Cellular Respiration and Fermentation

Overview

Cellular respiration and fermentation are essential metabolic processes that allow cells to extract energy from organic molecules. This energy is stored in the form of ATP, which powers most cellular work. These processes are central to the flow of energy in living organisms and are tightly linked to photosynthesis in the global carbon and energy cycles.

Catabolic Pathways and Energy Yield

Catabolic Pathways: Breaking Down Organic Fuels

• Catabolic pathways involve the breakdown of complex molecules into simpler ones, releasing energy in the process.

• The breakdown of organic molecules is exergonic, meaning it releases energy.

• Aerobic respiration is a catabolic process that consumes organic molecules and oxygen (O2), yielding ATP.

• Fermentation is a partial degradation of sugars that occurs without the use of oxygen.

Key Terms:

• Exergonic reaction: A chemical reaction that releases energy.

• Aerobic respiration: Respiration that requires oxygen.

• Fermentation: An anaerobic process that allows ATP production without oxygen.

Redox Reactions in Cellular Respiration

Oxidation and Reduction

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

• Oxidation: Loss of electrons from a substance.

• Reduction: Gain of electrons by a substance (the amount of positive charge is reduced).

• In redox reactions, the substance that loses electrons is oxidized, and the substance that gains electrons is reduced.

• Not all redox reactions involve complete transfer of electrons; some involve changes in electron sharing in covalent bonds.

Example: In the reaction between sodium and chlorine:

• Sodium is oxidized (loses an electron), and chlorine is reduced (gains an electron).

Stages of Cellular Respiration

Overview of the Process

Cellular respiration consists of three main stages:

1. Glycolysis – Occurs in the cytoplasm; breaks down glucose into two molecules of pyruvate.

2. Citric Acid Cycle (Krebs Cycle) – Completes the breakdown of glucose by oxidizing pyruvate to carbon dioxide in the mitochondrial matrix.

3. Oxidative Phosphorylation – Includes the electron transport chain and chemiosmosis; occurs in the inner mitochondrial membrane and produces most of the cell's ATP.

Summary Equation for Cellular Respiration

The overall chemical equation for aerobic cellular respiration is:

Glycolysis

Process and Phases

Glycolysis is the first step in the breakdown of glucose, occurring in the cytoplasm. It does not require oxygen and consists of two phases:

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

• Energy Payoff Phase: 4 ATP and 2 NADH molecules are produced, along with 2 pyruvate and 2 H2O.

Net Reaction:

• Glucose 2 Pyruvate + 2 H2O

• 2 NAD+ + 4 e- + 4 H+ 2 NADH + 2 H+

• 4 ATP formed – 2 ATP used = 2 ATP net gain

Pyruvate Oxidation and the Citric Acid Cycle

Pyruvate Oxidation

In the presence of oxygen, pyruvate enters the mitochondrion and is converted to acetyl CoA, which enters the citric acid cycle.

• Pyruvate is oxidized, releasing CO2 and generating NADH.

• Acetyl CoA is the entry compound for the citric acid cycle.

Citric Acid Cycle (Krebs Cycle)

The citric acid cycle completes the breakdown of pyruvate, generating ATP, NADH, and FADH2 per turn.

• Each turn produces: 1 ATP, 3 NADH, 1 FADH2, and 2 CO2.

• Occurs in the mitochondrial matrix.

Oxidative Phosphorylation

Electron Transport Chain and Chemiosmosis

Oxidative phosphorylation is the process that generates most of the ATP during cellular respiration. It involves two main components:

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

• Chemiosmosis: The energy released from electron transfer is used to pump protons (H+) across the membrane, creating a proton gradient. ATP synthase uses this gradient to synthesize ATP from ADP and inorganic phosphate.

Key Equation:

About 34% of the energy in a glucose molecule is transferred to ATP, making about 32 ATP per glucose molecule; the rest is lost as heat.

Fermentation and Anaerobic Respiration

ATP Production Without Oxygen

When oxygen is not available, cells can produce ATP through fermentation or anaerobic respiration.

• Fermentation: Glycolysis plus reactions that regenerate NAD+ by transferring electrons to organic molecules.

• Anaerobic respiration: Uses an electron transport chain with a final electron acceptor other than oxygen.

Types of Fermentation

• 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 under low oxygen).

Comparison Table: Fermentation vs. Cellular Respiration

Connections to Other Metabolic Pathways

Integration of Metabolism

Glycolysis and the citric acid cycle are central to metabolism and connect to the breakdown and synthesis of carbohydrates, fats, and proteins.

• Other macromolecules can enter the pathways at various points after conversion to intermediates.

• These pathways are regulated by feedback mechanisms to maintain energy balance.

Summary Diagram: Energy Flow in Cells

• Photosynthesis in chloroplasts uses light energy to convert CO2 and H2O into organic molecules and O2.

• Cellular respiration in mitochondria breaks down organic molecules, generating ATP and releasing CO2 and H2O.

• ATP produced powers most cellular work; some energy is lost as heat.

Example: In both plant and animal cells, mitochondria are the site of cellular respiration, while only plant cells (and some protists) have chloroplasts for photosynthesis.