Physiology of Absorption

Question 1

How are carbohydrates absorbed into intestinal tract

Answer 1

sodium-glucose linked transporter 1 is the main mechanism that allows glucose and galactose to be absorbed

sodium-potassium ATPase moves Na+ out of the cell which lowers its conc gradient so Na+ moves in via the SGLT1. it also uses energy provided by this to move glucose and galactose in as well

fructose is moved into the cell by facillitated diffusion via GLUT 5 and then all of these sugars are moved out of the cell across the basolateral membrane via GLUT 2

Question 2

How are proteins absorbed into the GI tract

Answer 2

sodium-potassium ATPase moves Na+ out of the cell which lowers its conc gradient. this then provides energy to pump H+ out of the cell against conc gradient

tri/dipeptides use the energy provided by the H+ gradient to move into the cell and breakdown into individual amino acids as well using PEPT1. amino acids are then transported across basolateral membrane using another Na+ pump

individual amino acids are also absorbed into the cell via sodium linked transporters.
specifc amino acids can also be transported out of the basolateral membrane by specific transporters

Question 3

transfer of passive immunity

Answer 3

enterocytes use endocytosis to take up small amounts of whole proteins. these proteins are broken down by lysosomes into smaller proteins or amino acids and then release them into the basolateral surface

M-cells don’t contain lots of lysosomes so the just take up the proteins and release them into the basolateral surface. but they are taken up by lysosomes in the peyer’s patch which presents the antigens to mount an immune response to train the immune system

Question 4

lipid absorption

Answer 4

fats arrive at lumen surface in micelles acidic environment due to sodium-hydrogen pump causes micelle breakdown releasing fatty acids, bile, cholesterol. fatty acids could be absorbed through membrane directly or using transporter proteins

inside the cell fats esterify into triglycerides and cholesterol which are then packaged into chylomicrons to be transported around the body via lymphatic system and then enter the blood system through the thoracic duct then the left subclavian vain

Question 5

absorption of vitamin A,D,E,K

Answer 5

these vitamins are fat soluable so they use the same pathway as fats
conditions affecting fat absorption will also affect these vitamin absorption
this includes cystic fibrosis, diarrhoea pancreatic and gall bladder disorders

Question 6

absorption of vitamin B1 (Thiamine)

Answer 6

B1 enters lumen via active uptake in phosphorylated state and then dephosphorylated in duodenum

in the enterocyte B1 is rephosphorylated by thiamine diphosphokinase which is then ejected from the basolateral membrane using Na+ based pumps

alcoholics have less B1 due to excreting more B1 in urine and it reduces effect of thiamine diphosphokinase

Question 7

absorption of vitamin B12 (cobalamin)

Answer 7

B12 used to produce amino acid methionine and DNA nucleotides
sourced from meat, fish and milk

B12 reaches the stomach attached to proteins and is detached by combining it with pepsin and haptocorrin. intrinsic factor is secreted in the stomach but doesnt bind to B12

in the duodenum procreatic enzymes break the B12 and haptocorrin links and binds it to intrinsic factors. this then binds to cubulin receptor which leads to endocytosis and once in the cell B12 is detached again and secreted out the cell and it enters circulation

Question 8

iron absorption

Answer 8

iron is in ferrous and ferric forms
vitamin C and enzyme Dcytb turns ferric into ferrous
ferrous iron is taken into the enterocyte by DMT1 transporter.
it then attaches to mobilferrin which takes iron to basolateral membrane and then detaches and iron is taken across membrane to interstitium by ferroportin transporter
then ferrous iron turns to ferric iron and then can be transported in the blood