Ap Biology Unit 3 Practice Test

AP Biology Unit 3 Practice Test: Cellular Energetics – A Comprehensive Review

AP Biology Unit 3, focusing on cellular energetics, is a crucial section for success on the AP exam. This unit covers a broad range of topics, from the basics of enzyme function and cellular respiration to the complexities of photosynthesis and fermentation. This comprehensive practice test and review will help solidify your understanding of these key concepts and prepare you for the exam. Mastering this unit will significantly boost your overall AP Biology score.

I. Introduction to Cellular Energetics

Cellular energetics revolves around the flow of energy within and between cells. Living organisms require a constant input of energy to maintain their complex organization and perform various life processes. This energy is primarily obtained through the breakdown of organic molecules, a process that is tightly regulated by enzymes. Understanding the fundamental principles of energy transfer, redox reactions, and enzyme kinetics is crucial for grasping the intricacies of cellular respiration and photosynthesis. This unit also introduces the concept of chemiosmosis, a critical mechanism driving ATP synthesis in both these processes.

Key Concepts:

• Energy and Metabolism: Understand the definitions of energy, metabolism, catabolism, anabolism, kinetic energy, potential energy, free energy (Gibbs free energy), and enthalpy. Be able to apply these concepts to biological systems.
• Enzymes: Know the function of enzymes as biological catalysts, the concept of activation energy, and the factors affecting enzyme activity (temperature, pH, substrate concentration, inhibitors). Understand the different types of enzyme inhibition (competitive, non-competitive, allosteric).
• Redox Reactions: Grasp the concept of oxidation and reduction reactions (redox reactions) and their importance in energy transfer. Understand how electron carriers like NAD+ and FAD function.

II. Cellular Respiration: Harvesting Energy from Glucose

Cellular respiration is the central process by which cells extract energy from glucose. This metabolic pathway involves a series of carefully orchestrated reactions that ultimately convert the chemical energy stored in glucose into ATP (adenosine triphosphate), the cell's primary energy currency. The process can be broadly divided into four main stages: glycolysis, pyruvate oxidation, the Krebs cycle (citric acid cycle), and oxidative phosphorylation (electron transport chain and chemiosmosis).

A. Glycolysis:

Glycolysis is an anaerobic process (occurs without oxygen) that takes place in the cytoplasm. It involves the breakdown of glucose into two molecules of pyruvate, generating a small net gain of ATP and NADH.

B. Pyruvate Oxidation:

Pyruvate, produced during glycolysis, is transported into the mitochondria, where it is oxidized to acetyl-CoA. This process releases CO2 and produces NADH.

C. Krebs Cycle (Citric Acid Cycle):

The Krebs cycle is a cyclical series of reactions that further oxidizes acetyl-CoA, releasing CO2 and producing ATP, NADH, and FADH2. These electron carriers will be crucial in the next stage.

D. Oxidative Phosphorylation:

This is the most significant ATP-producing stage. It involves the electron transport chain (ETC) embedded in the inner mitochondrial membrane and chemiosmosis. Electrons from NADH and FADH2 are passed along the ETC, releasing energy used to pump protons (H+) across the inner mitochondrial membrane, creating a proton gradient. This gradient drives ATP synthesis through chemiosmosis as protons flow back across the membrane through ATP synthase. Oxygen acts as the final electron acceptor, forming water.

Key Terms and Concepts:

• Substrate-level phosphorylation: ATP synthesis directly from a substrate.
• Chemiosmosis: ATP synthesis driven by a proton gradient.
• Electron transport chain: A series of protein complexes that transfer electrons, generating a proton gradient.
• ATP synthase: An enzyme that synthesizes ATP using the energy from the proton gradient.
• Anaerobic respiration: Cellular respiration in the absence of oxygen.

III. Fermentation: Anaerobic Energy Production

When oxygen is limited or absent, cells can resort to fermentation to generate ATP. Fermentation is less efficient than cellular respiration, producing far less ATP. There are two main types: lactic acid fermentation and alcoholic fermentation.

A. Lactic Acid Fermentation:

In lactic acid fermentation, pyruvate is reduced to lactate, regenerating NAD+ which is essential for glycolysis to continue. This process occurs in muscle cells during strenuous exercise and in some bacteria.

B. Alcoholic Fermentation:

In alcoholic fermentation, pyruvate is converted to ethanol and CO2, also regenerating NAD+. This process is used by yeast and some bacteria.

Key Differences between Cellular Respiration and Fermentation:

IV. Photosynthesis: Capturing Light Energy

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy in the form of glucose. This process is crucial for sustaining most life on Earth as it forms the base of most food chains. Photosynthesis occurs in two main stages: the light-dependent reactions and the light-independent reactions (Calvin cycle).

A. Light-Dependent Reactions:

These reactions occur in the thylakoid membranes of chloroplasts. Light energy is absorbed by chlorophyll and other pigments, exciting electrons. These energized electrons are passed along an electron transport chain, generating ATP and NADPH. Water is split (photolysis) to replace the electrons lost by chlorophyll and release oxygen as a byproduct.

B. Light-Independent Reactions (Calvin Cycle):

These reactions occur in the stroma of chloroplasts. ATP and NADPH produced during the light-dependent reactions are used to fix CO2 from the atmosphere into organic molecules, such as glucose. This process involves a series of enzyme-catalyzed reactions.

Key Concepts:

• Chlorophyll: The primary pigment that absorbs light energy.
• Photosystems: Complexes of proteins and pigments involved in light absorption and electron transfer.
• Photolysis: The splitting of water molecules to release electrons, protons, and oxygen.
• Carbon fixation: The incorporation of CO2 into organic molecules.
• RuBisCO: The enzyme that catalyzes the initial step of carbon fixation.

V. Comparison of Cellular Respiration and Photosynthesis

Both cellular respiration and photosynthesis are crucial metabolic pathways, but they have opposite functions. Cellular respiration breaks down glucose to produce ATP, while photosynthesis uses light energy to synthesize glucose. They are intricately linked, with the products of one serving as the reactants of the other. This cyclical relationship is essential for the flow of energy through ecosystems.

VI. Practice Questions

Now that we have reviewed the key concepts of AP Biology Unit 3, let's test your knowledge with some practice questions. Remember to try to answer these without referring back to the text.

1. What is the primary function of ATP in cellular processes?

2. Describe the role of enzymes in metabolic reactions.

3. Explain the difference between competitive and non-competitive enzyme inhibition.

4. What are the four main stages of cellular respiration? Briefly describe each stage.

5. What is the role of oxygen in cellular respiration?

6. What are the two main types of fermentation? How do they differ?

7. What are the two main stages of photosynthesis? Where do they occur?

8. Explain the role of chlorophyll in photosynthesis.

9. What is the relationship between cellular respiration and photosynthesis?

10. How does chemiosmosis contribute to ATP synthesis in both cellular respiration and photosynthesis?

VII. Answers and Explanations

1. ATP serves as the primary energy currency of cells, providing the energy needed for various cellular processes.

2. Enzymes act as biological catalysts, speeding up metabolic reactions by lowering the activation energy. They do this without being consumed in the reaction.

3. Competitive inhibition occurs when an inhibitor competes with the substrate for the active site of an enzyme. Non-competitive inhibition occurs when an inhibitor binds to a site other than the active site, altering the enzyme's shape and reducing its activity.

4. The four main stages of cellular respiration are: (a) glycolysis (breakdown of glucose in the cytoplasm); (b) pyruvate oxidation (conversion of pyruvate to acetyl-CoA in the mitochondria); (c) Krebs cycle (oxidation of acetyl-CoA in the mitochondria); and (d) oxidative phosphorylation (electron transport chain and chemiosmosis in the mitochondria).

5. Oxygen acts as the final electron acceptor in the electron transport chain of cellular respiration, allowing for the continuous flow of electrons and the generation of a proton gradient crucial for ATP synthesis.

6. The two main types of fermentation are lactic acid fermentation (produces lactate) and alcoholic fermentation (produces ethanol and CO2). Both regenerate NAD+ allowing glycolysis to continue in the absence of oxygen.

7. The two main stages of photosynthesis are: (a) light-dependent reactions (occur in the thylakoid membranes of chloroplasts), which convert light energy into chemical energy (ATP and NADPH); and (b) light-independent reactions (Calvin cycle; occur in the stroma of chloroplasts), which use ATP and NADPH to fix CO2 into glucose.

8. Chlorophyll is the primary pigment that absorbs light energy, initiating the process of photosynthesis.

9. Cellular respiration and photosynthesis are interconnected processes. Photosynthesis produces glucose and oxygen, which are used in cellular respiration. Cellular respiration produces CO2 and water, which are used in photosynthesis.

10. Chemiosmosis is a crucial mechanism driving ATP synthesis in both cellular respiration and photosynthesis. A proton gradient is established across a membrane (inner mitochondrial membrane in respiration, thylakoid membrane in photosynthesis). The flow of protons back across the membrane through ATP synthase generates ATP.

VIII. Conclusion

Mastering cellular energetics is vital for success in AP Biology. This comprehensive review has covered the essential concepts of enzyme function, cellular respiration, fermentation, and photosynthesis. By understanding these fundamental principles and practicing with questions, you will be well-prepared to tackle the challenges of the AP Biology exam and achieve a high score. Remember to consult your textbook and class notes for further in-depth information and clarification. Good luck with your studies!