Ions, Vitamins and Minerals Flashcards
How have multicellular organisms adapted for efficient diffusion?
They have circulatory systems to bring individual cells within diffusion range
What is the role of the cell membrane in diffusion?
Acts as a barrier enabling cells to maintain cytoplasmic concentration of substances different from extracellular concentration
What kind of molecules cross the membrane most easily?
Lipid soluble (non-polar) molecules cross more easily than water soluble (polar) molecules
What is osmosis?
The movement of water from a hypotonic to hypertonic medium
What are the 2 transport systems used to transport molecules across the epithelium and into the bloodstream?
Paracellular transport: through tight junctions and lateral intercellular spaces
Transcellular transport: through epithelial cells
By what methods can solutes cross the cell membrane?
Simple diffusion
Facilitated transport
Active transport
What are two types of transport proteins?
Channel proteins: form aqueous pores allowing specific solutes to pass across membrane
Carrier proteins: bind solute and undergo conformational change to transport it
Channel proteins allow faster transport that carrier proteins
What are different types of channels?
Voltage- gated Ligand gated (extracellular ligand) Ligand gated (intracellular ligand) Mechanically gated
What are different types of active transport?
Require energy
Primary active transport: linked directly to cellular metabolism (uses ATP)
Secondary active transport: derives energy from the concentration gradient of another substance that’s actively transported
What is facilitated transport/ facilitated diffusion?
Enhances the rate a substance can flow down its conc. gradient
This tends to equilibrate the substance across the membrane and does not require energy
What are examples of transporters?
Primary active transport:
- Na+/K+ ATPase (pancreatic HCO3-)
- H+/K+ ATPase (stomach parietal cells)
Secondary active transport:
- SGLT-1 co-transport (small bowel absorption of monosaccharides)
- HCO3-/ Cl- counter transport (pancreatic HCO3- secretion)
- Na+/ H+ counter transport (pancreatic HCO3- secretion)
Facilitated diffusion/ facilitated transport:
GLUT-5, GLUT-2 (small bowel absorption of monosaccharides)
What transporter is the absorption of glucose and galactose by?
Secondary active transport (carrier protein and electrochemical gradient) by a SGLT-1 carrier protein on the apical membrane
SGLT-1 can transport glucose uphill against ints conc. gradient (so effective when glucose levels in the lumen are below those in enterocytes)
How is fructose transported?
Facilitated diffusion by a GLUT-5 carrier protein on the apical membrane
Its effective at relatively low conc. of fructose in the lumen as tissue and plasma levels are low
How does glucose exit at the basolateral membrane?
Facilitated diffusion by carrier protein GLUT-2
GLUT-2 is high capacity but low affinity
Glucose between plasma and tissue/enterocyte are generally equilibrated
How is water absorbed generally?
99% of water in GI tract is absorbed
Absorption of water is powered by absorption of ions
Greatest amount of water is absorbed in small intestine, esp. in jejunum
Many ions are slowly absorbed by passive diffusion
Calcium and iron are completely absorbed and this absorption is regulated
How much water do we absorb a day?
approx. 8L of water a day absorbed in small bowel
approx. 1.4L of water a day absorbed in large bowel
Where is water produced in the body?
Ingested 2L Saliva 1.2L Gastric secretions 2L Bile 0.7 L Pancreas 1.2L Intestinal 2.4L
How is water absorbed through standing gradient osmosis?
Driven by Na+
Transport of Na+ from lumen into enterocyte- complex and varies between species
This becomes more efficient as you travel down the intestine:
- Counter transport in exchange for H+ (proximal bowel)
- Co-transport with amino acids, monosaccharides (jejunum)
- Co- transport with Cl- (ileum)
- Restricted movement through ion channels (colon)
How is chlorine transported?
Cl- is cotransported with Na+ (ileum) or exchanged with HCO3- (colon) into enterocytes
These processes are both secondary active transport
How is K+ transported in the colon and small intestine?
K+ diffuses via paracellular pathways in small intestine
K+ leaks out between cells in the colon- passive transport
What happen to intracellular sodium?
Active transport of Na+ into the lateral intercellular spaces by Na+K+ATPase transport in lateral plasma membrane
What drives the movement of water into intercellular spaces?
High concentrations of ions in the intercellular spaces causes the fluid there to be hypertonic
This causes an osmotic flow of water from the gut lumen via adjacent cells and 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 capillaries
How is calcium absorbed?
Duodenum and ileum absorb Ca2+
Ca2+ deficient diet increases gut’s ability to absorb
Vit D and parathyroid hormone stimulate absorption
We get 1-6g a day from our diet, secrete 0.6g and absorb 0.7g
How much calcium do we have in our bodies?
We have low intracellular calcium- approx. 100nM but this can increase by 10-100 fold during various cellular functions
We have high extracellular fluid Ca2+- approx. 1-3mM:
- plasma Ca2+ is approx. 2.2-2.6mM
- Luminal Ca2+ varies in nM range
