Cellular Respiration and Fermentation

Overview of Energy Flow in Cells

Cells require energy to perform work, and this energy is primarily supplied by the molecule ATP, which is generated through the breakdown of organic molecules. The processes of cellular respiration and fermentation are central to how cells harvest and utilize chemical energy from food.

• Photosynthesis captures light energy to produce organic molecules and oxygen from carbon dioxide and water.

• Cellular respiration breaks down organic molecules in the presence of oxygen to generate ATP, releasing carbon dioxide and water as waste products.

• Energy enters ecosystems as light and exits as heat, while chemical elements are recycled.

Catabolic Pathways and Energy Production

Catabolic Pathways: Definition and Importance

Catabolic pathways are metabolic routes that break down complex molecules into simpler ones, releasing energy in the process. This energy is used to synthesize ATP, the main energy currency of the cell.

• The breakdown of organic molecules is exergonic (releases energy).

• Electron transfer from food molecules to other molecules is a key mechanism in energy release.

• Catabolic pathways do not directly power cellular work; instead, they are linked to work by ATP.

Types of Catabolic Pathways

• Fermentation: Partial degradation of sugars without oxygen.

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

• Anaerobic respiration: Similar to aerobic respiration but uses compounds other than oxygen as final electron acceptors.

• Cellular respiration: Includes both aerobic and anaerobic processes, but often refers specifically to aerobic respiration.

Cellular Respiration Equation

The overall process of cellular respiration can be summarized by the following chemical equation:

• Glucose is the primary fuel, but fats and proteins can also be used.

Redox Reactions: Oxidation and Reduction

Principles of Redox Reactions

Redox (reduction-oxidation) reactions are chemical processes that involve the transfer of electrons between molecules, playing a central role in energy extraction from food.

• Oxidation: Loss of electrons from a substance.

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

• The substance that donates electrons is the reducing agent; the one that accepts electrons is the oxidizing agent.

• Redox reactions can involve complete transfer of electrons (as in ionic bonds) or changes in electron sharing (as in covalent bonds).

Example: In the reaction , sodium is oxidized and chlorine is reduced.

Redox in Cellular Respiration

• Organic molecules with many hydrogen atoms are excellent sources of high-energy electrons.

• During cellular respiration, glucose is oxidized and oxygen is reduced.

• Energy is released as electrons move from high-energy states (in glucose) to lower-energy states (in oxygen).

Stepwise Energy Harvest via NAD+ and the Electron Transport Chain

Role of Electron Carriers

Electrons from food are transferred in a series of steps, often involving the coenzyme NAD+ (nicotinamide adenine dinucleotide), which acts as an electron acceptor and oxidizing agent.

• Enzymes called dehydrogenases remove hydrogen atoms from substrates, transferring electrons and protons to NAD+ to form NADH.

• NADH carries electrons to the electron transport chain, a series of molecules embedded in the inner mitochondrial membrane (or plasma membrane in prokaryotes).

• Electrons are passed through the chain in a series of redox reactions, releasing energy in small, manageable amounts.

• Oxygen is the final electron acceptor, forming water.

Stages of Cellular Respiration

Overview of the Three Main Stages

• Glycolysis: Breaks down glucose into two molecules of pyruvate in the cytoplasm; does not require oxygen.

• Pyruvate oxidation and the citric acid cycle: Completes the breakdown of glucose to carbon dioxide in the mitochondrial matrix (in eukaryotes).

• Oxidative phosphorylation: Includes the electron transport chain and chemiosmosis, producing most of the cell's ATP.

ATP Production Mechanisms

• Substrate-level phosphorylation: Enzyme transfers a phosphate group directly from a substrate to ADP to form ATP (occurs in glycolysis and the citric acid cycle).

• Oxidative phosphorylation: ATP is synthesized using energy released by the electron transport chain and chemiosmosis (accounts for ~90% of ATP produced by cellular respiration).

Summary Table: Comparison of Catabolic Pathways