Hello Lydia,

Q43
The key to this is understanding what they were saying about the conditions of the amino acids while inside the active site. The passage states specifically that the amino acids must be deprotonated in order to coordinate with Zn2+. For that to happen, we need a negative charge. Without knowing too much about cysteine, we can consider histidine, and know that as a basic amino acid, it generally likes to abstract protons and become positively charged (ie. -NH2 --> NH3). For it to be negatively charged and deprotonated (ie. -NH2-->NH-), its going against what it likes to do, thereby becoming destabilized. Let's rule out answers B and C.

Now about the pKa; pKa increases if something is becoming more basic, and decreases if something is becoming more acidic. For the amino acids to deprotonate, it would not desperately want to be protonated due to its negative charge, driving it to be more acidic in nature. Therefore, the pKa would be decreased.

Q49
In this case, we also have to consider the evidence present in all other lanes as well. In PNS and Sub, we do not see the same increase. So we cannot say that A is correct.

Q10
This was a trickster! Basically whats going on is you are given two solutions with a semi-permeable membrane which mimics biological membranes. You are now given two solutions that appears to be at two different concentrations. 100 mM Na3PO4 and 200 mM NaCl. Your instinct might be to say "well, water will be moving towards the more concentrated salt solution, so it will move to the right to equilibriate that concentration" and if it was the same salt, you would be correct. However, what's key to notice is that you are working with different number of ions in each salt solution, and that movement to equilibrium will be dictated by the concentration of ionized particles.

Every salt in solution becomes ionized. So, when you have Na3PO4, you are ending up with 3 ions of Na+ and 1 ions of PO4^3- for a total of 4 ions. Therefore even though you have 100 mM of Na3PO4, you actually have a total of 400 mM of ions. In a similar logic, with 200 mM NaCl, you have 2 ions dissociated in solution, Na+ and Cl-. So even though you have 200 mM NaCl, you actually have 400 mM of Na+ and Cl- after dissociation.

Since the two solutions on either side are isotonic, there will be no net movement of water.

Q11
Its true that with normal cell replication that we would result in the formation of tetrads. However, if we remove the DNA replication step, and we move forwards towards the division step, we would not have a tetrad and instead have our normal number of 23 homologous pairs with each chromosome containing only one chromatid (no sister chromatids), which means monads.

Hope this helps, happy studying!

Molly