Specific Issues in Alimentary Absorption Flashcards
What is the role of tight junctions with regards to plasma membrane proteins?
They form a fence so that the plasma proteins that are on the apical membrane can’t cross over to the basolateral membrane. This establishes polarity.
What’s the difference between carrier proteins and channel proteins?
Carrier proteins bind to the molecule and then undergo a conformational change that transports the molecule across the membrane.
Channel proteins are just aqueous pores allowing ions through.
Through which transporter does glucose and galactose absorption take place?
SGLT-1
Through which facilitated diffusion channel down fructose absorption take place?
Glut-5
Through which facilitated diffusion channel does exit of glucose into the blood take place?
Glut-2
What percentage of water presented to the GI tract is absorbed?
99%
Where is most water absorbed? State the amount absorbed by the small intestine and that large intestine.
Most water is absorbed in the small intestine
Small intestine = 8 L/day
Large intestine = 1.4 L/day
State some of the sources of the extra 8 L of water that is reabsorbed by the gut every day.
Bile, saliva, pancreatic secretions, intestinal secretions, gastric secretions
Name the process by which water is absorbed by the gut.
Standing Gradient Osmosis
Describe the transport of Na+ in different areas of the gut.
Jejunum - Na+ is co-transported with amino acids
Ileum - Na+ is co-transported with Cl-
Proximal Colon - Na+ is counter-transported with H+
Colon - Na+ moves in via ion channels
Describe the movement of K+ throughout the intestines.
K+ diffuses IN via paracellular pathways in the small intestine
K+ leaks out between cells in the colon
Describe the movement of Cl- throughout the intestines.
Cl- is cotransported with Na+ in the ileum
Cl- is exchanged with HCO3- in the colon
Describe the process of standing gradient osmosis.
Absorption of Na+ creates a high intracellular Na+ concentration
Na+/K+ channel pumps Na+ ions into the intercellular space
This creates an electrochemical gradient that draws anions into the intercellular space
You then get a concentration ionic solution surrounding the cells, which helps draw water via paracellular pathways into the intercellular space
As the water moves in you get an increase in hydrostatic pressure, which then forces water and ions to move across the membrane and into the blood stream.
Which parts of the small intestines absorb calcium?
Duodenum and Ileum
State two products that stimulate absorption of calcium.
Vitamin D
Parathyroid Hormone
How much calcium is contained in the diet and how much is absorbed?
Diet = 1-6 g/day Secreted = 0.6 g/day Absorbed = 0.7 g/day
What is the concentration of calcium intracellularly, extracellularly and in the blood plasma?
Intracellular = 0.1 mcM Extracellular = 1-3 mM Blood = 2.2-2.6 Mm
What two channels transport calcium into the cell from the lumen?
IMcal - intestinal calcium binding protein (facilitated diffusion)
Ion channel
What binds to and transport calcium within the cell to prevent it from triggering various intracellular pathways?
Calbindin
What two channels transport Ca2+ out of the basolateral membrane and how are they different?
PMCA - Plasma Membrane Calcium ATPase
Ca2+/Na+ Exchanger
PMCA - high affinity + low capacity
Ca2+/Na+ Exchanger - low affinity + high capacity
What effects does vitamin D3 have on absorption of Ca2+?
Increases calcium absorption by enhancing transport through the cytoplasm, increasing levels of calbindin and increasing Ca2+ ATPase
What is the normal dietary intake of iron and how much is absorbed?
15-20 mg/day
0.5-1.5 mg/day
What is iron present in the diet as?
Inorganic ions - Fe2+ and Fe3+
Heme
What effect does vitamin C have on iron?
It reduces Fe3+ to Fe2+
How is heme absorbed and how is its Fe2+ group liberated?
Heme is absorbed in tact through heme carrier protein (HCP-1)
Fe2+ group is liberated by heme oxygenase
What reduces Fe3+ to Fe2+ in the duodenum?
Duodenal cytochrome B
What transporter allows Fe2+ into the enterocyte?
Divalent Metal Transporter - DMT-1 (H+ cotransport)
Through which channel does the Fe2+ enter the blood?
Ferroportin
What transmembrane protein converts Fe2+ back into Fe3+ on the basolateral membrane?
Hephaestin
What does Fe3+ bind to in the blood to travel?
Apotransferrin - forms transferrin
What is the major iron regulating protein and what does it do?
Hepcidin - it suppresses ferroportin function and hence decreases iron absorption
Describe the storage of iron.
Fe2+ in the cell can bind to apoferritin forming a ferritin micelle
Fe2+ is oxidised to Fe3+ which crystallises within the protein shell
A single ferritin molecule can store up to 4000 iron ions
What happens to the ferritin?
The ferritin is removed when the enterocytes die and are excreted in the faeces
What is the main mechanism for absorption of vitamins?
Passive diffusion
How are fat soluble vitamins (ADEK) absorbed? How is Vitamin K, in particular, absorbed?
They are packaged into micelles and taken to the brush border.
Vitamin K is taken up by active transport.
How big is the liver’s store of vitamin B12?
2-5 mg
Describe the transport of Vitamin B12 from the stomach to the distal ileum.
Vitamin B12 is released by the action of acid and pepsin
Free B12 then binds to R protein (which is in the saliva and produced by parietal cells). The R protein prevents denaturation of free B12 in the stomach.
R protein is digested in the small intestine
B12 binds to intrinsic factor and is transported down the small intestine
The B12-intrinsic factor complex is then absorbed by the enterocytes by binding to cubulin receptors
What does Vitamin B12 bind to for transport to the liver?
Transcobalamin II
