MCAT

Hypoxia refers to a physiological condition in which the body lacks sufficient oxygen for normal cellular functioning. Prolonged hypoxia generally leads to an inhibition of mental capacity and a reduction in the work capacity of muscle. Severe cases of hypoxia can lead to coma or even death. Depending on the cause, hypoxia can be classified into four general types: Hypoxic hypoxia is a type of hypoxia that occurs when the partial pressure of oxygen in the blood is too low. For example, climbers at high altitude, where the air contains less oxygen, might experience hypoxic hypoxia because the partial pressure of oxygen in the air inhaled is very low, leading to insufficient partial pressure of oxygen in the blood. Anemic hypoxia describes a diminished ability of the blood to transport oxygen. Several factors can influence the oxygen-carrying capacity of the blood. Primary causes of anemic hypoxia include a lower than normal number of functional erythrocytes or an insufficient quantity of hemoglobin, the oxygen- carrying molecules of the blood. Abnormal hemoglobin can also decrease the blood's capacity to carry oxygen and lead to anemic hypoxia. Ischemic hypoxia is caused by a decreased delivery of blood to the tissues. Localized circulatory deficiencies, such as blood clots, and global circulatory deficiencies, such as heart failure, decrease the delivery of blood to the tissues, and can therefore cause ischemic hypoxia. Histotoxic hypoxia results from the inability of cells to utilize the oxygen available in the blood. Causes of histotoxic hypoxia include the poisoning of cellular enzymes involved in aerobic respiration, as well as the decreased metabolic capacity of the oxidative enzymes due to vitamin deficiency. Cyanide poisoning causes histotoxic hypoxia by blocking the action of cytochrome oxidase in the electron transport chain so that tissues cannot use oxygen even though it is available. Carbon monoxide binds to hemoglobin as shown in the dissociation curve below. The dissociation curve for oxygen is also shown. These dissociation curves indicate that:

Hypoxia refers to a physiological condition in which the body lacks sufficient oxygen for normal cellular functioning. Prolonged hypoxia generally leads to an inhibition of mental capacity and a reduction in the work capacity of muscle. Severe cases of hypoxia can lead to coma or even death. Depending on the cause, hypoxia can be classified into four general types: Hypoxic hypoxia is a type of hypoxia that occurs when the partial pressure of oxygen in the blood is too low. For example, climbers at high altitude, where the air contains less oxygen, might experience hypoxic hypoxia because the partial pressure of oxygen in the air inhaled is very low, leading to insufficient partial pressure of oxygen in the blood. Anemic hypoxia describes a diminished ability of the blood to transport oxygen. Several factors can influence the oxygen-carrying capacity of the blood. Primary causes of anemic hypoxia include a lower than normal number of functional erythrocytes or an insufficient quantity of hemoglobin, the oxygen- carrying molecules of the blood. Abnormal hemoglobin can also decrease the blood's capacity to carry oxygen and lead to anemic hypoxia. Ischemic hypoxia is caused by a decreased delivery of blood to the tissues. Localized circulatory deficiencies, such as blood clots, and global circulatory deficiencies, such as heart failure, decrease the delivery of blood to the tissues, and can therefore cause ischemic hypoxia. Histotoxic hypoxia results from the inability of cells to utilize the oxygen available in the blood. Causes of histotoxic hypoxia include the poisoning of cellular enzymes involved in aerobic respiration, as well as the decreased metabolic capacity of the oxidative enzymes due to vitamin deficiency. Cyanide poisoning causes histotoxic hypoxia by blocking the action of cytochrome oxidase in the electron transport chain so that tissues cannot use oxygen even though it is available. Hypoxia can often be treated by ventilation with pure oxygen. The increased in the alveoli will lead to an increased in the blood. Treatment with pure oxygen is LEAST effective in treating which of the following types of hypoxia?

Hypoxia refers to a physiological condition in which the body lacks sufficient oxygen for normal cellular functioning. Prolonged hypoxia generally leads to an inhibition of mental capacity and a reduction in the work capacity of muscle. Severe cases of hypoxia can lead to coma or even death. Depending on the cause, hypoxia can be classified into four general types: Hypoxic hypoxia is a type of hypoxia that occurs when the partial pressure of oxygen in the blood is too low. For example, climbers at high altitude, where the air contains less oxygen, might experience hypoxic hypoxia because the partial pressure of oxygen in the air inhaled is very low, leading to insufficient partial pressure of oxygen in the blood. Anemic hypoxia describes a diminished ability of the blood to transport oxygen. Several factors can influence the oxygen-carrying capacity of the blood. Primary causes of anemic hypoxia include a lower than normal number of functional erythrocytes or an insufficient quantity of hemoglobin, the oxygen- carrying molecules of the blood. Abnormal hemoglobin can also decrease the blood's capacity to carry oxygen and lead to anemic hypoxia. Ischemic hypoxia is caused by a decreased delivery of blood to the tissues. Localized circulatory deficiencies, such as blood clots, and global circulatory deficiencies, such as heart failure, decrease the delivery of blood to the tissues, and can therefore cause ischemic hypoxia. Histotoxic hypoxia results from the inability of cells to utilize the oxygen available in the blood. Causes of histotoxic hypoxia include the poisoning of cellular enzymes involved in aerobic respiration, as well as the decreased metabolic capacity of the oxidative enzymes due to vitamin deficiency. Cyanide poisoning causes histotoxic hypoxia by blocking the action of cytochrome oxidase in the electron transport chain so that tissues cannot use oxygen even though it is available. A patent foramen ovale occurs when an infant's foramen ovale does not close completely at birth. Based on the information presented in the passage, this can lead to:

In the small intestine, most amino acids are transported from the lumen of the gut into the epithelial cells against their concentration gradient. This movement of amino acids is coupled to the movement of sodium ions down their concentration gradient in a form of transmembrane movement called:

Which of the following functional groups are NOT found on the side chains of any of the naturally occurring amino acids?

When humans are submerged in water, the mammalian dive reflex acts to alter circulation. Heart rate decreases, blood flow to the extremities is reduced, and mean arterial blood pressure is increased. These accomodations lead to:

In 1972, Georges Ungar reported the discovery of a peptide that appeared to transfer learning. Ungar's claim was based on experiments in which rats placed in a chamber with specially designed dark and light regions were trained to avoid the dark regions of the chamber. Following their training, the rats were killed and brain extracts were prepared. These brain extracts were injected into naive rats which were then observed to acquire the fear of darkness without training. Two hypotheses were proposed to explain these remarkable results: Hypothesis 1 Ungar concluded that the extracts contained some chemical that transmitted the learned fear of darkness to the naive rats. A fifteen amino-acid polypeptide was isolated from the brain extracts and sequenced. Ungar claimed that this peptide, called scotophobin, was a chemical transmitter of learning. The peptide had the primary structure shown below: C-ser-asp-asn-arg-gln-gln-gly-lys-ser-ala-arg-gln-glygly-tyr-N scotophobin Hypothesis 2 Other researchers, who tested scotophobin but could not reproduce Ungar's results, argued that scotophobin did not transfer the learned fear of darkness. Instead, they suggested that scotophobin, which is structurally similar to ACTH and vasopressin, acted to increase stress in the rats. Since stress increases sympathetic nervous activity, rats injected with scotophobin would become hyperactive and tend to spend less time in the dark regions of the experimental chamber. They argued that such stress responses in the rats could be misinterpreted as a fear of darkness. Ungar's claim was further weakened by chemical analysis in which both the scotophobin extracts which Ungar had injected into the naive rats and a sample of synthesized scotophobin peptide were subjected to SDS polyacrylamide gel electrophoresis, as shown in Figure 1. Figure 1 Hydrolytic enzymes cleave polypeptides at specific amino acid residues. Which of the following hydrolytic enzymes could be used to cleave scotophobin into three fragments?

The hydrogens of alkanes have p values that are over 30 or 40. In contrast, the -hydrogens of aldehydes and ketones have p values that range from 19 to 21. These fairly acidic -hydrogens can be removed by strong bases to form anions called enolates. The enolate ions are strongly stabilized by resonance. Protonation of the enolate at oxygen produces an enol. Interconversion between the keto and enol forms is called tautomerization and is illustrated in Figure 1. The keto form is usually highly favored. Figure 1 Keto-enol tautomerization has some interesting consequences. For example, if a ketone is treated with acid or base in a solvent of O (heavy water), all of the - hydrogens will be exchanged for deuterium. This reaction is shown in Figure 2. Figure 2 Another consequence of keto-enol tautomerization is the racemization of chiral -carbons. In the enol form, the -carbon adopts a planar configuration and is no longer chiral. Tautomerization back to the ketone produces a racemic mixture of products. This is shown in Figure 3. Figure 3 A scientist attempts to follow the progress of the -deuteration shown in Figure 2 using proton NMR. Which of the following would be the best indicator that the reaction has proceeded to completion?

The hydrogens of alkanes have p values that are over 30 or 40. In contrast, the -hydrogens of aldehydes and ketones have p values that range from 19 to 21. These fairly acidic -hydrogens can be removed by strong bases to form anions called enolates. The enolate ions are strongly stabilized by resonance. Protonation of the enolate at oxygen produces an enol. Interconversion between the keto and enol forms is called tautomerization and is illustrated in Figure 1. The keto form is usually highly favored. Figure 1 Keto-enol tautomerization has some interesting consequences. For example, if a ketone is treated with acid or base in a solvent of O (heavy water), all of the - hydrogens will be exchanged for deuterium. This reaction is shown in Figure 2. Figure 2 Another consequence of keto-enol tautomerization is the racemization of chiral -carbons. In the enol form, the -carbon adopts a planar configuration and is no longer chiral. Tautomerization back to the ketone produces a racemic mixture of products. This is shown in Figure 3. Figure 3 The IUPAC name for the reactant in Figure 3 is:

The hydrogens of alkanes have p values that are over 30 or 40. In contrast, the -hydrogens of aldehydes and ketones have p values that range from 19 to 21. These fairly acidic -hydrogens can be removed by strong bases to form anions called enolates. The enolate ions are strongly stabilized by resonance. Protonation of the enolate at oxygen produces an enol. Interconversion between the keto and enol forms is called tautomerization and is illustrated in Figure 1. The keto form is usually highly favored. Figure 1 Keto-enol tautomerization has some interesting consequences. For example, if a ketone is treated with acid or base in a solvent of O (heavy water), all of the - hydrogens will be exchanged for deuterium. This reaction is shown in Figure 2. Figure 2 Another consequence of keto-enol tautomerization is the racemization of chiral -carbons. In the enol form, the -carbon adopts a planar configuration and is no longer chiral. Tautomerization back to the ketone produces a racemic mixture of products. This is shown in Figure 3. Figure 3 Which of the following ketones will have the most acidic -hydrogen: