Cellular Respiration and Fermentation

Introduction to Cellular Respiration

Cellular respiration is a fundamental metabolic process by which cells extract energy from organic molecules, primarily glucose, to produce ATP, the main energy currency of the cell. Both plant and animal cells utilize cellular respiration, which occurs mainly in the mitochondria.

• ATP (Adenosine Triphosphate): The molecule that stores and transfers energy for most cellular processes.

• Energy Flow: Energy enters ecosystems as light, is stored in organic molecules, and exits as heat.

• Recycling of Elements: Chemical elements essential to life are recycled, while energy flows through the system.

Energy and Work in Cells

Cellular Work and Energy Requirements

Cells require energy to perform work, including assembling polymers, transporting substances across membranes, moving, and reproducing. Animals obtain energy by consuming other organisms, while plants capture energy via photosynthesis.

• Cellular Work: Includes biosynthesis, active transport, movement, and cell division.

• Source of Energy: Organic molecules (e.g., glucose) are broken down to release energy.

Catabolic Pathways and ATP Production

Overview of Catabolic Pathways

Catabolic pathways break down complex molecules into simpler ones, releasing energy that is used to synthesize ATP. The main catabolic pathway is cellular respiration, which can be aerobic (using oxygen) or anaerobic (without oxygen).

• Fermentation: Partial degradation of sugars without oxygen.

• Aerobic Respiration: Consumes organic molecules and oxygen, yielding ATP.

• Anaerobic Respiration: Similar to aerobic but uses electron acceptors other than oxygen.

Redox Reactions in Cellular Respiration

Oxidation and Reduction

Cellular respiration involves a series of oxidation-reduction (redox) reactions, where electrons are transferred from one molecule to another, releasing energy.

• Oxidation: Loss of electrons from a substance.

• Reduction: Gain of electrons by a substance.

• Redox Example: In the reaction Na + Cl → Na+ + Cl-, sodium is oxidized and chlorine is reduced.

• Electron Carriers: Molecules like NAD+ and FAD accept electrons and transfer them to the electron transport chain.

Stages of Cellular Respiration

Overview of the Three Main Stages

Cellular respiration consists of three main stages: glycolysis, the citric acid cycle, and oxidative phosphorylation.

1. Glycolysis: Occurs in the cytosol; breaks down glucose into two molecules of pyruvate.

2. Citric Acid Cycle (Krebs Cycle): Completes the breakdown of glucose to CO2 in the mitochondrial matrix.

3. Oxidative Phosphorylation: Includes the electron transport chain and chemiosmosis; produces most of the cell's ATP.

Glycolysis

Glycolysis is the first step in cellular respiration and occurs in the cytosol. It does not require oxygen and results in the partial oxidation of glucose.

• Energy Investment Phase: 2 ATP are used to phosphorylate glucose intermediates.

• Energy Payoff Phase: 4 ATP and 2 NADH are produced, resulting in a net gain of 2 ATP per glucose.

• Net Reaction:

• Fate of Pyruvate: In the presence of oxygen, pyruvate enters the mitochondrion for further oxidation.

Pyruvate Oxidation and the Citric Acid Cycle

Pyruvate is converted to acetyl CoA, which enters the citric acid cycle. This cycle completes the oxidation of glucose derivatives, generating NADH and FADH2 for the electron transport chain.

• Products per Glucose: 2 ATP (substrate-level phosphorylation), 6 NADH, 2 FADH2, and 4 CO2 (waste).

• Key Steps: Acetyl CoA combines with oxaloacetate to form citrate, which is then oxidized in a series of steps.

Oxidative Phosphorylation: Electron Transport and Chemiosmosis

Most ATP is produced during oxidative phosphorylation, which involves the electron transport chain (ETC) and chemiosmosis.

• Electron Transport Chain: A series of protein complexes in the inner mitochondrial membrane transfer electrons from NADH and FADH2 to oxygen, the final electron acceptor.

• Proton Gradient: Energy from electron transfer pumps protons (H+) into the intermembrane space, creating a proton-motive force.

• ATP Synthase: Protons flow back into the matrix through ATP synthase, driving the phosphorylation of ADP to ATP.

• Equation:

• ATP Yield: Up to 32 ATP per glucose molecule (actual yield may vary).

Fermentation and Anaerobic Respiration

Fermentation

Fermentation is an anaerobic process that allows glycolysis to continue by regenerating NAD+ in the absence of oxygen. It produces a small amount of ATP (2 per glucose).

• Alcohol Fermentation: Pyruvate is converted to ethanol and CO2 (e.g., in yeast).

• Lactic Acid Fermentation: Pyruvate is reduced to lactate (e.g., in muscle cells, some bacteria).

Anaerobic Respiration

Some prokaryotes use electron acceptors other than oxygen (e.g., sulfate) in their electron transport chains, allowing ATP production without oxygen.

Comparing Fermentation, Anaerobic, and Aerobic Respiration

• Fermentation: Only glycolysis; 2 ATP per glucose; final electron acceptor is an organic molecule.

• Anaerobic Respiration: Includes glycolysis, citric acid cycle, and ETC; final electron acceptor is not oxygen.

• Aerobic Respiration: Includes glycolysis, citric acid cycle, and ETC; final electron acceptor is oxygen; up to 32 ATP per glucose.

Catabolism of Other Molecules

Versatility of Catabolism

Cells can use carbohydrates, fats, and proteins as fuel for cellular respiration. These molecules enter the metabolic pathways at different points.

• Carbohydrates: Broken down to glucose or intermediates of glycolysis.

• Proteins: Digested to amino acids; amino groups are removed (deamination) and excreted as nitrogenous waste.

• Fats: Broken down to glycerol (enters glycolysis) and fatty acids (converted to acetyl CoA via beta oxidation).

• ATP Yield: Fats produce more than twice as much ATP per gram as carbohydrates.

Regulation of Cellular Respiration

Feedback Mechanisms

Cellular respiration is tightly regulated by feedback inhibition, primarily at the level of glycolysis. High levels of ATP inhibit key enzymes, while low levels stimulate them.

• Phosphofructokinase: A key regulatory enzyme in glycolysis; inhibited by ATP and citrate, stimulated by AMP.

Summary Table: ATP Yield from Cellular Respiration

Key Points

• Cellular respiration efficiently converts the chemical energy of glucose into ATP.

• Fermentation allows ATP production in the absence of oxygen but is much less efficient.

• Metabolic pathways are interconnected and regulated to meet the cell's energy needs.