Science Questions: Beyond the Scope of the MCAT

[Nutmeg]

It's the BSM thread! This is the place where people can seek help with their homework or ask for help understanding questions where the answer isn't necessarily expected of people taking the MCAT. Since most premeds have similar sorts of undergraduate majors, hopefully if anyone needs help in their undergraduate homework or wants further clarification, then they can ask the question here and hope for some help. Of course, there's no guarantee that someone can answer any given question, but it's here now, and you can give it a try.

Acceptable topics include:

• Advanced science questions in bio, physics, chemistry, etc
  Mathematics
  Engineering
  Psychology
  Cognitive science

Unacceptable topics include:

• Anything actually required for the MCAT
  Non-science questions

My undergraduate majors were chemical engineering and molecular and cell biology (neuro emphasis). Hopefully some of the bright people on these boards will drop in regularly. Even if you can't help, you might learn something, and that's basically what this thread is all about.

Cheers!

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[deleted106503]

I wonder if it has anything to do with climate. Gases compress a lot due to decrease in temperature (Charles's Law ) I'm from FL, and I'd never heard of using nitrogen to fill your tires until you brought up this question. It just isn't an issue here because it basically doesn't ever get cold here. I can't even tell you the last time I had to fill my tires; usually they just check the pressure when I get my oil changed every four months and they leave well enough alone. You're from MA, so you could have some issues due to temperature swings, but no doubt the changes you get are nowhere near as severe as the kind of climate they have in Winnipeg.

My friend gets Nitrogen filled in his tires from Costco here in Orlando.

 

[QofQuimica]

My friend gets Nitrogen filled in his tires from Costco here in Orlando.

Dude, they sell beach sand to tourists in Orlando. I wouldn't judge the rest of the state based on what they do in Orlando.

 

[deleted106503]

Dude, they sell beach sand to tourists in Orlando. I wouldn't judge the rest of the state based on what they do in Orlando.

True!

 

[UMP]

My friend gets Nitrogen filled in his tires from Costco here in Orlando.

they do it around here too... i think Bell Tire does it... it sounds like a bull**** promotional ploy

 

[deleted106503]

they do it around here too... i think Bell Tire does it... it sounds like a bull**** promotional ploy

It was free, but I would never pay for it either if they charged. Can't say I care enough for it.

 

[Dave_D]

Anyways, I bet you could have nearly 100% N2 in your tires with just using compressed air. The first set will have 70% N2. Then as Oxygen diffuses out faster, you put in more compressed air. So the volume of new "displacing" air is 70% N2 instead of 0% N2 that left (I'm doing net out, which is the diff between O2 and N2 out). So each time you refill your tires with just a little more air, you're adding more N2.

Actually I just realized you're on to something. If diffusion is an issue (like they claim it is.) then they're missing a piece of the puzzle. Diffusion goes from regions of high concentration to low. So if you were to fill your tires with nitrogen O2 should diffuse in until equilibrium is reached and that would be the same level if you added extra O2 and let it diffuse out. So if we do like you suggest I'd expect O2 levels to be the same after a while in the air filed vs. nitrogen filled tires.(Of course if diffusion isn't a big deal then of course there'd be a difference in gas composition in the air vs. N2 filled tires but then the whole point of doing N2 goes away.)

I'm guessing that F1 teams actually have a real reason but for the average person it's pointless.(It might be as simple as a compressor might break at the wrong time but a bunch of gas cylinders rarely do.)

 

[ih8mcat]

This is a relativity problem, I'm supposed to start by finding the velocity, but i'm not sure if I use the earth's circumference as the distance for the v=d/t...ok here's the problem:

A clock is placed in a satellite that orbits the earth with a period of 119 min. By what time interval (in seconds) will this clock differ from an identical clock on earth after 1 y? (Assume that special relativity applies and neglect general relativity.)

Any help would be greatly appreciated, thanks!

 

[Smooth Operater]

How come cysteine is polar, but hydrophobic? I thought polar molecules are always hydrophilic? thanks!

 

[Captain Fantastic]

"[This] study has shown that free cysteine (Cys_SH) residues, as well as disulfide-bonding cystines (Cys_SS), behave like strongly hydrophobic residues in proteins. This apparent hydrophobic nature of Cys_SH seems to conflict with the polarized nature of the sulfhydryl group. However, we need to consider the fact that the sulfhydryl group is inactive toward water molecules. More specifically, unlike the hydroxyl group (-OH), the sulfhydryl group (-SH) has essentially no ability to form a hydrogen bond with water. This may explain why the -SH group is both active in metal binding and disulfide bond formation, and is hydrophobic in water."

From Strong hydrophobic nature of cysteine residues in proteins by Nagano, Ota, and Nishikawa.

 

[QofQuimica]

bump

 

[Shrike]

Aw... I'm so disappointed that this thread seems to have died.

Comments about the nitrogen-filled tires: I did plenty of research to confirm the following: it's BS. The decreased rate of effusion of nitrogen is nowhere near worth the expense, if there is expense. (More on this later.) Other oft-cited rationales: different heating characteristics (total BS); reduced tendency to oxidize tires (irrelevant if you actually use your tires, because they'll wear out long before it matters); decreased flammability (apparently confusing compressed air with compressed oxygen).

What it comes down to is that compressed nitrogen is available cheap (in big quantities), at high pressures; also, the separation process dries it. Compressed air is much harder to get, partly because the presence of water in it tends to rust the inside of steel containers (just ask a scuba diver). For a Formula One team, it's far easier to use tanks of whatever-the-heck than to have a compressor; the cheapest whatever-the-heck is nitrogen. The stuff they spout about more consistent heating? Who knows, they may believe it, but it's utter tripe.

Aircraft also use nitrogen, because their tires are inflated to high enough pressures that it's impractical for them to use anything other than bottled gas. Again, nitrogen's the cheapest option.

 

[Shrike]

This is a relativity problem, I'm supposed to start by finding the velocity, but i'm not sure if I use the earth's circumference as the distance for the v=d/t...ok here's the problem:

A clock is placed in a satellite that orbits the earth with a period of 119 min. By what time interval (in seconds) will this clock differ from an identical clock on earth after 1 y? (Assume that special relativity applies and neglect general relativity.)

First, solve for v. It's not OK to use Earth's radius; the altitude is such that the centripetal force mv^2/r is equal to the gravitational force GMm/r^2. Use 119 x 60 x v = 2(pi)r to get r = about 1200v, then plug that back in. I get v = about 7000 m/s, though I could easily be off.

Then, time is dilated by the Lorenz factor, square root of (1 - v^2/c^2), or about .9999999995. The change is a factor of around 5e(-10). A year has somewhat over 30,000,000 seconds, so I think your clock will be slow by something on the order of 0.001 seconds.

That's the I-can't-find-a-pencil, I refuse to use a calculator version. Am I at all close?

 

[gridiron]

First, solve for v. It's not OK to use Earth's radius; the altitude is such that the centripetal force mv^2/r is equal to the gravitational force GMm/r^2. Use 119 x 60 x v = 2(pi)r to get r = about 1200v, then plug that back in. I get v = about 7000 m/s, though I could easily be off.

Then, time is dilated by the Lorenz factor, square root of (1 - v^2/c^2), or about .9999999995. The change is a factor of around 5e(-10). A year has somewhat over 30,000,000 seconds, so I think your clock will be slow by something on the order of 0.001 seconds.

That's the I-can't-find-a-pencil, I refuse to use a calculator version. Am I at all close?

That sounds right.

 

[CATallergy]

I read that in atomic hydrogen with empty 2s, 2px, 2py, and 2pz orbitals, these are all degenerate. Why would this be? it seems like the lower energy of hybridized sp orbitals would not compensate for the increase in 2s without more electrons (like even at least one p electron).

thanks

 

[soonereng]

Comments about the nitrogen-filled tires: I did plenty of research to confirm the following: it's BS. The decreased rate of effusion of nitrogen is nowhere near worth the expense, if there is expense. (More on this later.) Other oft-cited rationales: different heating characteristics (total BS);

Actually there would be different heating characteristics between pure N2 vs N2+O2, but it is probably negligible. N2 requires greater heat input (1.039 kJ/kg*K) to increase 1 degree than O2 (.915 kJ/kg*K). Since PV=nRT, the tire pressure would have greater consitancy as the tire heated up than with air, and we all know tire pressure is very important in contact area and suspension dynamics.

decreased flammability (apparently confusing compressed air with compressed oxygen).

No O2=no explosion (compressed air has 20% O2 in it).

I have an even more far fetched reason: mass. N2 weighs less than air. If you have ever held an F1 wheel and tire, they are super light (but not because of the gas difference).

 

[unsung]

Okay, so these aren't really "advanced" biology Qs, but merely Qs that came to mind as I was reviewing information on the endocrine system. I'm not in school right now (self-study), so hopefully someone can help out!

1. If a deficiency in insulin results in diabetes, can an excess of glucagon also result in diabetes? (I.e. if there is a chronic excess of glucagon that results in chronically high blood sugar levels, can that "overwhelm" insulin production, in a sense?)

2. Is Addison's Disease the result of a deficiency in glucocorticoids, mineralocorticoids, or gonadocorticoids? (i.e. which one(s)?)

3. Relatedly, is Cushing's disease caused by a deficiency in glucocorticoids, mineralocorticoids, or gonadocorticoids? (i.e. which one(s)?)

4. An excess of insulin or a deficiency of glucagon both result in hypoglycemia. However, the former can also be described as "insulin shock". Is there an analogous alternate term describing the latter?

5. What are the effects of excess cholecystokinin? (And what are the effects of a deficiency in cholecystokinin?)


Thanks in advance!

 

[neurodoc]

1. If a deficiency in insulin results in diabetes, can an excess of glucagon also result in diabetes? (I.e. if there is a chronic excess of glucagon that results in chronically high blood sugar levels, can that "overwhelm" insulin production, in a sense?)

"Real World" Diabetes can be due to two things: 1) insulin deficiency; and, 2) insulin resisitance. Glucagon will increase blood glucose levels, but that in itself will not vreate diabetes.

2. Is Addison's Disease the result of a deficiency in glucocorticoids, mineralocorticoids, or gonadocorticoids? (i.e. which one(s)?)

Glucocorticoids

3. Relatedly, is Cushing's disease caused by a deficiency in glucocorticoids, mineralocorticoids, or gonadocorticoids? (i.e. which one(s)?)

Too much (not deficiency of) glucocorticoids

4. An excess of insulin or a deficiency of glucagon both result in hypoglycemia. However, the former can also be described as "insulin shock". Is there an analogous alternate term describing the latter?

"Hypoglucagonergic hypoglycemia" might work...

5. What are the effects of excess cholecystokinin? (And what are the effects of a deficiency in cholecystokinin?)

Don't know. Maybe some problems with pancreatic and gall bladder function


Thanks in advance!

See above inserts.

 

[Shrike]

Actually there would be different heating characteristics between pure N2 vs N2+O2, but it is probably negligible. N2 requires greater heat input (1.039 kJ/kg*K) to increase 1 degree than O2 (.915 kJ/kg*K). Since PV=nRT, the tire pressure would have greater consitancy as the tire heated up than with air, and we all know tire pressure is very important in contact area and suspension dynamics.

Actually, check your math. As you point out below, nitrogen is slightly lighter than oxygen, while as you point out here, nitrogen requires greater heat per unit mass. This works out to have nitrogen's and oxygen's heat capacity per unit volume, which is what's relevant here, within 1% of each other.

No O2=no explosion (compressed air has 20% O2 in it).

Do you read a lot about explosions resulting from combustion involving compressed air? I'm thinking not.

I have an even more far fetched reason: mass. N2 weighs less than air. If you have ever held an F1 wheel and tire, they are super light (but not because of the gas difference).

That's actually less far-fetched than the others, but it's still pretty silly. F1 tires are probably inflated to a gauge pressure of about 3atm, and so might contain something on the order of 20 moles of gas. But the resulting difference of 80 grams doesn't mean what you think -- Formula One cars must meet a certain minimum weight, so the savings comes only in potential change in the ratio of sprung to unsprung weight, plus a negligible change in the rotational inertia.

No, I think I'm going with my original interpretation that it's all BS.

 

[soonereng]

Actually, check your math.

If you read my original post, I said the difference was probably negligible. You just took the time to calculate it out and proved my assumption correct.

 

[Shrike]

If you read my original post, I said the difference was probably negligible. You just took the time to calculate it out and proved my assumption correct.

I did read your post; I think you're being disingenuous now. You gave the values for heat per kg, not heat per mole, because you, uh, wanted to emphasize, uh, ... say, you think maybe you forgot that the relevant value was per mole (and per unit volume)? When you said "N2 requires greater heat input than O2," the numbers you gave there weren't meant to suggest that the difference was 14%, as it appeared to be -- you figured everyone would realize that those numbers you gave were actually misleading, and that the real difference was about 0.65%, even when contrasting against pure oxygen (and therefore about a tenth of a percent for air)? When you said "the tire pressure would have greater consitancy as the tire heated up than with air, and we all know tire pressure is very important," you really meant that the tire pressure wouldn't have greater consistency?

Yes, you said it was probably a negligible effect. Every word was right, except the "probably." But the thrust of your post was that there might actually be some difference.

But you're right: I just did the math you should have done and proved that you were correct. Uh-huh.

 

[soonereng]

I did read your post; I think you're being disingenuous now....When you said "N2 requires greater heat input than O2," the numbers you gave there weren't meant to suggest that the difference was 14%, as it appeared to be -- you figured everyone would realize that those numbers you gave were actually misleading, and that the real difference was about 0.65%, even when contrasting against pure oxygen (and therefore about a tenth of a percent for air)?

I am not being disingenuous. All of my statements were correct. I just gave some numbers that I had easily accessible to me for illustration. The heating characteristics ARE different between N2 and air, but as I originally said without crunching the numbers they are probably negligible. I'm typically not going to waste my time calculating out something if I believe it to be a secondary effect, but to appease you, I went ahead and did the calculation, and the difference at 100 F between the specific heat on a molar basis at constant pressure for air (not 02) and N2 is 0.4%.

Yes, you said it was probably a negligible effect. Every word was right, except the "probably." But the thrust of your post was that there might actually be some difference.

Why would I start my post with a statement saying that all the numbers I am giving below probably have no effect if I didn't mean it? I was sincere in my original statement, and I think you are reading too much into this.

 

[Dave_D]

Well might as well bump this one with a new question. (BTW, Cecil answered my nitrogen question ) Anyway I was listening to people talk about CocaCola and this and that about how many issues are caused when sweetened with high fructose corn syrup vs sucrose. Anyway from my orgo class I learned enough that I figured that an acid would probably catalyze the hydrolysis of sucrose so it probably doesn't matter which they use. (I have found on line articles that do infact state this.) So quick question, suppose I went into a store and bought a bottle of Coke or Pepsi at room temperature and it was originally sweetened with succose. If I were to test it would I find it fairly close to equilibrium with regards to the equation

sucrose <-> fructose + glucose

or would I still find significant amounts of sucrose?(If I knew some of the numbers like the concentration of the sugar and the pH I could probably figure it out.)

 

[DiverDoc]

Heres to hoping that this thread is still alive.

The question reads: the different isotypes of immunologlobin are identified based on their?

a) variable region
b) heavy chain constant region
c) light chain constant region

My textbook says "constant region", but I am thinking the answer is C, because to me, to have it on the heavy chain would be of no use.

 

[Dave_D]

Heres to hoping that this thread is still alive.

The question reads: the different isotypes of immunologlobin are identified based on their?

a) variable region
b) heavy chain constant region
c) light chain constant region

My textbook says "constant region", but I am thinking the answer is C, because to me, to have it on the heavy chain would be of no use.

According to this wiki article the answer is B
Short thing on isotypes

Anybody know more about this?(I'm seeing that there's only 2 types of light chains but 5 types of heavy.)

 

[liverotcod]

Heavy chains have an antigen-binding region, Fab (arms of the Y) as do light chains, but also have an Fc (base of the Y) region that corresponds to the non-antigen-specific functionality of the immunoglobulin molecule. There are five types: gamma, delta, alpha, mu, and epsilon. The five isotypes of antibodies each have the matching heavy chain: IgG, IgD, IgA, IgM, IgE. They each have a different immunologic function.
IgG: Uses its Fc region to opsonize pathogens for phagocytosis and activate the complement pathway.
IgD: Not very important or well-understood.
IgA: Fc region forms dimer, active in mucosal secretions
IgM: Expressed on surface of B cells and also secreted as the earliest specific response to an antigen (production of IgM doesn't require helper T-cell activity to switch to the other types)
IgE: Fc region attaches to receptor on mast cells; antigen binding causes mast cell degranulation -> inflammation. Classic allergic reaction is IgE mediated.