TBR test 2 q's

[2010premed]

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How would you answer these?

Which form of thermoregulation is MOST reduced in a humid climate?
A. Heat loss via convection
B. Heat loss via conduction
C. Heat loss via radiation
D. Heat loss via vaporization

Aspirin, an acid, has a pKa close to 3. The lumen of the stomach has a pH close to 2. Aspirin is effective when taken orally because it:

A. becomes a charged molecule, easily passing over the cell membranes of the stomach lining to enter the blood.
B. becomes a charged molecule that does not pass over the cell membranes of the stomach lining, so that it remains in the digestive tract.
C.
remains a neutral molecule, able to traverse the cell membranes of the stomach lining to enter the blood.D. remains a neutral molecule, unable to traverse the cell membrane, so that it remains in the digestive tract.

 

[BerkReviewTeach]

How would you answer these?

Which form of thermoregulation is MOST reduced in a humid climate?
A. Heat loss via convection
B. Heat loss via conduction
C. Heat loss via radiation
D. Heat loss via vaporization

This can be solved by knowing that we perspire to regulate our temperature, and a humid environment reduces the ability for moisture to evaporate from our skin. At the very least, we know that humidity is a measure of the vapor pressure of water in the air relative to its max vapor pressure at a given temperature. Vapor pressure depends on vaporization, so choice D is the answer the writer is rewarding us for picking.

Aspirin, an acid, has a pKa close to 3. The lumen of the stomach has a pH close to 2. Aspirin is effective when taken orally because it:

A. becomes a charged molecule, easily passing over the cell membranes of the stomach lining to enter the blood.
B. becomes a charged molecule that does not pass over the cell membranes of the stomach lining, so that it remains in the digestive tract.
C. remains a neutral molecule, able to traverse the cell membranes of the stomach lining to enter the blood.
D. remains a neutral molecule, unable to traverse the cell membrane, so that it remains in the digestive tract.

Assuming we know nothing about the structure of aspirin, we are forced to use either small clues from orgo or from physiology. To traverse a cell membrane, the structure must be able to pass through the lipid bilayer, so it should be uncharged and preferably planar. We know that aspirin can impact pain at various locations in our body (it's a prostaglandin inhibitor), so it must be able to travel through our body. This means it must go into the blood, so only choice C looks to fit our observations.

The hint we get from chemistry lies in the pKa value. Carboxylic acids are the common functional group in that range. If the pH is 2 and the pKa is 3, then the compound will be protonated (pH is lower than pKa). Carboxylic acids are neutral in their protonated state, so we have some support form o chem that our reasoning in choosing C is backed up by the numbers given.

 

[loveoforganic]

1) D. - Increased humidity in the air means your sweat isn't going to readily evaporate, due to LeChatlier's.

2) C. - Let's use some process of elimination. pH < pKa, so your aspirin is protonated. Cross of A and B. Now, without even needing to know that nonpolar substances pass through the cell membrane, just think logically. Where is a drug going to be distributed to the body? Through the blood or through the digestive tract? While it's possible that you could eventually get some absorption through the digestive tract, you're ultimately still going to be ending up in the blood, which is already in choice C, making it the better answer.

 

[BerkReviewTeach]

1) D. - Increased humidity in the air means your sweat isn't going to readily evaporate, due to LeChatlier's.

2) C. - Let's use some process of elimination. pH < pKa, so your aspirin is protonated. Cross of A and B. Now, without even needing to know that nonpolar substances pass through the cell membrane, just think logically. Where is a drug going to be distributed to the body? Through the blood or through the digestive tract? While it's possible that you could eventually get some absorption through the digestive tract, you're ultimately still going to be ending up in the blood, which is already in choice C, making it the better answer.

LOL! 9:40 versus 9:41 after this question sat untouched for hours. I like that I got in one minute ahead of you, so it looks like you copied me and therefore your cyber reputation takes a hit.

BTW, excellent reasoning on your part as usual!

 

[Rabolisk]

Think of humidity and sweating.

The acidic functional group in aspirin is a carboxylic acid.

 

[loveoforganic]

LOL! 9:40 versus 9:41 after this question sat untouched for hours. I like that I got in one minute ahead of you, so it looks like you copied me and therefore your cyber reputation takes a hit.

Glad you're back from hiatus, now lose the 'tude! I'm expanding/structuring the MCAT program using TBR as the class books, but we can take our business elsewhere! 😛

 

[2010premed]

How do I use Le Chatelier's for this?

If there's humidity, that means that there is increased water vapor in the air
evaporation of sweating (liquid to gas), so I can't add the water vapor to neither the reactant nor the product side of the reaction

 

[2010premed]

all these really help, thank you =)

 

[2010premed]

Here are 3 more bio q's

1.. .In reacting triphenyl phosphine with 2-bromopentane, what is true for the stereochemistry of 2-bromopentane?
Answer: The stereochemistry at carbon 2 is irrelevant, because it is lost in the Wittig reagent.

..​

They give me the wittig rexn example in the passage, but I can't c n p it

I don’t see how C 2 is lost?
2.. .The term .amphiphatic .refers to alpha helices that have "sidedness." Sidedness results when one face of a helix is composed of hydrophilic R groups, while another face of the same helix is composed of hydrophobic R groups. How does this affect the tertiary structure of a protein?
A.. .It allows for stronger tertiary interactions between hydrophilic secondary structural units.
B.. .It causes weaker tertiary interactions between hydrophobic secondary structural units.
C.. .It forces alpha helices that do not exhibit amphiphatic behavior to uncoil and then fold around an alpha helix that does exhibit amphiphatic behavior.
D.. .It denatures the portion of the protein that is not in the alpha helix and that exhibits amphiphatic behavior.
How would you approach this?
3.. .Which of the following statements supports the idea that DNA is made up of nucleic acids?
A.. .When the pKa < pH, the molecule is neutral.
B.. .When the pKa < pH, the molecule is negative.
C.. .When the pKa > pH, the molecule is positive.
D.. .When the pKa > pH, the molecule is neutral.
Answer: B
I just answered this based on that DNA is negatively charged, was this the right way to go about it?

 

[loveoforganic]

How do I use Le Chatelier's for this?

If there's humidity, that means that there is increased water vapor in the air
evaporation of sweating (liquid to gas), so I can't add the water vapor to neither the reactant nor the product side of the reaction

H2O(l) -> H2O(g)

Humidity means there's a bunch of H2O(g).

 

[BerkReviewTeach]

Here are 3 more bio q's

1. In reacting triphenyl phosphine with 2-bromopentane, what is true for the stereochemistry of 2-bromopentane?

• In the formation of the Wittig intermediate, the phosphine attacks carbon 2 and displaces the Br leaving group, so the stereochemistry would seem to matter at that point. But in the next step, carbon 2 loses it's proton and the electron pair forms a pi-bond (back bond) to phosphorus, making the carbon sp2-hybridized. This means that the carbon loses its original chirality, so it makes no difference whether you started with (R)-2-bromopentane or (S)-2-bromopentane.

2. The term amphiphatic refers to alpha helices that have "sidedness." Sidedness results when one face of a helix is composed of hydrophilic R groups, while another face of the same helix is composed of hydrophobic R groups. How does this affect the tertiary structure of a protein?

A. It allows for stronger tertiary interactions between hydrophilic secondary structural units.
B. It causes weaker tertiary interactions between hydrophobic secondary structural units.
C. It forces alpha helices that do not exhibit amphiphatic behavior to uncoil and then fold around an alpha helix that does exhibit amphiphatic behavior.
D. It denatures the portion of the protein that is not in the alpha helix and that exhibits amphiphatic behavior.

How would you approach this?

• Keep in mind that the basic rule behind protein folding is to maximize the attractive interactions and minimize repulsive interactions. The H-bonding that holds together and alpha helix will not unfold because of weaker tertiary interactions, so C is out right away. Also, it's not going to denature another part of the protein-it will simply fold to increase attractive interactions, so D is highly unlikely. The answer has to be either A or B, because they are opposites. By having one face (half of the alpha helix's external side) all hydrophilic, it will be able to form stronger hydrophilic interactions with other hydrophilic sites (another alpha helix for instance), which by definition is a tertiary interaction. Tertiary interactions will be stronger, because attractive forces between sidechains has increased. A is the only answer that fits this idea.

3. Which of the following statements supports the idea that DNA is made up of nucleic acids?
A. When the pKa < pH, the molecule is neutral.
B. When the pKa < pH, the molecule is negative.
C. When the pKa > pH, the molecule is positive.
D. When the pKa > pH, the molecule is neutral.

I just answered this based on that DNA is negatively charged, was this the right way to go about it?

• While your logic is perhaps only scratching the surface, it is valid and applicable and given the answer choices, it's all you need. So yes, you got the best answer with valid reasoning.
  
  In a little more depth, this requires knowing something about functional groups. First off, when pH > pKa, the environment feels basic to the compound, so it is deprotonated. When pH < pKa, the environment feels acidic to the compound, so it is protonated.
  
  If you think of carboxylic acids, they go from neutral to negatively charged when deptotonated. So if pH > pKa for an acid, then it will be negatively charged. If pH < pKa for an acid, then it will be neutral. If you think of amines, they go from positively charged to neutral when deptotonated. So if pH > pKa for an amine, then it will be neutral. If pH < pKa for an amine, then it will be positively charged.
  
  You knew that DNA was negative charged, so the only thing that could explain this would be pH > pKa for an acidic molecule, thus giving it a negative charge.
  
  To be completely honest, I read through the answer explanation at the CBT site and I don't think it helped much. Hopefully this explanation is better, but if not, please ask for some clarity.

 

[2010premed]

thx! The only things that's confusing about # 3 is that both B and D are true, but I guess that's when you have to know DNA is negative charged. I get 2 now with the definition of tertiary interactions

 

[2010premed]

ah, ok. I was thinking of water vapor as H2O (l)... is water vapor always = H2O (g)?

 

[loveoforganic]

Yep. Generally, vapor refers to gaseous water that's a result of evaporation and steam refers to gaseous water that's a result of boiling, but I don't think that's a hard rule.

 

[BerkReviewTeach]

Yep. Generally, vapor refers to gaseous water that's a result of evaporation and steam refers to gaseous water that's a result of boiling, but I don't think that's a hard rule.

Sounds like a solid rule to me. It's a vapor if it's below the boiling point (and therefore in equilibrium with the liquid form) and a gas if it's above the boiling point.