Cellular Respiration

Introduction to Cellular Respiration

Cellular respiration is a fundamental metabolic pathway that allows cells to harvest energy from organic molecules, primarily glucose, to produce ATP. This process is essential for all aerobic organisms and connects energy and carbon cycling within ecosystems.

• Ultimate Source of Energy: The sun is the ultimate energy source for most ecosystems. Plants convert solar energy into chemical energy via photosynthesis, producing glucose.

• Energy and Carbon Cycling: Energy flows into ecosystems as sunlight and leaves as heat. Carbon cycles as CO2 is fixed by photosynthesizers and released by consumers during respiration.

• Catabolic Pathway: Cellular respiration is a catabolic pathway, breaking down glucose and other molecules to release energy.

• Aerobic Process: Cellular respiration requires oxygen (O2), making it an aerobic process.

• Overall Equation: The simplified equation for cellular respiration is:

• Oxidation and Reduction: In this equation, glucose is oxidized (loses electrons), and oxygen is reduced (gains electrons).

• Major Stages: Cellular respiration consists of glycolysis, the citric acid cycle, and oxidative phosphorylation.

Redox Reactions in Cellular Respiration

Redox reactions are central to cellular respiration, involving the transfer of electrons and energy release.

• Oxidation: Loss of electrons from a molecule.

• Reduction: Gain of electrons by a molecule.

• NAD+ and FAD: Coenzymes that accept electrons during oxidation, becoming NADH and FADH2.

• Electron Transport Chain: NADH and FADH2 carry high-energy electrons to the electron transport chain, where ATP is synthesized.

Stages of Cellular Respiration

Glycolysis

Glycolysis is the first stage of cellular respiration, occurring in the cytoplasm and breaking down glucose into pyruvate.

• Inputs: Glucose, 2 ATP, 2 NAD+

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

• Energy Investment Phase: 2 ATP are used to activate glucose.

• Energy Payoff Phase: 4 ATP are produced, and NAD+ is reduced to NADH.

• Oxygen Requirement: Glycolysis does not require oxygen and can occur anaerobically.

Pyruvate Oxidation and Citric Acid Cycle

Pyruvate oxidation links glycolysis to the citric acid cycle, which completes the breakdown of glucose in the mitochondrial matrix.

• Pyruvate Oxidation: Pyruvate is converted to acetyl-CoA, producing NADH and CO2.

• Citric Acid Cycle: Acetyl-CoA combines with oxaloacetate to form citrate. The cycle turns twice per glucose molecule, generating:

  • 4 CO2, 2 ATP, 6 NADH, 2 FADH2 per glucose

• ATP Production: 2 ATP per glucose via substrate-level phosphorylation.

• Location: Mitochondrial matrix

• Cycle Nature: Oxaloacetate is regenerated, making the process cyclical.

Oxidative Phosphorylation

Oxidative phosphorylation is the final stage, coupling electron transport and chemiosmosis to produce most of the cell's ATP.

• Electron Transport Chain: Located in the inner mitochondrial membrane; electrons from NADH and FADH2 are passed through protein complexes (including cytochromes).

• Oxygen: Final electron acceptor, forming water.

• Chemiosmosis: Energy from electron transport pumps H+ into the intermembrane space; H+ flows back through ATP synthase, driving ATP production.

• ATP Yield: Up to 32 ATP per glucose molecule.

• Efficiency: About 34% of glucose energy is transferred to ATP; the rest is lost as heat.

Fermentation and Anaerobic Respiration

Fermentation

Fermentation allows cells to produce ATP without oxygen, using glycolysis and reactions that regenerate NAD+.

• Alcohol Fermentation: Pyruvate is converted to ethanol and CO2; NAD+ is regenerated.

• Lactic Acid Fermentation: Pyruvate is reduced to lactate; NAD+ is regenerated. Used by muscle cells and some bacteria.

• ATP Yield: Only 2 ATP per glucose (from glycolysis).

• Obligate Anaerobes: Cannot survive in O2; rely on fermentation or anaerobic respiration.

• Facultative Anaerobes: Can use either fermentation or aerobic respiration.

Metabolic Pathways and Regulation

Connecting Metabolic Pathways

Cellular respiration is interconnected with other metabolic pathways, allowing the breakdown and synthesis of various biomolecules.

• Proteins: Digested to amino acids; after deamination, can enter glycolysis or the citric acid cycle.

• Fats: Digested to glycerol (used in glycolysis) and fatty acids (broken down by beta oxidation to acetyl-CoA).

• Beta Oxidation: Process by which fatty acids are broken down to produce acetyl-CoA, NADH, and FADH2.

• ATP Yield: Fats produce more ATP per gram than carbohydrates.

• Feedback Inhibition: High levels of ATP and citrate inhibit glycolysis, regulating cellular respiration.

Evolutionary Significance of Glycolysis

Ancient Pathway

Glycolysis is considered an ancient metabolic pathway, used by early prokaryotes before oxygen was abundant in the atmosphere. It occurs in the cytosol and does not require membrane-bound organelles.

• Universality: Found in nearly all living organisms.

• Adaptation: Allows cells to generate ATP under anaerobic conditions.