Week 11- Ions, Vitamins and Minerals Flashcards
How does diffusion differ over micro vs macroscopic distances/
Occurs rapidly over microscopic distances, but slowly over macroscopic distances
How does water move between hyper and hypotonic mediums
Hypo to hyper tonic
Describe electrolyte conc in hyper and hypotonic mediums
Highest in hypertonic mediums
What is paracellular transport?
Through tight junctions and lateral intercellular spaces
What is transcellular transport?
Through the epithelial cells
How can solutes cross cell membranes?
Simple diffusion
Facilitated transport
Active transport
What are the 2 types of transport proteins?
Carrier and channel proteins
Describe channel proteins
Form aqueous pores allowing specific solutes to pass across the membrane.
Describe carrier proteins
Bind to the solute and undergo a conformational change to transport it across the membrane
Out of carrier and channel proteins what allows faster transport?
Channel
What are the different types of transport proteins?
Voltage gated Ligand gated (extracellular) Ligand gated (intracellular) Mechanically gated
What are the types of carrier mediated transport?
Uniport
Symport
Antiport
Describe primary active transport
Linked directly to cellular metabolism (uses ATP to power the transport)
Describe secondary active transport
Derives energy from the concentration gradient of another substance that is actively transported
Describe facilitated diffusion
Enhances the rate a substance can flow down its concentration gradient. This tends to equilibrate the substance across the membrane and does not require
energy
How are glucose and galactose absorbed?
By secondary active transport (carrier protein & electrochemical gradient). Carrier protein = SGLT-1 on apical membrane
How is SGLT 1 special?
Can transport glucose uphill against its
concentration gradient
How is fructose absorbed?
By facilitated diffusion, carrier protein = GLUT-5 on apical membrane
How does glucose exit the basolateral membrane?
By facilitated diffusion, carrier protein = GLUT-2, a high-capacity, low affinity facilitative transporter
How much of total water is reabsorbed?
99%
Where is most water reabsorbed?
In the small intestine, esp the jejunum
How are most ions absorbed?
Slowly absorbed by passive diffusion
What ions are notably incompletely absorbed?
Calcium and iron
How much water is absorbed in the large vs small bowel per day?
Approximately 8 litres of water a day absorbed in the small bowel.
Approximately 1.4 litres of water a day absorbed in the large bowel
Where does the 9L of water come from if you don’t drink that much?
Ingest 2L Saliva 1.2L Gastric secretions 2L Bile 0.7L Pancreas 1.2L Intestinal 2.4L
What happens to intracellular sodium?
Active transport of Na+ into the lateral
intercellular spaces by Na+K+ATPase
transport in the lateral plasma membrane
How are Cl- and HCO3 transported into the intercellular space?
Due to electrical potential created by the Na+ transport
Describe overall standing gradient osmosis
Osmotic flow of water from the gut lumen via adjacent cells, tight junctions into the
intercellular space.
Water distends the intercellular channels and causes increased hydrostatic pressure.
Ions and water move across the basement membrane of the epithelium and are carried
away by the capillaries
Where is calcium absorbed?
Duodenum and ileum
What stimulates calcium absorbtion?
Calcium deficiency, PTH and vit D
How much calcium is eaten, secreted and absorbed daily?
Diet 1-6g/day
Secretions 0.6g
Absorb 0.7g
What is calcium conc intra and extra cellularly?
Low intracellular [Ca2+] approx 100 nM (0.1µM)
High extracellular fluid [Ca2+] approx 1-3mM
How is calcium carried across the apical membrane?
Intestinal calcium-binding protein (IMcal)- facilitated diffusion.
Ion channel
How is calcium carried across the basolateral membrane?
By plasma membrane Ca2+ ATPase (PMCA) against concentration gradient.
Also pumped across basolateral membrane by plasma membrane Na+/Ca2+ exchanger against concentration gradient.
Describe how Ca2+ ATPase (PMCA) works
PMCA has a high affinity for Ca2+ (but low capacity).
Maintains the very low concentrations of calcium normally
observed within a cell
Describe how Na+/Ca2+ exchanger works
The Na+/Ca2+ exchanger has a low affinity for Ca2+ but a high capacity. Requires larger concentrations of Ca2+ to be effective
How much iron is digested and absorbed daily?
Ingests approx 15-20mg/day but absorbs only 0.5-1.5mg/day
What are the 2 ways iron is present in the body?
Inorganic iron (Fe3+ ferric, Fe2+ ferrous) As part of haem group (haemoglobin, myoglobin and cytochromes).
What form of iron can we absorb and not absorb?
Cannot absorb Fe3+, only Fe2+.
What does Fe3+ make insolube salts with?
Hydroxide
Phosphate
HCO3-
What reduces Fe3+ to Fe2+ usually?
Vit C
What reduces Fe3+ to Fe2+ in the duodenum?
Duodenal cytochrome B (Dcytb)
How are fat soluble vitamins transported?
Vitamins A, D, E, K transported to brush border in micelles. K taken up by active transport
How much B12 does the liver store?
2-5mg
What happens if B12 absorbtion is disrupted? What is the name of the condition that arises
Slows the maturation of red blood cells - pernicious anaemia
How is most b12 in food found?
Bound to proteins
How is free b12 released?
In the stomach, low pH and the digestion of proteins by pepsin releases it .
What does HCl do to b12?
Denatures it
What is intrinsic factor (IF)?
Vit B12 binding glycoprotein secreted by parietal cells
Describe the effects of IF
Vit B12/IF is resistant to digestion.
No IF then no absorption of vit B12
Vit B12/IF complex binds to cubilin receptor, taken up in distal ileum
Where is the b12/IF complex broken down?
In cells
What happens to b12 in cells?
B12 binds to protein transcobalamin II (TCII), crosses basolateral membrane by
unknown mechanism
Travels to liver bound to TCII.
TCII receptors on cells allow them to uptake complex.
Proteolysis then breaks down TCII inside the cell.
