Sodium transport - (Jun/06/2006 )

Hi there can someone pllease help me with this question I have an exam tomorrow and am unsure of the answer

The diffusion of sodium across a cell membrane:
1-is directed into the cell
2-involves the direct hydrolyses of ATP
3-is an example if faciliated diffusion
4-uses the engery stored in ion gradients
5-is coupled by the removal of potassium ions from the cell

I think 1 3 and 4 are clearly wrong as doesn't Na+ create an ion gradient and it uses a transport protein and as the membrane is impermeable to charged ions it can't be directed into it so my guess would be 2 or 5 but not really sure

Please help!!!!
biology_06er

1 CORRECT diffusion is a non nergy consumoing process. So it goes from the more concentrated to the less concentrated compartment. Hence, as [Na+] outside > [Na+] inside, sodium ions goes into the cell.

2 FALSE diffusion does not implies ATP hydrolysis.

3 not too sure abou that point. i know that channels from inward diffusion does not take in count the na+ but due to the high gradient of concentration of Na+, the Na+ goes through these channels...

4 : tha


5 : false. It's the opposite. Exchange Na+/K+

Thanks fred_33

haha I didn't even realise that the homework part of this site was new till I saw mine was the first post... shaaame....anyway thanks again...

I'll take a swing at this one (spoiler: I come to a different conclusion than Fred did...).

Most people are clear on the concept of diffusion (drop some ink in a bowl of water), but remember -- we've got to cross a membrane in this case. If the membrane is permeable to the solute, then the solute can enter the cell by simple diffusion. If it is not permeable to the cell, then it must be helped across the membrane. If this helping process takes place without the consumption of cellular energy, then it is called "facilitated diffusion" -- it is still passive transport (requiring no energy), but it is not simple diffusion, because the membrane is impermeable to the substance.

The key to the question is the use of the word "diffusion". No diffusion process uses energy, so number 2 and number 4 are out right off the bat. The sodium-potassium pump operates to bring potassium *into* the cell, and thus number 5 is wrong (plus, this pump uses energy and thus not be considered diffusion).

Now it gets a bit trickier -- is number 1 correct, or number 3? Number 1 uses the word "directed", which implies an energy consuming process, and a particular direction -- into the cell. Diffusion works both ways equally well -- the solute will go from a higher concentration to a lower concentration regardless of which side of the membrane has the lower concentration. If it does anything other than this, it is not diffusion, and can thus be eliminated as the correct answer.

So, the correct answer is number 3.

The membrane does, as you noted, resist the passage of charged ions. Ion channels, however, overcome this natural tendency for the lipid bilayer to resist the passage of charged ions by providing a hydrophilic pore. Sodium is allowed to diffuse across the membrane with the help of a membrane protein that forms a hole (or channel) in the membrane -- one that is for example, too small for potassium to pass through, and too negatively charged to let chloride through. As an aside, lithium can zip right through the sodium ion channel, and that's why it's useful for the treatment of bipolar disorder, which is caused by sodium channel problems...

BTW, biology_06er -- your question was not only the first in our new homework question forum, but was also just the kind we're looking for -- it is obvious from your question you have some understanding of what behind the question, and have thought about it yourself...

I've been thinking a bit more about this question, and concluded that an argument could be made for picking both answers 1 and 3 together. It hinges on the meaning of the word "directed".

The passive transport via facilitated diffusion of sodium ions across the membrane can theoretically occur in both directions -- into and out of the cell. However, in reality, this diffusion occurs almost exclusively in one direction -- into the cell. Why? Because the cell maintains an artifically low sodium concentration inside the cell, through the action of the sodium-potassium pump. By this active (energy consuming) transport mechanism, the cell expels three sodium ions in exchange for bringing two potassium ions into the cell.

Thus, because the intracellular sodium ion concetration is kept artifically low, diffusion through the ion channel has only one choice -- into the cell. One could say the cell thus forces or "directs" the diffusion of sodium ions into the cell by virtue of its maintaining an artifical concentration gradient, albeit through a seperate active transport mechanism.

But the diffusion of these ions, although it's artifically influenced to proceed in one direction only -- into the cell -- is clearly a facilitated diffusion, thus answer three is more correct, though answer one could also be thought of as correct by the logic outlined above.

The ambiguous wording of answer one should be corrected, and, if you answered the question by selecting answer one and got it wrong, you might be able to use such an argument to regain some points...

Hey there

Thanks alot for that HomeBrew ...yay finished ONE bio test today (the hardest one still to come *sniff*)...For the above question as you say you could make an argument for it...the thing is as it's multi choice we have a system called "scantron" not sure where else in the world it's used (I'm in NZ) which is colour in the box eg A B C D E and then it is marked electronically so we will never know if we are right or not, but thanks again for the reply/s ..much appreciated

biology_06er