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Unit 3

3.9 MC Answers and Review

6 min readdecember 10, 2021


AP Biology 🧬

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Answers and Review for Multiple Choice Practice on Cellular Energetics

STOP ⛔ Before you look at the answers, make sure you gave this practice quiz a try so you can assess your understanding of the concepts covered in Unit 3. Click here for the practice questions: AP Bio Unit 3 Multiple Choice Questions.
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Facts about the test: The AP Biology exam has 60 multiple choice questions and you will be given 1 hour 30 minutes to complete the section. That means it should take you around 15 minutes to complete 10 questions.
The following questions were not written by College Board and although they cover information outlined in the AP Biology Course and Exam Description the formatting on the exam may be different.

1. The building blocks of enzymes are:
A. Amino Acids
B. Monosaccharides
C. Nucleotides
D. Fatty Acids
Answer: Enzymes are proteins! Amino acids build protein. 
📄 Study AP Biology, Unit 3.1: Enzyme Structure

2. The most important part of an enzyme's structure is the:
A. Sequence of hydrogen bonding
B. Sequence of nucleotides
C. Allosteric site
D. Active site
Answer: Each enzyme contains a structural component called the active site. This structure specifically "fits" to a substrate much like a lock and key.
📄 Study AP Biology, Unit 3.1: Enzyme Structure

3. Noncompetitive inhibitors bind to the:
A. Sympatric site
B. Allopatric site
C. Allosteric site
D. Active site
Answer: Competitive inhibitors bind to the active site, allopatric and sympatric terms come from types of speciation!
  📄 Study AP Biology, Unit 3.2: Enzyme Catalysis

4. Enzymes operate their best at optimal environmental conditions specific for the type of enzyme. Pepsin, an enzyme found in the stomach most likely operates best at a pH of:
A. 1-3
B. 6-7
C. 9-11
D. 13-14
Answer: In an acidic environment like the stomach, the pH is likely to be low, between 1-3.
📄 Study AP Biology, Unit 3.3: Environmental Impacts on Enzyme Function

5. Which best describes what happens when an enzymes gets denatured?
A. The peptide bonds between amino acids break
B. The enzyme unfolds as secondary, tertiary, and quaternary structures become disrupted
C. The primary structure unravels
D. Only the quaternary structures separate
Answer: The covalent bonds between amino acids do not break when an enzyme unfolds.
📄 Study AP Biology, Unit 3.3: Environmental Impacts on Enzyme Function

6. Students disagree on whether or not photosynthesis occurs in organisms other than plants. Choose the correct understanding of this:
A. Only plants can perform photosynthesis
B. Plants, fungi, and all protists can undergo photosynthesis
C. Plants and animals can both perform photosynthesis
D. Plants are NOT the only organism that can undergo photosynthesis. Photosynthetic bacteria like cyanobacteria were among the first organisms to harvest light energy to make organic molecules
Answer: Photosynthetic bacteria like cyanobacteria have been around longer than plants! Certain protists like algae can also perform photosynthesis.
📄 Study AP Biology, Unit 3.5: Photosynthesis

7. Redox reactions involve a transfer of electrons where one molecule loses electrons as another gains electrons. In photosynthesis which reactant is oxidized?
A. Water
B. Oxygen
C. Carbon Dioxide
D. Sugar
Answer: Carbon dioxide and water are not reactants of photosynthesis. Oxidation refers to the loss of electrons. During the light reactions of photosynthesis, water is split, causing it to lose electrons.
📄 Study AP Biology, Unit 3.5: Photosynthesis

8. All organisms can use this process to generate ATP
A. ETC
B. Glycolysis
C. Aerobic cellular respiration
D. Krebs cycle
Answer: Glycolysis occurs in the cytoplasm of all organisms and does not require the use of oxygen or mitochondria.
📄 Study AP Biology, Unit 3.6: Cellular Respiration

9. Biologically speaking, the purpose of fermentation is
A. to regenerate ATP
B. to reduce NAD+ and FADH
C. to recycle NAD+ and FADH
D. to hydrolyze ATP
Answer: In the absence of oxygen, glycolysis can occur to generate ATP. Organisms undergo fermentation after glycolysis to oxidize NADH and FADH2, so that NAD+ and FADH can be used again in glycolysis
📄 Study AP Biology, Unit 3.6: Cellular Respiration

10. In an incredible process, water is split (breaking a strong polar covalent bond) in order to replace electrons during:
A. Krebs cycle
B. glycolysis and Krebs cycle
C. the citric acid cycle
D. the light reactions
Answer: Water is split during photosynthesis after chlorophyll absorbs light energy and loses electrons. Also fun fact, Krebs cycle and citric acid cycle are the same thing so you should know those answers are incorrect!
📄 Study AP Biology, Unit 3.5: Photosynthesis

11. Where does the electron transport chain occur in photosynthesis and cellular respiration respectively?
A. mitochondrial matrix: thylakoid
B. stroma: mitochondrial matrix
C. thylakoid membrane: mitochondrial cristae
D. stomata: mitochondria
Answer: The electron transport chain occurs within a phospholipid membrane. In photosynthesis it is the thylakoid, during cellular respiration it is the inner mitochondrial membrane also known as the cristae.
📄 Study AP Biology, Unit 3.5: Photosynthesis
Study AP Biology, Unit 3.6: Cellular Respiration

12. Where is the pH the lowest in a mitochondria during cellular respiration?
A. the matrix
B. the stroma
C. the thylakoid
D. the intermembrane space
Answer: Hydrogen ions are pumped across the inner mitochondrial membrane into the intermembrane space during cellular respiration. pH is a measure of the concentration of H+. With more H+ in the intermembrane space, the pH is lower or more acidic.
📄 Study AP Biology, Unit 3.6: Cellular Respiration

13. Oxidative phosphorylation generates the most ATP for an anaerobically respiring cell. What two events occur during this process?
A. Krebs cycle and ETC
B. ETC and chemiosmosis
C. Glycolysis and Krebs cycle
D. Glycolysis and Citric acid cycle
Answer: A commonly mistaken concept, oxidative phosphorylation occurs during cellular respiration and includes the process of the electron transport chain AND chemiosmosis. Chemiosmosis refers to the process of hydrogen ions moving down their electrochemical gradient through ATP synthase which generates ATP. The ETC is needed to create that electrochemical gradient.
📄 Study AP Biology, Unit 3.6: Cellular Respiration

14. The fitness of an organism is directly related to
A. How well it can survive and reproduce in its environment
B. How well it can metabolize organic materials from the environment
C. How well it can adapt to fit it's environment
D. How well it can evade predators and hunt
Answer: The option about "adapting" to the environment is a trap. No single organism can adapt to the environment. Fitness strictly deals with EXISTING variation among a population and how variations can improve reproduction and survival. Not all organisms rely on hunting for food!
📄 Study AP Biology, Unit 3.7: Fitness

15. Plants contain a variety of pigments capable of absorbing light. Why is this biologically beneficial?
A. It allows the ETC during the Calvin cycle to improve efficiency
B. it creates an environment of lower pH to establish the electrochemical gradient
C. different pigments produce beautiful colors in autumn
D. different pigments absorb varying wavelengths of light, giving plants a broader range for light absorption
Answer: In a question like this, try to answer it on your own before looking at answer options--you know this stuff! Plants contain a variety of pigments like chlorophyll a, chlorophyll b, and carotenoids. With this variety, a plant is able to harvest a broader range of wavelengths for photosynthesis.
📄 Study AP Biology, Unit 3.5: Photosynthesis

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