Absorption

Question 1

what disaccharidases are localised to the membrane of enterocytes

Answer 1

• Maltase (breaks down maltose), sucrase (splits sucrose into glucose and fructose) as they can cleave internal and terminal 1-4
• isomaltose (can cleave alpha limit dextrin’s as it cleaves 1-6)
  Lactase which breaks down lactose

Question 2

what are the 2 stages of carbohydrate digestion?

Answer 2

Intraluminal digestion (salivary enzymes and pancreatic enzymes in the GI tract) and membrane digestion (microvilli on the brush border convert oligosaccharides to monosaccharides)

Question 3

where are the disaccharidases in the small intestine concentrated?

Answer 3

jejunum

Question 4

how does intraluminal digestion occur for proteases?

Answer 4

Intraluminal digestion occurs via proteases excreted by the gastric and pancreatic cells (secreted as proenzymes activated by low PH and enterokinases on the brush border respectively)

Question 5

what are the 2 types of peptidases?

Answer 5

endopeptidases (cleave after specific amino acids within polypeptide chain)
exopeptidase (cleave specific peptide bond adjacent to the C terminus)

Question 6

how are neonates an exception to protein absorption?

Answer 6

can absorb intact antibodies from mothers breast milk via endocytosis
otherwise peptides have to be oligopeptides or amino acids to be absorbed

Question 7

where are peptidases present?

Answer 7

secreted from chief cells in the stomach, or acinar cells in the pancreas where they are activated in the small intestine, membrane of enterocytes (membrane digestion)

Question 8

give 3 examples of proteins transporters in enterocytes?

Answer 8

PepT1- H+ cotransporter (oligopeptides and H+ into cells), Na+ cotransporter of AA, Na+ independent AA transporters on the basal side

Question 9

what is cystinuria?

Answer 9

cysteine stone formation in renal system due to mutated channels leading to deficient cysteine uptake in the intestine

Question 10

what is vitamin A for?

Answer 10

• Retinol (animal form), Beta Carotene (Plants)
• Needed to form rhodopsin a key component of the eye for detecting light
  Deficiencies lead to light blindness

Question 11

explain vitamin E

Answer 11

• Also know as alpha-tocopherol
• Major source is oily food
• Antioxidant, turn of oxidants produced by immune cells to kill pathogens
• Deficiencies damage caused in many areas of the body / poor immune system function due to the build-up of oxidants

Question 12

Explain vitamin D source and effects of poor vitamin D

Answer 12

• Can get it from sunlight, if lack sufficient sunlight need to consume it in our diet
• Vitamin D deficiency = bone softening also known as osteomalacia also have demineralisation

Question 13

explain vitamin K

Answer 13

• Green leafy vegetables are a major source
• Produced by macrobacteria in the gut
• Deficiencies are uncommon but supplements are given to babies
• Needed for prothrombin formation so deficiencies = excessive bleeding

Question 14

how are vitamins absorbed?

Answer 14

dissolved within micelles in the small intestine to aid adsorption

Question 15

what are the functions of dietary lipids?

Answer 15

energy storage molecule that provides a large source of energy and insulation
essential for membrane formation as essential fatty acids cannot be endogenously produced

Question 16

what are the 3 sources of lipases?

Answer 16

There are lingual, gastric (from chief cells) and pancreas lipases

Question 17

how does bile aid lipid digestion?

Answer 17

emulsifies fat globules (micelle) this gives a large surface area for lipase to work
provides optimum PH for lipases

Question 18

explain the action of colipase

Answer 18

come from the pancreas and anchors the lipase to the micelle so catalytic activity can occur without the lipase being washed off by bile salts

Question 19

when are lingual lipases most active?

Answer 19

at low PH’s they work more optimally when they reach the stomach
they work minimally in the mouth at neutral PH

Question 20

how are triglycerides broken down

Answer 20

diglycerides = single fatty acids and monoacylglycerol

Question 21

how are short medium fatty acids absorbed?

Answer 21

passive diffusion through enterocytes

Question 22

explain the degradation of micelles

Answer 22

as triglycerides are broken down longer fatty acids remain with the lipid droplet shorter ones can diffuse into the solution and across the gastric mucosa
a new core lipid will replace what is lost to be broken down and the micelle will get smaller and smaller

Question 23

how are the longer FA remaining in the micelles absorbed?

Answer 23

protonated at the low PH of the enterocyte brush border
then absorbed by diffusion, carrier mediated transport or integrated into the cell membrane

Question 24

how is the low PH at the brush border created?

Answer 24

Na+/H+ antiporter in the enterocyte

Question 25

what happens to FA once inside the enterocytes

Answer 25

fatty acids are converted back into triglycerides in the smooth ER, leading to the formation of fat droplets Apoproteins are then made in the rough ER and they associate with the lipids in the SER This forms chylomicrons and VLDL (very low density lipoproteins)

Question 26

what happens to chylomicrons and VLDL?

Answer 26

The VLDL and chylomicrons leave via exocytosis, they enter the lymphatic system and then blood via the thoracic duct Once in the blood, chylomicrons are broken down into FA and glycerol by lipoprotein lipase present on endothelial cells surface Glycerol can be absorbed by the cell, FA can go to muscle or adipocyte cells to store fat

Question 27

what are the 5 types of cell in the small intestine?

Answer 27

goblet, stem, Paneth cells (produce antibacterial proteins e.g. lysosomes for protection), villous enterocytes (absorptive function), crypt enterocytes (secretory function)

Question 28

describe the fold structure of the small intestine

Answer 28

covered in circular folds called plicae of circulares which have microvilli present on them to increase surface area for absorption

Question 29

what are the 2 main routes of ion and water absorption?

Answer 29

transcellular and paracellular

Question 30

how is paracellular movement into enterocytes driven?

Answer 30

Paracellular transport is driven by the pumping of the ions (mainly Na,Cl into the interstitial fluid, making it hypertonic near the apical membrane and drawing water through tight junctions into the gut, a similar process occurs for transcellular movement

Question 31

what are the 4 mechanisms of sodium absorption?

Answer 31

SGTL1 sodium glucose transporter AA / galactose symporters also exist H+/Na antiporter (often coupled with HCO3/Cl antiporter to maintain PH) ENaC (epithelial channels absorb Na+ against conc particularly in colon )

Question 32

how do mineralocorticoids effect sodium reabsorption?

Answer 32

e.g. Aldosterone bind to the cells causing increased opening of ENaC channels, increased fusing of channels from vesicles to the membrane and increased synthesis of Na channels and Na/K pumps

Question 33

how does E.coli act on sodium pumps?

Answer 33

it binds to the enterocytes and releases toxins that they take-up via endocytosis. This interacts with G proteins increasing the cAMP levels This causes increased Cl- secretion and it block Cl and Na uptake, this results in low electrolytes being absorbed and osmotic diarrhoea

Question 34

when is CL/HCO3 transporter uncoupled from the Na/H+ transporter?

Answer 34

small intestine and ileum to neutralise stomach acid and combat the effects of bacteria making faeces more acidic

Question 35

what is Congenital chloridorrhea

Answer 35

absence of CL-HCO3 exchanger, results in diarrhoea, dehydration, low electrolyte levels and failure to thrive

Question 36

which 2 ways can calcium be absorbed?

Answer 36

paracellular (passively) and transcellularly

Question 37

how is Ca absorbed via the transcellular route?

Answer 37

binds to TRVP6 receptor, Calbindin-D 9K binds to Ca2+ to reduce it toxicity, Calcium Is removed from enterocyte via Na/Ca antiporter and the PCMA Ca membrane transporter

Question 38

how is vitamin D linked to Ca absorption?

Answer 38

Binding of Vitamin D to the VD nuclear receptor increases transcription of the TRPV6 gene, promoting Ca uptake. Therefore deficiencies in VD have the same symptoms / cause calcium deficiency