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It is critical for the human body blood to maintain its pH at approximately 7.4. Decreased or increased blood pH

are called acidosis and alkalosis respectively; both are serious metabolic problems that can cause death. The

table below lists the major buffers found in the blood and/or kidneys.

pKa of a typical conjugate acid:* MCAT-part-3-page300-image127

Histidine side chains

N-terminal amino groups

6.1

6.3

6.8

7.0

8.0

9.2

*For buffers in many of these categories, there is a range of actual pKa values.

The relationship between blood pH and the pKa of any buffer can be described by the Henderson-Hasselbalch

equation:

pH = pKa + log([conjugate base] / [conjugate acid])

Equation 1

Bicarbonate, the most important buffer in the plasma, enters the blood in the form of carbon dioxide, a

byproduct of metabolism, and leaves in two forms: exhaled CO2 and excreted bicarbonate. Blood pH can be

adjusted rapidly by changes in the rate of CO2 exhalation. The reaction given below, which is catalyzed by

carbonic anhydrase in the erythrocytes, describes how bicarbonate and CO2 interact in the blood.

Reaction 1

What would be the order of conjugate acid strength in the following buffers?

Section: Physical Sciences

Histidine side chains = organic phosphates > NH4

+ > organic phosphates > histidine side chains

Histidine side chains > organic phosphates > NH4

+ > organic phosphates = histidine side chains

If the distance between two bodies each 1 Kg is 1 meter, then the force of attraction between these two bodies

Section: Physical Sciences

Although nihilism is commonly defined as a form of extremist political thought, the term has a broader meaning.

Nihilism is in fact a complex intellectual stance with venerable roots in the history of ideas, which forms the

theoretical basis for many positive assertions of modern thought. Its essence is the systematic negation of all

perceptual orders and assumptions. A complete view must account for the influence of two historical

crosscurrents: philosophical skepticism about the ultimacy of any truth, and the mystical quest for that same

pure truth. These are united by their categorical rejection of the “known”.

The outstanding representative of the former current, David Hume (1711-1776), maintained that external reality

is unknowable, since sense impressions are actually part of the contents of the mind. Their presumed

correspondence to external “things” cannot be verified, since it can be checked only by other sense

impressions. Hume further asserts that all abstract conceptions turn out, on examination, to be generalizations

from sense impressions. He concludes that even such an apparently objective phenomenon as a cause-andeffect relationship between events may be no more than a subjective fabrication of the observer. Stanley Rosen

notes: “Hume terminates in skepticism because he finds nothing within the subject but individual impressions

For mystics of every faith, the “experience of nothingness” is the goal of spiritual practice. Buddhist meditation

techniques involve the systematic negation of all spiritual and intellectual constructs to make way for the

apprehension of pure truth. St. John of the Cross similarly rejected every physical and mental symbolization of

God as illusory. St. John’s spiritual legacy is, as Michael Novak puts it, “the constant return to inner solitude, an

unbroken awareness of the emptiness at the heart of consciousness. It is a harsh refusal to allow idols to be

placed in the sanctuary. It requires also a scorching gaze upon all the bureaucracies, institutions, manipulators,

and hucksters who employ technology and its supposed realities to bewitch and bedazzle the psyche.”

Novak’s interpretation points to the way these philosophical and mystical traditions prepared the ground for the

political nihilism of the nineteenth and twentieth centuries. The rejection of existing social institutions and their

claims to authority is in the most basic sense made possible by Humean skepticism. The political nihilism of the

Russian intelligentsia combined this radical skepticism with a near mystical faith in the power of a new

beginning. Hence, their desire to destroy becomes a revolutionary affirmation; in the words of Stanley Rosen,

“Nihilism is an attempt to overcome or repudiate the past on behalf of an unknown and unknowable, yet hopedfor, future.” This fusion of skepticism and mystical re-creation can be traced in contemporary thought, for

example as an element in the counterculture of the 1960s.

The passage implies that the two strands of nihilist thought

Section: Verbal Reasoning

are combined in nineteenth and twentieth century political nihilism.

remained essentially separate after the eighteenth century.

are derived from distinct Eastern and Western philosophical traditions.

The nuclei of certain unstable isotopes will spontaneously decay, producing a more stable nucleus and

releasing a particle or quantity of energy. Alpha decay releases a helium nucleus, beta decay emits an electron,

while gamma decay is the emission of a high energy photon. Each type of radioactive decay is characterized, in

part, by the half-life of the radioactive material – the time required for half of the nuclei in a sample to undergo

decay. Examples of such decays are shown in Figure 1.

Figure 1

A Geiger counter can be used to detect the decay of radioactive materials. A simple Geiger counter consists of

a hollow metal cylinder with a wire along its axis. The cylinder is filled with low pressure argon gas and a high

voltage difference is applied between the wire and the cylinder. When alpha, beta, or gamma radiation passes

through the cylinder, it interacts with the gas particles and leads to the formation of ions which cause a

discharge between the wire and the cylinder. The consequent current may be used to drive a speaker,

producing the characteristic clicking sound of the Geiger counter each time a pulse of current occurs. The

Geiger counter circuitry is shown in Figure 2.

Figure 2

A Geiger counter is best suited for which of the following applications:

Section: Physical Sciences

comparing the relative magnitudes of radioactivities of two nuclear waste depositories.

calculating the total energy of a radioactive particle.

When food is scarce, tool use among non-human primates does not increase. This counterintuitive finding

leads researchers to suggest that the driving force behind tool use is ecological opportunity – and that the

environment shapes development of culture. Whether you’re a human being or an orangutan, tools can be a

big help in getting what you need to survive. However, a review of current research into the use of tools by nonhuman primates suggests that ecological opportunity, rather than necessity, is the main driver behind primates

such as chimpanzees picking up a stone to crack open nuts.

An opinion piece by Dr Kathelijne Koops of the University of Cambridge and others, published today (12

November 2014) in Biology Letters, challenges the assumption that necessity is the mother of invention. She

and her colleagues argue that research into tool use by primates should look at the opportunities for tool use

provided by the local environment.

Science Daily and University of Cambridge, Tools and primates: Opportunity, not necessity, is the mother of

This two-paragraph passage is the introduction to the article cited below the passage. From just this

introduction, it can be assumed that the author will most likely use the following technique to support the

premise of the article:

Section: Verbal Reasoning

The author will use data from a variety of studies to support the title statement.

The author will use a list of cause-and-effect relationships to support the title statement.

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