MCAT Biology Related Questions

[tms]

Hi everyone,

 

I am going through the biology material and doing some practice problems...there are some concepts/questions that I am unsure of and was wondering if someone can explain them/help me out.

 

Some questions I have so far (may post more as I continue studying):

 

- if an animal cell is placed in 1M of substance A or 1M of substance B, does it matter what these substances are or would the cell shrink in both cases?

 

- genetics problem: A couple is heterozygous for 2 autosomal recessive diseases. What is the probability that their first child will have either of those diseases?

 

This is the way I solved the problem (but I am getting the wrong answer....I am not sure how to go about this question).

 

I am thinking of the dihybrid cross (AaBb X AaBb) and the 9:3:3:1 ratio, the probability of having either of the disease is then 3/16 + 3/16 = 6/16 = 3/8.

 

Can someone please explain these to me.....thanks!

[bored]

answers are below

 

Hi everyone,

 

I am going through the biology material and doing some practice problems...there are some concepts/questions that I am unsure of and was wondering if someone can explain them/help me out.

 

Some questions I have so far (may post more as I continue studying):

 

- if an animal cell is placed in 1M of substance A or 1M of substance B, does it matter what these substances are or would the cell shrink in both cases?

 

Yes it does matter what the substance. it only shrinks or bursts if it is higher or lower in concentration relative to the cell. so if the outside is 2 M of salt which the cell is 200 M of nacl, than water would move in, and the cell will burst.

 

- genetics problem: A couple is heterozygous for 2 autosomal recessive diseases. What is the probability that their first child will have either of those diseases?

 

stats!

 

ok, if the couple is heterozygous for both the diseases, than the chance of the kid getting the disease is 0.25, or 25%. the chance that the kid will have one of the disease is 25%, the chance that the kid will have the other disease is also 25%. I would say that the kid has 25% chance of having either of the disease, just by thinking about it for a while.. I didn't use any stats.. perhaps someone can varify?

 

This is the way I solved the problem (but I am getting the wrong answer....I am not sure how to go about this question).

 

I am thinking of the dihybrid cross (AaBb X AaBb) and the 9:3:3:1 ratio, the probability of having either of the disease is then 3/16 + 3/16 = 6/16 = 3/8.

 

Can someone please explain these to me.....thanks!

[DaKirbster]

First question:

 

It depends on the concentration of A or B *inside* the cell as well as outside. Saying that a cell is put into a solution containing 1M of solute A is meaningless if we don't know the concentration of A is inside the cell. If the cell is hypotonic (less concentrated, eg. 0.1M) to the solution, then water will leave the cell to try to reach a concentration equilibrium on each side of the membrane. If the cell is hypertonic (more concentration, eg. 5M) to the solution, water will enter the cell for the same reason.

 

Second question:

 

I am assuming that each person in the couple is heterozygous for both diseases, and that they are carried on separate alleles, ie. AaBb x AaBb. Also, I'm assuming the question is asking the probability that the child will get at least one of the diseases. Personally, I would treat each disease separately and calculate each outcome:

 

Aa x Aa --> There is a 25% chance that the child will be homozygous recessive and be diseased. The same applies for the other disease (Bb x Bb).

 

The possibilities are:

 

The child gets the aa disease, the bb disease, both, or neither.

 

The fast way to get the answer is to calculate the probability that the child will get neither and take the opposite probabilty [100% - p(neither)], since in every other outcome the child has at least one disease.

 

The probability that the child will get neither disease is 75% (for AA or Aa) times 75% (BB or Bb) = 9/16.

 

100% - 9/16 = 7/16.

 

Therefore, the probability that the child will get at least one disease is 7/16.

 

You can confirm this by calculating the probability the normal way.

 

Probability that the child will get both diseases = 25% * 25% = 1/16

Probability that the child will get only disease aa = 75% * 25% = 3/16

Probability that the child will get only disease bb = 25% * 75% = 3/16

 

1/16 + 3/16 + 3/16 = 7/16.

 

Hope that helps.

[bored]

Hi everyone,

 

I am going through the biology material and doing some practice problems...there are some concepts/questions that I am unsure of and was wondering if someone can explain them/help me out.

 

Some questions I have so far (may post more as I continue studying):

 

- if an animal cell is placed in 1M of substance A or 1M of substance B, does it matter what these substances are or would the cell shrink in both cases?

 

- genetics problem: A couple is heterozygous for 2 autosomal recessive diseases. What is the probability that their first child will have either of those diseases?

 

This is the way I solved the problem (but I am getting the wrong answer....I am not sure how to go about this question).

 

I am thinking of the dihybrid cross (AaBb X AaBb) and the 9:3:3:1 ratio, the probability of having either of the disease is then 3/16 + 3/16 = 6/16 = 3/8.

 

Can someone please explain these to me.....thanks!

 

for question 2, the above answer looks correct, I guess you forgot to add the possibility of getting aabb in your calculation. so it would be 3/16 + 3/16+ 1/16

[tms]

Thanks for the response!

 

The first question is clarified but I am still a little confused about the genetics problem.

 

The problem I was working on said what is the probability that the first child with have either disease A or disease B. That's why I did not add the probability (1/16) for having both diseases. The solution says 1/2 (I do not have the full explanations for the solutions so I am not sure how they are getting that value).

[markov79]

kay, if the answer is 1/2, i think it'd just be like this:

 

EACH parent is heterozygous for BOTH diseases.

you then have parent A having a 1/2 shot of passing on disease X, and 1/2 shot of passing on disease Y. the same goes for parent B.

 

to find the probability of the child having disease X, multiply the probabilities of each parent passing on disease X. parent A has a 1/2 probability, multiply that by the 1/2 chance that parent B will also pass on: 1/4.

 

do the same thing for disease Y. 1/4.

 

since it's the probability of the child inheriting EITHER disease X OR disease Y, we add the probabilities: 1/4 + 1/4 = 2/4 = 1/2.

 

tbh, the wording of that question wasn't that great, which probably is the reason you had to even ask, and i derived my solution solely based on the answer.

[thehumanmacbook]

For the genetics problem, you would need more information.

 

For instance, linkage vs law of independent assortment would change how you would derive the answer. If both heterozygous diseases were close to each other on the chromosome, the chances of receiving both would be different if you assumed the law of independent assortment to be in affect, where the chances would probably be additive (either).

[tms]

Thanks markov79! Yeah, the wording of the question is not clear, I copied it here exactly as it was in the problem I was looking at, anyways, the way you solved it makes sense. Thanks

[markov79]

For the genetics problem, you would need more information.

 

For instance, linkage vs law of independent assortment would change how you would derive the answer. If both heterozygous diseases were close to each other on the chromosome, the chances of receiving both would be different if you assumed the law of independent assortment to be in affect, where the chances would probably be additive (either).

 

LOL. that stuff totally slipped my mind. i guess this is goodbye, grade 11/12, first/second year biology............. maybe i'll see you again some day.

[tms]

Another question:

 

I keep getting confused with what upstream versus downstream means. Can someone please explain this?

[kpm]

Upstream means more 5'. Downstream means more 3'.

[tms]

Thanks!

 

A -> B -> C -> D

 

Can I say that B is upstream of D? and C is downstream of A? Is that correct?

[repede]

Thanks!

 

A -> B -> C -> D

 

Can I say that B is upstream of D? and C is downstream of A? Is that correct?

 

That is correct. In a pathway, "upstream" means a process/substance that comes before the point of reference, and "downstream" means a process/substance that comes after the point of reference.

 

So for example, if you're talking about a neurotransmitter binding to a receptor, the signalling cascade that occurs as a result is called the "downstream signalling cascade".

[Let'sGo1990]

I have a question.

 

If an inhibitor of the Na/K ATPase is added to cells, which of the following may occur?

 

a) the cell will shrink and lose water.

the interior of the cell will become less negatively charged.

c) secondary active transport processes will compensate for the loss of primary active transport.

d) the cell will begin to proliferate.

 

Obviously c) and d) are incorrect.

 

If the ATPase were to stop, K would leave through the leak channels, and the cell would become more negatively charged.

 

If K conc in the cell decreases, water will want to move out of the cell, so it would shrink and lose water.

 

So I'd answer a), but the book says the answer is , and offers the following reason:

 

"The Na/K ATPase is required to establish the resting membrane potential in which the cellular interior has a negative charge. It pumps out one net positive ion. If this net positive ion stays inside the cell, the resting potential becomes less negative (choice B is correct). Since the interior of the cell is now more charged, the cell will have a tendency to take on water by osmosis, and will swell (choice A is wrong)".

 

I guess I can see their reasoning, but I don't understand why my reasoning doesn't work.

[aaronjw]

And this is why I loathe multiple choice because it's not that your answer is wrong but that it's not the most correct answer.

 

MC shouldn't have right and most correct answer sets. There should be right and wrong, not frigging shades of grey.

 

I have a question.

 

If an inhibitor of the Na/K ATPase is added to cells' date=' which of the following may occur?

 

a) the cell will shrink and lose water.

the interior of the cell will become less negatively charged.

c) secondary active transport processes will compensate for the loss of primary active transport.

d) the cell will begin to proliferate.

 

Obviously c) and d) are incorrect.

 

If the ATPase were to stop, K would leave through the leak channels, and the cell would become more negatively charged.

 

If K conc in the cell decreases, water will want to move out of the cell, so it would shrink and lose water.

 

So I'd answer a), but the book says the answer is , and offers the following reason:

 

"The Na/K ATPase is required to establish the resting membrane potential in which the cellular interior has a negative charge. It pumps out one net positive ion. If this net positive ion stays inside the cell, the resting potential becomes less negative (choice B is correct). Since the interior of the cell is now more charged, the cell will have a tendency to take on water by osmosis, and will swell (choice A is wrong)".

 

I guess I can see their reasoning, but I don't understand why my reasoning doesn't work.[/quote']

[markov79]

I have a question.

 

If an inhibitor of the Na/K ATPase is added to cells' date=' which of the following may occur?

 

a) the cell will shrink and lose water.

the interior of the cell will become less negatively charged.

c) secondary active transport processes will compensate for the loss of primary active transport.

d) the cell will begin to proliferate.

 

Obviously c) and d) are incorrect.

 

If the ATPase were to stop, K would leave through the leak channels, and the cell would become more negatively charged.

 

If K conc in the cell decreases, water will want to move out of the cell, so it would shrink and lose water.

 

So I'd answer a), but the book says the answer is , and offers the following reason:

 

"The Na/K ATPase is required to establish the resting membrane potential in which the cellular interior has a negative charge. It pumps out one net positive ion. If this net positive ion stays inside the cell, the resting potential becomes less negative (choice B is correct). Since the interior of the cell is now more charged, the cell will have a tendency to take on water by osmosis, and will swell (choice A is wrong)".

 

I guess I can see their reasoning, but I don't understand why my reasoning doesn't work.[/quote']

 

i thought leak channels were generally there, but that the Na/K pump was there to stabilize all forces. ie you have action potential and leak channels operating all the time. (also note that i'm not entirely sure that your book would even touch on leak channels... would it? i think the question depends on how much the book expects you to know). anyway, with the APs and leak channels happening, the Na/K pump operates to stabilize all this, obviously by pumping out a net charge of +1. if you just remove that, all other crap is going to happen, but the net current of +1 to the outside won't. so, relative to before, it'll become less negative. it kind of looks like your book isn't relying on leak channels to be part of the question, though. but even if it is, i think it'd just make for a more "chaotic" cell environment, since again no stablizing factor. are there leak channels for sodium? if so, that might counteract the potassium one. as always, there's a possibility this is wrong.

[Let'sGo1990]

And this is why I loathe multiple choice because it's not that your answer is wrong but that it's not the most correct answer.

 

MC shouldn't have right and most correct answer sets. There should be right and wrong, not frigging shades of grey.

 

Right on haha. Although I still prefer MC over short answer b/c short answer can be a lot heavier on the subjectivity.

[Let'sGo1990]

i thought leak channels were generally there, but that the Na/K pump was there to stabilize all forces. ie you have action potential and leak channels operating all the time. (also note that i'm not entirely sure that your book would even touch on leak channels... would it? i think the question depends on how much the book expects you to know). anyway, with the APs and leak channels happening, the Na/K pump operates to stabilize all this, obviously by pumping out a net charge of +1. if you just remove that, all other crap is going to happen, but the net current of +1 to the outside won't. so, relative to before, it'll become less negative. it kind of looks like your book isn't relying on leak channels to be part of the question, though. but even if it is, i think it'd just make for a more "chaotic" cell environment, since again no stablizing factor. are there leak channels for sodium? if so, that might counteract the potassium one. as always, there's a possibility this is wrong.

 

I'm using the TPR bio book. They mentioned the potassium leak channels in the book themselves, but didn't say anything about Na leak channels. I remember learning that the amount of Na leaking in was negligible compared to the amount of K leaving though.

[markov79]

I'm using the TPR bio book. They mentioned the potassium leak channels in the book themselves' date=' but didn't say anything about Na leak channels. I remember learning that the amount of Na leaking in was negligible compared to the amount of K leaving though.[/quote']

 

kay then yeah i'm not sure LOL. shows you how much i learned this year... LOL.

[bpatient]

Oh come on Phys 3120 ftw!

[thehumanmacbook]

I'm using the TPR bio book. They mentioned the potassium leak channels in the book themselves' date=' but didn't say anything about Na leak channels. I remember learning that the amount of Na leaking in was negligible compared to the amount of K leaving though.[/quote']

 

This is a bit of a poorly worded question. Remember that this may not be a neuron/muscle cell, and we're just looking at Resting Membrane Potential (RMP). Let's assume we don't have the ligand/ion gated channels on this cell, but we do have leak channels. Basically what Na+/K+ ATPases helps to do is keep the net positive charge outside of the cell to build both a conc'n and potential gradient. once Na+/K+ ATPase is inhibited, the net (+) is inside the cell, in the form of a Na+ ion.

 

At this point you're probably wondering "why doesn't the K+ just leak out of the cell to restore Potential?" That's b/c of eq'm potential, where there is a certain point of eq'm where it's actually less favourable for K+ to leave. Thus K+ is more/less trapped within the cell, and Na+ is trapped as well because of the impereability of the membrane. Because of these trapped molecules, the interior is less negatively charged. Also, the cell would swell a little bit more because its relative intracellular conc'n would increase compared to when Na/K ATPase is working (b/c of the trapped Na), so there would be a bit more water inside. B is the best answer.

[tms]

This is a chemistry question (but I didn't want to start a thread), hopefully someone can help me understand this.

 

Which one is least reactive?

 

I narrow the choices down to Mg(s) and Cu(s).

 

I am thinking that the one that is most stable in terms of electron configuration. The one that is most stable will be the least reactive. So, I thought that it would be Mg(s) since it has a full s orbital but the answer is Cu(s). Doesn't Cu(s) have [Ar] 3d10 4s1 configuration? Given that it will lose the 4s electron before the 3d electrons and since the s orbital is not full, would it not be more reactive than Mg(s)?

 

Can someone please explain this?

 

Thanks!

[markov79]

if you drop magnesium in heated water, it freaks out.

if you drop copper in heated water, it rusts over like 100 years.

 

magnesium is the winner.

 

i'm just saying that tongue-in-cheek. i actually don't know why magnesium is more reactive, but it makes sense (see above).

 

edit: frig. i knew it had to have something to do with energy levels.

[DaKirbster]

http://wiki.answers.com/Q/Why_is_magnesium_more_reactive_than_copper

[tms]

Thanks DaKirbster!

 

Just so I am understanding this correctly, in terms if energy of orbitals, it's s<p<d<f. Since Mg(s) have the reactive electron in the lower energy orbital, it more likely to lose it and since Cu(s) has the reactive electrons in s and d orbitals, it is less likely to lose it. I am also assuming that this thinking can be applied to other examples. Am I on the right path?