MCAT-Section-2-Biological-Sciences Section Two : Biological Sciences - Practice Questions

Question 4

The human kidney can be damaged from a number of causes resulting in a patient’s inability to filter toxins (i.e. urea) from the body which could result in death. Complete kidney failure is usually first treated with dialysis which “cleans” the blood.

Figure 1: Dialysis. The “dialyzer” is a glass container that has 3 main components: (1) blood percolating through to be cleaned; (2) a dividing membrane; and (3) the dialysate. The latter is liquid containing chemicals used to draw fluids and toxins out of the bloodstream and supply electrolytes and other chemicals to the bloodstream. (kidney.niddk.nih.gov) Considering the information provided, which of the following is NOT consistent with the process of dialysis?

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The membrane in the dialyzer separating the dialysate and the blood must be semipermeable.
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The concentration of toxins in the dialysate must increase during dialysis.
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The concentration of glucose and vital minerals in the dialysate must be similar to that of the patient’s blood.
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Excessive minerals and toxins cross the membrane in the dialyzer by osmosis.

Reference / correct answer:

Excessive minerals and toxins cross the membrane in the dialyzer by osmosis.

On the Surface: Dialysis only refers to the transfer of the solute (diffusion), while the transfer of the solvent is called osmosis.

Going Deeper: Diffusion is the movement of a substance down its concentration gradient (from a region of high concentration to one of lower concentration). This is the process that must be occurring in the dialyzer to remove toxins from the blood (because the dialysate, logically, would have no toxins so the toxins defuse out of the blood into the dialysate). Note that vital substances in the blood should be kept in the blood: options would include having a dialysate with the same concentration of those substances (so they don’t diffuse out of the blood) or by adjusting the permeability of the membrane.

Osmosis is the movement of a solvent across a semi-permeable membrane from a region of low solute concentration to a region of high solute concentration. In Biology and Biochemistry, that solvent is, of course, water. In this context, the easier way to imagine osmosis is that it is simply the diffusion of water across a semi-permeable membrane from a region of high water concentration (i.e. low solute concentration) to a region of low water concentration (i.e. high solute concentration).

So, in summary, osmosis deals with how much water passes through the membrane, while dialysis deals with what type of solute (i.e., sodium, protein, glucose) passes through by simple diffusion.

N.B. Notice the link between diffusion and osmosis and the Second Law of Thermodynamics (Physics). All spontaneous change leads to an increase in the entropy (randomness) of the universe. Of course, biological systems exist because processes which increase entropy can be coupled with processes which increase order, as long as the overall (net) entropy of the universe increases.

Question 5

Hemophilia is a genetically inherited disease that causes the synthesis of an abnormal clotting factor. As a result, hemophiliacs bleed excessively from the slightest injury. The figure below is a partial pedigree for the hemophilia trait in Queen Victoria’s descendants. The pedigree indicates no history of hemophilia for either parent prior to the F1 generation.

If Beatrice had married a hemophiliac and had a son, what is the probability that the son would have been a hemophiliac?

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0%
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25%
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50%
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100%

Question 6

All of the following are example of sensory, or neural, adaptation EXCEPT:

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After putting on a shirt, you eventually no longer feel the sensation of the fabric on your back.
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After first walking into a crowded room, you no longer are distracted by the buzz of conversation around you.
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After first walking outside on a sunny day, you no longer are blinded by the initial brightness of the light.
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After first walking into an anatomy lab, you no longer notice the smell of formaldehyde.

MCAT-Section-2-Biological-Sciences – Section Two : Biological Sciences