Gastro - Ions Flashcards
What is diffusion, what decides its rate and why do mammals evolve circulatory systems?
In general, diffusion is the process whereby atoms and molecules intermingle because of their random thermal motion. Diffusion occurs rapidly over microscopic distances, but slowly over macroscopic distances. Multicellular organisms, such as humans, evolve circulatory systems to bring individual cells within the diffusion range.
What does the cell membrane act as? which types of molecules are easily diffused?
The cell membrane acts as a diffusion barrier, enabling cells to maintain cytoplasmic concentration of substances different from their extracellular concentrations. Lipid soluble molecules can cross more easily than water soluble molecules.
Define osmosis
The definition of osmosis is the diffusion of water from a hypotonic to a hypertonic medium
how can molecules cross the epithelium to enter the bloodstream?
In general, molecules can cross either from the paracellular transport through tight junctions and lateral intercellular spaces, or via transcellular transport through the epithelial cells In general concepts, the solutes can cross the cell membrane by simple diffusion, facilitated transport or active transport. And it can be transported either via channel proteins or carrier proteins.
Differences between channel and carrier proteins
Channel proteins form aqueous pores allowing specific solutes to pass across the membrane, whereas carrier proteins bind to the solute and undergo a conformational change to transport the solute across the membrane. In general, channel proteins allow much faster transport and then carrier proteins. A carrier protein has a solute binding site which then transforms in shape and allows it to open up and go across the lipid bilayer, whereas a channel protein opens up to a specific solute and allows it to go across a gradient.
Uniport vs couples transport
you could either have an uniport carrier transport or coupled transport in which another ion is co-transported with the solute. In uniport it only involves one solute across the membrane, whereas in coupled transport it involves another ion. And that could be either a symport in which both the ion and the coupled ion go in the same direction, or antiport in which the ion and the coupled ion go in an opposite direction across the membrane.
Types of AT
Active transport requires energy. It can be either primary active transport, which is linked directly to cellular metabolism and uses ATP. Or secondary active transport, which derives the energy from the concentration gradient of another substance that is actively transported.
Examples of 2AT, FT
Examples of secondary active transporters include SGLT-1, bicarbonate and chloride counter transport, and sodium and hydrogen counter transport involved in pancreatic bicarbonate secretion. And examples of facilitated transport includes GLUT-5 and GLUT-2 in small bowel absorption of monosaccharides
Absorption of carbohydrates involved which carrier proteins
The absorption of carbohydrates involving glucose and galactose is secondary active transport done by a carrier protein SGLT-1 on the apical membrane. This protein can transport glucose uphill against its concentration gradient, so it’s effective when glucose levels in the lumen are below those in the enterocyte, for example. Fructose, or the absorption of fructose, is by facilitated diffusion, and this is via the GLUT- 5 on the apical membrane This is effective at relatively low concentrations of fructose in the lumen as tissue and plasma levels are generally low.
How is the exit of glucose facilitated at the BLM
Then at the basolateral area, the exit of glucose is by facilitated diffusion using GLUT- 2 which is a high capacity, low affinity facilitative transporter, and glucose between plasma and tissue
enterocytes are generally equilibrated that way.
How is water absorbed in the GI tract?
Ninety nine percent of water that is presented to the GI tract is absorbed. The absorption of water is powered by the absorption of ions. The greatest amount of water is absorbed in the small intestine, especially in the jejunum, which contains a large volume of microvilli.
Many ions are slowly absorbed by passive diffusion in this process.
How much water is absorbed? `How is this possible when only a little is ingested?
In general, there is approximately 8 litres of water a day absorbed in the small bowel and approximately 1 . 4 litres of water a day is absorbed in the large bowel. we probably have a maximum of two litres ingested a day, but we produce about a litre of saliva a day, in addition to two litres of gastric secretions. And to add to that, we have bile secretion, pancreatic secretions and intestinal secretions, which contributes to a large volume of water in our intestines.
This in turn, is then absorbed in the small bowel, as we said, in about 8 litres, and the colon absorbs the rest of the water we have.
How is water absorbed? How is sodium absorbed?
The absorption of water is through a standing gradient osmosis, which is driven by sodium. Transport of sodium from lumen into the enterocytes is usually complex and varies between species, and it varies also along the line of the small bowel. It involves counter-transport in exchange for hydrogen in the proximal bowel,
co-transport with amino acids and monosaccharides in the jejunum, co-transport with chloride in the ileum,
How is chloride co transpoted? How is potassium diffused?
Chloride is co-transported with sodium and is exchanged with bicarbonate in the colon into enterocytes. Both of these happen as a secondary active transport. Potassium diffuses in via paracellular pathways in the small intestine, and leaks out between cells in colon, which is a passive transport process.
what happens to this intracellular sodium?
The active transport of sodium into the lateral intracellular spaces by sodium, potassium ATPase transport in the lateral plasma membrane Chloride and bicarbonate is transported then into the intercellular spaces due to the electrical potential created by the sodium transport.
