The Official 3/23/13 MCAT Thread

[Redpancreas]

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Figured I'd start this one out. I'll be registering for the MCAT first thing tomorrow. Who else is taking it on this date? How do you all plan on studying over the school year?

 

[gettheleadout]

Actin filaments are involved in exocytosis so I don't know if you could say they're less important than microtubules in NT release.

Also for 1 I can't picture the setup you're describing. Are you saying an isotonic solution would draw the contents out of your micropipette? Ones I have used keep their solution in due to a difference in pressures on either side of the liquid and wouldn't be affected by osmolarity, but you may be describing something different. If the sol'n in the pipette is more elctrolytic than the cytosol than it would have to be hypertonic, but I don't know if that would be necessary for the setup you described.

Well, if you want to get that specific about the filaments, the synapsins and other proteins that hold the vesicles in places interact with both microtubules and actin filaments in the cytoskeleton. But consider you assume the role of actin filaments in exocytosis to be more important. Lack of actin filaments would then result in no synaptic transmission at all, which would simply be inactivity and not fit the definition of neural fatigue. Further, if this is something which is interpretable at this level of detail, then the question set up (with B and C being equivalent and thus both false) tells you which way to interpret it.

I'm talking about whole-cell patch-clamp (http://en.wikipedia.org/wiki/Patch_clamp#Whole-cell_recording_or_whole-cell_patch). I suppose my explanation for a hypotonic case is off because of the applied suction (forgot about that) but either way the solution should be isotonic. I'm not saying that in an isotonic case the solution would be drawn out of the pipette, but that it would be stable when contiguous with the cytosol. If it were hypotonic, acknowledging the applied suction, it would actually result in diffusion of cellular contents into the pipette tip, which is not ideal.

 

[slz1900]

Well, if you want to get that specific about the filaments, the synapsins and other proteins that hold the vesicles in places interact with both microtubules and actin filaments in the cytoskeleton. But consider you assume the role of actin filaments in exocytosis to be more important. Lack of actin filaments would then result in no synaptic transmission at all, which would simply be inactivity and not fit the definition of neural fatigue. Further, if this is something which is interpretable at this level of detail, then the question set up (with B and C being equivalent and thus both false) tells you which way to interpret it.

I'm talking about whole-cell patch-clamp (http://en.wikipedia.org/wiki/Patch_clamp#Whole-cell_recording_or_whole-cell_patch). I suppose my explanation for a hypotonic case is off because of the applied suction (forgot about that) but either way the solution should be isotonic. I'm not saying that in an isotonic case the solution would be drawn out of the pipette, but that it would be stable when contiguous with the cytosol. If it were hypotonic, acknowledging the applied suction, it would actually result in diffusion of cellular contents into the pipette tip, which is not ideal.

Sorry I meant ENDOcytosis with respect to fatigue 😳 Since endocytotic uptake of nts is important for preventing fatigue and actin is the main player in forming those vesicles I just don't think you can rank microtubules vs actin in terms of importance in the overall process.

As for the experiment, that makes more sense after the article. Thanks

 

[gettheleadout]

GLUT4 expression is decreased in adipocytes in nearly all insulin resistant states in humans and rodents, but the mechanism by which this contributes to systemic insulin resistance has not been clear. Researchers hypothesize that elevated serum RBP4 might be a mechanistic link by which downregulation of GLUT4 in adipocytes contributes to the development or worsening of systemic insulin resistance. The results of this study show that RBP4 elevation is a widespread abnormality in insulin-resistant states of various etiologies. Serum levels and/or urinary excretion of RBP4 have been reported to be elevated in humans with type 2 diabetes.

In the figure (attached), different groups of mice were injected with insulin to test the Rbp4’s effect on insulin activity in muscle cells and liver cells.

(I had a hard time trying to squeeze the figure into the SDN attachment dimensions so it's pretty small but you can zoom in hopefully)

QUESTIONS:
1. After a large meal, which of the following hormones would be expected to be in high levels in the blood?
a. Glucagon
b. Cortisol
c. Insulin
d. Epinephrine

2. Which of the following is a likely consequence of someone with low insulin sensitivity?
a. Increased secretion of glucagon
b. Increased urinary output
c. Stomach ulcers
d. Increased blood pH

3. Based on figure 1, which of the following is true?
a. Rbp4 has a causal role in insulin resistance in hepatocytes only
b. Rbp4 has a causal role in insulin resistance in muscle cells only
c. Rbp4 has a causal role in both hepatocytes and muscle cells
d. Rbp4 does not have a causal role in either hepatocytes or muscle cells.

4. Rosiglitazone is an anti-diabetic agent that improves insulin sensitivity. Rosiglitazone’s effect is most likely seen in:
a. Liver cells
b. Muscle cells
c. Both liver and muscle cells
d. Neither liver nor muscle cells

5. Based on the information in the passage, what is the expected level of Rbp4 mRNA in liver cells of Glut4-/- mice, compared to a wild type control group of mice?
a. Greater Rbp4 mRNA level than control
b. Smaller Rbp4 mRNA level than control
c. Same Rbp4 mRNA level as control

Have fun 🙂

My answers in white below:

1. C
2. D
3. B
4. B
5. A

 

[gettheleadout]

Sorry I meant ENDOcytosis with respect to fatigue 😳 Since endocytotic uptake of nts is important for preventing fatigue and actin is the main player in forming those vesicles I just don't think you can rank microtubules vs actin in terms of importance in the overall process.

As for the experiment, that makes more sense after the article. Thanks

Ah, I see. I actually thought about mentioning reuptake of neurotransmitters as a factor in fatigue in the answer explanation, but I thought about synapses where cleft enzymes degrade a portion of what's released and considered it would make more sense to focus on the axonal transport role.

That's a really good point though, reuptake does play a large role in fatigue, and I didn't consider the actin fibers' participation in that.

Nonetheless, I think if they could be ranked, even if this information weren't expected of us to be known, the question format implies the correct "ranking."

In any case, yours is a good thought for everyone to consider material-wise. If a question of this nature were on the MCAT, looking in the choices for a clue would be a good strategy.

 

[Entadus]

GLUT4 expression is decreased in adipocytes in nearly all insulin resistant states in humans and rodents, but the mechanism by which this contributes to systemic insulin resistance has not been clear. Researchers hypothesize that elevated serum RBP4 might be a mechanistic link by which downregulation of GLUT4 in adipocytes contributes to the development or worsening of systemic insulin resistance. The results of this study show that RBP4 elevation is a widespread abnormality in insulin-resistant states of various etiologies. Serum levels and/or urinary excretion of RBP4 have been reported to be elevated in humans with type 2 diabetes.

In the figure (attached), different groups of mice were injected with insulin to test the Rbp4's effect on insulin activity in muscle cells and liver cells.

(I had a hard time trying to squeeze the figure into the SDN attachment dimensions so it's pretty small but you can zoom in hopefully)

QUESTIONS:
1. After a large meal, which of the following hormones would be expected to be in high levels in the blood?
a. Glucagon
b. Cortisol
c. Insulin
d. Epinephrine

2. Which of the following is a likely consequence of someone with low insulin sensitivity?
a. Increased secretion of glucagon
b. Increased urinary output
c. Stomach ulcers
d. Increased blood pH

3. Based on figure 1, which of the following is true?
a. Rbp4 has a causal role in insulin resistance in hepatocytes only
b. Rbp4 has a causal role in insulin resistance in muscle cells only
c. Rbp4 has a causal role in both hepatocytes and muscle cells
d. Rbp4 does not have a causal role in either hepatocytes or muscle cells.

4. Rosiglitazone is an anti-diabetic agent that improves insulin sensitivity. Rosiglitazone's effect is most likely seen in:
a. Liver cells
b. Muscle cells
c. Both liver and muscle cells
d. Neither liver nor muscle cells

5. Based on the information in the passage, what is the expected level of Rbp4 mRNA in liver cells of Glut4-/- mice, compared to a wild type control group of mice?
a. Greater Rbp4 mRNA level than control
b. Smaller Rbp4 mRNA level than control
c. Same Rbp4 mRNA level as control

Have fun 🙂

1. C - In the fed state, the pancreas releases insulin to facilitate uptake of glucose by the cells.
2. B - Low insulin sensitivity will decrease glucose uptake by the cells. This will increase serum glucose concentration, thereby increasing urinary output to compensate.
3. B - This is a guess - not sure what RBP4-Tg is supposed to mean
4. C - Insulin is active in both muscle and liver tissues. The question does not mention that Rosiglitazone acts on RBP4 or Glut4, only that it improves insulin sensitivity
5. C - The passage implies that RBP4 elevation leads to down-regulation of Glut4, NOT that lack of Glut4 expression will lead to elevated RBP4

 

[gettheleadout]

Tg means transgenic, but it doesn't matter because we're not concerned with graphs A and C.

 

[osprey099]

I will post the answers with explanations tomorrow around noon so more people get the opportunity to answer it since it's relatively late tonight.

 

[gettheleadout]

I will post the answers with explanations tomorrow around noon so more people get the opportunity to answer it since it's relatively late tonight.

Very good. 👍

 

[NuttyEngDude]

ugg i suck at bio, lol:

my osprey passage answers:
1. c
2. b
3. d
4. c
5. c (glut4 is a symptom not a cause)

 

[gettheleadout]

Man I feel like an idiot now haha... Oh well, I've never done an experimental passage before, so I now see the mindset with which I need to approach such problems.

 

[NuttyEngDude]

Man I feel like an idiot now haha... Oh well, I've never done an experimental passage before, so I now see the mindset with which I need to approach such problems.

are you assuming your answers are wrong?

 

[gettheleadout]

are you assuming your answers are wrong?

Basically yes, I just agree more with Entadus' reasoning than my own (which I now think was simply too shallow, I didn't approach the questions with the precision needed for these sorts of experimental passages. But, I'm glad I see that now.)

Edit: But like I said, this only makes me feel better because no doubt much of MCAT prep is practice, and I'd rather be exposed to this now. I'm also glad I have the TRPH Science Workbook, which supposedly contains a lot of experimental passages like this (some very difficult), and I'm hopeful it will be excellent help.

 

[Doctor Dapper]

Just bombed a physics practice quiz... Blah. My retinas have had enough for today. Good night.

 

[Entadus]

Basically yes, I just agree more with Entadus' reasoning than my own (which I now think was simply too shallow, I didn't approach the questions with the precision needed for these sorts of experimental passages. But, I'm glad I see that now.)

Edit: But like I said, this only makes me feel better because no doubt much of MCAT prep is practice, and I'd rather be exposed to this now. I'm also glad I have the TRPH Science Workbook, which supposedly contains a lot of experimental passages like this (some very difficult), and I'm hopeful it will be excellent help.

I've been doing the TRPH SW bio passages, they're great 🙂 I have a ton left still, I'm hoping the practice will count come test day.

Just bombed a physics practice quiz... Blah. My retinas have had enough for today. Good night.

Night 😴 Review it in the morning

 

[NuttyEngDude]

doing work for classes now (oh yeah i have those 😛 )

i'll write a physical science passage tomorrow, i wouldnt do a bio passage justice if i tried, lol.

 

[Captain Sisko]

doing work for classes now (oh yeah i have those 😛 )

i'll write a physical science passage tomorrow, i wouldnt do a bio passage justice if i tried, lol.

Looking forward to it. I might take on a ps myself, maybe bring in some of my day job 🙂

 

[osprey099]

ANSWERS & EXPLANATIONS:

1. After a large meal, which of the following hormones would be expected to be in high levels in the blood?
a. Glucagon
b. Cortisol
c. Insulin
d. Epinephrine

Insulin is right because after a large meal, your blood will have an elevated level of glucose and will need insulin for cells to uptake that glucose. Glucagon and cortisol both elevate blood glucose so A and B are wrong. Epinephrine release is the result of sympathetic nerve stimulation, which is not the case here since after a large meal, your body is in the "rest & digest" state rather than the "fight or flight" state.

2. Which of the following is a likely consequence of someone with low insulin sensitivity?
a. Increased secretion of glucagon
b. Increased urinary output
c. Stomach ulcers
d. Increased blood pH

Someone with low insulin sensitivity (type 2 diabetes) will have a high leve of blood glucose, resulting in more water to diffuse from the interstitial space into blood vessels to balance the high osmotic pressure in the blood. This results in a higher glomerular filtration rate and ultimately, an increased amount of urine. Glucagon will even FURTHER increase blood glucose so A is wrong. Stomach ulcers have nothing to do with insulin insensitivity so C is wrong. Increased blood pH is not a result of insulin insensitivity either, so D is wrong.

3. Based on figure 1, which of the following is true?
a. Rbp4 has a causal role in insulin resistance in hepatocytes only
b. Rbp4 has a causal role in insulin resistance in muscle cells only
c. Rbp4 has a causal role in both hepatocytes and muscle cells
d. Rbp4 does not have a causal role in either hepatocytes or muscle cells.

Figure 1b shows that following injection of insulin, Rbp4+/- (heterozygotic mice) and Rbp4-/- (homozygous mice) result in an equal increase in insulin mediated activity, compared to the control group of mice (the wild type). Also, figure 1a shows that Rbp4-TG (transgenic = rbp4 is over-expressed) mice have lowered insulin mediated activity following insulin injection, compared to WT mice (WT mice have regular Rbp4 levels). These 2 graphs support the finding that Rbp4 serves some sort of an inhibitory role in insulin activity in muscle cells. Now we have to look at figures 1c and 1d, the same experiments as 1a and 1b but done in hepatocytes of the liver. As you can see from 1c, Rbp4-Tg and WT mice show no significant difference in insulin activity following insulin injection. Also, WT, Rbp4+/-, and Rbp4-/- all show almost the same amount of insulin activity following insulin injection. This implies that Rbp4 has an unsignificant effect in liver cells.

4. Rosiglitazone is an anti-diabetic agent that improves insulin sensitivity. Rosiglitazone’s effect is most likely seen in:
a. Liver cells
b. Muscle cells
c. Both liver and muscle cells
d. Neither liver nor muscle cells

See explanation for number 3. If Rbp4 has inhibitory effects in muscle cells only, then any drug targeted towards the downstream effects of Rbp4 will have have effect in muscle cells only.

5. Based on the information in the passage, what is the expected level of Rbp4 mRNA in liver cells of Glut4-/- mice, compared to a wild type control group of mice?
a. Greater Rbp4 mRNA level than control
b. Smaller Rbp4 mRNA level than control
c. Same Rbp4 mRNA level as control

The question asks what is the expected level of Rbp4-mRNA in LIVER cells of Glut4-/- mice. The passage states that Glut4 is downstream of Rbp4. From figures 1c and 1d, we concluded that Rbp4 does not play a significant role in the insulin sensitivity of liver cells. Thus, the presence or absence of Glut4 does not affect the hepatocytes' ability to uptake glucose. Thus, the same level of Rbp4 mRNA will be expected in liver cells of Glut4-/- mice as in liver cells of Glut4+/+ (control) mice.


Takeaways:
You should know some common experiment jargon. If you are not a big fan of research, like me, you should still be able to read through these passages with relative ease. Here's some stuff that will help you read through future experimental passages:
* Whenever there is a -/- following a gene name, like rbp4-/-, this means that the gene is knocked out (aka absent from the cells). On the other hand, rbp4-TG means that the gene is transgenic, which basically means that the gene is over-expressed. WT refers to normal levels of the gene. This is common to ALL research studies.
* Know what is downstream of what. In the above passage, Glut4 is downstream of Rbp4. That means something that happens to Rbp4 will probably affect Glut4, unless the researchers demonstrate otherwise.
* Draw maps. The above passage did not require you to do this but if you see a passage with 3-4 different genes or proteins downstream of this and that, take a good amount of time to draw a very coherent map because this will save you so much more time on the questions.

Hope this helps 🙂

 

[slz1900]

Insulin insensitivity increases protein metabolism which could cause alkalosis and also can prevent ulcers from healing, so 2 has a few right answers imo.

 

[Entadus]

Insulin insensitivity increases protein metabolism which could cause alkalosis and also can prevent ulcers from healing, so 2 has a few right answers imo.

Insulin insensitivity would more likely cause acidosis (for example diabetic ketoacidosis, as the body begins to burn other fuels besides glucose since the cells cannot uptake glucose. The production of ketone bodies will thereby lower pH)

Thanks for the passage and answers, Osprey 🙂 I disagree with #4 since the drug you mentioned does not necessarily target anything in the RBP4 pathway... B would only be true if that was the only protein affecting insulin sensitivity. Surely there must be some other pathway involved that could potentially affect the liver as well? For example, what if the drug's mechanism of action involves the up-regulation of insulin receptors or glucose transporters?

 

[slz1900]

Insulin insensitivity would more likely cause acidosis (for example diabetic ketoacidosis, as the body begins to burn other fuels besides glucose since the cells cannot uptake glucose. The production of ketone bodies will thereby lower pH)

Thanks for the passage and answers, Osprey 🙂 I disagree with #4 since the drug you mentioned does not necessarily target anything in the RBP4 pathway... B would only be true if that was the only protein affecting insulin sensitivity. Surely there must be some other pathway involved that could potentially affect the liver as well? For example, what if the drug's mechanism of action involves the up-regulation of insulin receptors or glucose transporters?

Gah you are right about protein degradation causing acidosis. I always get it mixed up since urea is basic. I stand by ulcers being an answer as well though, since diabetic patients can get peptic ulcers.

 

[osprey099]

Insulin insensitivity would more likely cause acidosis (for example diabetic ketoacidosis, as the body begins to burn other fuels besides glucose since the cells cannot uptake glucose. The production of ketone bodies will thereby lower pH)

Thanks for the passage and answers, Osprey 🙂 I disagree with #4 since the drug you mentioned does not necessarily target anything in the RBP4 pathway... B would only be true if that was the only protein affecting insulin sensitivity. Surely there must be some other pathway involved that could potentially affect the liver as well? For example, what if the drug's mechanism of action involves the up-regulation of insulin receptors or glucose transporters?

Hmm you're right. I should have stated in my question that it is specific to Rbp4

 

[Captain Sisko]

Here's a ps passage. Enjoy!

Modern aircraft are powered by jet engines, which draw in atmospheric air at a lower velocity and expel it behind them at a higher velocity, creating thrust. Early jet powered aircraft (1930's to 1960's) were powered mostly by turbojets, while later aircraft were and are powered by turbofans. The difference between a turbojet and a turbofan is that while a turbojet has one inlet and one exit, a turbofan has one inlet and two exits, where some of the ingested air is drawn into its turbojet-like "core" while the majority is sent around the core and through the "bypass." The bypass air is worked upon by a large diameter fan and then expelled through an outer exhaust nozzle that is coaxial with the inner exhaust nozzle, which expels core air.

Thrust T, which is a force, is generally defined as T=W*V, where W is the mass flow rate of air and V is the velocity of the air, in m/s. However, there are other forces that act upon a jet engine. One must consider the air entering the engine as a force pushing against the forward thrust from the exhaust. Because the velocity of the exhaust is significantly higher than that at the inlet, there is a net forward thrust. The thrust on a turbojet is shown as:

T_turbojet = W_exit * V_exit -W_inlet * V_inlet

Another force that acts upon the engine is gravity, which is usually downwards depending on the spatial orientation of the aircraft. The engine is held in place by a mount on the aircraft, which also acts on it. Another force to be considered is loading caused by aircraft maneuvers.

1) What are the units of W?

a. L/sec

b. Kg^-1*sec

c. Kg*sec^-1

d. Joules

2) What would be the equation for thrust produced by a turbofan?

a. T_tf = W_bypass * V_bypass -W_core * V_core + (W_core + W_bypass) * V_inlet

b. T_tf = W_bypass * V_bypass + W_core * V_core - (W_core + W_bypass) * V_inlet

c. T_tf = -W_bypass * V_bypass - W_core * V_core + (W_core + W_bypass) * V_inlet

d. T_tf = W_core * V_core -W_inlet * V_inlet

3) Mach number is defined as the ratio of a velocity to the speed of sound, where the speed of sound a is defined as a = sqrt(gamma R T) where gamma is the ratio of specific heats, generally constant for a set mixture of gases, and R is the gas constant. As an aircraft increases in altitude but maintains a constant speed, its Mach number:

a. Increases

b. Decreases

c. Remains Constant

d. Cannot be determined with the information given

4) There is a change in kinetic energy of air as it passes through the jet engine. What is the source of the added energy?

a. The jet engine has an electric motor, which is powered by batteries aboard the aircraft

b. Gravitational potential energy is converted into kinetic energy as the aircraft decreases in altitude

c. Fuel is burned inside the engine, and its chemical energy is eventually converted into the exhaust gases' kinetic energy

d. None of the above. Kinetic energy is actually conserved because the mass of the air decreases while the velocity increases.

5) Jet engines make noise, which is discernible at a level that is inversely proportional to the distance from the source to a microphone. An aircraft flies on a path that takes it directly over Washington Dulles International Airport, entering DC airspace from the Northwest and leaving to the Southeast. Ignoring wind and interference from other sources, the pitch of the aircraft heard when it is 6 miles to the Northwest, compared to that when it is 5 miles to the Southeast, is:

a. Higher

b. Lower

c. The same

d. Cannot be determined with the information given

6) The Lift force vector produced by an aircraft's wings is generally normal to the direction of motion. If it is assumed that the lifting force magnitude does not change with the angle of the vector, and the aircraft weighs 10,000 kg with a maximum engine thrust is 60,000 N, what is the approximate maximum sustained angle to the horizontal at which it can fly?

a. 30 deg

b. 45 deg

c. 60 deg

d. 90 deg

 

[Entadus]

Here's a fun friday biochem passage courtesy of your friendly neighborhood Entadus 🙂 Good luck everybody!

Passage 1

Serine proteases are enzymes found in both prokaryotes and eukaryotes, and serve to cleave peptide bonds in proteins. In humans, serine proteases are involved in various physiological functions, including hemostasis, digestion, and immune response. A general mechanism for serine proteases has been proposed to involve Histidine and Serine residues found near each other in the enzyme's catalytic domain. See figure 1.
Figure 1: http://upload.wikimedia.org/wikipedia/commons/1/17/Serine_protease_mechanism_by_snellios.png
Two broad categories of serine proteases exist: chymotrypsin-like (trypsin-like) and subtilisin-like. Trypsin-like proteases cleave peptide bonds following a positively charged amino acid. *Subtilisin is a serine protease in prokaryotes. Subtilisin is evolutionary unrelated to the chymotrypsin-clan, but shares the same catalytic mechanism utilising a catalytic triad, to create a nucleophilic serine. This is the classic example used to illustrate convergent evolution, since the same mechanism evolved twice independently during evolution. (*Credit: Wikipedia)

1. What is the catalytic role of Histidine in Serine proteases?

A. The nitrogen acts as a nucleophile.
B. The aromatic side-chain exhibits Van der Waal interaction with the substrate.
C. The side-chain acts as an acid, destabilizing the substrate.
D. The side-chain acts as a base, activating a nearby Serine residue.

2. Researchers working on novel therapeutics for immune disorders decide to create a drug that will act as a competitive inhibitor of human chymotrypsin. The drug will be a small molecule that will most closely resemble:

A. The proposed tetrahedral intermediate
B. Histidine-Serine dipeptide
C. Substrate protein
D. Lysine

3. The specificity of Trypsin-like Serine proteases for positively charged regions of the subsrate is theorized to be due to characteristics of the S1 pocket within the active site. This specificity is most likely due to which possible combination of amino acid residues in the substrate and S1 pocket, respectively?

A. Lysine, Arginine
B. Arginine, Glutamate
C. Histidine, Serine
D. Glycine, Asparagine

4. Serine proteases in the digestive tract are often released as zymogens. Which of the following is NOT an example of a zymogen?

A. Pepsinogen
B. Proelastease
C. Trypsin
D. Chymotrypsinogen

5. The cleavage of a Gly-Leu dipeptide could be accomplished by a similar mechanism to Serine protease using which reagent available to a synthetic chemist?

A. Hydrogen with Palladium on Carbon
B. Sodium hydroxide in water, reflux conditions
C. Lithium Aluminum Hydride
D. Sodium Borohydride

6. Convergent evolution could be responsible for which of the following phenomena?

A. Dolphins and humans display similar vertebral structures, especially when either group is compared with birds.
B. Birds and mammals typically both have 4 chambered hearts, and this is the result of separate evolutionary processes.
C. Mitochondria and bacteria both have circular genomes, though mitochondria have far fewer genes than any autonomous bacteria.
D. Bats' wings and flying squirrels' wings are used for different purposes, even though both groups are descended from a common mammalian ancestor.

 

[Captain Sisko]

You know, when all is said and done we should get the mods to compile all these into our own mcat.

 

[Captain Sisko]

Here's a fun friday biochem passage courtesy of your friendly neighborhood Entadus 🙂 Good luck everybody!

Passage 1

Serine proteases are enzymes found in both prokaryotes and eukaryotes, and serve to cleave peptide bonds in proteins. In humans, serine proteases are involved in various physiological functions, including hemostasis, digestion, and immune response. A general mechanism for serine proteases has been proposed to involve Histadine and Serine residues found near each other in the enzyme's catalytic domain. See figure 1.
Figure 1: http://upload.wikimedia.org/wikipedia/commons/1/17/Serine_protease_mechanism_by_snellios.png
Two broad categories of serine proteases exist: chymotrypsin-like (trypsin-like) and subtilisin-like. Trypsin-like proteases cleave peptide bonds following a positively charged amino acid. *Subtilisin is a serine protease in prokaryotes. Subtilisin is evolutionary unrelated to the chymotrypsin-clan, but shares the same catalytic mechanism utilising a catalytic triad, to create a nucleophilic serine. This is the classic example used to illustrate convergent evolution, since the same mechanism evolved twice independently during evolution. (*Credit: Wikipedia)

1. What is the catalytic role of Histadine in Serine proteases?

A. The nitrogen acts as a nucleophile.
B. The aromatic side-chain exhibits Van der Waal interaction with the substrate.
C. The side-chain acts as an acid, destabilizing the substrate.
D. The side-chain acts as a base, activating a nearby Serine residue.

2. Researchers working on novel therapeutics for immune disorders decide to create a drug that will act as a competitive inhibitor of human chymotrypsin. The drug will be a small molecule that will most closely resemble:

A. The proposed tetrahedral intermediate
B. Histidine-Serine dipeptide
C. Substrate protein
D. Lysine

3. The specificity of Trypsin-like Serine proteases for positively charged regions of the subsrate is theorized to be due to characteristics of the S1 pocket within the active site. This specificity is most likely due to which possible combination of amino acid residues in the substrate and S1 pocket, respectively?

A. Lysine, Arginine
B. Arginine, Glutamate
C. Histidine, Serine
D. Glycine, Asparagine

4. Serine proteases in the digestive tract are often released as zymogens. Which of the following is NOT an example of a zymogen?

A. Pepsinogen
B. Proelastease
C. Trypsin
D. Chymotrypsinogen

5. The cleavage of a Gly-Leu dipeptide could be accomplished by a similar mechanism to Serine protease using which reagent available to a synthetic chemist?

A. Hydrogen with Palladium on Carbon
B. Sodium hydroxide in water, reflux conditions
C. Lithium Aluminum Hydride
D. Sodium Borohydride

6. Convergent evolution could be responsible for which of the following phenomena?

A. Dolphins and humans display similar vertebral structures, especially when either group is compared with birds.
B. Birds and mammals typically both have 4 chambered hearts, and this is the result of separate evolutionary processes.
C. Mitochondria and bacteria both have circular genomes, though mitochondria have far fewer genes than any autonomous bacteria.
D. Bats' wings and flying squirrels' wings are used for different purposes, even though both groups are descended from a common mammalian ancestor.

I get
d d b c b b

 

[gettheleadout]

Here's a ps passage. Enjoy!

Modern aircraft are powered by jet engines, which draw in atmospheric air at a lower velocity and expel it behind them at a higher velocity, creating thrust. Early jet powered aircraft (1930's to 1960's) were powered mostly by turbojets, while later aircraft were and are powered by turbofans. The difference between a turbojet and a turbofan is that while a turbojet has one inlet and one exit, a turbofan has one inlet and two exits, where some of the ingested air is drawn into its turbojet-like "core" while the majority is sent around the core and through the "bypass." The bypass air is worked upon by a large diameter fan and then expelled through an outer exhaust nozzle that is coaxial with the inner exhaust nozzle, which expels core air.

Thrust T, which is a force, is generally defined as T=W*V, where W is the mass flow rate of air and V is the velocity of the air, in m/s. However, there are other forces that act upon a jet engine. One must consider the air entering the engine as a force pushing against the forward thrust from the exhaust. Because the velocity of the exhaust is significantly higher than that at the inlet, there is a net forward thrust. The thrust on a turbojet is shown as:

T_turbojet = W_exit * V_exit -W_inlet * V_inlet

Another force that acts upon the engine is gravity, which is usually downwards depending on the spatial orientation of the aircraft. The engine is held in place by a mount on the aircraft, which also acts on it. Another force to be considered is loading caused by aircraft maneuvers.

1) What are the units of W?

a. L/sec

b. Kg^-1*sec

c. Kg*sec^-1

d. Joules

2) What would be the equation for thrust produced by a turbofan?

a. T_tf = W_bypass * V_bypass -W_core * V_core + (W_core + W_bypass) * V_inlet

b. T_tf = W_bypass * V_bypass + W_core * V_core - (W_core + W_bypass) * V_inlet

c. T_tf = -W_bypass * V_bypass - W_core * V_core + (W_core + W_bypass) * V_inlet

d. T_tf = W_core * V_core -W_inlet * V_inlet

3) Mach number is defined as the ratio of a velocity to the speed of sound, where the speed of sound a is defined as a = sqrt(gamma R T) where gamma is the ratio of specific heats, generally constant for a set mixture of gases, and R is the gas constant. As an aircraft increases in altitude but maintains a constant speed, its Mach number:

a. Increases

b. Decreases

c. Remains Constant

d. Cannot be determined with the information given

4) There is a change in kinetic energy of air as it passes through the jet engine. What is the source of the added energy?

a. The jet engine has an electric motor, which is powered by batteries aboard the aircraft

b. Gravitational potential energy is converted into kinetic energy as the aircraft decreases in altitude

c. Fuel is burned inside the engine, and its chemical energy is eventually converted into the exhaust gases' kinetic energy

d. None of the above. Kinetic energy is actually conserved because the mass of the air decreases while the velocity increases.

5) Jet engines make noise, which is discernible at a level that is inversely proportional to the distance from the source to a microphone. An aircraft flies on a path that takes it directly over Washington Dulles International Airport, entering DC airspace from the Northwest and leaving to the Southeast. Ignoring wind and interference from other sources, the pitch of the aircraft heard when it is 6 miles to the Northwest, compared to that when it is 5 miles to the Southeast, is:

a. Higher

b. Lower

c. The same

d. Cannot be determined with the information given

6) The Lift force vector produced by an aircraft's wings is generally normal to the direction of motion. If it is assumed that the lifting force magnitude does not change with the angle of the vector, and the aircraft weighs 10,000 kg with a maximum engine thrust is 60,000 N, what is the approximate maximum sustained angle to the horizontal at which it can fly?

a. 30 deg

b. 45 deg

c. 60 deg

d. 90 deg

My answers in white below:

1. C
2. B
3. A
4. C
5. A
6. B

 

[gettheleadout]

Here's a fun friday biochem passage courtesy of your friendly neighborhood Entadus 🙂 Good luck everybody!

Passage 1

Serine proteases are enzymes found in both prokaryotes and eukaryotes, and serve to cleave peptide bonds in proteins. In humans, serine proteases are involved in various physiological functions, including hemostasis, digestion, and immune response. A general mechanism for serine proteases has been proposed to involve Histadine and Serine residues found near each other in the enzyme's catalytic domain. See figure 1.
Figure 1: http://upload.wikimedia.org/wikipedia/commons/1/17/Serine_protease_mechanism_by_snellios.png
Two broad categories of serine proteases exist: chymotrypsin-like (trypsin-like) and subtilisin-like. Trypsin-like proteases cleave peptide bonds following a positively charged amino acid. *Subtilisin is a serine protease in prokaryotes. Subtilisin is evolutionary unrelated to the chymotrypsin-clan, but shares the same catalytic mechanism utilising a catalytic triad, to create a nucleophilic serine. This is the classic example used to illustrate convergent evolution, since the same mechanism evolved twice independently during evolution. (*Credit: Wikipedia)

1. What is the catalytic role of Histadine in Serine proteases?

A. The nitrogen acts as a nucleophile.
B. The aromatic side-chain exhibits Van der Waal interaction with the substrate.
C. The side-chain acts as an acid, destabilizing the substrate.
D. The side-chain acts as a base, activating a nearby Serine residue.

2. Researchers working on novel therapeutics for immune disorders decide to create a drug that will act as a competitive inhibitor of human chymotrypsin. The drug will be a small molecule that will most closely resemble:

A. The proposed tetrahedral intermediate
B. Histidine-Serine dipeptide
C. Substrate protein
D. Lysine

3. The specificity of Trypsin-like Serine proteases for positively charged regions of the subsrate is theorized to be due to characteristics of the S1 pocket within the active site. This specificity is most likely due to which possible combination of amino acid residues in the substrate and S1 pocket, respectively?

A. Lysine, Arginine
B. Arginine, Glutamate
C. Histidine, Serine
D. Glycine, Asparagine

4. Serine proteases in the digestive tract are often released as zymogens. Which of the following is NOT an example of a zymogen?

A. Pepsinogen
B. Proelastease
C. Trypsin
D. Chymotrypsinogen

5. The cleavage of a Gly-Leu dipeptide could be accomplished by a similar mechanism to Serine protease using which reagent available to a synthetic chemist?

A. Hydrogen with Palladium on Carbon
B. Sodium hydroxide in water, reflux conditions
C. Lithium Aluminum Hydride
D. Sodium Borohydride

6. Convergent evolution could be responsible for which of the following phenomena?

A. Dolphins and humans display similar vertebral structures, especially when either group is compared with birds.
B. Birds and mammals typically both have 4 chambered hearts, and this is the result of separate evolutionary processes.
C. Mitochondria and bacteria both have circular genomes, though mitochondria have far fewer genes than any autonomous bacteria.
D. Bats' wings and flying squirrels' wings are used for different purposes, even though both groups are descended from a common mammalian ancestor.

My answers in white below:

1. D
2. A
3. B
4. C
5. C
6. B

 

[gettheleadout]

You know, when all is said and done we should get the mods to compile all these into our own mcat.

Well I'm here haha

 

[Entadus]

Here's a ps passage. Enjoy!

Modern aircraft are powered by jet engines, which draw in atmospheric air at a lower velocity and expel it behind them at a higher velocity, creating thrust. Early jet powered aircraft (1930's to 1960's) were powered mostly by turbojets, while later aircraft were and are powered by turbofans. The difference between a turbojet and a turbofan is that while a turbojet has one inlet and one exit, a turbofan has one inlet and two exits, where some of the ingested air is drawn into its turbojet-like "core" while the majority is sent around the core and through the "bypass." The bypass air is worked upon by a large diameter fan and then expelled through an outer exhaust nozzle that is coaxial with the inner exhaust nozzle, which expels core air.

Thrust T, which is a force, is generally defined as T=W*V, where W is the mass flow rate of air and V is the velocity of the air, in m/s. However, there are other forces that act upon a jet engine. One must consider the air entering the engine as a force pushing against the forward thrust from the exhaust. Because the velocity of the exhaust is significantly higher than that at the inlet, there is a net forward thrust. The thrust on a turbojet is shown as:

T_turbojet = W_exit * V_exit -W_inlet * V_inlet

Another force that acts upon the engine is gravity, which is usually downwards depending on the spatial orientation of the aircraft. The engine is held in place by a mount on the aircraft, which also acts on it. Another force to be considered is loading caused by aircraft maneuvers.

1) What are the units of W?

a. L/sec

b. Kg^-1*sec

c. Kg*sec^-1

d. Joules

2) What would be the equation for thrust produced by a turbofan?

a. T_tf = W_bypass * V_bypass -W_core * V_core + (W_core + W_bypass) * V_inlet

b. T_tf = W_bypass * V_bypass + W_core * V_core - (W_core + W_bypass) * V_inlet

c. T_tf = -W_bypass * V_bypass - W_core * V_core + (W_core + W_bypass) * V_inlet

d. T_tf = W_core * V_core -W_inlet * V_inlet

3) Mach number is defined as the ratio of a velocity to the speed of sound, where the speed of sound a is defined as a = sqrt(gamma R T) where gamma is the ratio of specific heats, generally constant for a set mixture of gases, and R is the gas constant. As an aircraft increases in altitude but maintains a constant speed, its Mach number:

a. Increases

b. Decreases

c. Remains Constant

d. Cannot be determined with the information given

4) There is a change in kinetic energy of air as it passes through the jet engine. What is the source of the added energy?

a. The jet engine has an electric motor, which is powered by batteries aboard the aircraft

b. Gravitational potential energy is converted into kinetic energy as the aircraft decreases in altitude

c. Fuel is burned inside the engine, and its chemical energy is eventually converted into the exhaust gases' kinetic energy

d. None of the above. Kinetic energy is actually conserved because the mass of the air decreases while the velocity increases.

5) Jet engines make noise, which is discernible at a level that is inversely proportional to the distance from the source to a microphone. An aircraft flies on a path that takes it directly over Washington Dulles International Airport, entering DC airspace from the Northwest and leaving to the Southeast. Ignoring wind and interference from other sources, the pitch of the aircraft heard when it is 6 miles to the Northwest, compared to that when it is 5 miles to the Southeast, is:

a. Higher

b. Lower

c. The same

d. Cannot be determined with the information given

6) The Lift force vector produced by an aircraft's wings is generally normal to the direction of motion. If it is assumed that the lifting force magnitude does not change with the angle of the vector, and the aircraft weighs 10,000 kg with a maximum engine thrust is 60,000 N, what is the approximate maximum sustained angle to the horizontal at which it can fly?

a. 30 deg

b. 45 deg

c. 60 deg

d. 90 deg

my answers in white are below,
1 c - easy dimensional analysis
2 b - close reading of the passage indicates this
3 a - i reason that at increased altitude, the speed of sound will be slower (since the air is less dense), thereby increasing mach number
4 c - intuition
5 a - classic doppler question
6 a - vectors and trig

 

[NuttyEngDude]

My answers to sisko's questions:

1. D
2. D
3. A
4. E
5. A
6. D

My answers to Entadus's questions:
1. D
2. B
3. A
4. B
5. D
6. B

 

[Jr MD]

I need some serious physics help ...and im not loling haha i feel confident in bio gchem and ochem and verbal if im lucky lol but physics?? Ugh...ill have to find another method than TBR

Sent from my T-Mobile myTouch Q using SDN Mobile

 

[Albein]

Was just clobbered by the 2/3 of sound waves and Doppler passages. Judging from most of the mistakes, going to attribute it to being burnt out.


-

"To laugh often and much; to win the respect of intelligent people and the affection of children...to leave the world a better place...to know even one life has breathed easier because you have lived. This is to have succeeded."
-Ralph Waldo Emerson

 

[Jr MD]

Was just clobbered by the 2/3 of sound waves and Doppler passages. Judging from most of the mistakes, going to attribute it to being burnt out.


-

"To laugh often and much; to win the respect of intelligent people and the affection of children...to leave the world a better place...to know even one life has breathed easier because you have lived. This is to have succeeded."
-Ralph Waldo Emerson

Some one explain the concept if being burnt out please...im sure I've felt it but my freshman premeds i mentor want a formal example lol

Sent from my T-Mobile myTouch Q using SDN Mobile

 

[Albein]

Some one explain the concept if being burnt out please...im sure I've felt it but my freshman premeds i mentor want a formal example lol

Sent from my T-Mobile myTouch Q using SDN Mobile

In my opinion, it's different for everyone and what they go through/feel. For me, I notice it when i start making mistakes that aren't common such as not realizing which numbers are smaller or simple computational mistakes. Another thing for me is having no energy or desire to keep going.


-

"To laugh often and much; to win the respect of intelligent people and the affection of children...to leave the world a better place...to know even one life has breathed easier because you have lived. This is to have succeeded."
-Ralph Waldo Emerson

 

[NuttyEngDude]

My Physical Science Section questions.... Enjoy 🙂



-------------------------------
The idea of a hypothetical planet always on the other side of the sun from the Earth has been a recurring theme in science fiction, fiction, and UFO claims. A planet with the same orbiting speed/path as Earth could (according to the theory) stay 180 degrees from Earth, if it were the same distance from the sun and the same mass as Earth, which would allow it to have the same (or very similar) planetary atmospheric pressure and surface temperature range. Thus, what would make the planet undetectable to astronomers would also allow it to be habitable to beings at least similar to humans. The idea is said to have been particularly popular in 1950s science fiction.

However, if such a planet actually existed it would have to be detectable from Earth, according to present scientific cosmology, for a number of reasons.
** Note the above is from Wikipedia, I am crediting the source ***

1. Which is the best reason that it would have to be detectable?
   • It would never be possible for a Counter-Earth to permanently stay 180 degrees from Earth .
   • It would be unlikely for the Counter-Earth to have the same number of moons as the current Earth thereby changing its angular momentum
   • Jupiter which protected our planet from the Late Heavy Bombardment would not have been in the same orbital position required to protect the Counter-Earth.
   • The Counter-Earth would cause constructive interference with the orbital wave of the Earth causing both planets to lose orbit.
   -
2. Assume a Counter-Earth were to exist but subsequently break apart due to continual collisions with asteroids from the Late Heavy Bombardment. If one of these smaller pieces were survive until today and fly near enough to the Earth's moon causing it to break orbit, crash into the earth and to add into the earth's crust, what would be the new velocity of the Earth?
   Assume Mm = Mass of the moon, Vm = Velocity of the moon, Me = Mass of the Earth, Ve = Velocity of the Earth
   • sqrt (2Mm + 2Me)
   • sqrt (2(Me+Mm)
   • (MmVm + MeVe)/(Mm+Me)
   • (Mm+Me)/(MmVm + MeVe)
   -
3. The Doctor sees the inevitable amount of destruction that a Moon crashing into the Earth could cause to his favorite race in the universe decides to save the planet by towing it with his Tardis. If he moves the planet closer to the sun by 20 million km, what is the fastest transmission time Houston could issue new move orders to the mars rover?
   Assume the distance from the sun to the Earth is 150 million km, the distance from the sun to mars is 230 million km and the distance of the Tardis spaceship from the sun is 50 million km
   • 0.33 seconds
   • 267 seconds
   • 333 seconds
   • 0.267 seconds
   -
4. If the earth were to shine a green light towards Mars as the doctor was towing the planet, what color would this light appear to someone on Mars assuming Mars was, for the duration of light transmission time, stationary?
   • The color would be more greenish than someone would notice on Earth
   • The color would be more blueish than someone would notice on Earth
   • The color would be more yellowish than someone would notice on Earth
   • The color would be distorted by the induced magnetic field due to planetary travel
   -
5. Assuming the Doctor was successful in saving the planet by moving it closer to the sun. The side effects would be a drop in ocean levels by roughly a meter. If a light beam were to strike the ocean at 45 degrees, how would the angle of refraction differ from normal conditions (normal orbit) as compared to these new conditions?
   • The angle would be smaller
   • The angle would be larger
   • The light would travel along the surface of the ocean
   • It would appear more red due to constructive interference from particulate matter
   -
6. Assuming the same conditions in the previous question, how would sound be affected in this new ocean?
   • It would change to a lower pitch
   • It would change to a higher pitch
   • It would travel slower than before
   • It would travel faster than before
   -
7. A container with a mixture of gases including CO2 has a small hole in it. As the planet is towed towards the sun and back to its original position, the amount of overall atmospheric CO2 has increased roughly 20% due to the additional heat from moving towards the sun. As the planet moves back to its original position, global temperatures return to normal but the additional atmospheric CO2 levels remain. How does the CO2 effusion rate from the container change during this time?
   
   • The effusion rate slows down then slows down more
   • The effusion rate speeds up then slows down slower than the original rate
   • The effusion rate speeds up then goes back to its original rate
   • The effusion rate speeds up then stops completely

 

[osprey099]

Just got 3/6 and 3/6 on 2 verbal passages. Fml, I was on something like a 40/45 streak 😡

 

[Albein]

Man I feel like an idiot now haha... Oh well, I've never done an experimental passage before, so I now see the mindset with which I need to approach such problems.

might not work for everyone, but i struggled with the experimental passages my first go around with the MCAT. i took a class this past semester that forced me to read 2 experimental papers and the change has been very significant with the experimental bio passages ive done. i recommend picking up some papers and reading them and paying attention to the graphs and the differences with the controls as opposed to the different treatments.

 

[NuttyEngDude]

Just got 3/6 and 3/6 on 2 verbal passages. Fml, I was on something like a 40/45 streak 😡

😱 40/45 is pretty good on verbal lol

 

[Entadus]

Great work everyone, I'll post the answers to my passage tomorrow to give more people a chance to respond. I know Friday isn't exactly peoples' most productive day...

 

[osprey099]

😱 40/45 is pretty good on verbal lol

I think it's skewed because I am basically re-doing TPRHL verbal workbook since I did it last summer already but didn't mark anything. I got a 8 on the verbal section last September so yea..

 

[NuttyEngDude]

I think it's skewed because I am basically re-doing TPRHL verbal workbook since I did it last summer already but didn't mark anything. I got a 8 on the verbal section last September so yea..

I'm back on daily verbal practice as of today so I'll be feeling the pain too....

 

[osprey099]

Is it just me or are EK1001 Chem problems extremely difficult?

 

[NuttyEngDude]

Is it just me or are EK1001 Chem problems extremely difficult?

Havent worked them all but I didnt think so particularly. Which section are you working on?

 

[Entadus]

Solutions below! (spoilers)

Here's a fun friday biochem passage courtesy of your friendly neighborhood Entadus 🙂 Good luck everybody!

Passage 1

Serine proteases are enzymes found in both prokaryotes and eukaryotes, and serve to cleave peptide bonds in proteins. In humans, serine proteases are involved in various physiological functions, including hemostasis, digestion, and immune response. A general mechanism for serine proteases has been proposed to involve Histidine and Serine residues found near each other in the enzyme's catalytic domain. See figure 1.
Figure 1: http://upload.wikimedia.org/wikipedia/commons/1/17/Serine_protease_mechanism_by_snellios.png
Two broad categories of serine proteases exist: chymotrypsin-like (trypsin-like) and subtilisin-like. Trypsin-like proteases cleave peptide bonds following a positively charged amino acid. *Subtilisin is a serine protease in prokaryotes. Subtilisin is evolutionary unrelated to the chymotrypsin-clan, but shares the same catalytic mechanism utilising a catalytic triad, to create a nucleophilic serine. This is the classic example used to illustrate convergent evolution, since the same mechanism evolved twice independently during evolution. (*Credit: Wikipedia)

1. What is the catalytic role of Histidine in Serine proteases?

A. The nitrogen acts as a nucleophile.
B. The aromatic side-chain exhibits Van der Waal interaction with the substrate.
C. The side-chain acts as an acid, destabilizing the substrate.
D. The side-chain acts as a base, activating a nearby Serine residue. - See figure 1. Pretty easy if you can understand arrow pushing mechanisms.

2. Researchers working on novel therapeutics for immune disorders decide to create a drug that will act as a competitive inhibitor of human chymotrypsin. The drug will be a small molecule that will most closely resemble:

A. The proposed tetrahedral intermediate - This would probably be the hardest question for anyone who has not studied the design of competitive enzyme inhibitors. However you should understand that the drug would be designed to fit into the catalytic domain (active site) of the enzyme, and bind competitively where the substrate would normally bind. Now imagine the normal functioning enzyme. Substrate binds because it fits well into the active site. The enzyme then quickly brings the substrate towards products VIA THE TRANSITION STATE. This is key. The transition state is only formed because it is more stable (LOWER FREE ENERGY) than substrate while it is inside the enzyme. Therefore, the transition state (tetrahedral intermediate) is thermodynamically favored over substrate inside the enzyme. When designing a drug to fit inside, we should choose one that binds as well as possible to the active site. The more stable the drug-enzyme complex (lower free energy), the better binding we will observe.
B. Histidine-Serine dipeptide - distractor. His and Ser are residues in the enzyme itself, not part of a small molecule that would bind inside.
C. Substrate protein - Second best answer. While something that mimicked the substrate would fit into the active site and compete with natural substrate, this is not the best choice.
D. Lysine - distractor. It would be positively charged, however a lone lysine amino acid would not fit well or have any specificity for the active site.

3. The specificity of Trypsin-like Serine proteases for positively charged regions of the subsrate is theorized to be due to characteristics of the S1 pocket within the active site. This specificity is most likely due to which possible combination of amino acid residues in the substrate and S1 pocket, respectively?

A. Lysine, Arginine - Positive, Positive - Explains the positivity of the substrate but not WHY the substrate would bind to the S1 pocket.
B. Arginine, Glutamate - Positive, Negative - This one was a little tricky, it relied on an understanding of why a substrate will bind in the pocket of an enzyme. You needed to realize that the substrate needed to be positively charged at physiologic pH (Arginine or Lysine would work), however the reason WHY the positively charged substrate binds so well is that the S1 pocket is NEGATIVELY charged to attract it. So the enzyme will have something negative inside (Glutamate, Asparagine). Answer choice B is the only one with Positive, Negative amino acids.
C. Histidine, Serine - Distractor... His and Ser are residues found in the catalytic site, NOT residues that are in the substrate or S1 pocket.
D. Glycine, Asparagine - Neutral, Negative

4. Serine proteases in the digestive tract are often released as zymogens. Which of the following is NOT an example of a zymogen?

A. Pepsinogen - Coverted to Pepsin by HCl in the stomach
B. Proelastease - Coverted to Elastease
C. Trypsin - The zymogen of Trypsin is Trypsinogen!
D. Chymotrypsinogen - Coverted to Chymotrypsin

5. The cleavage of a Gly-Leu dipeptide could be accomplished by a similar mechanism to Serine protease using which reagent available to a synthetic chemist?

A. Hydrogen with Palladium on Carbon - Would only reduce a C=C double bond
B. Sodium hydroxide in water, reflux conditions - Base catalyzed hydrolysis of amide to form Carboxylic acid and Amine
C. Lithium Aluminum Hydride - Wrong! LAH would only REDUCE the amide bond to form an Amine - however it would not cleave the C-N bond!
D. Sodium Borohydride - A weaker reducing agent, see LAH explanation above.

6. Convergent evolution could be responsible for which of the following phenomena?

A. Dolphins and humans display similar vertebral structures, especially when either group is compared with birds.
B. Birds and mammals typically both have 4 chambered hearts, and this is the result of separate evolutionary processes.
C. Mitochondria and bacteria both have circular genomes, though mitochondria have far fewer genes than any autonomous bacteria.
D. Bats' wings and flying squirrels' wings are used for different purposes, even though both groups are descended from a common mammalian ancestor.

 

[gettheleadout]

Solutions below! (spoilers)

Sweet! I only missed one and it's because I haven't reviewed the Nitrogen Compounds chapter of TBR Organic yet. 😀

 

[osprey099]

Havent worked them all but I didnt think so particularly. Which section are you working on?

I was just randomly flipping through pages and doing problems. I finished review a while ago and I've gotten 80-90% in TBR gen chem passages but the EK1001's that I've attempted so far are pretty hard in terms of asking what happens under very specific conditions other detailed information.

 

[gettheleadout]

My Physical Science Section questions.... Enjoy 🙂



-------------------------------
The idea of a hypothetical planet always on the other side of the sun from the Earth has been a recurring theme in science fiction, fiction, and UFO claims. A planet with the same orbiting speed/path as Earth could (according to the theory) stay 180 degrees from Earth, if it were the same distance from the sun and the same mass as Earth, which would allow it to have the same (or very similar) planetary atmospheric pressure and surface temperature range. Thus, what would make the planet undetectable to astronomers would also allow it to be habitable to beings at least similar to humans. The idea is said to have been particularly popular in 1950s science fiction.

However, if such a planet actually existed it would have to be detectable from Earth, according to present scientific cosmology, for a number of reasons.
** Note the above is from Wikipedia, I am crediting the source ***

1. Which is the best reason that it would have to be detectable?
   • It would never be possible for a Counter-Earth to permanently stay 180 degrees from Earth .
   • It would be unlikely for the Counter-Earth to have the same number of moons as the current Earth thereby changing its angular momentum
   • Jupiter which protected our planet from the Late Heavy Bombardment would not have been in the same orbital position required to protect the Counter-Earth.
   • The Counter-Earth would cause constructive interference with the orbital wave of the Earth causing both planets to lose orbit.
   -
2. Assume a Counter-Earth were to exist but subsequently break apart due to continual collisions with asteroids from the Late Heavy Bombardment. If one of these smaller pieces were survive until today and fly near enough to the Earth’s moon causing it to break orbit, crash into the earth and to add into the earth’s crust, what would be the new velocity of the Earth?
   Assume Mm = Mass of the moon, Vm = Velocity of the moon, Me = Mass of the Earth, Ve = Velocity of the Earth
   • sqrt (2Mm + 2Me)
   • sqrt (2(Me+Mm)
   • (MmVm + MeVe)/(Mm+Me)
   • (Mm+Me)/(MmVm + MeVe)
   -
3. The Doctor sees the inevitable amount of destruction that a Moon crashing into the Earth could cause to his favorite race in the universe decides to save the planet by towing it with his Tardis. If he moves the planet closer to the sun by 20 million km, what is the fastest transmission time Houston could issue new move orders to the mars rover?
   Assume the distance from the sun to the Earth is 150 million km, the distance from the sun to mars is 230 million km and the distance of the Tardis spaceship from the sun is 50 million km
   • 0.33 seconds
   • 267 seconds
   • 333 seconds
   • 0.267 seconds
   -
4. If the earth were to shine a green light towards Mars as the doctor was towing the planet, what color would this light appear to someone on Mars assuming Mars was, for the duration of light transmission time, stationary?
   • The color would be more greenish than someone would notice on Earth
   • The color would be more blueish than someone would notice on Earth
   • The color would be more yellowish than someone would notice on Earth
   • The color would be distorted by the induced magnetic field due to planetary travel
   -
5. Assuming the Doctor was successful in saving the planet by moving it closer to the sun. The side effects would be a drop in ocean levels by roughly a meter. If a light beam were to strike the ocean at 45 degrees, how would the angle of refraction differ from normal conditions (normal orbit) as compared to these new conditions?
   • The angle would be smaller
   • The angle would be larger
   • The light would travel along the surface of the ocean
   • It would appear more red due to constructive interference from particulate matter
   -
6. Assuming the same conditions in the previous question, how would sound be affected in this new ocean?
   • It would change to a lower pitch
   • It would change to a higher pitch
   • It would travel slower than before
   • It would travel faster than before
   -
7. A container with a mixture of gases including CO2 has a small hole in it. As the planet is towed towards the sun and back to its original position, the amount of overall atmospheric CO2 has increased roughly 20% due to the additional heat from moving towards the sun. As the planet moves back to its original position, global temperatures return to normal but the additional atmospheric CO2 levels remain. How does the CO2 effusion rate from the container change during this time?
   
   • The effusion rate slows down then slows down more
   • The effusion rate speeds up then slows down slower than the original rate
   • The effusion rate speeds up then goes back to its original rate
   • The effusion rate speeds up then stops completely

My answers in white below:

1. A
2. C
3. C
4. C
5. A or B - I haven't learned any optics at all
6. C
7. B

 

[Jr MD]

Just wanted to make an announcement while km doing my second 1/3s on this lovely afternoon...

DO NOT EVER EVER EVER CHANGE YOUR FIRST ANSWER ...

i did it multiple times and my first answer id always correct 🙁

That is all... Lol

Sent from my T-Mobile myTouch Q using SDN Mobile

 

[DrBTS]

On page 56 of chapter 4 of Physics EK theres a chart and a graph for elastic/inelastic/partially inelastic collisions. What does this even signify, so lost, can you guys make sense out of these equations and equivalencies for v? unbelievable!

 

[NuttyEngDude]

don't have that book, can you post a pic of the chart?