Chapter 9: Cellular Respiration

Overview of Cellular Respiration

Cellular respiration is a fundamental metabolic process by which cells convert biochemical energy from nutrients into adenosine triphosphate (ATP), releasing waste products. It is essential for providing energy to support cellular activities in both plants and animals.

• Main Goal: The primary goal of cellular respiration is to produce ATP, the energy currency of the cell.

• General Chemical Reaction: The overall reaction for aerobic cellular respiration is:

• Exergonic Process: Cellular respiration is an exergonic process, meaning it releases energy.

• Redox Process: It is a redox (oxidation-reduction) process. Glucose is oxidized (loses electrons), and oxygen is reduced (gains electrons).

Main Stages of Cellular Respiration

Cellular respiration occurs in several stages, each with specific roles and locations within the cell.

• Glycolysis: Occurs in the cytoplasm; breaks down glucose into two molecules of pyruvate, producing a small amount of ATP and NADH.

• Pyruvate Oxidation: Pyruvate is transported into the mitochondria and converted into acetyl-CoA, producing NADH and releasing CO2.

• Citric Acid Cycle (Krebs Cycle): Takes place in the mitochondrial matrix; acetyl-CoA is oxidized, generating NADH, FADH2, ATP, and CO2.

• Oxidative Phosphorylation: Involves the electron transport chain and chemiosmosis; occurs in the inner mitochondrial membrane, producing the majority of ATP.

Summary Table: Stages of Cellular Respiration

ATP Production in Cellular Respiration

ATP is produced at different stages of cellular respiration through substrate-level phosphorylation and oxidative phosphorylation.

• Stage 1 (Glycolysis) and Stage 2 (Citric Acid Cycle): ATP is produced by substrate-level phosphorylation.

• Stage 3 (Oxidative Phosphorylation): Most ATP is produced here via chemiosmosis, driven by the electron transport chain.

• ATP Synthase: The enzyme responsible for synthesizing ATP from ADP and inorganic phosphate, powered by a proton gradient.

Redox Reactions in Cellular Respiration

Redox reactions involve the transfer of electrons between molecules, crucial for energy extraction in respiration.

• Oxidation: Loss of electrons from a molecule (e.g., glucose is oxidized).

• Reduction: Gain of electrons by a molecule (e.g., oxygen is reduced to water).

• Electron Carriers: NAD+ and FAD are reduced to NADH and FADH2, which transport electrons to the electron transport chain.

Fermentation vs. Cellular Respiration

Cells can generate ATP in the absence of oxygen through fermentation, but this process is less efficient than aerobic respiration.

• Fermentation: Anaerobic process; produces ATP by substrate-level phosphorylation only, yielding lactic acid or ethanol as end products.

• Cellular Respiration: Aerobic process; produces much more ATP by utilizing the electron transport chain and oxygen as the final electron acceptor.

Summary Table: Comparison of Aerobic Respiration and Fermentation

Types of Metabolism: Anaerobic vs. Aerobic

Cells can use different metabolic strategies depending on oxygen availability.

• Aerobic Respiration: Requires oxygen; yields high amounts of ATP.

• Anaerobic Respiration: Does not require oxygen; yields less ATP.

• Facultative Anaerobes: Organisms that can switch between aerobic and anaerobic metabolism depending on oxygen availability.

Macromolecules and Cellular Respiration

Various macromolecules can serve as fuel for cellular respiration, not just glucose.

• Carbohydrates: Broken down into glucose and enter glycolysis.

• Fats: Broken down into glycerol and fatty acids; fatty acids enter the citric acid cycle as acetyl-CoA.

• Proteins: Broken down into amino acids; after deamination, carbon skeletons enter glycolysis or the citric acid cycle.

Additional info: Intermediates from cellular respiration can be used to synthesize other macromolecules, demonstrating the interconnectedness of metabolic pathways.

Key Terms and Definitions

• ATP (Adenosine Triphosphate): The main energy carrier in cells.

• Glycolysis: The first step in cellular respiration, breaking down glucose into pyruvate.

• Citric Acid Cycle (Krebs Cycle): A series of reactions that generate electron carriers and CO2.

• Electron Transport Chain: A sequence of proteins that transfer electrons and pump protons to generate a proton gradient for ATP synthesis.

• Fermentation: An anaerobic process that allows glycolysis to continue by regenerating NAD+.

• Redox Reaction: A chemical reaction involving the transfer of electrons.