BIO123.ch6.OER.Liz.notes.blanks

Chapter 6 - Introduction to Cellular Respiration

Chapter Objectives

• Explain what a redox reaction is.

• Know the overall equation for aerobic cellular respiration and be able to explain which molecules are reduced or oxidized into which molecules.

• Identify ATP and describe how it is involved in energy transfers within cells.

• Describe the basic steps of glycolysis.

• Know the starting reactants and final products of glycolysis.

• Identify which organisms are capable of glycolysis and where they carry these out in the cell.

• Describe the location of pyruvate oxidation in the cell.

• Explain what happens during pyruvate oxidation, including the starting reactants and final products.

• Describe the location of the citric acid cycle in the cell.

• Explain what happens during the citric acid cycle, including the starting reactants and final products.

• Describe the location of oxidative phosphorylation in the cell.

• Describe the overall outcome of oxidative phosphorylation in terms of the products of each stage.

• Describe the relationships of glycolysis, the citric acid cycle, and oxidative phosphorylation in terms of their ATP outputs.

• Describe the relationships of glycolysis, citric acid cycle, and oxidative phosphorylation in terms of their electron carriers.

• Describe the fundamental difference between anaerobic cellular respiration and fermentation.

• Describe the types of fermentation that readily occur and the conditions that initiate that fermentation.

• Discuss how metabolic pathways, such as glycolysis and the citric acid cycle, can use sugars other than glucose to generate ATP.

• Discuss how proteins and lipids can be used to generate ATP by entering glycolysis, pyruvate oxidation, and the citric acid cycle as intermediates.

• Be able to define and explain all bolded terms.

Keywords

• Cellular respiration, redox reaction, ATP, glycolysis, pyruvate oxidation, citric acid cycle, oxidative phosphorylation, fermentation, substrate-level phosphorylation.

6.1 Energy in Living Systems

Cellular Respiration

• All living things perform cellular respiration, which can be:

  • Aerobic (requires oxygen)

  • Anaerobic (does not require oxygen)

Aerobic Cellular Respiration

• Utilizes potential energy to drive the synthesis of ATP with the help of oxygen.

Oxidation and Reduction Reactions

• Redox reactions are chemical reactions where electrons are transferred from one molecule to another.

  • Reduced: Molecules that gain electron(s).

  • Oxidized: Molecules that lose electron(s).

• Aerobic cellular respiration exemplifies a reduction reaction.

• Reduction and oxidation usually occur together.

Electron Carriers

• Molecules like Nicotinamide adenine dinucleotide (NAD) shuttle electrons to the electron transport chain, crucial for ATP production.

ATP in Living Systems

• ATP provides much of the energy necessary for cellular processes.

• Hydrolysis of ATP provides energy for coupled endergonic reactions: ATP → ADP + Pi (exergonic).

  • ATP can be regenerated via the following:

    • Substrate-level phosphorylation

    • Oxidative phosphorylation

6.2 Glycolysis

Overview

• Glycolysis is the first metabolic pathway used to metabolize glucose, requiring no oxygen.

• It occurs in the cytoplasm of eukaryotic cells.

• Small amounts of ATP are generated using substrate-level phosphorylation (intermediate reactant transfers phosphate to ADP).

6.3 Citric Acid Cycle

Overview

• Also known as the Krebs cycle, occurs in the mitochondrial matrix.

• Acetyl CoA is transferred to oxaloacetate to form citrate.

• Throughout the cycle:

  • Citrate is oxidized, producing:

    • 3 NADH

    • 1 FADH2

    • 2 CO2

    • 1 ATP

• The cycle runs continuously in the presence of sufficient reactants, returning to oxaloacetate.

Outputs from Glycolysis and Citric Acid Cycle

• ATP: 4 ATP total (2 from glycolysis, 2 from citric acid cycle)

• CO2: 6 CO2 (2 from pyruvate oxidation, 4 from citric acid cycle)

• NADH: 10 NADH (total from all phases)

• FADH2: 2 from citric acid cycle

• At the end of the citric acid cycle, glucose is fully oxidized.

6.4 Oxidative Phosphorylation

Overview

• Comprises two parts: electron transport chain (ETC) and chemiosmosis, both occurring in the inner mitochondrial membrane.

• Most ATP produced in cellular respiration stems from oxidative phosphorylation.

Electron Transport Chain (ETC)

• A sequence of electron carriers (proteins) embedded in the mitochondrial membrane.

• NADH and FADH2 are oxidized while electrons are transferred to oxygen (O2) as the terminal electron acceptor, forming H2O.

• Energy released from these transfers is used to pump protons (H+) across the membrane, creating a gradient.

Chemiosmosis

• Uses kinetic energy derived from the proton gradient to generate ATP by moving H+ ions down their gradient, forming ATP from ADP and Pi.

6.5 Fermentation

Overview

• Occurs in the absence of oxygen, where glycolysis continues but additional pathways regenerate NAD+.

• Two primary types of fermentation:

  • Lactic Acid Fermentation: Occurs in muscle cells and some bacteria; NADH is oxidized, producing lactate while regenerating NAD+.

  • Alcohol Fermentation: Performed by yeast; converts pyruvate into ethanol and CO2, regenerating NAD+.

6.6 Connections to Other Metabolic Pathways

Metabolism of Organic Molecules

• Various organic molecules (e.g., carbohydrates, proteins, lipids) can feed into glycolysis, pyruvate oxidation, and the citric acid cycle to generate ATP.

Chapter Conclusion

• All cells use cellular respiration to synthesize ATP through redox reactions, employing both substrate-level and oxidative phosphorylation.

• Glycolysis, pyruvate oxidation, citric acid cycle, and oxidative phosphorylation collectively manage the metabolism of glucose, while the capacity to utilize proteins and lipids illustrates the versatility of these metabolic pathways.