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Chapter 6: Glycolysis, Aerobic Respiration, and Fermentation

6.1 Overview of Cellular Respiration

• Cellular Respiration: The process by which macromolecules are catabolized and oxidized to produce ATP.

• A series of enzymatic reactions that sequentially breaks down glucose, removing electrons and utilizing their energy to establish a proton (H+) gradient across the inner mitochondrial membrane.

• The proton gradient powers ATP synthase to convert ADP to ATP.

• Energy Measurement: Energy from diet is measured in Calories (1 Cal = energy required to raise 1 liter of water by 1°C).

• Organic Molecules: For heterotrophs, diets contain 4 major macromolecules and cofactors necessary for life.

ATP Cycle and Transformation

• ATP Hydrolysis: ATP is converted to ADP during energy usage and is recycled by mitochondrial enzyme ATP synthase.

• Living organisms constantly transform energy:

  • Catabolism: Breakdown of fuel to release energy.

  • Anabolism: Building macromolecules, requiring energy input.

Aerobic vs. Anaerobic Respiration

• Aerobic Respiration: Uses oxygen to convert food's potential chemical energy into usable ATP.

• Most energy transformations occur in the mitochondria, utilizing oxygen as the final electron acceptor in the Electron Transport Chain (ETC).

• Anabolism (Photosynthesis): Involves building larger molecules from smaller ones and is endergonic (requires energy).

• Catabolism (Glycolysis & Respiration): Involves breaking down molecules, which is exergonic (releases energy).

• Photosynthesis and aerobic respiration are complementary processes.

Electron Carriers in Cellular Respiration

• High-energy electrons from food are transferred to electron carriers (NAD+/NADH & FAD/FADH2) which move electrons to the mitochondrial ETC.

• Cellular respiration involves redox reactions where oxidation (loss of electrons) and reduction (gain of electrons) occur.

Energy Efficiency and Production

• Energy release in respiration is gradual to prevent uncontrolled oxidation (explosions).

• Not only heterotrophs perform this; all eukaryotes with mitochondria rely on these processes for ATP production.

Learning Outcomes for 6.1

• Compare and contrast anabolism and catabolism.

• Explain the link between anabolism, catabolism, photosynthesis, and respiration.

• Diagram free energy graphs for both photosynthesis and respiration.

Section 6.2: Glycolysis

Overview of Glycolysis

• The first step in cellular respiration, occurring in the cytoplasm.

• Glycolysis splits one glucose (6C) molecule into two pyruvate (3C) molecules, producing ATP and NADH.

• Glycolysis can function in both aerobic and anaerobic conditions.

Phases of Glycolysis

• Energy Investment Phase:

  • 2 ATPs are used to prime glucose, making it more reactive.

  • Glucose is split into two 3-carbon molecules.

• Energy Harvest Phase:

  • The 3-carbon molecules are oxidized, reducing NAD+ to NADH.

  • Phosphates are added and ATP is produced, resulting in net production of 2 ATP and 2 NADH.

Summary: Inputs and Outputs of Glycolysis

• Major Inputs:

  • 1 glucose molecule (6C).

  • 2 ATP molecules.

  • 2 NAD+.

• Outputs:

  • 4 ATP (net gain of 2 ATP).

  • 2 NADH.

  • 2 pyruvate.

Learning Outcomes for 6.2

• Understand inputs and outputs of glycolysis.

• Diagram where glycolysis occurs within the cellular context.

Section 6.3: Pyruvate Oxidation

Overview

• Occurs in the mitochondrial matrix following glycolysis.

• Pyruvate, a 3C molecule, is oxidized to Acetyl CoA.

• CO2 is released as waste during the oxidation process.

Inputs and Outputs of Pyruvate Oxidation

• Inputs:

  • 2 Pyruvate.

  • 2 NAD+.

  • 2 Coenzyme A.

• Outputs:

  • 2 NADH.

  • 2 CO2.

  • 2 Acetyl CoA.

Learning Outcomes for 6.3

• Diagram pyruvate oxidation in the cellular context.

• Identify oxidation and reduction components.

Section 6.4: The Krebs Cycle

Overview

• Also known as the Citric Acid Cycle, operates in the mitochondrial matrix.

• Acetyl-CoA delivers 2C to undergo a series of redox reactions, forming a 6C citrate molecule.

• Each Acetyl CoA generates:

  • 1 ATP.

  • 3 NADH.

  • 1 FADH2.

  • 2 CO2 waste products.

Summary

• For each glucose (yielding 2 Acetyl CoA), the Krebs cycle produces:

  • 2 ATP, 6 NADH, 2 FADH2, and liberates 4 CO2.

• All carbon atoms of glucose have converted to CO2 by the end of the cycle.

Learning Outcomes for 6.4

• Understand and diagram the Krebs cycle's inputs and outputs.

Section 6.5: Electron Transport Chain (ETC) and ATP Synthase

Overview

• The ETC is the final step in cellular respiration, located within the inner mitochondrial membrane.

• It consists of a series of redox reactions where NADH and FADH2 donate electrons to regenerate their oxidized forms, creating a proton gradient.

Key Processes in the ETC

• Function of ETC:

  • Transfer of electrons reduces energy status as they pass through protein complexes.

  • Some complexes act as proton pumps, moving H+ ions from the matrix to the intermembrane space, generating a proton gradient.

• Final Electron Acceptor: Oxygen combines with electrons and H+ to form water.

ATP Production

• The generated proton gradient drives ATP synthesis via ATP synthase, producing approximately 36 ATP from one glucose molecule (actual yield closer to 30 due to transport costs and inefficiencies).

Learning Outcomes for 6.5

• Diagram ETC's location and processes.

Section 6.6: Fermentation

Overview

• Fermentation pathways allow cells to recycle NADH to NAD+ without oxygen, enabling continued glycolysis under anaerobic conditions.

• Useful for anaerobic organisms and situations where oxygen is limited.

Types of Fermentation

• Lactic Acid Fermentation: Pyruvate converted to lactic acid; NADH recycled to NAD+.

• Alcoholic Fermentation: Pyruvate converted to ethanol and CO2; NADH recycled to NAD+.

Efficiency and Importance

• Fermentation is much less efficient (only a fraction of ATP compared to aerobic respiration) but is critical for survival under low oxygen conditions.

• Energy efficiency of cellular respiration is ~37%, with glycolysis alone operating at only 2% efficiency.

Learning Outcomes for 6.6

• Diagram where fermentation occurs in the cell and its position in respiration process.