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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[Shrike]

What fun this will be. OK, me first. No rhetorical questions from me -- I'll ask stuff I really want to know.

What is the modification to the Bernoulli equation for compressible materials? Under what conditions does the compressibility become significant? (We neglect it all the time, conceptually, when dealing with air -- how far off are we?)

What happens to the Bernoulli equation at supersonic speeds? An engineer student of mine once told me that, at supersonic speeds in air, as pressure decreases speed increases -- thus the shape of rocket engine nozzles, which increase the speed of the exhaust flow. Assuming this is true, why?

Why does Bernoulli not work for blood flow through our circulatory system -- is it energy loss due to vicosity, vessel wall elasticity, or something else?

 

[liverotcod]

Why are we here? What is the meaning of life?

(or would these be more relevant to the Verbal thread?)

 

[liverotcod]

What fun this will be. OK, me first. No rhetorical questions from me -- I'll ask stuff I really want to know.
What happens to the Bernoulli equation at supersonic speeds? An engineer student of mine once told me that, at supersonic speeds in air, as pressure decreases speed increases -- thus the shape of rocket engine nozzles, which increase the speed of the exhaust flow. Assuming this is true, why?

Not a complete answer, but a fascinating discussion:
http://www.engapplets.vt.edu/fluids/CDnozzle/cdinfo.html

 

[Shrike]

Not a complete answer, but a fascinating discussion:
http://www.engapplets.vt.edu/fluids/CDnozzle/cdinfo.html

Thanks for the link -- it answers some of my questions, and as you said is fascinating. But I still want to know why supersonic flow accelerates as it expands.

 

[liverotcod]

Thanks for the link -- it answers some of my questions, and as you said is fascinating. But I still want to know why supersonic flow accelerates as it expands.

According to the article, it accelerates because it can. The driver is the pressure differential between the engine and the ambient pressure. The expansion permits continued accceleration along the differential rather than an abrupt shift to subsonic due to the shock wave.

 

[Twitch]

An arrow leaves a bow at a speed of 43.6 m/s. If the bow exerted an average force of 95N on the arrow, how far did the archer pull back the bow string?

 

[liverotcod]

An arrow leaves a bow at a speed of 43.6 m/s. If the bow exerted an average force of 95N on the arrow, how far did the archer pull back the bow string?

You need the mass of the arrow, or an approximation.

 

[Twitch]

You need the mass of the arrow, or an approximation.

That's what I'm saying.... just curious if anyone knew any other way.

 

[liverotcod]

That's what I'm saying.... just curious if anyone knew any other way.

Or the amount of time that the arrow + string is accelerating.

 

[Twitch]

Or the amount of time that the arrow + string is accelerating.

That would help. Unfortunately that's all the question says.

Another Question:
A boy jumps vertically upwards from the top of a platform and then drops on a trampoline below which is 3m below his starting point.

If the trampoline has an effective spring constant of 5.2*10^4 N/m and is depressed 0.35m , calculate the boy's initial upward velocity.

 

[liverotcod]

That would help. Unfortunately that's all the question says.

Another Question:
A boy jumps vertically upwards from the top of a platform and then drops on a trampoline below which is 3m below his starting point.

If the trampoline has an effective spring constant of 5.2*10^4 N/m and is depressed 0.35m , calculate the boy's initial upward velocity.

Also insoluble without the mass of the boy. You could assume, say, 25 kg and work through it.

 

[liverotcod]

Molecular self-assembly has always been a mystery to me. I mean, I know how it works in theory, but I have a difficult time wrapping my mind around it. It doesn't seem real to me. Can anyone help?

Along the same lines, I am mystified by cellular processes' self-activation. We anthropomorphize these processes and pretend that there's someone pulling the strings, but in reality it all just sorta happens in order to satisfy the 2nd law of thermodynamics, right? Or am I missing something.

And on a larger scale, this all ties into theories of consciousness and free will. If everything we do is essentially a response to thermodynamic pressure - is all an exercise in physics - then where do consciousness and free will come in? I mean that in theory given sufficient computational power and knowledge of biological systems, one could predictively compute whether I'm going to succeed in medical school, or fail. But it *feels* like it's going to depend more on whether I bother to get off my a$$ on Saturday mornings to go study than on some predetermined physical equation. I keep coming back to the insufficiency of science to explain the human condition. As I am an atheistic-leaning secular humanist, this inability to accept these implications of the "scientific" explanation of the cosmos really bothers me.

OK, so this question is moving into metaphysics, but at least it's beyond the scope of the MCAT, right?

 

[Nutmeg]

Molecular self-assembly has always been a mystery to me. I mean, I know how it works in theory, but I have a difficult time wrapping my mind around it. It doesn't seem real to me. Can anyone help?

Do you mean protein folding?

Along the same lines, I am mystified by cellular processes' self-activation. We anthropomorphize these processes and pretend that there's someone pulling the strings, but in reality it all just sorta happens in order to satisfy the 2nd law of thermodynamics, right? Or am I missing something.

Yeah, pretty much. It's easier to imagine, say, water running down hill. It's easy to look at a patch of ground in some random part of the St Lawrence River watershed, and say: "okay... If I dump some water right here, or right there, with all the possible directions it can possibly go, it's going to end up passing Quebec City, and getting dumped into the Atlantic at the precise location of the St Lawrence delta. Yeah, right." It's a bit easier for us to imagine this sort of thing, because we have an innate understanding of the differences between going uphill and going downhill. We have a harder time imagining the difficulty of going against a chemical potential gradient, or an electric gradient. It's easy for us to imagine throwing a bunch of hockey pucks across a frozen pond, and them all ending up stuck in the snow along the sides, "miraculously" being self-segregating from the slick surface; but it's harder to imagine a Van der Waals interaction forcing the self assembly of a lipid bilayer. But in the long run, they're really all the same.

Chemicals are banging around at high speeds. They make a billion wrong collisions, but when they make that one right collision, the action happens, and the wrong collisions don't even matter anymore.

On some drops of water in the watershep might end up in some dead-end pond somewhere, yes. And some proteins don't fold right, and sometimes an ion channel just gets bumped open at the wrong time, or fails to open at the right time. But in the long run, you end up with a statistical average that acheives the needed result. Send a man to war against an army, he will die. Send a million men, and many will die, but the army will be defeated, and the losses lose significance in the long run. We know only the name of Waterloo as the place where Wellington defeated Napolean, but we do not know the name of the individual Brittish soldier who was defeated by a Frenchman that day.

The law of mass action: thermodynamics, sociology, economics, and evolution are all similarly governed.

And on a larger scale, this all ties into theories of consciousness and free will. If everything we do is essentially a response to thermodynamic pressure - is all an exercise in physics - then where do consciousness and free will come in? I mean that in theory given sufficient computational power and knowledge of biological systems, one could predictively compute whether I'm going to succeed in medical school, or fail. But it *feels* like it's going to depend more on whether I bother to get off my a$$ on Saturday mornings to go study than on some predetermined physical equation. I keep coming back to the insufficiency of science to explain the human condition. As I am an atheistic-leaning secular humanist, this inability to accept these implications of the "scientific" explanation of the cosmos really bothers me.

OK, so this question is moving into metaphysics, but at least it's beyond the scope of the MCAT, right?

I copy/pasted my free will argument from an old thred. Basically, I regard the question as a false dichotomy, and as a question that is intrinsically meaningless--akine to "what color is the sound of the wind blowing through the trees?" You might come up with a poetic answer, but not a meaningful one.

Either the universe is deterministic, and everything will occur as a direct, unchangeable product of laws (be they scientific principles or the word of God), or there is something called free will wherein individuals are able to make choices, and they may make any choice they want, and physical laws only affect their manifestation of those choices, but the choices themselves are determined by something beyond the deterministic machine. This is a false dichotomy, and the two options give the exact same results anyways.

The whole thing seems to hinge upon the outcome of a make believe scenario. God comes down from the heavens, (or a super-genius with eight billion Cray supercomputers wired together chugging all the data of the velocity and location of all matter), and says: "I have seen the future, and in my infinite wisdom, I can see that you are going to lose $50 betting on the Super Bowl this year." And then we all wonder, can you decide to not bet that money so that you will not meet the deterministic fate?

The question becomes "can an entity observe two parallel universes that are completely identical in every physical aspect (molecular location and momentum, etc), and can two different outcomes result from a non-physical difference between the two, where an individual "wills" different activity? It might as well be "can God microwave a burrito so hot, even He couldn't eat it?"

Well, when that happens, I'll start to worry about free will. In the mean time, it's pretty clear that it's a moot question, with no discernable differences in the outcomes I'm capable of perceiving.

 

[liverotcod]

Nutmeg, I really like your answer to the second of my questions. For the first, I was thinking not just of protein folding, but of all biological processes where atoms seem to just "migrate" to their expected locations in a molecule. I'm not saying this well. But I think your answer to my second question answers this as well.

As for your answer to my third question, I must say your pragmatic approach makes sense, but I still find it unsatisfying. Here's an interesting passage on scientific determinism, from here:

Modern perspectives on determinism

Scientific determinism and first cause
Since the early twentieth century when astronomer Edwin Hubble first hypothesized that red shift shows the universe is expanding, prevailing scientific opinion has been that the universe started with a Big Bang, and therefore has a finite age. Different astrophysicists hold different views about precisely how the universe originated (Cosmogony), but a consistent viewpoint is that scientific determinism has held at the macroscopic level since the universe came into being.


Determinism and generative processes
In emergentist or generative philosophy of cognitive sciences and evolutionary psychology, free will is the generation of infinite behaviour from the interaction of finite-deterministic set of rules and parameters. Thus the unpredictability of the emerging behaviour from deterministic processes leads to a perception of free will, though free will as an ontological entity does not exist.

As an illustration, the strategy board-games chess and Go have rigorous rules in which no information (such as cards' face-values) is hidden from either player and no random events (such as dice-rolling) happen within the game. Yet, chess and especially Go with its extremely simple determinstic rules, can still have an extremely large number of unpredictable moves. By analogy, emergentists or generativists suggest that the experience of free will emerges from the interaction of finite rules and deterministic parameters that generate infinite and unpredictable behaviour.

Dynamical-evolutionary psychology, cellular automata and the generative sciences, model emergent processes of social behaviour on this philosophy, showing the experience of free will as essentially a gift of ignorance or as a product of incomplete information.

I like the Go analogy in particular.

 

[Nutmeg]

Nutmeg, I really like your answer to the second of my questions. For the first, I was thinking not just of protein folding, but of all biological processes where atoms seem to just "migrate" to their expected locations in a molecule. I'm not saying this well. But I think your answer to my second question answers this as well.

As for your answer to my third question, I must say your pragmatic approach makes sense, but I still find it unsatisfying. Here's an interesting passage on scientific determinism, from here:

Modern perspectives on determinism

Scientific determinism and first cause
Since the early twentieth century when astronomer Edwin Hubble first hypothesized that red shift shows the universe is expanding, prevailing scientific opinion has been that the universe started with a Big Bang, and therefore has a finite age. Different astrophysicists hold different views about precisely how the universe originated (Cosmogony), but a consistent viewpoint is that scientific determinism has held at the macroscopic level since the universe came into being.


Determinism and generative processes
In emergentist or generative philosophy of cognitive sciences and evolutionary psychology, free will is the generation of infinite behaviour from the interaction of finite-deterministic set of rules and parameters. Thus the unpredictability of the emerging behaviour from deterministic processes leads to a perception of free will, though free will as an ontological entity does not exist.

As an illustration, the strategy board-games chess and Go have rigorous rules in which no information (such as cards' face-values) is hidden from either player and no random events (such as dice-rolling) happen within the game. Yet, chess and especially Go with its extremely simple determinstic rules, can still have an extremely large number of unpredictable moves. By analogy, emergentists or generativists suggest that the experience of free will emerges from the interaction of finite rules and deterministic parameters that generate infinite and unpredictable behaviour.

Dynamical-evolutionary psychology, cellular automata and the generative sciences, model emergent processes of social behaviour on this philosophy, showing the experience of free will as essentially a gift of ignorance or as a product of incomplete information.

I like the Go analogy in particular.

As for molecular movement, one must regard that the Universe doesn't care which atom does what. With diffusion don a chemical gradient, for instance, there really isn't a driving force for change, because there really isn't any real thing that's changing other than a statistical average. If you put 158 preschoolers on a footbal field blind folded, and you have them all in a 10' radius circle, and you tell them all to run around for five minutes, they'll end up scattered all over the field. But if you have them all scattered all over the field, and you tell them to run around, it is highly unlikely that they will end up in the circle.

But let's think about reaction at a catalyst. Let's say that all the kids are swinging wiffle ball bats wildly, and they tend to get away from one another and spread out as much as they can. If you then say that anyone who ends up in the 10' circle will immediately be removed from the field, then you will create a void around the circle. This means that there's a better chance statistically of going from being not near the circle to being near it, than there is to go from being near the circle to being not near it, for the simple reason that if you are near the cirlce, you have a chance of being in the circle soon after and getting whisked away. Kids can go from where they are to where they're not, but they can't go from where they're not to where there are already kids. Chemicals work in the same way.

My problem with the quoted bit is that it might try to refute determinism, but it doesn't refute or confirm free will. If there is randomness--as quantum mechanics strongly suggests--then that does not necessarily mean that the open space for change without direct physical causation is necessarily filled by a non-physical entity called "free will."

So, is free will the manifestation of physical law, or is free will a non-physical dynamic that influences physical phenomena? And what happens when the immovable object is struck by the unstoppable force?

 

[Caboose]

SO here's my boggle: I was on another forum and someone asked,

"I am confused over entropy in relation to equilibrium. Is the change in S of the system 0 at equilibrium? or does the change in entropy of the universe equal 0 at equlibrium? I am also unsure of the equation change in S= dqrev./ T. When does this equation apply?"

To which I responded:

Since the deltaS = S(final-initial) the CHANGE in S will be 0 @ equil.

Maximum entropy:

deltaS of the universe = 0 at equilibrium, where we all want to be.
This means that the change in both the environment and the system are… well, not changing and = 0

When you’re not at equilibrium, somebody has got to be changing so you can get there. This is when a reaction is spontaneously heading to equilibrium and it looks like this:

DeltaS universe = deltaS system + deltaS surroundings > 0


I don't know what to do with the heat added to the system over absolute T as I don't really get it. ...?
Also, I think that saying that the equation spontaniously heads to equilibrium is misleading as the entire process is measured by deltaG. Is it o.k. to say that IF it were only up to the change in entropy, it would be spontanious?
What a debacle!

 

[QofQuimica]

I don't know what to do with the heat added to the system over absolute T as I don't really get it. ...?
Also, I think that saying that the equation spontaniously heads to equilibrium is misleading as the entire process is measured by deltaG. Is it o.k. to say that IF it were only up to the change in entropy, it would be spontanious?

I am going to move this discussion to the non-MCAT science questions since this is way beyond what you need to know for MCAT-level general chemistry.

The equation deltaS = qrev/T is the definition of entropy for a reversible reaction according to the Second Law of Thermodynamics. The First Law tells us that we have two forms of energy: heat, which is not directed, and which therefore increases entropy (qrev = heat due to a reversible process) and work, which is directed, and therefore does not increase entropy (that's why work does not appear in the definition of entropy).

You are correct that spontaneity is measured by deltaG. The value of deltaS can be positive or negative for a spontaneous reaction, depending on the values of deltaH and T, the absolute temperature. I think what you said is probably ok given your caveat about assuming you can neglect the enthalpy.

 

[Nutmeg]

Why does Bernoulli not work for blood flow through our circulatory system -- is it energy loss due to vicosity, vessel wall elasticity, or something else?

Which version of the Bernoulli Equation are you talking about? In mass transfer, we used something called "the Engineering Bernoulli," which was big, ugly, and accounted for all sorts of crazy crap. However, I do know that blood flow must take into account viscous losses, and the viscosity is related to the sheer rate (ie mu =/= constant).

Here's one link I managed to find: http://advan.physiology.org/cgi/content/full/25/1/44

I'm also thinking that there may be a problem with the irrotational requirement for the Bernoulli equation. Since blood contains particles with much higher rotaional inertia than the average molecule, there's a good chance that there would be a need to consider curl, which even the Engineering Bernoulli doesn't do.

 

[Nutmeg]

Alright, question for shrike or some other physics pro out there:

I was adding water to a tank on a scale, and I added the water at an angle so that the water was spinning furiously inside the tank. The dude I was working with asked if that wouldn't effect the weight, becuase the added rotational inertia. I said no, it wouldn't because gravitation effects mass and it doesn't increase the mass. Besides that, by experience I know that the weight shown on the scale doesn't drop when the water stops spinning, although the spinning water definitely makes the tank harder to push.

But I got to thinking about this. The force of gravity is proportional to the mass of the object, and the acceleration of the object is a equally proportional to the mass. In my physics class, I remember this being stated as the reason why two objects of different masses fall at the same rate: they experience different forces that differ in proportion to the degree to which the forces make the masses accelerate. Hence, they accelerate at the same rate because the mass of the falling object cancels out of the equation.

But what about two objects of the same mass, one spinning, one not? They both have the same gravitational attraction, but one has more inertia. Yet I would expect that they would fall at the same rate, wouldn't they?

What gives???

 

[GCT]

But what about two objects of the same mass, one spinning, one not? They both have the same gravitational attraction, but one has more inertia. Yet I would expect that they would fall at the same rate, wouldn't they?

It'll be best to consider this in light of relativity, if you're referring to a situation in a vacuum then I would imagine that they would fall with the same accerelation. Einstein thought about similar situations, except that he eventually figured that space is curved from such propositions

 

[N1DERL&]

Weird question.... warning!

Why is it that when you're eating wasabi peas and you bite into that really potent one, fire seems to shoot up through your nose and shoot out flames?? Physiologically, what is going on? Curoius minds, want to know..!

 

[novawildcat]

How is this possible????? How can .99999.........=1.0000000......?????


Don't believe me?

let x=0.9999999............

10x=9.9999999999..........
-x=-0.9999999999.........
--------------------------
9x=9.000000000000000.....


x=1, therefore 1=0.9999999................

 

[Nutmeg]

That's one way to look at it. Another is to say that

(1/3) = 0.333333333

3 * 0.333333333 = 0.99999999999
and yet
3 * (1/3) = 1.0000000000000

Or, if you want something more formal, I found this:

http://mathforum.org/dr.math/faq/faq.0.9999.html

 

[novawildcat]

How about this one. Draw a right triangle with sides of length 1 and hypotenuse square root of 2. Obviously the length of the hypotenuse is greater than the length of each side. however, draw a straight line from the hypotenuse perpendicular to the side opposite the hypotenuse. Do this as many times as you wish. What you effectively have done is made a function from the set of all points making up the hypotenuse to the set of all points making up the side of length 1. How is this significant? You just matched two infinite sets with each other and showed that they were the same size. There are as many points in the interval [0,1] as there are many points in the interval [0,2]! Try this, try to think of a way to match all the integers {......-2,-1,0,1,2.......} to the infinite infinte amount of points in the interval [0,1]. Once you have exhausted yourself, you will realize it can't be done! There are obviously an infinite amount of integers, as well as an infinite amount of numbers between 0 and 1, so why can't it be done? here's why--there are actually different sizes of infinity! in fact, there are an infinite amount of different sizes of inifinty!

 

[Nutmeg]

How about this one. Draw a right triangle with sides of length 1 and hypotenuse square root of 2. Obviously the length of the hypotenuse is greater than the length of each side. however, draw a straight line from the hypotenuse perpendicular to the side opposite the hypotenuse. Do this as many times as you wish. What you effectively have done is made a function from the set of all points making up the hypotenuse to the set of all points making up the side of length 1. How is this significant? You just matched two infinite sets with each other and showed that they were the same size. There are as many points in the interval [0,1] as there are many points in the interval [0,2]! Try this, try to think of a way to match all the integers {......-2,-1,0,1,2.......} to the infinite infinte amount of points in the interval [0,1]. Once you have exhausted yourself, you will realize it can't be done! There are obviously an infinite amount of integers, as well as an infinite amount of numbers between 0 and 1, so why can't it be done? here's why--there are actually different sizes of infinity! in fact, there are an infinite amount of different sizes of inifinty!

My Calc I professor referred to two different types of infinity: the "countably infinite" (like the number of positive intigers) and the "incountably infinite" (like the number of irrational numbers between 0 and 1). The proof he gave for the fact that there are more numbers between 0 and 1 than there are positive intgers is this: For any positive integer you can name, there is a number between 0 and 1 that looks like that same integer with a decimal point in front of it. That's one-to-one. But for each of those numbers, you could put an infinite number of zeros between the decimal and the first digit.

 

[Caboose]

Howdy SuperTroopers,

There I lay last night, "WHY?!" my mind screamed out in the darkness. Why is the work created by friction = (Fd cos theta)?

I tried to derive it from the opposite side, (created by F cos theta), but then I ended up with W= Fd as W=d(d) which I think is going in the wrong direction anyway. Actually, I'm a ninny - none of the above makes sense.

So, I need to know how you get the force acting on the adjacent side, (friction), when I have the hypotenuse F, the adjacent d and the angle between. How is such an equation made?

Caboose.

 

[Caboose]

THEN...

I was looking at viruses. I'm convinced they are man-made disease carriers from the future to control populations designed by someone who played a lot of virtual Dungeons and Dragons. What odd little packages they are.

So, some viruses contain enzymes to digest the host cell membrane and all contain either DNA or RNA. Their capsid is a mere shell of love that is sometimes cuddled by a lipid rich envelope. Why is it so difficult for us to "kill" the things? Endospores must engulf themselves in layers and layers of membrane in order to remain dormant for hundreds of years, but viruses can withstand ALL SORTS of torture and happily go about their business with one layer.

If, somehow, one removes the harmful DNA from these capsids, the spike proteins on the empty virus can be used in vaccines. How are these proteins not denatured under harsh conditions?
(Is AIDS a rapidly mutating virus being that they can't make a vaccine?)

Just curious.
Caboose.

 

[Willow]

Why is it so difficult for us to "kill" the things? Endospores must engulf themselves in layers and layers of membrane in order to remain dormant for hundreds of years, but viruses can withstand ALL SORTS of torture and happily go about their business with one layer.

Do they though? Viruses are generally pretty easy to kill outside the body, for the short time they survive. What makes them so tough is that we are trying to kill something that's residing in our own cells, so we have to selectively kill them without killing ourselves.

When we do find a way to get them (e.g. reverse transcriptase inhibitors) they, being such rapidly mutating little buggers, quickly evolve a new version of whatever's been targeted.

(Is AIDS a rapidly mutating virus being that they can't make a vaccine?)

Yes. Crazy fast.

How are these proteins not denatured under harsh conditions?

You mean in vaccine prep? It may be the conditions are optimised so that the protiens stay intact or in some cases they may be so simple that they'll refold properly after denaturation. More than likely, the proteins are totally denatured but it doesn't matter. Antibodies are created to epitopes (small regions of antigenicity) not necesarily the entire protein. Indeed, some vaccines are peptides -- just small portions of the antigenic proteins.

 

[novawildcat]

Howdy SuperTroopers,

There I lay last night, "WHY?!" my mind screamed out in the darkness. Why is the work created by friction = (Fd cos theta)?

I tried to derive it from the opposite side, (created by F cos theta), but then I ended up with W= Fd as W=d(d) which I think is going in the wrong direction anyway. Actually, I'm a ninny - none of the above makes sense.

So, I need to know how you get the force acting on the adjacent side, (friction), when I have the hypotenuse F, the adjacent d and the angle between. How is such an equation made?

Caboose.

I'm not quite clear on what you are asking. Anyway, work is defined as the force vector dotted into the directional vector. Mathematically the dot product of two vectors is defined as the magnitude of the 1st vector times teh magnitude of the 2nd vector times the cosine of the angle between them.

 

[Caboose]

Do they though? Viruses are generally pretty easy to kill outside the body, for the short time they survive. What makes them so tough is that we are trying to kill something that's residing in our own cells, so we have to selectively kill them without killing ourselves.

When we do find a way to get them (e.g. reverse transcriptase inhibitors) they, being such rapidly mutating little buggers, quickly evolve a new version of whatever's been targeted.

Yes. Crazy fast.

You mean in vaccine prep? It may be the conditions are optimised so that the protiens stay intact or in some cases they may be so simple that they'll refold properly after denaturation. More than likely, the proteins are totally denatured but it doesn't matter. Antibodies are created to epitopes (small regions of antigenicity) not necesarily the entire protein. Indeed, some vaccines are peptides -- just small portions of the antigenic proteins.

Quick and helpful - very nice! Thanks!

Novawildcat - indeed, that is the product of Caboose chasing her tail. I don't understand where the force of friction comes from - how it is derived from the vectors. I'm not very up on the physics yet, so perhaps I can study a little and add onto this question to make it make sense.
Thanks for the dot product info, perhaps I can investigate...
Caboose.

 

[novawildcat]

The force due to friction can only be determined experimentally. F=uN where u=coefficient of friction and N is the normal force (this is the definition of friction). u is determined through experimentation. If we think about this in vector terms your have the force vector due to friction acting in one direction and the object moving in some other direction s (which is the displacement vector). By definition the work done by friction would be the magnitude of F times the magnitude of s times the cosine of the angle between those vectors.

 

[mercaptovizadeh]

Alright, question for shrike or some other physics pro out there:

I was adding water to a tank on a scale, and I added the water at an angle so that the water was spinning furiously inside the tank. The dude I was working with asked if that wouldn't effect the weight, becuase the added rotational inertia. I said no, it wouldn't because gravitation effects mass and it doesn't increase the mass. Besides that, by experience I know that the weight shown on the scale doesn't drop when the water stops spinning, although the spinning water definitely makes the tank harder to push.

But I got to thinking about this. The force of gravity is proportional to the mass of the object, and the acceleration of the object is a equally proportional to the mass. In my physics class, I remember this being stated as the reason why two objects of different masses fall at the same rate: they experience different forces that differ in proportion to the degree to which the forces make the masses accelerate. Hence, they accelerate at the same rate because the mass of the falling object cancels out of the equation.

But what about two objects of the same mass, one spinning, one not? They both have the same gravitational attraction, but one has more inertia. Yet I would expect that they would fall at the same rate, wouldn't they?

What gives???

Someone correct me if I am wrong, but I think that there are higher order terms in relativistic gravitational that are due to things like rotation.

 

[Caboose]

I was watching this neurosurgery where the surgeon broke through the dura and cerebrospinal fluid started to pump out. What drives this? If there is such a force driving the circulation of the CB fluid, why is it necessary for ciliated ependymal cells?

Simple?

Caboose.

 

[Caboose]

no love for neuro.
Caboose.

 

[QofQuimica]

no love for neuro.
Caboose.

Sorry, caboose, I don't know the answer to this one. Maybe you could ask in the surgery or neurology residency forums?

 

[Caboose]

Sorry, caboose, I don't know the answer to this one. Maybe you could ask in the surgery or neurology residency forums?

Oooh! Thanks for answering. I thought the thread might have died and no one told me. I never thought to venture over there.

I got another one, wanna try?
It was dark and sultry... it was the jejunum.
The triglycerides were ripped apart violently in the lumen by the evil Lipase. The monoglycerides and fatty acids huddled together in a small micelle, emulsified in bile. I don't know if you've ever been emulsified in bile, but it's not pleasant and the fats raced to the membrane to save themselves. They diffused through the nonpolar bilayer and popped into the enterocyte who immediately put them back together into tryglycerides and shipped them off to the s.ER, to the Golgi and eventually spit them out into the interstitial space as chylomicrons.

So... why does lipase have to break up triglycerides into monoglycerides for shipment into the cell where the reverse reaction will take place immediately anyway? Why are triglycerides not able to form micelles and diffuse as they are? Are the triglycerides modified in the enterocyte so that they are different from those still in the lumen?
What did you eat for lunch?

Caboose.

 

[Caboose]

Sorry, caboose, I don't know the answer to this one. Maybe you could ask in the surgery or neurology residency forums?

Look what Nate gave me!

CSF is made mainly by secretary cells in the choroid plexus within the 3rd ventricle of the brain (deep above the spinal cord). From there it travels down the aqueduct of Sylvius (great vacation spot ) and down the central canal of the spinal cord and out holes into the subarachnoid space. It then is able to pass through one-way valves into venous sinuses where it is removed into circulation. So, there is a constant formation and removal of CSF that creates a flow. The fluid also protects the brain from the pulsing blood vessels around it which would squish the brain with each systole. Kind of like a hydraulic shock absorber. This is probably what creates the "pulsing" ejection of fluid in a wound.
Anyway, that is my interpretation and I hope that is all correct since I have an exam on it in a week.
Peachy keen!!
Caboose.

 

[QofQuimica]

lol, no, the forum is still going. But I've never studied neurosurgery, so I wouldn't even know where to start with that other one. BTW, there is a separate neurosurgery residency forum. You might also try asking in the MSTP forum if the surgeons and neurologists can't help you.

Your new question is interesting (and if nothing else, you can always become a mystery writer if you change your mind about med school!). I can answer part of it: triglycerides aren't polar like glycerol is, and they don't have a charged polar end like their fatty acid precursors do. So triglycerides can't form micelles. But as to why the triglycerides don't just diffuse across the membrane on their own, I don't know. Ditto about whether the enterocytes modify them in some way when re-assembling them. Any of you cell bio people around?

P.S. It's only 11AM, so I didn't eat lunch yet. But when I do, I plan to have some chocolate cake. It's chock-full of triglycerides, which will be dismembered, and I won't feel the least bit sorry about it.

 

[QofQuimica]

Look what Nate gave me!

CSF is made mainly by secretary cells in the choroid plexus within the 3rd ventricle of the brain (deep above the spinal cord). From there it travels down the aqueduct of Sylvius (great vacation spot ) and down the central canal of the spinal cord and out holes into the subarachnoid space. It then is able to pass through one-way valves into venous sinuses where it is removed into circulation. So, there is a constant formation and removal of CSF that creates a flow. The fluid also protects the brain from the pulsing blood vessels around it which would squish the brain with each systole. Kind of like a hydraulic shock absorber. This is probably what creates the "pulsing" ejection of fluid in a wound.
Anyway, that is my interpretation and I hope that is all correct since I have an exam on it in a week.
Peachy keen!!
Caboose.

cool. Thanks for sharing.

 

[Caboose]

P.S. It's only 11AM, so I didn't eat lunch yet. But when I do, I plan to have some chocolate cake. It's chock-full of triglycerides, which will be dismembered, and I won't feel the least bit sorry about it.

Poor little guys probably never know what hit 'em.
Evidently, triglycerides would have to be transported via proteins which would be about as easy as birthing.

I'll be back soon.
Thank you so much for your help!!!
Caboose.

 

[Lindyhopper]

It seems as if gene therapy is again advancing inti clinical trials. Does anyone have any experience/info on adenoviruses & why they seem to be a promising transfer vector.

 

[PneoDr]

Tonight I was talking with a friend. Long story short: I said that when an iceberg melts, the water level will be the same before and after. He said he dont buy it because most of the iceberg is below the surface of the water and solid water takes up more volume than liquid water. He thinks that water levels will decrease. I couldnt figure out a good way to explain to him why this is not true. It is just something I THOUGHT i always knew. After searching the internet, I have found many scientists who say water levels would increase, some who say it would decrease, and others who say it would stay the same. Explain it with ice in a glass. I still think water level will stay the same. Using density of water solid and liquid i get that 90 percent of the ice will be submerged and 10% above water line. However, liquid water is 10% more dense than solid water. I want to say that the 10% of ice above the water level compensates to exactly equal the 10% difference in volumes of liquid and solid water. Am I wrong??? I am starting to wonder, and I may be wrong. I just want to understand and if im right to be able to explain it better.

 

[Nutmeg]

Tonight I was talking with a friend. Long story short: I said that when an iceberg melts, the water level will be the same before and after. He said he dont buy it because most of the iceberg is below the surface of the water and solid water takes up more volume than liquid water. He thinks that water levels will decrease. I couldnt figure out a good way to explain to him why this is not true. It is just something I THOUGHT i always knew. After searching the internet, I have found many scientists who say water levels would increase, some who say it would decrease, and others who say it would stay the same. Explain it with ice in a glass. I still think water level will stay the same. Using density of water solid and liquid i get that 90 percent of the ice will be submerged and 10% above water line. However, liquid water is 10% more dense than solid water. I want to say that the 10% of ice above the water level compensates to exactly equal the 10% difference in volumes of liquid and solid water. Am I wrong??? I am starting to wonder, and I may be wrong. I just want to understand and if im right to be able to explain it better.

No, you're exactly right. When something floats, it displaces a volume of water which is the volume of water that would be needed to get a mass equal to the floating object. If an icebrg is floating, there should be no effect on the volume of the water level after it melts, apart from a teeny tiny difference in the volume of water at 0 C vs 5 C. But assuming that you've got ice water at 0 degrees in equilibrium with floating ice, and if just enough energy was added to the system to make the ice melt while keeping he temperature at 0 degrees, then the water level should remain unchanged.

 

[Nutmeg]

Biology question not related to the MCAT..........

Why does milk calm the stomach down when you have an upset stomach??? Does it have something that is basic in it to counter the acidity???

Milk is slightly acidic, but the calcium in milk acts as a buffer. That brings up stomach pH rapidly when you drink it.

It has been suggested that this buffering effect, while it helps the short-term effects of heartburn, is bad in the long-term because the stomach should be acidic and need sto be for proper digestion. However, the site that says that is sponsored by Nexium, so I'd take that with a grain of salt (washed down with a big glass of milk, of course). I know I drink a lot of milk, but I only seem to have acid problems only when I have stress problems, and I tend to drink milk primarily with foods that are highly acidic--for instance when eating lots o' tomatoes.

 

[Fermata]

Milk is slightly acidic, but the calcium in milk acts as a buffer. That brings up stomach pH rapidly when you drink it.

It has been suggested that this buffering effect, while it helps the short-term effects of heartburn, is bad in the long-term because the stomach should be acidic and need sto be for proper digestion. However, the site that says that is sponsored by Nexium, so I'd take that with a grain of salt (washed down with a big glass of milk, of course). I know I drink a lot of milk, but I only seem to have acid problems only when I have stress problems, and I tend to drink milk primarily with foods that are highly acidic--for instance when eating lots o' tomatoes.

I'd be careful.

It's not a wise idea to mix something highly acidic(like orange juice) with milk.

Instant curdling.

 

[Pharmwannab]

This is borderline MCAT related/non MCAT related heh. What would be the geometry of an sp7 hybridized atom, assuming no lone pairs?

 

[QofQuimica]

This is borderline MCAT related/non MCAT related heh. What would be the geometry of an sp7 hybridized atom, assuming no lone pairs?

That has to be a typo. There are only three p-orbitals in an atom for a given principle quantum level. Beyond that, to exceed the octet, the atom will use d-orbitals if it has any available.

 

[QofQuimica]

bump

 

[NapeSpikes]

I have a question. I'm taking a Genetics night class and we are studying chromosomal alterations, specifically uniparental disomy.

Why does non-disjunction in meiosis I create a gamete with two homologous non-identical chromosomes, while non-disjunction in meiosis II creates a gamete with identical ones? It seems like both would create a gamete with identical homologous chromosomes. What am I missing here about meiosis?

I understand that these scenarios both result in UPD, the first in heterodisomy and the second case in isodisomy, but I don't understand why the nonidentical/identical chromosome situations arise in the first place.