In the 1980’s, an epidemic of bovine spongiform encephalopathy, or mad cow disease, swept through cattle herds in the United Kingdom. Scientists and veterinarians were troubled and had a difficult time managing the disease because it spread from one animal to another, and behaved differently than other diseases in the past. 

When infectious material from affected animals was treated with high levels of radiation, for example, the material remained infectious. All known bacteria or viruses that carry disease would have been killed by such a treatment. Additionally, some animals developed the disease without first being exposed to sick animals. Perhaps most frustratingly, among those animals that are exposed before becoming sick, it can take many years after exposure for illness to appear.

There quickly emerged two distinct explanations for the disease. 

Scientist 1:

Mad cow disease is unlike any disease we have handled before. It is increasingly clear that the best explanation for the disease’s dynamics involve proteins, called the protein-only hypothesis. These protein molecules are likely causative of the disease, and they lack any DNA or RNA. It is damage to these DNA or RNA molecules that kills bacteria or viruses when exposed to high levels of radiation. The most important observations that made scientists consider a unique, protein-only model for this disease involved its resistance to radiation. Remarkably, this would be the first example of an infectious agent copying itself without DNA or RNA to mediate the process.

Moreover, some animals develop the disease spontaneously, without physically being infected by another animal. This suggests that internal disorder among protein molecules is a potential route to developing disease, and may be accelerated by exposure to other sick animals.

In fact, this is consistent with the proposed mechanism. It is likely that proteins fold incorrectly, and then influence proteins around them to take on this errant conformation. Some proteins may fold incorrectly by chance, which explains spontaneous disease development. It also explains the long course of disease, as it takes many years for enough proteins to fold incorrectly and result in observable disease.

Scientist 2:

The suggestion that mad cow disease is caused exclusively by protein, in the absence of DNA or RNA, is such a dramatic departure from accepted biological processes that it warrants careful scrutiny. Additionally, other more conventional explanations should be thoroughly investigated before coming to such a conclusion.

Some scientists have shown that very small particles resembling viruses are visible in infectious material under powerful microscopes. Additionally, these viruses are consistent in size and shape with known, highly radiation-resistant viruses called polyomaviruses. It takes much higher-than-typical doses of radiation to cause enough DNA damage to inactivate these viruses.

The observation that mad cow disease occurs spontaneously in some animals is also explained by the viral explanation. Many viruses exist in animals and humans for years, undetected and not causing any observable disease. Sickness or stress can make these viruses reactivate, offering the illusion of spontaneous illness. All of these observations are consistent with the viral hypothesis.

Which of the following facts is an implicit assumption in the argument of Scientist 1?

If a drug is administered intravenously (i.e. injected into a vein), then the concentration of the drug in the blood will rise and then reach a plateau. After some time, the body will begin to eliminate this drug from the bloodstream. During this elimination phase, ion pumps will actively transport some drugs from the blood into the tubules of the kidneys. Organic anion transporters (OAT) transport acids into the tubules of the kidney while organic cation transporters (OCT) transport bases into the tubules of the kidney. This mechanism, known as secretion, depends entirely on transporters and allows both acidic and basic drugs to be eliminated from the body.

Another process known as reabsorption can also occur in the kidneys. During reabsorption, drugs can be transported from the tubules of the kidney back into the blood. This process depends on the pH of the fluid in kidney tubules (e.g. the urine). At a low pH (i.e. acidic environment), acidic drugs are best reabsorbed. Conversely, at high pH (i.e. basic environment), basic drugs are best reabsorbed.

Experiment 1

The same drug was administered to various individuals. After some time elapsed, the rate of renal clearance (i.e. the rate of urine elimination from the body) was measured in each of the individuals. Subsequently, the pH of the urine was measured for each individual. The rate of renal clearance versus urinary pH was then plotted in the provided figure (each black dot represents a different individual).

Experiment 2

A scientist was able to engineer a mouse kidney that lacked organic anion transporters. She administered various drugs to the mouse and measured the rates of secretion. The data collected is located in the provided table.

Suppose an individual who had taken the drug administered in Experiment 1 exhibited the following urinary pH:

Would this drug have stayed in the bloodstream for an extended period of time?

The chart above shows the height growth of three different plant species after a period of 2 weeks. Each plant species was grown in 4 different soil mediums. All the plants were grown in the same environment with equal amounts of light, water, and oxygen. 

What factor would weaken the design of this experiment?

The chart above shows the height growth of three different plant species after a period of 2 weeks. Each plant species was grown in 4 different soil mediums. All the plants were grown in the same environment with equal amounts of light, water, and oxygen.

Based on the chart, soil Medium C has what effect on plant growth?

Mitochondria make 90% of the energy needed by the body to sustain life. The Mitochondrial Free Radical Theory of Aging (MFRTA) theorizes that individuals who live longest produce fewer mitochondrial oxygen reactive species than individuals that have a shorter life span. Therefore, lifespan will increase if fewer mtROS are produced, and lifespan will decrease if more mtROS are produced. An experiment was done to test this theory, and the results are shown in the chart below. Four test groups of flies were involved, two groups consisted of females, and two groups consisted of males.

Does this data support or dispute the MFRTA theory? 

Mitochondria make 90% of the energy needed by the body to sustain life. The Mitochondrial Free Radical Theory of Aging (MFRTA) theorizes that individuals who live longest produce fewer mitochondrial oxygen reactive species than individuals that have a shorter life span. Therefore, lifespan will increase if fewer mtROS are produced, and lifespan will decrease if more mtROS are produced. An experiment was done to test this theory, and the results are shown in the chart below. Four test groups of flies were involved, two groups consisted of females, and two groups consisted of males.

What information would weaken the experiment's results?

Mitochondria make 90% of the energy needed by the body to sustain life. The Mitochondrial Free Radical Theory of Aging (MFRTA) theorizes that individuals who live longest produce fewer mitochondrial oxygen reactive species than individuals that have a shorter life span. Therefore, lifespan will increase if fewer mtROS are produced, and lifespan will decrease if more mtROS are produced. An experiment was done to test this theory, and the results are shown in the chart below. Four test groups of flies were involved, two groups consisted of females, and two groups consisted of males.

What, if added, would strengthen the results of the experiment? 

An experiment was done to test the antibiotic resistance of the bacteria Pseudomonas flourescens and Escherichia coli by using the antibiotic disk sensitivity method. Four different antibiotics were tested on each bacterium. Media were prepared with each type of bacterium and a drop of each antibiotic was added to each corner of the plates, a different antibiotic in each corner. The cultures were observed for eighteen hours. After this period of time, the zones of inhibition (where the bacteria was not able to grow due to the antibiotic) were measured. The table below shows the results of the four antibiotics on the two bacteria, Pseudomonas flourescens and Escherichia coli.

Which of the following changes would weaken the experimental results?

An experiment was done to test the antibiotic resistance of the bacteria Pseudomonas flourescens and Escherichia coli by using the antibiotic disk sensitivity method. Four different antibiotics were tested on each bacterium. Media were prepared with each type of bacterium and a drop of each antibiotic was added to each corner of the plates, a different antibiotic in each corner. The cultures were observed for eighteen hours. After this period of time, the zones of inhibition (where the bacteria was not able to grow due to the antibiotic) were measured. The table below shows the results of the four antibiotics on the two bacteria, Pseudomonas flourescens and Escherichia coli.

A biologist wanted to do an experiment involving two species of large cat (species A and species B) around a potential new food source.

He isolated the populations of each cat in a location that closely approximated their natural environment, except this potential new food source was the only food source available to them. 

Experiment 1:

When species A was left alone with the new food source, their numbers decreased rapidly until none were left.

Experiment 2:

When species B was left alone with the new food source, their numbers increased and the species flourished.

Experiment 3:

When an equal number of species A and B were placed with the new food source, both species maintained relatively constant numbers.

Which of the following changes would most alter the results of the experiments?