Cellular Respiration

Overview of Cellular Respiration

Cellular respiration is a fundamental metabolic process by which cells convert biochemical energy from nutrients into adenosine triphosphate (ATP), releasing waste products. It consists of several stages, including glycolysis, the citric acid cycle, and oxidative phosphorylation.

• Purpose: To generate ATP, the energy currency of the cell.

• Main Stages: Glycolysis, Citric Acid Cycle (Krebs Cycle), Oxidative Phosphorylation.

• Location: Glycolysis occurs in the cytoplasm; the citric acid cycle and oxidative phosphorylation occur in the mitochondria.

Key Terminologies

Understanding the following terms is essential for mastering cellular respiration:

• Substrate-level phosphorylation: Direct synthesis of ATP from ADP and a phosphorylated substrate.

• Oxidative phosphorylation: ATP synthesis powered by the transfer of electrons through the electron transport chain to oxygen.

• Electron transport chain (ETC): Series of protein complexes in the inner mitochondrial membrane that transfer electrons and pump protons.

• Chemiosmosis: Movement of ions across a semipermeable membrane, down their electrochemical gradient, coupled to ATP synthesis.

• ATP synthase: Enzyme that synthesizes ATP using the proton gradient.

• Proton motive force: The force generated by the transmembrane proton gradient.

• NADH & FADH2: Electron carriers that donate electrons to the ETC.

• Anaerobic respiration: Respiration without oxygen, using alternative electron acceptors.

Stages of Cellular Respiration

Glycolysis

Glycolysis is the first stage of cellular respiration, occurring in the cytoplasm. It breaks down glucose into pyruvate, generating ATP and NADH.

• Energy Payoff Phase (Steps 6-10): ATP and NADH are produced.

• Key Features:

  • Does not require oxygen (anaerobic process).

  • Produces 2 ATP and 2 NADH per glucose molecule.

  • End product: 2 pyruvate molecules.

Citric Acid Cycle (Krebs Cycle)

The citric acid cycle is a series of chemical reactions used by all aerobic organisms to generate energy through the oxidation of acetyl-CoA.

• Location: Mitochondrial matrix.

• Inputs: Acetyl-CoA, NAD+, FAD, ADP, and H2O.

• Outputs: CO2, NADH, FADH2, ATP (or GTP).

• Cycle: Each turn produces 3 NADH, 1 FADH2, 1 ATP (or GTP), and 2 CO2.

Oxidative Phosphorylation

Electron Transport Chain (ETC)

The ETC is a series of protein complexes embedded in the inner mitochondrial membrane. Electrons from NADH and FADH2 are transferred through these complexes, ultimately reducing oxygen to water.

• Stages:

  1. Electrons are transferred from NADH and FADH2 to the ETC.

  2. Energy from electron transfer pumps protons (H+) into the intermembrane space, creating a proton gradient.

  3. Oxygen acts as the final electron acceptor, forming water.

• Key Features:

  • Establishes a proton gradient across the inner mitochondrial membrane.

  • Complexes I-IV are involved in electron transfer and proton pumping.

Chemiosmosis and ATP Synthase

Chemiosmosis is the process by which the energy stored in the proton gradient is used to synthesize ATP via ATP synthase.

• Proton Motive Force: The gradient of protons across the membrane creates potential energy.

• ATP Synthase: Protons flow back into the mitochondrial matrix through ATP synthase, driving the phosphorylation of ADP to ATP.

• Analogy: ATP synthase functions like a turbine, similar to a hydroelectric plant where water flow generates electricity.

Equation for ATP Synthesis

The general equation for oxidative phosphorylation is:

ATP Yield from Cellular Respiration

The total ATP yield from one molecule of glucose during aerobic respiration is approximately 30-32 ATP.

• Glycolysis: 2 ATP (substrate-level phosphorylation)

• Citric Acid Cycle: 2 ATP (substrate-level phosphorylation)

• Oxidative Phosphorylation: ~26-28 ATP

ATP Production Table

Anaerobic Respiration and Fermentation

What if Oxygen is Unavailable?

Cells can switch to anaerobic pathways when oxygen is not available. These include lactic acid fermentation (in animals) and alcoholic fermentation (in yeast and some plants).

• Anaerobic Respiration: Uses alternative electron acceptors (e.g., nitrate, sulfate).

• Fermentation: Regenerates NAD+ for glycolysis, producing lactate or ethanol.

• ATP Yield: Much lower than aerobic respiration (2 ATP per glucose).

Metabolic Integration

Linkage to Other Metabolic Pathways

Respiration is interconnected with other metabolic pathways, allowing cells to utilize various biomolecules for energy.

• Catabolism: Breakdown of carbohydrates, fats, and proteins feeds into cellular respiration.

• Anabolism: Intermediates from respiration are used for biosynthesis of amino acids, nucleotides, etc.

Summary Table: Key Features of Oxidative Phosphorylation and Chemiosmosis

Additional info:

• Some slides included analogies (e.g., hydroelectric plant) to help visualize ATP synthase function.

• In-class questions and objectives were present, but the notes above expand on the brief points for academic completeness.