which of the following is true of the krebs cycle

HESI A2

Biology HESI A2 Practice Test

1. Which of the following is true of the Krebs cycle?

A. It is a redox reaction involving proteins produced during glycolysis
B. It is a redox reaction involving sugars produced during glycolysis
C. Protons are passed along a gradient to produce ATP
D. It is also known as the glycolic acid cycle

Correct answer: B

Rationale: The Krebs cycle, also known as the citric acid cycle, involves a series of redox reactions that occur in the mitochondria. The cycle begins with the oxidation of acetyl CoA, which is derived from the breakdown of sugars produced during glycolysis. These sugars are broken down further in the Krebs cycle to produce ATP and reduce electron carriers such as NADH and FADH2. The cycle does not involve proteins produced during glycolysis. Protons are not passed along a gradient to produce ATP directly in the Krebs cycle; rather, they are used in the electron transport chain to generate ATP. The Krebs cycle is not known as the glycolic acid cycle; glycolysis is the metabolic pathway that produces pyruvate from glucose.

2. How does an enzyme work on a chemical reaction that occurs in a substrate?

A. An enzyme slows down the chemical reaction.
B. An enzyme speeds up the chemical reaction.
C. An enzyme has no effect on the chemical reaction.
D. An enzyme stops most chemical reactions.

Correct answer: B

Rationale: Enzymes are biological catalysts that facilitate chemical reactions by lowering the activation energy required for the reaction to occur. This allows the reaction to proceed more quickly and efficiently. Enzymes do not change the overall outcome of the reaction, but they significantly increase the rate at which it takes place. Therefore, choice B, 'An enzyme speeds up the chemical reaction,' is the correct answer. Choices A, C, and D are incorrect because enzymes do not slow down, have no effect, or stop chemical reactions; instead, they accelerate the process by lowering the activation energy.

3. What is the correct order of the hierarchy of levels in the biological classification of organisms?

A. Kingdom, phylum, class, order, family, genus, and species
B. Phylum, kingdom, class, order, family, genus, and species
C. Order, phylum, class, kingdom, family, genus, and species
D. Kingdom, phylum, order, class, family, genus, and species

Correct answer: A

Rationale: The correct order of the hierarchy of levels in the biological classification of organisms is Kingdom, Phylum, Class, Order, Family, Genus, and Species. This order is based on the Linnaean system of classification, with each level representing a progressively more specific grouping of organisms. Choice B is incorrect because it starts with Phylum instead of Kingdom. Choice C is incorrect as it places Order before Phylum. Choice D is incorrect because it does not follow the correct order of the biological classification hierarchy.

4. Which names a final step in protein synthesis?

A. DNA unzips.
B. Amino acids bond.
C. Transfer RNA bonds to messenger RNA.
D. Messenger RNA moves to ribosomes.

Correct answer: B

Rationale: The final step in protein synthesis is when amino acids bond together to form a protein chain. This process occurs during translation, where transfer RNA (tRNA) brings specific amino acids to the ribosome, and the ribosome catalyzes the formation of peptide bonds between the amino acids. This step ultimately leads to the synthesis of a complete protein based on the instructions from messenger RNA (mRNA). Choice A ('DNA unzips') is incorrect as it refers to the initiation of transcription, not the final step of protein synthesis. Choice C ('Transfer RNA bonds to messenger RNA') is incorrect as it describes the process of translation initiation rather than the final step. Choice D ('Messenger RNA moves to ribosomes') is also incorrect as mRNA is already present at the ribosomes throughout the translation process, not just in the final step.

5. Anaerobic respiration happens in _ because _ .

A. Animals; they need energy
B. Plants; they use sunlight
C. Bacteria; they live in environments with low oxygen
D. None of the above

Correct answer: C

Rationale: Anaerobic respiration happens in bacteria because they live in environments with low oxygen. Bacteria, unlike animals and plants, do not have easy access to oxygen for aerobic respiration and must find alternative ways to produce energy to survive. This makes anaerobic respiration essential for their metabolic processes. Choice A is incorrect because although animals need energy, they generally rely on aerobic respiration when oxygen is available. Choice B is incorrect because plants primarily perform photosynthesis using sunlight to produce energy. Choice D is incorrect as anaerobic respiration does occur in specific organisms like bacteria when oxygen is scarce.