May 13, 2016
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  1. Let's say you have a substrate that has an (R) configuration. Will an enzyme that acts on it necessarily have an (R) or (S) configuration?
  2. If glucose runs low, does the brain use ketone bodies or fatty acids? And why does it prefer one over the other?
  3. Under starvation conditions what is the order of things that are metabolized?
  4. What is the difference between iconic vs. echoic vs. sensory vs. episodic memory? To me it seems as if sensory memory includes iconic, echoic, and episodic memory and iconic memory=episodic memory. Is this right?
  5. What is the difference between working memory and short term memory?
  6. I remember seeing "electrically gated channels" as an answer choice somewhere. Do these actually exist and if they do what's the difference between electrically gated and voltage gated channels?
 

aldol16

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  • Let's say you have a substrate that has an (R) configuration. Will an enzyme that acts on it necessarily have an (R) or (S) configuration?
It will necessarily have a configuration - it's not simply R or S at the macromolecular level because there are simply too many stereocenters. At the protein level, there are so-called "L-peptides" and "D-peptides" corresponding to the L or D amino acids that make them up. So as you should know, nature exclusively uses L-amino acids. Researchers have taken advantage of this fact to think outside the box, so to speak, in order to develop novel therapeutic approaches that would be orthogonal to the body's waste-disposal systems (http://www.cvri.ucsf.edu/~dminor/pdf/03_26_96.pdf).

So briefly, an enzyme does have a specific configuration but it's not a simple R or S designation.

  • If glucose runs low, does the brain use ketone bodies or fatty acids? And why does it prefer one over the other?
Ketone bodies. Fatty acids are bound to serum albumin and thus cannot cross the blood-brain barrier.

  • Under starvation conditions what is the order of things that are metabolized?
I think overall, it goes glucose, glycogen, fatty acids, protein. Glucose first because it's the most obvious form of carbohydrate that's in the blood. Glycogen next because it's a reserve of glucose. Then fatty acids when the body just needs energy. Proteins are last because the body has few means of making amino acids de novo without food input.
 

aldol16

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  • What is the difference between working memory and short term memory?
I believe they are the same for the purposes of the MCAT but I'm no psychologist.

  1. I remember seeing "electrically gated channels" as an answer choice somewhere. Do these actually exist and if they do what's the difference between electrically gated and voltage gated channels?
I believe they are the same but I'm no psychologist.
 

theonlytycrane

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  1. If glucose runs low, does the brain use ketone bodies or fatty acids? And why does it prefer one over the other?
fatty acids can get metabolized (beta oxidation) to acetyl-coa which combine to form ketone bodies so its just another form of the same energy.
 
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Jul 15, 2015
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  • What is the difference between working memory and short term memory?
  • I remember seeing "electrically gated channels" as an answer choice somewhere. Do these actually exist and if they do what's the difference between electrically gated and voltage gated channels?
Working memory is a context that lets you navigate your surroundings and make decisions. It is memory that lets you make decisions on a small time scale. Short-term memory is encoding of information for a brief period of time without the ability to manipulate it in your mind. For example, short-term memory is what you'd use when someone tells you their phone number and you don't have a paper to jot it down. So what do you do? You can either repeat it to yourself over and over again to keep remembering it, or you can not repeat it and over time you'll forget it. Make sense? Working memory, on the other hand, would allow you to encounter something and manipulate it for a specific purpose. An example would be reading an MCAT passage. You're absorbing information, holding it in your mind for a brief period of time, and determining what to do with it.

Electrically gated channels are voltage-gated channels. I've never heard of "electrically gated channels" and a quick google search pulled up only voltage-gated channels. In my 8 years of studying and doing research in neuroscience, I've never heard anyone say "electrically gated."
 
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OP
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May 13, 2016
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Ketone bodies. Fatty acids are bound to serum albumin and thus cannot cross the blood-brain barrier.
Why are ketone bodies able to travel freely through the blood, but fatty acids have to be transported by a protein? Aren't both of these molecules polar?

Another random question: Say you have two liquids of differing densities in a beaker. If you pour the contents of the beaker into a graduated cylinder, will the less dense liquid come out of the beaker first?
 
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OP
K
May 13, 2016
77
6
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Pre-Medical
Working memory is a context that lets you navigate your surroundings and make decisions. It is memory that lets you make decisions on a small time scale. Short-term memory is encoding of information for a brief period of time without the ability to manipulate it in your mind. For example, short-term memory is what you'd use when someone tells you their phone number and you don't have a paper to jot it down. So what do you do? You can either repeat it to yourself over and over again to keep remembering it, or you can not repeat it and over time you'll forget it. Make sense? Working memory, on the other hand, would allow you to encounter something and manipulate it for a specific purpose. An example would be reading an MCAT passage. You're absorbing information, holding it in your mind for a brief period of time, and determining what to do with it.

Electrically gated channels are voltage-gated channels. I've never heard of "electrically gated channels" and a quick google search pulled up only voltage-gated channels. In my 8 years of studying and doing research in neuroscience, I've never heard anyone say "electrically gated."
That makes so much more sense, thank you. I bet the "electrically gated channels" was probably a distractor.
 
Jul 15, 2015
101
38
Status
Pre-Medical
  1. Let's say you have a substrate that has an (R) configuration. Will an enzyme that acts on it necessarily have an (R) or (S) configuration?
  2. If glucose runs low, does the brain use ketone bodies or fatty acids? And why does it prefer one over the other?
  3. Under starvation conditions what is the order of things that are metabolized?
  4. What is the difference between iconic vs. echoic vs. sensory vs. episodic memory? To me it seems as if sensory memory includes iconic, echoic, and episodic memory and iconic memory=episodic memory. Is this right?
  5. What is the difference between working memory and short term memory?
  6. I remember seeing "electrically gated channels" as an answer choice somewhere. Do these actually exist and if they do what's the difference between electrically gated and voltage gated channels?
Echoic (Auditory) and Iconic (Visual, very brief) memory are types of sensory memory. Episodic memory is autobiographical in the sense that you remember events from the past and can explicitly state details about them. It's similar to declarative memory. For example, I remember when I was in middle school, 9/11 happened. I remember exactly where I was and what I was doing at the time. I remember the tv channel that was showing live footage of the buildings, and I remember exactly the moment when my mom shut off the tv right as one of the planes struck a tower. This is an episodic memory. It's not the reason for the memory that's particularly important, but knowing that you can explicitly state details about a past event that makes it episodic.
 
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aldol16

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Why are ketone bodies able to travel freely through the blood, but fatty acids have to be transported by a protein? Aren't both of these molecules polar?
You should review fatty acid transport. Fatty acids are not polar. They have polar head groups (the carboxylic acid group) but are not themselves polar. That's why fats don't dissolve in water. You can test this easily by throwing a soapy solution into water. The soap will remain in a separate layer. This is because soaps are just salts of fatty acids.

Ketone bodies are small and polar and so they can travel through the blood freely. Fatty acids must be transported by serum albumin. In fact, that's the job of serum albumin - to bind and transport fatty acids where they need to go.

Another random question: Say you have two liquids of differing densities in a beaker. If you pour the contents of the beaker into a graduated cylinder, will the less dense liquid come out of the beaker first?
If you do this very slowly, yeah. Because the less dense layer will simply be on top of the beaker. Once you get to the interface, physics takes over.
 
OP
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May 13, 2016
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You should review fatty acid transport. Fatty acids are not polar. They have polar head groups (the carboxylic acid group) but are not themselves polar. That's why fats don't dissolve in water. You can test this easily by throwing a soapy solution into water. The soap will remain in a separate layer. This is because soaps are just salts of fatty acids.

Ketone bodies are small and polar and so they can travel through the blood freely. Fatty acids must be transported by serum albumin. In fact, that's the job of serum albumin - to bind and transport fatty acids where they need to go.
Whoops forgot that fatty acids have hydrophobic tails. As for ketone bodies, why is a polar substance able to cross the blood brain barrier?
 

aldol16

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Whoops forgot that fatty acids have hydrophobic tails. As for ketone bodies, why is a polar substance able to cross the blood brain barrier?
It's not so much about their polarity as much as their small size.
 
Jul 15, 2015
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Whoops forgot that fatty acids have hydrophobic tails. As for ketone bodies, why is a polar substance able to cross the blood brain barrier?
Direct from Wikipedia: "The blood–brain barrier allows the passage of water, some gases, and lipid-soluble molecules by passive diffusion, as well as the selective transport of molecules such as glucose and amino acids that are crucial to neural function." https://en.wikipedia.org/wiki/Blood–brain_barrier

For the purposes of the MCAT, fatty acids cannot cross the BBB. However, recent studies have shown that some FAs can cross the barrier, but their metabolism in the brain isn't well understood.
 

aldol16

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Direct from Wikipedia: "The blood–brain barrier allows the passage of water, some gases, and lipid-soluble molecules by passive diffusion, as well as the selective transport of molecules such as glucose and amino acids that are crucial to neural function." https://en.wikipedia.org/wiki/Blood–brain_barrier

For the purposes of the MCAT, fatty acids cannot cross the BBB. However, recent studies have shown that some FAs can cross the barrier, but their metabolism in the brain isn't well understood.
Sometimes Wikipedia goes into too much detail and lists too many exceptions and sometimes it goes into too few details and lists too few exceptions. Just a caveat.

I suppose you're talking about something like this? http://pubs.acs.org/doi/full/10.1021/acs.molpharmaceut.5b00580. It's important to note that fatty acids still cannot cross the BBB by themselves by virtue of their polarity (or lack thereof). They need carrier proteins that bind to them and take them across.