides.
May be Na+-dependent, or Na+-independent