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Tutor profile: Hunter F.

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Hunter F.
Medical Student
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Questions

Subject: Spanish

TutorMe
Question:

Please describe the differences between the preterite and imperfect tense in Spanish.

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Hunter F.
Answer:

In Spanish, there are two forms of past tense that help us understand the general time frame of how an action has transpired. The preterite form is used for completed actions that occur at a relatively shorter time-frame. One example is "I ate the chicken," which is translated to "Yo comí el pollo." The verb in this case is highlighting the fact that the action was completed. This is contrasted with the imperfect tense, which is used to describe actions that were transgressing over a more prolonged period of time. For example, "As a child, I used to love swimming," would be translated as "Como un niño, me encantaba nadar." In this case, the verb encantar is translated in the imperfect tense to highlight that the action wasn't completed, but rather carried out over a longer period of time (childhood). It is important to consider how the meaning, or semantics, of a sentence is altered by changing this verb tense. If I were to change the example of the preterite tense that I used before for the imperfect tense, we can see how the meaning changes. The sentence "Yo comía el pollo." is translated as "I was eating the chicken" or "I used to eat the chicken." In Spanish, we also tend to combine the two verb tenses in the same sentence as demonstrated in the examples below: "Yo corría cuando resbalé." - "I was running when I slipped." "Ellos nadaban cuando su amiga llegó." - "They were swimming when their friend arrived." *Note - I am fluent in Spanish and, if necessary, I would encourage Spanish immersion as a way to teach it unless I need to explain more complicated grammar like above. In these cases, I will switch to English to explain.

Subject: MCAT

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Question:

Explain why cooperativity in hemoglobin is necessary for the optimum oxygen transport.

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Hunter F.
Answer:

Hemoglobin exhibits cooperativity, which is a biochemical phenomenon by which binding of a molecule to one subunit of a protein complex is not independent of binding to another. In the example of hemoglobin, when no oxygen molecules are bound to the heme prosthetic groups of the four subunits of hemoglobin, affinity for oxygen is highest. Physiologically, this is advantageous as deoxygenated blood (where hemoglobin's affinity for oxygen is highest) is located in the pulmonary capillaries where oxygen binding should be optimized. On the other hand, when all four oxygen molecules are bound to each of the four subunits of hemoglobin, its affinity for oxygen is lowest. This is also advantageous, as oxygenated blood located in the systemic capillaries are more likely to release oxygen where they are taken up by target tissues. The reason hemoglobin has varying affinities for oxygen is because of a conformational change from the "tight" or low affinity state, and the "relaxed" or high affinity state. If we were to graph oxygen binding to hemoglobin as a function of oxygen concentration, we will see that oxygen does not readily as well at lower concentrations. However, when concentrations of oxygen are highest (in the lungs) there is a much higher affinity for oxygen. The shape of this graph is characterized as 'sigmoidal.' This is contrasted with myoglobin's curve, which is hyperbolic. Since myoglobin consists of only one subunit, affinity for oxygen is fixed. If we overlap the two curves, we can see that myoglobin's binding is greater than hemoglobin at lower concentrations, which facilitates passage of oxygen from the carrier molecule of hemoglobin, to the myoglobin which is found in muscle tissue.

Subject: Anatomy

TutorMe
Question:

Outline the journey that an oxygen molecule takes places from air to brain cell in the right temporal lobe.

Inactive
Hunter F.
Answer:

Air at sea level is constituted of approximately 21% oxygen. When we inhale, the phrenic nerve stimulates the diaphragm to contract and paradoxically move downward doing the process of inhalation. This allows for a negative intrathoracic pressure. Since fluids (including air) move down a pressure gradient, air enters through our nares, nasopharynx, oropharynx, trachea, bronchi, bronchioles, respiratory ducts, and, finally, our lung parenchyma. Within the grape-like alveoli of our lungs, gas exchange takes place. Thin blood-filled capillaries line these alveoli. Hemoglobin within the red blood cells are in the "loose" state and have a higher affinity to bind oxygen. Oxygen, a nonpolar molecule diffuses through the alveolar membrane and into the blood. The blood returns to the left atrium of the heart via the pulmonary veins, which contain oxygenated blood. During the process of diastole, blood passes through the bicuspid (mitral) valve and into the left ventricle. During systole, contraction of the left ventricular myometrium results in an increased pressure in this ventricle and closure of the mitral valve. As the ventricle continues to contract isometrically, the pressure increases to such an extent that the aortic valve opens and blood passes into the aorta. At the aortic arch there are three branches: the brachiocephalic artery, the left common carotid, and the left subclavian artery. To reach the right brain, blood enters the brachiocephalic artery, which subsequently branches into the right common carotid artery. This artery then divides into the internal and external carotid artery. The external carotid artery supplies blood outside of the skull while the internal carotid supplies the brain inside of the skull. As we follow the blood into the right internal carotid artery, it then enters the Circle of Willis, an anatomic arterial structure from which blood supplies the brain. From here, blood enters the middle cerebral artery (MCA), which supplies the lateral-most parts of the cerebral cortex, including the temporal lobes. The MCA continues to divide into smaller and smaller arteries that are then called arterioles. At the capillary level, oxygen must pass through the blood-brain barrier, which protects the sensitive brain from polar toxins that might have bypassed the liver. An oxygen molecule, being extremely small and nonpolar, crosses this lipid barrier through simple diffusion. First, the oxygen crosses the endothelium of the capillary, then the extracellular matrix, and finally, the foot processes of glial cells called astrocytes. Once absorbed into the cerebrospinal fluid, oxygen then diffuses into the neurons where they are used to produce ATP in the mitochondria.

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