Chapter 7: Cellular Respiration – Mechanisms and Pathways

Chapter 7: Respiration

Introduction to Cellular Respiration

Cellular respiration is a fundamental metabolic process by which cells extract energy from organic molecules to produce ATP, the main energy currency of the cell. This process is essential for powering cellular work, including chemical, transport, and mechanical activities.

Energy Flow and Cellular Work

Cells operate as open systems, requiring a constant input of energy (E) to perform work. Energy flows into ecosystems as sunlight, is transformed by autotrophs into chemical energy, and is eventually used by cells to generate ATP. Some energy is lost as heat during these processes.

• Chemical work: Synthesis of macromolecules

• Transport work: Pumping substances across membranes

• Mechanical work: Movement, such as muscle contraction

Redox Reactions (Oxidation-Reduction)

Redox reactions are central to cellular respiration. In these reactions, electrons are transferred from one molecule (the electron donor) to another (the electron acceptor). Oxidation is the loss of electrons, while reduction is the gain of electrons.

• Oxidation: Loss of electrons

• Reduction: Gain of electrons

• Mnemonic: OiL RiG (Oxidation is Loss, Reduction is Gain)

Energy Harvest and Electron Carriers

During cellular respiration, energy is harvested in a controlled manner. Electrons from organic molecules are transferred to electron carriers such as NAD+, which shuttles them to the electron transport chain (ETC).

• NAD+: Electron acceptor, reduced to NADH

• NADH: Electron carrier, donates electrons to the ETC

Electron Transport Chain (ETC)

The ETC is a series of protein complexes embedded in the inner mitochondrial membrane. It enables the stepwise transfer of electrons from NADH and FADH2 to oxygen, the final electron acceptor, producing water and releasing energy used to generate ATP.

• Prevents explosive release of energy

• Allows controlled synthesis of ATP

Stages of Cellular Respiration

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

Stage 1: Glycolysis

Glycolysis is the first step in cellular respiration, occurring in the cytosol. It breaks down one molecule of glucose (6C) into two molecules of pyruvate (3C), producing 2 ATP and 2 NADH.

• Location: Cytosol

• Oxygen requirement: None (anaerobic)

• Net products per glucose: 2 ATP, 2 NADH, 2 pyruvate, 2 H2O

Substrate-Level Phosphorylation

ATP is generated directly in glycolysis by substrate-level phosphorylation, where an enzyme transfers a phosphate group from a substrate to ADP.

Glycolysis: Energy Investment and Payoff Phases

Glycolysis consists of two phases:

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

• Energy Payoff Phase: 4 ATP and 2 NADH are produced as glucose is split into pyruvate.

Glycolysis (Summary)

Overall, glycolysis yields a net gain of 2 ATP and 2 NADH per glucose molecule.

Stage 2: Pyruvate Oxidation and Citric Acid Cycle (Krebs Cycle)

Pyruvate produced in glycolysis is transported into the mitochondrion, where it is oxidized to acetyl CoA. Acetyl CoA enters the citric acid cycle, which completes the breakdown of glucose and generates electron carriers NADH and FADH2.

Mitochondrion Structure

The mitochondrion has a double membrane structure with an outer membrane, an inner membrane (folded into cristae), and a matrix where the citric acid cycle occurs.

Pyruvate Oxidation

Each pyruvate is converted to acetyl CoA, producing NADH and releasing CO2.

• Inputs: 2 pyruvate

• Outputs: 2 acetyl CoA, 2 NADH, 2 CO2

Citric Acid Cycle (Krebs Cycle)

The citric acid cycle is a series of enzyme-catalyzed reactions in the mitochondrial matrix that completes the oxidation of acetyl CoA. It generates ATP, NADH, FADH2, and CO2 as waste.

• Per glucose (2 cycles): 2 ATP, 6 NADH, 2 FADH2, 4 CO2

Stage 3: Oxidative Phosphorylation

Oxidative phosphorylation includes the electron transport chain and chemiosmosis. It occurs in the inner mitochondrial membrane and produces the majority of ATP during cellular respiration.

Electron Transport Chain (ETC)

The ETC consists of protein complexes that transfer electrons from NADH and FADH2 to oxygen. The energy released is used to pump protons (H+) across the inner mitochondrial membrane, creating a proton gradient.

• Final electron acceptor: O2, forming H2O

• Proton gradient: Drives ATP synthesis

Chemiosmosis and ATP Synthase

Protons flow back into the mitochondrial matrix through ATP synthase, which uses the energy of the proton gradient to phosphorylate ADP, forming ATP. This process is called chemiosmosis.

• ATP yield: 26–28 ATP per glucose

Summary Table: Cellular Respiration Pathways

Stage	Main Inputs	Main Outputs	Location	ATP Produced
Glycolysis	Glucose, 2 NAD+, 2 ADP	2 Pyruvate, 2 NADH, 2 ATP, 2 H2O	Cytosol	2
Pyruvate Oxidation	2 Pyruvate, 2 NAD+, 2 CoA	2 Acetyl CoA, 2 NADH, 2 CO2	Mitochondrial Matrix	0
Citric Acid Cycle	2 Acetyl CoA, 6 NAD+, 2 FAD, 2 ADP	4 CO2, 6 NADH, 2 FADH2, 2 ATP	Mitochondrial Matrix	2
Oxidative Phosphorylation	10 NADH, 2 FADH2, O2	H2O, ~26–28 ATP	Inner Mitochondrial Membrane	26–28