MCAT Question of the Day

Correct Answer: C. genes responsible for the promotion of apoptosis

Cancerogenesis is an evolutionary process. Cancer cells are under heavy selective pressure and only those cells that possess appropriate genetic characteristics will survive. All changes that result in increased angiogenesis, cell growth and expression of oncogenes favor cancer development. On the other hand, apoptosis (programmed cell death) represents the way in which an organism eliminates potential cancer cells. Silencing the genes responsible for promotion of apoptosis is an essential step in malignant tumor development.

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Correct Answer: D. The collision is inelastic, and the total kinetic energy in the system will be lower after the collision.

An elastic collision is one in which all of the kinetic energy is conserved in the collision. However, elastic collisions do not exist at large scale in the real world, and in this case the collision is not elastic. Instead, the collision is inelastic, meaning some kinetic energy is lost from the impact, usually as heat, sound, or vibration. The kinetic energy is lost as a result of the contact between the two balls, not because of the friction interaction between the grass and the ball, so answer B is not correct.

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Correct Answer: C. C2H2

In this question, we use the ideal gas law, PV = nRT. First of all, we calculate the number of mols in the volume of 11.2 L. n = PV/RT, or n = (1*11.2)/(0.082*298). This gives us roughly n= 0.5 mol. You can bypass this by remembering that there are 22.4 L in a gaseous mol at STP. Thus, there is 0.5 mol of a substance that we know weighs 13g. The molar mass must be 26 g/mol then. With the knowledge that C = 12g/mol and H = 1g/mol (standard knowledge), we find that the hydrocarbon should be C2H2.

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Correct Answer: C. I and II

Solution: Recall power is energy over time. We will need to know what type of energy is used and then find it. Calculating the amount of energy comes down to remembering the formulas.

The Olympic cyclist exhibits kinetic energy.
[latex]K.E.=\frac{1}{2}mv^2,\ \Rightarrow K.E.=\left(\frac{1}{2}\right)\left(100\right){\left(\frac{200}{10}\right)}^2=20000\ {\rm J,\ }[/latex]
[latex]\Rightarrow {\rm P=}\frac{{\rm E}}{{\rm t}}?\left(\frac{{\rm 20000}}{{\rm 10}}\right){\rm =2000\ Watts}[/latex]

The Olympic weightlifter exhibits potential energy. Potential energy can be thought as the work of the force of weight. Work being distance times force.
[latex]P.E.=mgh,\ \Rightarrow P.E.=\left(200\right)\left(10\right)\left(4\right)=8000\ {\rm J}[/latex]
[latex]\Rightarrow {\rm P=}\frac{{\rm E}}{{\rm t}}?\left(\frac{{\rm 8000}}{{\rm 4}}\right){\rm =2000\ W}[/latex]

The laptop exhibit electrical power. Electrical power is current times voltage.
[latex]P=IV,\ V=IR\Rightarrow P=\frac{V^2}{R}\Rightarrow \frac{{20}^2}{4}=100\ {\rm W}[/latex]

Both Olympic athlete outperform the laptop at 2000 W.

2000 watts is thought to be the maximum power humans can achieve. Olympic level athletes push this limit every Olympic season.

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Correct Answer: B. Succinyl coenzyme A to succinate

There is only one step in the citric acid cycle that causes substrate level phosphorylation: the conversion of succinyl coenzyme A to succinate. GDP and inorganic phosphate combine to form GTP, which is roughly equivalent to the formation of one molecule of ATP. All others do not produce ATP. Oxaloacetate may be formed from pyruvate though it requires the dephosphorylation of one molecule of ATP. Moreover, this reaction is not considered one of the classic reactions in the citric acid cycle. In the conversion of malate to oxaloacetate, NAD+ loses a proton to form NADH.

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Correct Answer: B. 400 N

We can set up a free body diagram representing the situation.

To solve this problem you could use the conservation of work and find the required work to lift himself up against gravity. The distance will be the hypotenuse times the sine, soh-cah-toa.

[latex]W=\overrightarrow{F}\cdot \overrightarrow{d},\ \ W_g=W_F\Rightarrow mg\frac{L}{2}sin\theta =F\frac{7L}{8}sin\theta \Rightarrow[/latex]
[latex]F=mg\frac{4}{7}[/latex]

You could also solve this problem with the understanding of torque. The person is virtually a ridged body, with the point of rotation placed at his feet. While he does this pushup he will be in rotational equilibrium. That means the if his butt move up, his head will also move up at the same rate. With that said the torque on his weight must equal the torque he exerts.

[latex]\overrightarrow{\tau }{\rm =}\overrightarrow{{\rm r}}{\rm \times }\overrightarrow{{\rm F}},\ \ {|\overrightarrow{\tau }}_F|=|{\overrightarrow{\tau }}_w|\Rightarrow F\frac{7L}{8}=mg\frac{L}{2}\ \Rightarrow[/latex][latex]F=mg\frac{4}{7}\Rightarrow \left(70\right)\left(10\right)\left(\frac{4}{7}\right)=400\ {\rm N}[/latex]

It is interesting to note that you only lift a little bit more than half your weight when you execute a pushup.

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Correct Answer: B. Octane.

The boiling point of hydrocarbons increases as the number of carbons increases. Therefore, Heptane has a higher boiling point than hexane, which has a higher boiling point than pentane. This is because the intermolecular forces between molecules keep the hydrocarbons in solution, requiring more heat to overcome these forces and become a gas. In the case of branched molecules, the hydrocarbons are not able to “stack” as easily as straight-chain hydrocarbons. This reduces intermolecular interactions and, subsequently, the boiling point. Even though octane, 2,2-dimethylhexane, and 2-methylheptane have eight carbons, octane has the highest boiling point.

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Correct Answer: B. The solubility of Fe(OH)3 is pH-dependent owing to the presence of [OH-] in the solubility expression.

Note solubility, not solubility product, is hydroxide-dependent. D is wrong because [H+] and [OH-] are related.

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Correct Answer: A. 16 N-s

This question combines two principles, translational motion and impulse. The magnitude of impulse is given by the equation

J = Ft = m(vf-vi)

In this case, we are not given time (t) so to find the magnitude of impulse, we must focus on the change in momentum. The change in momentum is given with the equation

m(vf – vi) = 1 kg * (10 m/s – vi)

vf is the velocity directly after impact with the floor. vi is velocity directly before impact. It is important to note that vi is also the final velocity of the ball in the reference frame of throwing the ball to the ground. Thus,

vf² = vi² +2a?x = 4² + 2(10)(1) = 36

Our velocity right before impact is thus 6 m/s. Because velocity is a vector function, we must remember that this velocity is in the opposite direction of vf. Finally, we return to our previous impulse equation substituting
-6 m/s for vi and we see that the magnitude of impulse (J) is 16 N-s.

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Correct Answer: A. I only
3-bromo-1,1-dimethylcyclobutane structure
A saturated molecule is one where all the valences of the atoms are satisfied by single covalent bonds. In other words, there are no double bonds. Statement I is true.

A planar molecule is a flat one. In this case, we have two opposite corners in a tetrahedral shape. II is not true.

Ring strain is minimized in a cyclohexane and maximized in a cyclopropane, but it is always present in a cyclic molecule. III is not true. Thus the answer is A.

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