MCAT practice questions

[goldstandard_mcat]

The bacterium E. coli is normally killed by low concentrations of the antibiotic streptomycin. In a standard E. coli population, about 1 in 106 cells (0.0001%) are mutants that are resistant to streptomycin; that is, they can grow and divide in the presence of the antibiotic. Suppose that a rapidly growing population of E. coli is exposed to low levels of streptomycin. What percentage of the cells will be resistant to streptomycin after 20 generations in the presence of the antibiotic?
A) None of the cells
B) About 0.0001%
C) About 20%
D) About 100%

Answer: D. Tricky, tricky! In all likelihood, even after 1 or 2 generations all bacteria sensitive to the antibiotic will be dead leaving only the mutants (who are resistant to the antibiotic) to grow and divide! (BIO 2)

[goldstandard_mcat]

Which of the following best describes the process of oxidation?
A. The process in which electrons are added
B. The process in which oxygen is produced
C. The process in which hydrogen content increases
D. The process in which the oxidation number increases

[goldstandard_mcat]

A person stands on a scale in an elevator (true, it does not happen everyday but it is the sort of insanity that happens on the MCAT!). She notices that the scale is reading lower than her normal weight. Which of the following most reasonably describes the motion of the elevator?

A. It is moving down and slowing down.
B. It is moving down at constant speed.
C. It is moving up and speeding up.
D. It is moving up and slowing down.

[goldstandard_mcat]

Which of the following is an example of positive feedback?

A. A body temperature of 39 C causes a further increase
B. Elevated TSH results in elevated thyroxine
C. Calcitonin and parathormone regulate calcium levels
D. Increased TBG leads to an increase in TSH

[goldstandard_mcat]

Which of the following is an example of positive feedback?

A. A body temperature of 39 C causes a further increase
B. Elevated TSH results in elevated thyroxine
C. Calcitonin and parathormone regulate calcium levels
D. Increased TBG leads to an increase in TSH

Answer A

Endocrine glands are often linked to neural control centers by homeostatic feedback mechanisms. The two types of feedback mechanisms, which happen to be frequent visitors to the real MCAT, are negative feedback and positive feedback. Negative feedback decreases the deviation from the normal value, and is important in maintaining homeostasis and thus controls most endocrine glands.

The classic example of negative feedback is the thermostat in your home. As the temperature rises (deviation from the ideal normal value), the thermostat detects the change and triggers the air-conditioning to turn on and cool the house (or, it turns off the heating). Once the temperature reaches its thermostat setting (normal value), the air conditioning turns off.

Positive feedback controls self-perpetuating events that can be out of control and do not require continuous adjustment. For example, in positive feedback the original stimulus is promoted rather than negated, thus the deviation from the normal value increases. Unlike negative feedback that maintains hormone levels within narrow ranges, positive feedback is rarely used to maintain homeostatic functions.

If calcium decreases, the parathyroid glands sense the decrease and secrete more parathyroid hormone. The parathyroid hormone stimulates calcium release from the bones and increases the calcium uptake into the bloodstream from the collecting tubules in the kidneys. Conversely, if blood calcium increases too much, the parathyroid glands reduce parathyroid hormone production. Both responses are examples of negative feedback because in both cases the effects are negative (opposite) to the stimulus.

If your increased body temperature triggers a further increase in body temperature (getting further away from the normal), then this is a classic example of positive feedback.

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