Chapter 7: Cellular Respiration

Redox Reactions: Reduction and Oxidation

Redox reactions are fundamental to cellular respiration, involving the transfer of electrons between molecules. These reactions are essential for energy production in biological systems.

• Redox reaction: A chemical reaction in which electrons are transferred from one molecule (the reductant) to another (the oxidant).

• Oxidation: Loss of electrons from a molecule, atom, or ion.

• Reduction: Gain of electrons by a molecule, atom, or ion.

• In biological systems, hydrogen atom transfer (proton + electron) often accompanies electron transfer.

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

Principle of Redox in Biology

Redox reactions are central to energy metabolism, as they allow cells to extract energy from organic molecules.

• Organic molecules are oxidized, releasing electrons that are transferred to electron acceptors.

• Energy released during electron transfer is used to synthesize ATP.

• Free electrons are rarely transferred directly; instead, electron carriers facilitate the process.

Overview of Cellular Energy Metabolism

Cellular respiration and photosynthesis are linked by redox processes, enabling the flow of energy through ecosystems.

• Photosynthesis: Converts solar energy into chemical energy stored in glucose ().

• Cellular respiration: Breaks down glucose to release energy, producing and .

• Energy is lost as heat during these processes.

Catabolic Pathways and ATP Production

Catabolic pathways break down organic molecules, releasing energy that is captured in ATP.

• Exergonic reactions: Release energy by breaking down complex molecules.

• Electron transfer: Drives ATP synthesis through a series of redox reactions.

• Cellular respiration: The main process for ATP production in most organisms.

• Aerobic respiration: Uses oxygen as the final electron acceptor.

• Anaerobic respiration: Occurs without oxygen, includes fermentation.

• Glucose is the most efficient energy source.

Oxidation of Organic Fuel Molecules

During cellular respiration, organic fuels such as glucose are oxidized, and electrons are transferred to oxygen.

• Glucose oxidation:

• Oxygen is the ultimate electron acceptor, becoming reduced.

• Controlled electron transfer prevents energy loss as heat.

ATP: Structure and Hydrolysis

ATP (adenosine triphosphate) is the primary energy currency of the cell, providing energy for cellular processes.

• Structure: Composed of adenine, ribose, and three phosphate groups.

• Hydrolysis:

• Energy is released when the terminal phosphate bond is broken.

Generation of ATP: Two Mechanisms

Cells generate ATP through substrate-level phosphorylation and oxidative phosphorylation.

• Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP from a substrate.

• Oxidative phosphorylation: Uses energy from electron transport chain (ETC) and chemiosmosis.

• Oxidative phosphorylation produces the majority of ATP in cells.

Electron Carriers: NAD+ and FAD

Electron carriers transport electrons during cellular respiration, facilitating redox reactions.

• NAD+ (nicotinamide adenine dinucleotide): Accepts electrons and is reduced to NADH.

• FAD (flavin adenine dinucleotide): Accepts electrons and is reduced to FADH2.

• Electron carriers shuttle electrons to the electron transport chain.

Stages of Cellular Respiration

Cellular respiration consists of three main stages, each contributing to the breakdown of glucose and production of ATP.

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

• Citric acid cycle (Krebs cycle): Completes the breakdown of glucose in the mitochondrial matrix.

• Oxidative phosphorylation (ETC): Produces most ATP via electron transport and chemiosmosis.

Glycolysis

Glycolysis is the first stage of cellular respiration, converting glucose into pyruvate and generating ATP and NADH.

• Occurs in the cytosol.

• Consists of two phases: energy investment and energy payoff.

• Net products: 2 ATP, 2 NADH, 2 pyruvate per glucose molecule.

Overall reaction of glycolysis:

Energy Harvest via NAD+ and the Electron Transport Chain

Electrons from NADH and FADH2 are transferred to the electron transport chain, driving ATP synthesis.

• Electron transport chain is located in the inner mitochondrial membrane.

• Electrons move through a series of protein complexes, releasing energy.

• Energy is used to pump protons, creating a gradient for ATP synthesis.

Summary Table: Stages of Cellular Respiration

Additional info:

• Fermentation is an alternative pathway when oxygen is absent, producing less ATP and resulting in products like lactate or ethanol.

• ATP is a universal energy currency, used in nearly all cellular processes.

• Cellular respiration is one of the most ancient metabolic pathways, found in all domains of life.