MCAT Exam: Absolutely Free Online Practice

Free Test Prep MCAT Exam Questions

The Russian wheat aphid, Diuraphis noxia, is a small green insect discovered in southern Russia around the

turn of the century. Agricultural researchers are not quite sure, but they believe the Russian aphid adapted itself

to wheat about ten thousand years ago, when the crop was first domesticated by man. What is not in doubt is

the insect’s destructiveness. Spread by both wind and human transport, the Russian aphid has destroyed

wheat fields throughout Asia, Africa, and Latin America. Until a few years ago, the United States had been free

of this pest. But in the spring of 1986, a swarm of Russian aphids crossed the Mexican border and settled a few

hundred miles north, in central Texas. From there, it quickly spread to other Western states, destroying wheat

fields all along its path. In fact, the level of destruction has been so great over the past five years that

entomologists are calling the Russian aphid the greatest threat to American agriculture since the Hessian fly,

Phytophaga destructor, was inadvertently brought to the colonies on ships by German mercenary troops during

the Revolutionary War. A combination of several factors have made it particularly difficult to deal with the threat

posed by this aphid. First, Russian aphids reproduce asexually at a phenomenal rate. This process, known as

parthenogenesis, often results in as many as twenty generations of insects in a single year. Although most

generations remain in a limited geographic area because they have no wings, a few generations are born with

wings, allowing the insect to spread to new areas. Second, because wheat is a crop with a very low profit

margin, most American farmers do not spray it with pesticides; it simply is not economical to do so. And since

the Russian aphid has only recently entered the United States, it has no natural enemies among North

American insects or animals. As a result, there have been no man-made or natural obstacles to the spread of

the Russian aphid in the United States.

Agricultural researchers seeking to control the Russian aphid have looked to its place of origin for answers. In

the Soviet Union, the Russian aphid has been kept in check by predators which have evolved alongside it over

many thousands of years. One species of wasp seems to be particularly efficient at destroying the aphid. The

pregnant females of the species search the Russian aphid’s home, the interior of a wheat stalk, sting the aphid

into paralysis, and then inject an egg into its body. When the egg hatches the wasp larva feeds off of the aphid,

killing it in the process.

The introduction of predators like the wasp, coupled with the breeding of new strains of insect-resistant wheat,

may substantially curb the destructiveness of the Russian aphid in the future. For the time being, however,

American farmers are left to their own devices when it comes to protecting their wheat crops.

Which of the following statements would be most in agreement with the statements in the passage?

Section: Verbal Reasoning

It is more difficult to stop the spread of an insect that reproduces asexually than one that reproduces

Section: Biological Sciences

When softball players take batting practice, they often use a machine called an “automatic pitcher,” which is

essentially a cannon that uses air pressure to launch a projectile. In a prototype automatic pitcher, a softball is

loaded into the barrel of the cannon and rests against a flat disk. That disk is locked into place, and a high air

pressure is built up behind it. When the disk is released, the softball is pushed along the barrel of the cannon

and ejected at a speed of V0.

Figure 1 shows the batter and automatic pitcher. The angle of the barrel to the horizontal is θ. The unit vectors i

and j point in the horizontal and vertical directions respectively.

Figure 2

What is the ratio of the horizontal distance traveled by the softball at Point B to the horizontal distance traveled

at Point C?

Section: Physical Sciences

Arginine is one of the 20 most common natural amino acids. Most healthy people do not need to supplement

with arginine because the body usually produces sufficient quantities. The pathway for arginine synthesis was

studied using cells from a red bread mold. This natural form of arginine is illustrated below.

The red bread mold Neurospora crassa grows well on a cultural plate with "minimal" medium which is a fluid

containing only a few simple sugars, inorganic salts, and vitamin. Neurospora that grows normally in nature

(wild type) has enzymes that convert these simple substances into the amino acids necessary for growth.

Mutating any one of the genes that makes an enzyme can produce a Neurospora strain that cannot grow on

minimal medium. The mutant would only grow if the enzyme product were to be added as a supplement. On the

other hand, if a "complete" medium is provided, containing all required amino acids, then Neurospora would

grow, with or without mutation.

Figure 1 A synthesis pathway for the amino acid arginine. Each gene in italics in the diagram produces one

enzyme necessary for the synthesis of this essential amino acid required for growth.

Table 1 Growth response of mutant strains in “minimal” media with supplements (ornithine, citrulline,

argininosuccinate, and arginine) as indicated. Strain growth is indicated by (+) and no strain growth is indicated

by (−).

Which of the following is NOT an accurate description of naturally-occurring arginine?

Section: Physical Sciences

The magnetic field is produced by:

stationary electrons.

Free Test Prep MCAT Exam Questions

Absolute Free MCAT Exam Practice for Comprehensive Preparation