Chapter 7: Cellular Respiration & Fermentation

Introduction: Life is Work

All living organisms require energy to perform cellular work. This energy is obtained from external sources, either by consuming other organisms or by harnessing energy from sunlight (photosynthesis).

• Heterotrophs obtain energy by eating other organisms.

• Autotrophs (e.g., plants, algae) capture energy from sunlight.

Catabolic Pathways Yield Energy by Oxidizing Organic Fuel

Overview of Catabolic Pathways and ATP Production

Catabolic pathways break down energy-rich organic molecules, releasing potential energy stored in covalent bonds. This energy is used to generate ATP for cellular work, with some lost as heat.

• Catabolism: Degradation of molecules into simpler forms with less energy.

• Example: Glucose breakdown releases energy for ATP synthesis.

Types of Cellular Respiration

Cellular respiration includes aerobic and anaerobic processes, as well as fermentation. Each pathway differs in its use of oxygen and final products.

• Aerobic respiration: Complete breakdown of organic molecules using O2.

• Anaerobic respiration: Uses molecules other than O2 (e.g., sulfur, nitrogen).

• Fermentation: Partial breakdown without O2, producing little ATP and waste products.

General equation for aerobic respiration:

Redox Reactions in Cellular Respiration

Energy is released from organic molecules through oxidation-reduction (redox) reactions, which transfer electrons between molecules.

• Oxidation: Loss of electrons; molecule becomes less negative and less energized.

• Reduction: Gain of electrons; molecule becomes more negative and more energized.

• OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).

Oxidation of Organic Fuel Molecules

During cellular respiration, fuel molecules such as glucose are oxidized, and oxygen is reduced. Glucose is a highly reduced molecule, containing many hydrogen atoms that act as reducing agents.

• Glucose (C6H12O6): Highly energized due to abundant hydrogen atoms.

• Hydrogen transfer:

Electron Carriers and ATP Production

NAD+ and FAD as Electron Acceptors

Electrons from organic compounds are transferred to coenzymes such as NAD+ and FAD, which act as electron carriers.

• NAD+: Accepts electrons to become NADH (reduced form).

• NADH: Stores potential energy used to synthesize ATP.

• FAD: Accepts electrons to become FADH2.

Stages of Cellular Respiration

Overview of Stages

Cellular respiration occurs in several stages, each contributing to the breakdown of glucose and production of ATP.

• Glycolysis: Occurs in the cytosol; splits glucose into two pyruvate molecules.

• Pyruvate oxidation: Converts pyruvate to acetyl-CoA in mitochondria (eukaryotes).

• Citric acid cycle (Krebs cycle): Completes glucose catabolism, generating CO2, NADH, and FADH2.

• Oxidative phosphorylation: Uses electron transport chain and chemiosmosis to produce most ATP.

ATP Production Mechanisms

• Substrate-level phosphorylation: Direct transfer of phosphate to ADP during glycolysis and citric acid cycle; produces a small amount of ATP.

• Oxidative phosphorylation: Accounts for the bulk of ATP production; occurs in the inner mitochondrial membrane (eukaryotes) or cytoplasmic membrane (prokaryotes).

Glycolysis

Harvesting Chemical Energy by Oxidizing Glucose

Glycolysis is the process of splitting glucose into two pyruvate molecules, generating ATP and NADH. It does not require oxygen and occurs in the cytosol.

• Net ATP yield: 2 ATP per glucose molecule.

• NADH produced: High-energy electrons for later stages.

• Pyruvate: End product, enters mitochondria if O2 is present.

Pyruvate Oxidation and Citric Acid Cycle

Completing the Energy-Yielding Oxidation

In the presence of oxygen, pyruvate is oxidized to acetyl-CoA, which enters the citric acid cycle. This cycle generates CO2, NADH, and FADH2 for use in the electron transport chain.

• Inputs: Acetyl-CoA, NAD+, FAD

• Outputs: CO2, NADH, FADH2, ATP

• Occurs twice per glucose molecule (once for each acetyl-CoA).

Oxidative Phosphorylation and Chemiosmosis

Electron Transport Chain (ETC)

The ETC is a series of protein complexes in the mitochondrial membrane that transfer electrons from NADH and FADH2 to oxygen, generating a proton gradient.

• Proton gradient: Protons are pumped into the intermembrane space, creating potential energy.

• ATP synthase: Uses the proton-motive force to synthesize ATP from ADP and inorganic phosphate.

• Oxygen: Final electron acceptor, forming water as a byproduct.

ATP Yield

• Total ATP from one glucose: About 34% of glucose's potential energy is converted to ATP.

• Majority of ATP is produced during oxidative phosphorylation.

Fermentation and Anaerobic Respiration

ATP Production Without Oxygen

When oxygen is unavailable, cells can produce ATP through fermentation or anaerobic respiration.

• Anaerobic respiration: Uses molecules other than O2 as final electron acceptors (e.g., sulfur, nitrogen).

• Fermentation: Does not use ETC; partial breakdown of glucose, regenerates NAD+ for glycolysis.

Types of Fermentation

Applications: Alcohol fermentation is used in brewing, winemaking, and baking; lactic acid fermentation is important in cheese and yogurt production, and occurs in muscle cells during intense exercise.

Connections to Other Metabolic Pathways

Catabolic Pathways for Other Molecules

Glycolysis and the citric acid cycle are central hubs for the metabolism of carbohydrates, proteins, and fats.

• Carbohydrates: Hydrolyzed to glucose, enter glycolysis.

• Proteins: Digested to amino acids; amino groups removed (deamination), carbon skeletons enter glycolysis or citric acid cycle.

• Fats: Broken down to glycerol (enters glycolysis) and fatty acids (enter citric acid cycle via beta-oxidation).

ATP yield: Fats produce about twice as much ATP per gram as carbohydrates.

Anabolic Pathways (Biosynthesis)

Cells use intermediates from glycolysis and the citric acid cycle to synthesize new organic molecules for growth and repair.

• Excess nutrients: Used to synthesize fats stored in adipose tissue.

• Building blocks: Amino acids, nucleotides, and other molecules can be synthesized from metabolic intermediates.

Summary Table: Comparison of Energy Pathways

Additional info: These notes expand on the original slides by providing definitions, examples, and structured tables for comparison. All major topics from the provided materials are covered and organized for exam preparation.