Unit 7 Ap Biology Review

Unit 7 AP Biology Review: Cellular Respiration & Fermentation: A Deep Dive

This comprehensive review covers AP Biology Unit 7, focusing on cellular respiration and fermentation. Understanding these processes is crucial for success on the AP Biology exam, as they represent a cornerstone of biological energy production. We’ll explore the intricacies of glycolysis, the Krebs cycle, oxidative phosphorylation, and fermentation, providing a detailed overview alongside helpful strategies for mastering this complex unit.

Introduction: Harnessing Energy from Food

Living organisms require a constant supply of energy to power essential life functions, from muscle contraction and protein synthesis to maintaining cellular integrity. This energy is derived primarily from the breakdown of glucose, a simple sugar, through a series of metabolic pathways collectively known as cellular respiration. Cellular respiration is an aerobic process, meaning it requires oxygen. However, when oxygen is limited, organisms can utilize fermentation, an anaerobic pathway that produces less ATP but allows for continued energy production. This unit will delve into the specifics of both respiration and fermentation, examining the individual steps, energy yields, and regulatory mechanisms involved.

1. Glycolysis: The First Step

Glycolysis, meaning "sugar splitting," is the initial stage of both cellular respiration and fermentation. It occurs in the cytoplasm and doesn't require oxygen. This anaerobic process involves a series of ten enzyme-catalyzed reactions that convert one molecule of glucose (a six-carbon sugar) into two molecules of pyruvate (a three-carbon compound).

• Key Events in Glycolysis:
  • Energy Investment Phase: The initial steps require the input of two ATP molecules to phosphorylate glucose, making it more reactive.
  • Energy Payoff Phase: Subsequent reactions generate four ATP molecules and two NADH molecules (an electron carrier).
  • Net Gain: The net gain of glycolysis is 2 ATP and 2 NADH per glucose molecule.

2. The Krebs Cycle (Citric Acid Cycle): Oxidizing Pyruvate

If oxygen is present, pyruvate enters the mitochondria and undergoes a series of reactions in the Krebs cycle (also known as the citric acid cycle). This cycle takes place in the mitochondrial matrix and plays a vital role in oxidizing pyruvate, releasing carbon dioxide and generating high-energy electron carriers.

• Pyruvate Oxidation: Before entering the Krebs cycle, pyruvate is converted into acetyl-CoA, releasing one molecule of CO2 and generating one NADH per pyruvate molecule.
• Krebs Cycle Reactions: The acetyl-CoA enters a cyclical series of eight reactions, producing:
  • 2 CO2 molecules per acetyl-CoA (4 CO2 per glucose)
  • 3 NADH molecules per acetyl-CoA (6 NADH per glucose)
  • 1 FADH2 molecule per acetyl-CoA (2 FADH2 per glucose)
  • 1 ATP molecule per acetyl-CoA (2 ATP per glucose)

3. Oxidative Phosphorylation: Electron Transport Chain & Chemiosmosis

Oxidative phosphorylation is the final stage of cellular respiration and the most significant ATP producer. It occurs in the inner mitochondrial membrane and involves two tightly coupled processes: the electron transport chain (ETC) and chemiosmosis.

• Electron Transport Chain (ETC): The high-energy electrons carried by NADH and FADH2 are passed along a series of protein complexes embedded in the inner mitochondrial membrane. As electrons move down the chain, energy is released and used to pump protons (H+) from the mitochondrial matrix into the intermembrane space, creating a proton gradient.

• Chemiosmosis: The proton gradient generated by the ETC drives the synthesis of ATP via chemiosmosis. Protons flow back into the matrix through ATP synthase, an enzyme that uses the proton motive force to phosphorylate ADP to ATP. This process is called chemiosmosis because the synthesis of ATP is coupled to the movement of protons across a membrane.

• Oxygen's Role: Oxygen acts as the final electron acceptor in the ETC, forming water. Without oxygen, the ETC would halt, and ATP production would significantly decrease.

4. Fermentation: Anaerobic Energy Production

When oxygen is absent, cells resort to fermentation to generate ATP. Fermentation is a less efficient process than cellular respiration, yielding only 2 ATP molecules per glucose molecule (from glycolysis). There are two main types of fermentation:

• Lactic Acid Fermentation: Pyruvate is reduced to lactic acid, regenerating NAD+ which is essential for glycolysis to continue. This type of fermentation is common in muscle cells during strenuous exercise and in certain bacteria.

• Alcoholic Fermentation: Pyruvate is converted to acetaldehyde, which is then reduced to ethanol, regenerating NAD+. This type of fermentation is used by yeast and some bacteria in the production of alcoholic beverages and bread.

5. Regulation of Cellular Respiration

Cellular respiration is a highly regulated process, ensuring that ATP production matches the energy demands of the cell. Several factors influence the rate of respiration, including:

• Availability of Substrate: The concentration of glucose and other substrates directly affects the rate of glycolysis and the Krebs cycle.

• Allosteric Regulation: Enzymes involved in glycolysis and the Krebs cycle are subject to allosteric regulation, meaning their activity is modulated by the binding of molecules to sites other than the active site. For example, ATP inhibits several enzymes in glycolysis, slowing down respiration when ATP levels are high.

• Feedback Inhibition: The end product of a metabolic pathway can inhibit an earlier enzyme in the pathway, preventing overproduction of the end product.

6. Comparing Cellular Respiration and Fermentation

7. Connections to Other AP Biology Topics

Understanding cellular respiration and fermentation is crucial for grasping many other concepts in AP Biology, including:

• Photosynthesis: The products of photosynthesis (glucose and oxygen) are the reactants of cellular respiration. These two processes are interconnected and essential for the flow of energy through ecosystems.

• Enzyme Function: Cellular respiration involves numerous enzymes, highlighting the importance of enzyme activity in metabolic processes.

• Membrane Transport: The proton gradient established during oxidative phosphorylation exemplifies the role of membrane transport in energy production.

• Evolution: The evolution of cellular respiration and its efficiency has been a significant factor in the diversification of life on Earth.

8. Frequently Asked Questions (FAQ)

• Q: What is the difference between substrate-level phosphorylation and oxidative phosphorylation?

  • A: Substrate-level phosphorylation involves the direct transfer of a phosphate group from a substrate to ADP, generating ATP. This occurs during glycolysis and the Krebs cycle. Oxidative phosphorylation involves the use of a proton gradient to drive ATP synthesis via ATP synthase, occurring during the electron transport chain.

• Q: Why is oxygen so important in cellular respiration?

  • A: Oxygen acts as the final electron acceptor in the electron transport chain, allowing the continuous flow of electrons and the generation of a proton gradient necessary for ATP synthesis. Without oxygen, the ETC would stop, severely limiting ATP production.

• Q: What are the roles of NADH and FADH2?

  • A: NADH and FADH2 are electron carriers that transport high-energy electrons from glycolysis and the Krebs cycle to the electron transport chain. These electrons drive the pumping of protons, creating the proton gradient essential for ATP synthesis.

• Q: How is cellular respiration regulated?

  • A: Cellular respiration is regulated through various mechanisms, including substrate availability, allosteric regulation of enzymes, and feedback inhibition. These mechanisms ensure that ATP production matches the energy needs of the cell.

9. Conclusion: Mastering Cellular Respiration and Fermentation

This comprehensive review covers the key aspects of cellular respiration and fermentation, essential topics for the AP Biology exam. Remember to focus on understanding the individual steps, energy yields, and connections between these processes. By thoroughly grasping these concepts, you will be well-prepared to tackle any questions related to cellular energy production on the AP Biology exam and beyond. The understanding of these pathways is not merely rote memorization; it is the key to understanding the fundamental processes that sustain life itself. Through diligent study and practice, you can achieve mastery of this critical unit and enhance your overall understanding of biology. Good luck!