Cellular Respiration

Overview of Cellular Respiration

Cellular respiration is a series of metabolic processes by which cells convert biochemical energy from nutrients into adenosine triphosphate (ATP), releasing waste products. It is essential for the production of energy in both prokaryotic and eukaryotic organisms.

• Purpose: To extract energy from organic molecules (mainly glucose) and store it in the form of ATP.

• Main Stages: Glycolysis, Pyruvate Oxidation, Krebs Cycle, and Oxidative Phosphorylation.

• Location: Cytoplasm and mitochondria (in eukaryotes).

Redox Reactions in Cellular Respiration

Definition and Importance

Redox (reduction-oxidation) reactions involve the transfer of electrons between molecules. These reactions are fundamental to cellular respiration, as they allow the controlled release of energy from glucose.

• Oxidation: Loss of electrons from a molecule.

• Reduction: Gain of electrons by a molecule.

• Mnemonic: "OIL RIG" – Oxidation Is Loss, Reduction Is Gain (of electrons).

• Example: In cellular respiration, glucose is oxidized and oxygen is reduced.

Steps of Cellular Respiration

Glycolysis

Glycolysis is the first step of cellular respiration, occurring in the cytoplasm. It breaks down one molecule of glucose into two molecules of pyruvate.

• Location: Cytoplasm

• Inputs: Glucose, 2 NAD+, 2 ATP, 4 ADP, 4 Pi

• Outputs: 2 Pyruvate, 2 NADH, 4 ATP (net gain: 2 ATP)

• Equation:

Pyruvate Oxidation

Pyruvate oxidation links glycolysis to the Krebs cycle. Each pyruvate is transported into the mitochondrion and converted into acetyl-CoA.

• Location: Mitochondrial matrix (eukaryotes)

• Inputs: 2 Pyruvate, 2 NAD+, 2 CoA

• Outputs: 2 Acetyl-CoA, 2 NADH, 2 CO2

• Equation:

Krebs Cycle (Citric Acid Cycle)

The Krebs cycle completes the oxidation of glucose derivatives, generating electron carriers for the next stage.

• Location: Mitochondrial matrix

• Inputs (per glucose): 2 Acetyl-CoA, 6 NAD+, 2 FAD, 2 ADP, 2 Pi

• Outputs (per glucose): 4 CO2, 6 NADH, 2 FADH2, 2 ATP (or GTP)

• Equation:

Oxidative Phosphorylation

This stage includes the electron transport chain (ETC) and chemiosmosis, producing the majority of ATP during cellular respiration.

• Location: Inner mitochondrial membrane

• Electron Transport Chain (ETC): Electrons from NADH and FADH2 are transferred through protein complexes, ultimately reducing O2 to H2O.

• Inputs: 10 NADH, 2 FADH2, 6 O2, ADP, Pi

• Outputs: ~28 ATP, 6 H2O

• Chemiosmosis: The ETC pumps protons (H+) into the intermembrane space, creating a proton gradient. ATP synthase uses this gradient to synthesize ATP from ADP and Pi.

• Equation:

Anaerobic Respiration and Fermentation

Fermentation Pathways

When oxygen is not available, cells can generate ATP through fermentation. This process regenerates NAD+ for glycolysis but produces much less ATP than aerobic respiration.

• Alcoholic Fermentation: Occurs in yeast and some bacteria. Pyruvate is converted to ethanol and CO2.

• Lactic Acid Fermentation: Occurs in animal muscle cells and some bacteria. Pyruvate is reduced to lactic acid.

• Equation (Alcoholic):

• Equation (Lactic Acid):

Evolutionary Perspective

Prokaryotes and Glycolysis

Glycolysis is an ancient metabolic pathway, present in nearly all organisms. It likely evolved early in the history of life, before the presence of oxygen in Earth's atmosphere.

• Significance: Glycolysis does not require oxygen and occurs in the cytoplasm, making it suitable for both prokaryotes and eukaryotes.

• Evolution: The universality of glycolysis suggests it was present in the last universal common ancestor (LUCA).

Metabolism of Other Fuels

Pathways for Carbohydrates, Lipids, and Proteins

Besides glucose, cells can metabolize other macromolecules for energy. These molecules enter cellular respiration at various points.

Additional info: The flexibility of cellular respiration allows organisms to adapt to varying nutrient availability by metabolizing different types of organic molecules.