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BRS > Ions, Vitamins and Minerals > Flashcards

Ions, Vitamins and Minerals Flashcards

1
Q

How have multicellular organisms adapted for efficient diffusion?

A

They have circulatory systems to bring individual cells within diffusion range

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2
Q

What is the role of the cell membrane in diffusion?

A

Acts as a barrier enabling cells to maintain cytoplasmic concentration of substances different from extracellular concentration

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3
Q

What kind of molecules cross the membrane most easily?

A

Lipid soluble (non-polar) molecules cross more easily than water soluble (polar) molecules

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4
Q

What is osmosis?

A

The movement of water from a hypotonic to hypertonic medium

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5
Q

What are the 2 transport systems used to transport molecules across the epithelium and into the bloodstream?

A

Paracellular transport: through tight junctions and lateral intercellular spaces
Transcellular transport: through epithelial cells

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6
Q

By what methods can solutes cross the cell membrane?

A

Simple diffusion
Facilitated transport
Active transport

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7
Q

What are two types of transport proteins?

A

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

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8
Q

What are different types of channels?

A 
Voltage- gated
Ligand gated (extracellular ligand)
Ligand gated (intracellular ligand)
Mechanically gated
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9
Q

What are different types of active transport?

A

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

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10
Q

What is facilitated transport/ facilitated diffusion?

A

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

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11
Q

What are examples of transporters?

A

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)

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12
Q

What transporter is the absorption of glucose and galactose by?

A

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)

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13
Q

How is fructose transported?

A

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

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14
Q

How does glucose exit at the basolateral membrane?

A

Facilitated diffusion by carrier protein GLUT-2
GLUT-2 is high capacity but low affinity

Glucose between plasma and tissue/enterocyte are generally equilibrated

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15
Q

How is water absorbed generally?

A

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

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16
Q

How much water do we absorb a day?

A

approx. 8L of water a day absorbed in small bowel

approx. 1.4L of water a day absorbed in large bowel

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17
Q

Where is water produced in the body?

A 
Ingested 2L
Saliva 1.2L
Gastric secretions 2L
Bile 0.7 L
Pancreas 1.2L
Intestinal 2.4L
18
Q

How is water absorbed through standing gradient osmosis?

A

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)

19
Q

How is chlorine transported?

A

Cl- is cotransported with Na+ (ileum) or exchanged with HCO3- (colon) into enterocytes
These processes are both secondary active transport

20
Q

How is K+ transported in the colon and small intestine?

A

K+ diffuses via paracellular pathways in small intestine

K+ leaks out between cells in the colon- passive transport

21
Q

What happen to intracellular sodium?

A

Active transport of Na+ into the lateral intercellular spaces by Na+K+ATPase transport in lateral plasma membrane

22
Q

What drives the movement of water into intercellular spaces?

A

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

23
Q

How is calcium absorbed?

A

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

24
Q

How much calcium do we have in our bodies?

A

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
25
Q

How does calcium cross the memebrane?

A

Calcium crosses the apical membrane by:
1.Intestinal calcium-binding protein (IMcal) through facilitated diffusion- here Ca2+ acts as an intracellular signalling molecule
Once Ca2+ has entered cell, it binds to calbindin in cytosol which prevents the presence of high conc. of Ca2+ in the cell- prevents its action as an intracellular signal
Ca2+ is then pumped across the basolateral membrane by Plasma Membrane Ca2+ ATPase (PMCA)
PMCA has a high affinity for Ca2+ but a low capacity
This process maintains the low concentration of calcium, normally observed within a cell
Ca2+ is also pumped across the basolateral membrane by plasma membrane Na+/Ca2+ exchanger against conc. gradient
Na+/Ca2+ exchanger has a low affinity for Ca2+ but a high capacity. Requires larger conc. of Ca2+ to be effective
2. Ion channel (paracellular channels)

26
Q

What processes in the body is iron essential for?

A

Oxygen transport and oxidative phosphorylation

27
Q

What are the dangers of having too much iron in the body?

A

Iron is an electron donor and electron acceptor

Iron is toxic in excess- body has no mechanism to actively excrete it

28
Q

How much iron does the body absorb a day?

A

Adults ingest approx. 15-20mg/day but absorb only 0.5-1.5mg/day

Iron presents in the diet as:

a) inorganic iron (Fe3+ ferric, Fe2+ ferrous)
b) as part of heme group (haemoglobin, myoglobin and cytochromes)- heme is a small part of diet but more readily absorbed

Fe3+ cannot be absorbed so is reduced to Fe2+ - it’s reduced by vitamin C

29
Q

What does Fe3+ form insoluble salts with?

A

Hydroxide
Phosphate
HCO3-

30
Q

How is heme transported into cells?

A

Dietary haem is highly bioavailable
Heme is absorbed intact into the enterocyte
This occurs via heme carrier protein 1 (HCP-1) and via receptor mediated endocytosis
Fe2+ is liberated by Heme oxygenase

31
Q

How is iron taken up by the body?

A

Duodenal cytochrome B (Dcytb) catalyses the reduction of Fe3+ to Fe2+ in the process of iron absorption in the duodenum
Fe2+ transported via divalent metal transporter 1 (DMT-1), a H+- coupled co-transporter
Fe2+ binds to unknown factors and is carried across the basolateral membrane via ferroportin ion channel into blood
Hephaestin is a transmembrane copper- dependent ferroxidase that converts Fe2+ to Fe3+
Fe3+ binds to apotransferrin and travels in blood as transferrin
OR
Fe2+ binds to apoferritin to form ferritin micelle and Fe2+ is oxidised to Fe3+ which crystallises within protein shell
A single ferritin molecule can store up to 4000 iron ions

32
Q

What is the major iron regulating protein?

A

Hepcidin- suppresses ferroportin function to decrease iron absorption

33
Q

What happens to ferritin stores in enterocytes?

A

It prevents absorption of too much iron

Increase in iron concentration in cytosol increases ferritin synthesis

34
Q

What is the role of ferritin?

A

Irreversibly binds to iron in epithelial cells
Iron/Ferritin is not available for transport in plasma
Iron/ferritin is lost in the intestinal lumen and excreted in faeces

35
Q

How are vitamins transported in the body?

A

Passive diffusion predominant mechanism
Fat soluble vitamins: (A,D,E,K) transported to brush border in micelles. K taken up by active transport

Specific transport mechanisms for vitamin C (ascorbic acid), folic acid, vitamin B 1 (thiamine), vitamin B12

36
Q

Where is vitamin B12 mostly stored?

A

Liver contains large store (2-5mg)

Most vit B is bound to proteins

37
Q

What are consequences associated with vitamin B12 deficiency?

A

Prevents the maturation of RBCs- pernicious anaemia

38
Q

How is denaturation of B12 in the stomach avoided?

A

In the stomach low pH and the digestion of proteins releases free vit B12
However B12 is easily denatured by HCl
To prevent this B12 binds to R protein (haptocorrin) released in saliva and from parietal cells
R proteins are digested in the duodenum

39
Q

What is the role of intrinsic factor in the absorption of vitamin B 12?

A

Vitamin B12 binding glycoprotein (Intrinsic Factor) is secreted by parietal cells
Vitamin B12-IF complex is resistant to digestion
Without IF there is no absorption of Vit B12
This Vit B12-IF complex binds to cubilin receptor and is taken up in distal ileum

40
Q

What happens to vitamin B12 once it’s in the ileum?

A

Once in the cell the Vit B12-IF complex is broken- possibly in mitochondria
B12 binds to protein transcobalamin II (TCII) and crosses the basolateral membrane
It travels to the liver bound to TCII
TCII receptors on cells allow them to uptake the complex
Proteolysis then breaks down TCII in the cell

41
Q

What is the role of vitamin D?

A

Essential for normal Ca2+ absorption
1, 25 dihyroxycholecalciferol taken up by enterocytes:
- enhances transport of Ca2+ through cytosol
- increases levels of Calbindin
- increases rate of extrusion across basolateral membrane by increasing level of Ca2+ ATPase in membrane

BRS (60 decks)

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