Biochemistry and Metabolism

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MCAT Biology › Biochemistry and Metabolism

Questions 1 - 10
1

Acetyl-CoA is a react in the citric acid cycle, while and are products. If twelve molecules of are produced over a period of time, how many molecules are produced during this period?

Four

Twelve

Twenty-four

Two

Explanation

Each turn of the citric acid cycle is powered by one molecule of acetyl-CoA, resulting in three and one . The net reaction is:

Since twelve are produced, there must have been an input of four acetyl-CoA molecules and four total turns in the cycle. As a result, four molecules were produced.

2

Acetyl-CoA is a react in the citric acid cycle, while and are products. If twelve molecules of are produced over a period of time, how many molecules are produced during this period?

Four

Twelve

Twenty-four

Two

Explanation

Each turn of the citric acid cycle is powered by one molecule of acetyl-CoA, resulting in three and one . The net reaction is:

Since twelve are produced, there must have been an input of four acetyl-CoA molecules and four total turns in the cycle. As a result, four molecules were produced.

3

What is the purpose of the formation of lactic acid during anaerobic respiration?

It allows NAD+ to reform

It allows NADH to reform

It allows FAD to reform

It allows FADH2 to reform

It allows glucose to reform

Explanation

Cells need a constant supply of NAD+ to accept electrons during glycolysis in order to produce pyruvate from glucose.

4

The process of glycolysis is used by all cells of the body to turn glucose into ATP for cellular energy. When stores of glucose are low, however, the body can break down a form of stored glucose in the liver to increase glucose reserves. The supply of glycogen is limited, and eventually the body must break down free fatty acids (FFAs) through a process called beta-oxidation.

Which organ in the body cannot perform beta-oxidation, thus requiring the use of ketone bodies when stores of glucose are depleted?

Brain

Liver

Heart

Muscle

Explanation

The brain is unable to perform beta-oxidation of free fatty acids in the event of a prolonged fasting state. It is important to know this aspect of metabolism. In a fasting state, the liver beta-oxidizes free fatty acids into ketone bodies for the brain to use. Additionally, when energy demands are high, muscles can break down fat for additional ATP.

Unlike other organs in the body, the heart relies almost entirely on beta-oxidation for its energy needs.

5

What is the purpose of the formation of lactic acid during anaerobic respiration?

It allows NAD+ to reform

It allows NADH to reform

It allows FAD to reform

It allows FADH2 to reform

It allows glucose to reform

Explanation

Cells need a constant supply of NAD+ to accept electrons during glycolysis in order to produce pyruvate from glucose.

6

The process of glycolysis is used by all cells of the body to turn glucose into ATP for cellular energy. When stores of glucose are low, however, the body can break down a form of stored glucose in the liver to increase glucose reserves. The supply of glycogen is limited, and eventually the body must break down free fatty acids (FFAs) through a process called beta-oxidation.

Which organ in the body cannot perform beta-oxidation, thus requiring the use of ketone bodies when stores of glucose are depleted?

Brain

Liver

Heart

Muscle

Explanation

The brain is unable to perform beta-oxidation of free fatty acids in the event of a prolonged fasting state. It is important to know this aspect of metabolism. In a fasting state, the liver beta-oxidizes free fatty acids into ketone bodies for the brain to use. Additionally, when energy demands are high, muscles can break down fat for additional ATP.

Unlike other organs in the body, the heart relies almost entirely on beta-oxidation for its energy needs.

7

Most scientists subscribe to the theory of endosymbiosis to explain the presence of mitochondria in eukaryotic cells. According to the theory of endosymbiosis, early pre-eukaryotic cells phagocytosed free living prokaryotes, but failed to digest them. As a result, these prokaryotes remained in residence in the pre-eukaryotes, and continued to generate energy. The host cells were able to use this energy to gain a selective advantage over their competitors, and eventually the energy-producing prokaryotes became mitochondria.

In many ways, mitochondria are different from other cellular organelles, and these differences puzzled scientists for many years. The theory of endosymbiosis concisely explains a number of these observations about mitochondria. Perhaps most of all, the theory explains why aerobic metabolism is entirely limited to this one organelle, while other kinds of metabolism are more distributed in the cellular cytosol.

The primary purpose of the electron transport chain of mitochondria described in the passage is __________.

the generation of energy to sequester protons in the intermembrane space

to directly phosphorylate ADP

to directly phosphorylate AMP

to synthesize ATP synthase

to carry ADP into the mitochondrial matrix

Explanation

The electron transport chain serves to pump protons into the intermembrane space. The result is the buildup of the electrochemical gradient, and the passage of protons through ATP synthase. Essentially, the electron transport chain establishes the conditions for oxidative phosphorylation to occur.

8

During cellular respiration, where is NADH produced?

The cytosol and mitochondrial matrix

The nucleus

The endoplasmic reticulum

The cytosol

The mitochondrial intermembrane space

Explanation

NADH is produced during glycolysis, which occurs in the cytoplasm. NADH is also produced during the Krebs cycle, which occurs in the mitochondrial matrix. The protons generated in the production of NADH are later used in the intermembrane space to power ATP synthase during oxidative phosphorylation.

9

If the Krebs cycle is overstimulated, the body will produce too much of which of the following molecules?

Carbon dioxide

Oxygen

Glucose

Acetyl CoA

Pyruvate

Explanation

Of the answer choices, only carbon dioxide is a product of the Krebs cycle. If the cycle is overstimulated, too much of the products will be formed and the body will have too much carbon dioxide.

Glucose is the reactant that fuels glycolysis to produce pyruvate, which is then converted to acetyl CoA for the Krebs cycle. As such, each of these would be depleted as reactants fueling an overstimulation of the Krebs cycle.

10

Within the Krebs cycle, L-malate and NAD+ come together to form oxaloacetate, NADH, and H+. What type of chemical reaction is responsible for this step in the cycle?

Oxidation

Hydration

Decarboxylation

Dehydration

Explanation

In order for oxaloacetate to be formed, malate must lose electrons, which is the definition of an oxidation reaction. Alternately, NAD+ is reduced (gains electrons) to form NADH and H+.

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