MCAT Biology Related Questions

[Let'sGo1990]

This is a bit of a poorly worded question. Remember that this may not be a neuron/muscle cell, and we're just looking at Resting Membrane Potential (RMP). Let's assume we don't have the ligand/ion gated channels on this cell, but we do have leak channels. Basically what Na+/K+ ATPases helps to do is keep the net positive charge outside of the cell to build both a conc'n and potential gradient. once Na+/K+ ATPase is inhibited, the net (+) is inside the cell, in the form of a Na+ ion.

 

At this point you're probably wondering "why doesn't the K+ just leak out of the cell to restore Potential?" That's b/c of eq'm potential, where there is a certain point of eq'm where it's actually less favourable for K+ to leave. Thus K+ is more/less trapped within the cell, and Na+ is trapped as well because of the impereability of the membrane. Because of these trapped molecules, the interior is less negatively charged. Also, the cell would swell a little bit more because its relative intracellular conc'n would increase compared to when Na/K ATPase is working (b/c of the trapped Na), so there would be a bit more water inside. B is the best answer.

 

Just saw this. Thanks for the answer.

[tms]

I am still a little confused because of the explanation in the wiki answers link posted above. I thought that s orbital has the highest energy and that's why electrons are lost from the s orbital before a d orbital. However, in the explanation, it seems that the d orbital is of higher energy (as in the case of Cu) and hence, less likely to loss electrons.

 

Can someone please explain this? Thanks!

[thehumanmacbook]

However, chromium and copper have electron configurations [Ar] 3d5 4s1 and [Ar] 3d10 4s1 respectively, i.e. one electron has passed from the 4s-orbital to a 3d-orbital to generate a half-filled or filled subshell. In this case, the usual explanation is that "half-filled or completely filled subshells are particularly stable arrangements of electrons" - from http://en.wikipedia.org/wiki/Electron_configuration

 

if you read further, it states: The apparent paradox arises when electrons are removed from the transition metal atoms to form ions. The first electrons to be ionized come not from the 3d-orbital, as one would expect if it were "higher in energy", but from the 4s-orbital. The same is true when chemical compounds are formed. Chromium hexacarbonyl can be described as a chromium atom (not ion, it is in the oxidation state 0) surrounded by six carbon monoxide ligands: it is diamagnetic, and the electron configuration of the central chromium atom is described as 3d6, i.e. the electron which was in the 4s-orbital in the free atom has passed into a 3d-orbital on forming the compound. This interchange of electrons between 4s and 3d is universal among the first series of the transition metals.[16]

 

To be honest, it's really just an exemption that you have to remember, especially for Cu and Cr. The MCAT probably will try to "trick" you for this, but probably they won't ask for an explanation into the theory.

[tms]

Thanks!

 

What about Mn and Zn - are they also more stable that let's say Mg or Ca since they too have half filled and full filled d orbitals and are likely to lose the electrons from the s subshell first? or does the exception only apply to those with anomalous electron configurations?

[thehumanmacbook]

Thanks!

 

What about Mn and Zn - are they also more stable that let's say Mg or Ca since they too have half filled and full filled d orbitals and are likely to lose the electrons from the s subshell first? or does the exception only apply to those with anomalous electron configurations?

 

I don't think with just the e- configuration you would be able to tell reactivity for these two - because compared to Cu and Cr, Mn and Zn should be pretty happy with their e- configuration (full s, somewhat full d). Cu and Cr have only one e- in their s orbital, so they would probably want to get rid of it to achieve the nearest noble gas config.

[bpatient]

where is this chem question from, what book?

[tms]

It is from the TPR science workbook.

[dazzle]

The TPR is pretty unclear about this issue concerning crossing over during meiosis:

 

You have one maternal chromosome that is replicated during S phase and one paternal chromosome replicated as well. These two homologous pairs link up together during prophase I and "trade" genetic information in a process called crossing over.

 

Evidently, the homologous chromosomes, which were not identical to begin with (paternal vs maternal), are not identical after crossing over; they are not even identical to what they each were like before crossing over. But during the anaphase II separation of sister chromatids, are those paired sister chromatids identical to each other or has cross over scrambled everything, producing four varied copies of the chromosome in question? Please explain.

 

Thanks!

 

In the following: I sketch what is in a single cell at a time.

 

person: |(maternal) |(paternal)

S Phase: || ||

Prophase I (crossing over, two chromosome pairs cross over): ||||

Prophase II (after first cell division, haploid cell with replicated chromatids, are these identical?): ||

Anaphase/Telophase II (sister chromatids are separated to yield gamete): |

[Savac]

The sister chromatids will not be identical after crossing over has occurred. Crossing over can occur at several different locations on a chromosome. It is statistically improbable for both chromatids to end up identical.