MCAT practice questions

MCAT preparation and implications of MCAT scores.

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Re: MCAT practice questions

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.

Let’s randomly assign the upwards direction as positive. If up is positive, keeping in mind that her weight is directed down (i.e. towards the center of the Earth), the scale pushes up so that she does not go through the scale, and we know that the sum of forces is equal to ma from Newton’s 2nd Law, we get:
Fscale - weight = ma

Examine the preceding equation again keeping in mind the following interpretation: The scale reading is less than her normal weight, so ma and thus the acceleration must be negative; i.e., down. Downward acceleration means either: (1) something speeds up while moving down, or (2) something slows down while moving up (i.e. upward deceleration).

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Re: MCAT practice questions

The plasma proteins that bind thyroid hormones are albumin, a prealbumin called thyroxine-binding prealbumin (TBPA), and thyroxine-binding globulin (TBG). The free thyroid hormones in plasma are in equilibrium with the protein-bound thyroid hormones in the tissues. Free thyroid hormones are added to the circulating pool by the thyroid. It is the free thyroid hormones in plasma that are physiologically active (increasing the metabolic rate) and imbalances in these hormones result in thyroid disease. In thyroid storm, a form of hyperthyroidism, the normal body temperature of 37.5 C may rise to over 40 C.
Parathormone and the thyroid hormone calcitonin work antagonistically to regulate the plasma calcium and phosphate levels.

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

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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goldstandard_mcat
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Re: MCAT practice questions

Embryonic mesoderm in humans develops into which of the following structures?

A. Liver and pancreas
B. Brain
C. Lining of the respiratory tract
D. Skeleton

The mesoderm develops into muscle and connective tissue which includes blood and bone (skeleton!), as well as circulatory, reproductive and excretory organs. The ectoderm produces the epidermis and the nervous system including the brain. The endoderm will become the epithelial lining of the respiratory tract, and the digestive tract including the glands of the accessory organs (i.e. the liver and pancreas). The stage of the embryo where the 3 primary germ layers first develop is called the ‘gastrula’.

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goldstandard_mcat
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Re: MCAT practice questions

If the sum of all the forces acting on a moving object is zero, the object will:

A. decelerate and stop.
B. accelerate uniformly.
C. change the direction of its motion.
D. continue moving with constant velocity.

Explanation: For something to change direction or accelerate there must be a net force acting on it (Newton’s 2nd Law).
Also, Newton’s 1st Law states that an object in motion will stay in motion at constant velocity (a form of inertia) unless acted upon by a net (an unbalanced) force.

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Re: MCAT practice questions

Which amine has the highest boiling point?
Image: http://media-cache-ak0.pinimg.com/origi ... 4c32ec.jpg

The primary amines A and B can donate two hydrogen bonds and accept one (due to the lone pair on N). Generally, for compounds with the same functional group, the higher molecular weight compounds have a higher boiling point due to increased opportunity for intermolecular van der Waals (weak, but better than nothing) interactions. Of course, even though a periodic table is available during the exam with a click of a button, you do not need to calculate the molecular weight since you can quickly determine the number of carbons of the different N-containing structures which easily reveals B as the “heaviest”.

Helpful hint: If all other factors are the same, increased molecular weight means that it is harder to push that molecule into the vapor phase meaning the boiling point would be higher.
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