Test Prep MCAT

Compounds containing a hydroxyl group attached to a benzene ring are called phenols. Derivatives of phenols,

such as naphthols and phenanthrols, have chemical properties similar to those of phenols, as do most of the

many naturally-occurring substituted phenols. Like other alcohols, phenols have higher boiling points than

hydrocarbons of similar molecular weight. Like carboxylic acids, phenols are more acidic than their alcohol

counterparts. Phenols undergo a number of different reactions; both their hydroxyl groups and their benzene

rings are highly reactive. A number of chemical tests can be used to distinguish phenols from alcohols and

carboxylic acids.

Reaction B

Section: Biological Sciences 

Four major blood types exist in the human ABO blood system: types A, B, AB, and O; and there are three

alleles that code for them. The A and B alleles are codominant, and the O allele is recessive. Blood types are

derived from the presence of specific polysaccharide antigens that lie on the outer surface of the red blood cell

membrane. The A allele codes for the production of the A antigen; the B allele codes for the production of the B

antigen; the O allele does not code for any antigen.

While there are many other antigens found on red blood cell membranes, the second most important antigen is

the Rh antigen. Rh is an autosomally dominant trait coded for by 2 alleles. If this antigen is present, an

individual is Rh+; if it is absent, an individual is Rh−. For example, a person with type AB blood with the Rh

antigen is said to be AB+.

These antigens become most important when an individual comes into contact with foreign blood. Because of

the presence of naturally occurring substances that closely mimic the A and B antigens, individuals who do not

have these antigens on their red blood cells will form antibodies against them. This is inconsequential until

situations such as blood transfusion, organ transplant, or pregnancy occur.

Erythroblastosis fetalis is a condition in which the red blood cells of an Rh+ fetus are attached by antibodies

produced by its Rh− mother. Unlike ABO incompatibility, in which there are naturally occurring antibodies to

foreign antigens, the Rh system requires prior sensitization to the Rh antigen before antibodies are produced.

This sensitization usually occurs during the delivery of an Rh+ baby. So while the first baby will not be harmed,

any further Rh+ fetuses are at risk.

The Coombs tests provide a method for determining whether a mother has mounted an immune response

again her baby’s blood. The tests are based on whether or not agglutination occurs when Coombs reagent is

added to a sample. Coombs reagent contains antibodies against the anti-Rh antibodies produced by the

mother. The indirect Coombs test takes the mother’s serum, which contains her antibodies but no red blood

cells, and mixes it with Rh+ red blood cells. Coombs reagent is then added. If agglutination occurs, the test is

positive, and the mother must be producing anti-Rh antibodies. The direct Coombs test mixes the baby’s red

blood cells with Coombs reagent. If agglutination occurs, the test is positive, and the baby’s red blood cells

must have been attacked by its mother’s anti-Rh antibodies.

How might one most practically assess the risk of erythroblastosis fetalis in a pregnant woman?

Section: Biological Sciences 

Several techniques have been developed to determine the order of a reaction. The rate of a reaction cannot be

predicted on the basis of the overall equation, but can be predicted on the basis of the rate-determining step.

For instance, the following reaction can be broken down into three steps.

A + D → F + G

Step 1

A → B + C

(slow)

Step 2

B + D → E + F

(fast)

Step 3

E + C → G

(fast)

Reaction 1

In this case, the first step in the reaction pathway is the rate-determining step. Therefore, the overall rate of the

reaction must equal the rate of the first step, k1 [A] where k1 is the rate constant for the first step. (Rate

constants of the different steps are denoted by kx

, where x is the step number.)

In some cases, it is desirable to measure the rate of a reaction in relation to only one species. In a secondorder reaction, for instance, a large excess of one species is included in the reaction vessel. Since a relatively

small amount of this large concentration is reacted, we assume that the concentration essentially remains

unchanged. Such a reaction is called a pseudo first-order reaction. A new rate constant, k', is established, equal

to the product of the rate constant of the original reaction, k, and the concentration of the species in excess.

This approach is often used to analyze enzyme activity.

In some cases, the reaction rate may be dependent on the concentration of a short-lived intermediate. This can

happen if the rate-determining step is not the first step. In this case, the concentration of the intermediate must

be derived from the equilibrium constant of the preceding step. For redox reactions, the equilibrium can be

correlated with the voltage produced by two half-cells by means of the Nernst equation. This equation states

that at any given moment:

Section: Physical Sciences 

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.

Table 1

Buffer

pKa of a typical conjugate acid:*

Histidine side chains

Section: Physical Sciences

An individual is born with a mutation causing her to partially retain a form of fetal hemoglobin into adulthood.

Compared to a normal individual, this person would exhibit:

Section: Biological Sciences