MCAT Question of the Day

Correct Answer: A. A body temperature of 39 ^oC causes a further increase

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.

Today’s question has been brought to you by Gold Standard MCAT.

Get more passage-based questions at Gold Standard MCAT.

Subscribe below to receive the MCAT Question of the Day delivered straight to your inbox every morning.

Correct Answer: C. 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).

Today’s question has been brought to you by Gold Standard MCAT.

Get more passage-based questions at Gold Standard MCAT.

Subscribe below to receive the MCAT Question of the Day delivered straight to your inbox every morning.

Correct Answer: D. The process in which the oxidation number increases

Oxidation is the loss of electrons or an increase in oxidation state by a molecule, atom, or ion. The mnemonic LEO is A GERC stands for Loss Electrons Oxidation is Anode, Gain Electrons Reduction Cathode.

Today’s question has been brought to you by Gold Standard MCAT.

Get more passage-based questions at Gold Standard MCAT.

Subscribe below to receive the MCAT Question of the Day delivered straight to your inbox every morning.

Correct Answer: D. Less that 7.0 but the water is neutral.

It is not necessary to calculate anything to solve this problem but we will go through the steps.

The dissociation of water (note carefully that the ratio of the products is 1:1; also keep in mind that this is the basis of the neutrality of pure water: acid units = base units):

H2O + H2O <-> H3O⁺ + OH^–

The expression for the self-ionisation of water or the water dissociation constant:

Kw = [H3O⁺][OH^–] = [H⁺][OH^–]

At room temperature, keeping in mind that the dissociation of the ions is 1:1, we get: 10^-14 = [H⁺][OH^–] = [H⁺]²

And so [H⁺] = 10^-7 and then -log[H⁺] = 7 = pH

Doing the identical math but using Kw = 10^-13 yields a pH of 6.5 BUT because the ratio of the ions is still 1:1 (if not, you would not have done the calculation) thus it must be neutral.

Today’s question has been brought to you by Gold Standard MCAT.

Get more passage-based questions at Gold Standard MCAT.

Subscribe below to receive the MCAT Question of the Day delivered straight to your inbox every morning.

Correct Answer: D. 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:

On average, there is one question regarding diffusion or osmosis on the MCAT. Diffusion is the movement of a molecule across a semi-permeable barrier 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 dialyser 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 water – specifically water – across a semi-permeable membrane from a region of low solute concentration to a region of 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.

Today’s question has been brought to you by Gold Standard MCAT.

Get more passage-based questions at Gold Standard MCAT.

Subscribe below to receive the MCAT Question of the Day delivered straight to your inbox every morning.

Correct Answer: D. Camphor has two stereogenic centers and exists as a pair of enantiomers.

Position 1 and 4 as labeled in the diagram below (Ref: csi.chemie.tu-darmstadt.de) are the only 2 chiral (stereogenic) carbons in camphor since they are attached to 4 different substituents, each. Notice that because of the carbonyl group being closer to position 1, and because of the methyl substituent at position 1, the molecule is NOT symmetric when looking at it from above. Because a line of symmetry cannot be drawn, thus the molecule is NOT meso, and thus it has 2 non superposable (= non superimposable) mirror images (= enantiomers).

Additional learning points: Note that the maximum number of stereoisomers is given by 2 to the power of n, where n is equal to the number of chiral carbons. So the maximum for camphor, having 2 chiral carbons, would be 2 squared which is 4. However, camphor yields only two stereoisomers because the relative configuration of both centers is constrained in the bicyclic ring system (i.e. 2 closed rings in one system).

Also, note this Mnemonic: MeSo = Mirror of Symmetry.

Today’s question has been brought to you by Gold Standard MCAT.

Get more passage-based questions at Gold Standard MCAT.

Subscribe below to receive the MCAT Question of the Day delivered straight to your inbox every morning.

Correct Answer: A. decreases nonlinearly.

The only forces that act perpendicular to the incline are the normal force and the perpendicular component of weight. There is no acceleration perpendicular to the incline for this stationary object. The normal force is equal to the perpendicular component of the weight, which is given by mg cos. As the angle increases, the cosine of the angle must decrease. The graph of the normal force N vs. would show a curve, not a line and so the change is nonlinear.

Today’s question has been brought to you by Gold Standard MCAT.

Get more passage-based questions at Gold Standard MCAT.

Subscribe below to receive the MCAT Question of the Day delivered straight to your inbox every morning.

Correct Answer: D. B > A > D > C

When in aqueous solution, aldehyde and ketones groups can form hydrates: this is simply the result of addition of water to the carbonyl (the partial negative charge of the oxygen in water attacking the partial positive charge of the carbonyl group).

The ability to form a hydrate (addition of water across the C=O bond) can be seen as measure of the stability of a carbonyl group.

Factors affecting this are the electronic and steric effects of substitutents attached to the electrophilic carbonyl carbon. Hydrogen bond acceptors also aid hydrate formation.

Strained cyclopropanone B is the most ready to form a hydrate as the ring bond angle is far from the ideal trigonal planar angle. This strain is relieved somewhat upon formation of a near tetrahedral hydrate.

Formaldehyde A is more reactive (unstable) than aldehyde D which in turn is more reactive than ketone C. Across this sequence more +I methyl groups are being added to the electrophilic C=O carbon atom. In addition, the less hydrophobic the carbonyl compound (and the corresponding hydrate), the better the solvation through intermolecular hydrogen bonding (i.e., A > D > C).

Today’s question has been brought to you by Gold Standard MCAT.

Get more passage-based questions at Gold Standard MCAT.

Subscribe below to receive the MCAT Question of the Day delivered straight to your inbox every morning.

Correct Answer: C. 25 N

This is a ‘Double Block tackle’ and as there are four ropes lifting the 10 kg (100 N weight), each has a tension of 100/4 = 25 N.

The only catch is he needs to pull through four times the rope than would be required to simply lift the weight.

Subscribe below to receive the MCAT Question of the Day delivered straight to your inbox every morning.

Correct Answer: C. Newton’s Third Law of Motion

Newton’s Third Law of Motion states that: “When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction to that of the first body.”
Newton’s First Law of Motion deals with unbalanced forces or inertia and his second law with force, mass and acceleration, F = ma.
Note: Newton only had three Laws of Motion.

Subscribe below to receive the MCAT Question of the Day delivered straight to your inbox every morning.