Phil and I are breaking down passage 2 in Blueprint MCAT full-length one on nuclear physics. Plus, we’ll give tips for tackling high-level questions on the MCAT.
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[02:26] Passage 2
Nuclear power plants harness the energy of uranium 235 fission and convert it to electricity fission is initiated by bombarding uranium 235 with neutrons, which splits them into smaller atoms, which then induce fission of nearby uranium 235 causing a chain reaction that releases energy.
In 2011, power systems at Fukushima Daiichi NPP in Japan were disabled by a tsunami, causing the release of radiation and volatile radio nuclides, unstable isotopes that undergo decay into the surrounding environment. In Fukushima, there is concern about the effects of the radiation and radionuclide contamination on human animal and plant life. Nuclear radiation can damage DNA, induce cancer, and can cause cell death.
Scientists investigated two examples of DNA damage believed to be caused by the accident, one in Japanese fir trees and one in humans.
Researchers noticed that the Japanese for trees near the NPP had a higher rate of morphological defects and those in uncontaminated trees, including deletion of the leader shoots and buds produced after the accident.
This led to concern about the release of radionuclide iodine 131, an isotope with a half-life of eight days, and undergoes beta minus decay followed by gamma emission. Because the body can concentrate iodine in the thyroid, exposure to these radioactive isotopes can cause thyroid cancer and humans.
Researchers measured average 131 contamination levels in five areas A through E near the NPP. Each of which covered six square miles with similar populations.
Phil’s notes: There are probably going to be questions here. The MCAT is going to expect you to know about the alpha, beta, and gamma decay. And so you want to make sure that you have a good understanding of this. There will be some questions to assess the possibility that iodine 131 contamination was the cause of leader shoot deletion and thyroid cancer.
They mentioned the average number of leader shoot deletions of the 2012 buds and the incidence of pediatric thyroid cancer cases in the area between 2012 and 2014 the data is summarized below in table one.
[06:28] Table 1 Description
Table 1 is entitled I-131 Contamination, Morphological Defects, and Thyroid Cancer. It consists of five different areas here, labeled from highest to minimal contamination of this 131 iodine.
This leader shoot deletion rates per number of trees, which looking at this doesn’t show any correlation with highest to minimal as far as the deletion rate per 100 trees. The cancer incidents again look very similar in 2012 and the cancer incidents in 2013.
It looks there’s a potential correlation from highest to minimal with more than double the cancer incidents a year out.
And then the cancer incidents in 2014. Two more years, it’s five to six times higher. So obviously, looking at this, the deletion rate doesn’t look there’s a correlation potentially. But the cancer incidence obviously looks there’s some statistical significance as time goes on.
In 2012, they’re all pretty same within 100 of each other. And then in 2014, we have 2000 in the minimal contaminant area and 12,000 in the highly contaminated area. So something is definitely going on there.
Just side note, plants tend to not show mutations nearly as quickly as animals do, because our cell division occurs at a higher rate. But again, they’re not going to test us on botany, though. And so this that might be why we see this setup here.
[08:44] Question 6
Which of the following are products of the decay of Iodine-131 ?
A and B are both ionizing radiation
C and D are non-ionizing radiation
My thought process: To be cancerous and to cause effects, it has to be ionizing. So that’s just that, I think a knowledge thing right off the bat, so I’m not even going to waste my time with C and D. I’m going to stay with A and B because we know that this 131 is causing cancer.
So what we have here are Xenon-131 and Tellurium-131. If I go to my periodic table, and I look at Iodine, so Xenon is one up and Tellurium is one down or one to the left one to the right.
If I give up an electron, I’m assuming that’s giving up an electron, a product of that decay, then I’d go down to Tellurium-131.
[10:17] Phil Explains the Answer
Correct Answer: (A) Xenon-131
The passage tells us that we have a couple kinds of radiation that are occurring. We have gamma emissions or emitting light, we’re doing gamma decay. And we’re also doing beta minus decay.
In gamma decay, just the nucleus reorganizes slightly, and it becomes more stable. And so those from higher energy to low energy just emit energy because we can’t create or destroy energy.
So if it goes from high to low, it’s got to give off that energy somehow, and it gets it off as light. It just emits a photon to the highest energy photon that exists.
In terms of types, it’s not a radio wave or visible light, it’s going to be again ray, which is why it’s called gamma decay. And that’s got enough energy to knock an electron off of all sorts of molecules, which is why it’s damaging to the DNA.
The reason ionizing radiation is dangerous is that it’s ionizing our DNA and making it reactive by knocking an electron off of it.
The beta decay is a little bit trickier. There are two types of beta decay: beta plus and beta minus. In one case, a proton turns into a neutron. And in one case, a neutron turns into a proton.
Depending on the kind of decay we have, we’re going to either gain a proton or lose a proton, which is where the Tellurium and Xenon come in. We figure out which direction is going to go. The reaction has to be balanced.
And so if I have a proton turned into a neutron, that means I go from a positive thing to something with no charge. Then the decay product I get must be positively charged.
So you have a proton turns into a neutron at a positron or beta minus that’s a neutron turning into a proton and an electron. That way, it’s balanced in terms of charge.
Because we know we’re getting an electron, and we know the answer has got to be a or b because it’s ionizing, that means we must have a neutron turning into a proton. Because we’re also getting this negative charge as well with the electron. So that means we’re getting a proton. So iodine is going to turn into Xenon.
The easiest way is to just make sure that it’s a balanced reaction.
[13:22] Question 7
Why did the researchers choose to study pediatric rather than adult thyroid cancer cases?
(A) Thyroid cancer is not otherwise present in children.
(B) Thyroid cancer is not otherwise present in adults.
(C) Children receive a higher relative dose of iodine 131 at the same contamination levels
(D) Children receive a lower relative dose of iodine 131 at the same contamination.
[13:58] Thought Process
Correct Answer: C
They studied children because the children’s thyroid is taking up more iodine. Their metabolic level is higher versus adults where the metabolic level is a little bit lower. So we have a little bit less concern of that uptake.
Plus, it’s the idea that just children are smaller than adults so children have a higher dose of the same contaminate levels.
Even from the table, they show us that the incidence rates of thyroid cancer in the minimal area was 2000 in 2012, which is when the nuclear power plant started leaking.
The passage kind of tells us that A is not right because they tested the incidence of pediatric thyroid cancers and there was pediatric thyroid cancer. So it can’t be right because the past tells us there are cancers in the pediatric population.
[16:23] Question 8
Several families in areas four and five who read the results of the study believe that their children have no increased risk of developing thyroid cancer. Is this conclusion accurate?
Phil’s notes: Going back to areas four and five, we have the low and minimal exposure, that contamination of the iodine 131. The cancer incidents in 2012 was 2377 for low, 2344 for 2014. So over two years, the incidents did not increase in minimal. There was 2200 in the minimal area and still 2200 in the 2014 sampling, so it looks there’s no increase at all the incidence is the same across the board.
(A) No. The mutations caused by Iodine 131 exposure may take more than three years to manifest as cancer
(B) No. Iodine 131 has a half-life of only eight days.
(C) Yes. Table one shows that there was no increase in cancer in areas four and five in years 2013 and 2014.
(D) Yes. The radiation given off by iodine 131 is not mutagenic.
[17:44] Thought Process
Correct Anwer: A
Automatically D right off the bat. We know that that’s not true. The question is, “is the conclusion that these children have noticed increase risk true?” And it’s easy to jump to (C) yes, it must be true because the table says from 2012 to 2014. There’s no increased risk.
But what if we test it in 2015, and now all of a sudden, there’s a huge spike in these cancers and it just took longer to manifest.
And so the answer here is (A) No, the mutations caused by adding 131 exposure may take more than three years to manifest.
This is tricky because if you go just purely from the data on the table, C is the right answer. But that’s not the correct choice for this question.
This question is actually testing some outside knowledge of knowing that being exposed to something makes you more likely to get cancer later. That may not show up immediately. That may show up way down the line. There’s a combination of scientific reasoning and of not extrapolating too far.“The MCAT definitely wants you to understand what's going on with cancer biology. Cancer shows up on pretty much every exam.”Click To Tweet
It’s this idea that mutations happen. I may mutate my DNA, and not get cancer immediately. It takes an accumulation of lots of mutations in order to lead to cancer.
[20:35] Question 9
It’s found that small to moderate doses of iodine-131 are more likely to cause thyroid cancer than extremely large doses. Which of the following provides the best explanation for this observation?
(A) Small doses are not absorbed by the body.
(B) Extremely large doses of iodine 131 are not absorbed by the body
(C) The radiation released by large doses of iodine 131 is enough to damage DNA without killing cells.
(D) The amount of radiation released by small doses of iodine 131 is enough to damage DNA without killing cells.
[21:13] Thought Process
Correct Answer: D
A doesn’t make sense, because it just told us that small doses are more likely to cause cancer, so they have to absorb. So A is immediately gone.
It’s the idea that it does absorb if you put like five items, your body will pick up the five but if you put 50 it won’t pick any of them.
So D, just the high doses of iodine 131 basically just takes a bazooka to the DNA, which means it’s not going to turn into cancer because the cell is just dead.
The small mutations without killing the cell is what you kind of need in order to develop cancer here. This is an interesting scenario where the middle ground is actually the most dangerous. Lots of iodine will kill the cells. And where there’s no iodine, and you’re fine.