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RC - Supported Detail

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RC - Supported Detail [#permalink] New post 04 Jul 2012, 13:04
Biologists have long been fascinated with the ability of certain animal species to survive in the regions of the planet most hostile to life, through the process of biological adaptation. The deer mouse and the llama, which live in mountainous regions, and the Weddell Seal, which survives deep below the ocean surface without gills, face a particular challenge to their survival: maintaining respiration in low-oxygen environments.

For deer mice and llamas, the problem is dealing with hypoxia, the decreased availability of oxygen at high altitudes. These animals live in environments where the atmospheric pressure is too low to force sufficient amounts of oxygen into their lungs. Deer mice have developed an adaptation in response: Their blood has a decreased amount of DPG, an organic phosphate in mammals that affects the ability of hemoglobin to bind to oxygen. When less DPG is present, the hemoglobin, which delivers blood to the lungs, gains a higher blood-oxygen affinity; the hemoglobin is able to form tighter bonds with oxygen and deliver it to the lungs more effectively. However, if the deer mouse's DPG levels drop too low, the hemoglobin binds to oxygen so tightly that it is unable to release the oxygen into lung tissues.

Though llamas also have lowered levels of DPG, they have other adaptations that allow them to breathe normally at high altitudes. Llamas have red blood cells with higher concentrations of hemoglobin, allowing llamas to extract more oxygen from the air and transport that oxygen to the lungs with great efficiency. Furthermore, llamas' blood has a low pH level. Because llamas' blood is more acidic, the llama’s internal oxygen pressure increases, which makes up for the low pressure of the external environment.

The Weddell Seal faces the same problem of limited oxygen availability when it dives 700 meters below sea level in pursuit of prey. What is more, the Weddell Seal’s lungs collapse during these dives so as not to be crushed by the enormous pressure of the water at such depths. However, the seal’s muscle tissues contain high levels of oxygen-binding myoglobin. In essence, the seals store oxygen in their muscles instead of their lungs, allowing their oxygen supplies to diffuse more slowly and last for a much longer time.


According to the passage, the llama has adapted to breathe at high altitudes in which of the following ways?

I ) By developing a large number of red blood cells
II) By developing blood with a low pH level
III) By breathing in a more efficient manner



(A) I only

(B) II only correct

(C) I and II only

(D) II and III only your answer

(E) I, II and III
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Re: RC - Supported Detail [#permalink] New post 05 Jul 2012, 09:58
Expert's post
AbhiJ wrote:
Though llamas also have lowered levels of DPG, they have other adaptations that allow them to breathe normally at high altitudes. Llamas have red blood cells with higher concentrations of hemoglobin, allowing llamas to extract more oxygen from the air and transport that oxygen to the lungs with great efficiency. Furthermore, llamas' blood has a low pH level. Because llamas' blood is more acidic, the llama’s internal oxygen pressure increases, which makes up for the low pressure of the external environment.

According to the passage, the llama has adapted to breathe at high altitudes in which of the following ways?
I ) By developing a large number of red blood cells
II) By developing blood with a low pH level
III) By breathing in a more efficient manner

(A) I only
(B) II only
(C) I and II only
(D) II and III only
(E) I, II and III


Hi, there. I'm happy to help with this RC passage. :)

Notice, the third paragraph is the one that talks specifically about the adaptions of llama. Let's look at each one of the claims:

I ) By developing a large number of red blood cells --- the passage says explicitly "Llamas have red blood cells with higher concentrations of hemoglobin" Each RBC has more hemoglobin, but says nothing about having more RBC. This is not supported.

II) By developing blood with a low pH level --- there's direct textual evidence for this --- this is supported.

III) By breathing in a more efficient manner --- this is a tricky one, because we have to do some inference. What would it mean to breathe "more efficiently"? Well, the point of breathing, for us and for the llamas, is to get oxygen into the body, that is, to get it from the air into our bloodstream. If an animal can get oxygen from the air to their blood in greater quantities on each breath, to that extent they can be said to be breathing more efficiently. Well, we are explicitly told, llama have more hemoglobin, "allowing llamas to extract more oxygen from the air and transport that oxygen to the lungs with great efficiency." In other words, they breathe more efficiently. This one is supported.

[Reveal] Spoiler:
So, I is not supported, and II & III are. Answer = D


Does all this make sense? Here's a free lesson video on RC strategies that you might find helpful.
http://gmat.magoosh.com/lessons/321-gen ... strategies
Please let me know if you have any additional questions.

Mike :)
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Magoosh Test Prep

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Re: RC - Supported Detail   [#permalink] 05 Jul 2012, 09:58
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