Cellular Respiration

Introduction to Cellular Respiration

Cellular respiration is a fundamental metabolic process by which cells harvest chemical energy from organic molecules, such as glucose, to produce ATP (adenosine triphosphate). This process involves the breakdown of complex molecules into simpler waste products, releasing energy that cells use for work, with some energy lost as heat.

• Fermentation: An ATP-producing catabolic process that partially degrades sugars without oxygen.

• Cellular respiration: An ATP-producing catabolic pathway that completely degrades sugar with the help of oxygen.

• General equation:

• Three main stages of cellular respiration:

  1. Glycolysis

  2. Krebs Cycle (Citric Acid Cycle, TCA Cycle)

  3. Electron Transport Chain (ETC) and Oxidative Phosphorylation

Glycolysis

Overview of Glycolysis

Glycolysis is the first stage of cellular respiration and occurs in the cytosol. It involves the breakdown of one glucose molecule (6 carbons) into two molecules of pyruvate (3 carbons each).

• Each reaction is catalyzed by a specific enzyme in the cytosol.

• Does not require oxygen (anaerobic process).

• Occurs in two phases:

  1. Energy-investment phase: Uses 2 ATPs to phosphorylate intermediates of glycolysis.

  2. Energy-yielding phase: Produces 4 ATPs and 2 NADHs. Net gain is 2 ATPs and 2 NADHs per glucose.

• ATP production: Glycolysis produces a net of 2 ATPs and 2 NADHs per glucose.

Krebs Cycle (Citric Acid Cycle, TCA Cycle)

Overview of the Krebs Cycle

The Krebs cycle occurs in the mitochondrial matrix and completes the oxidation of glucose by breaking down pyruvate derivatives (acetyl CoA) into carbon dioxide.

• Each pyruvate (3C) is converted to Acetyl CoA (2C), releasing one CO2 and producing one NADH.

• For each Acetyl CoA entering the cycle:

  • 2 CO2 are released

  • 1 ATP, 3 NADH, and 1 FADH2 are produced

Electron Transport Chain (ETC) and Oxidative Phosphorylation

Overview of the Electron Transport Chain

The ETC is located on the inner mitochondrial membrane. It transfers electrons from NADH and FADH2 to oxygen, coupling this transfer to ATP synthesis.

• Each NADH yields 2-3 ATPs; each FADH2 yields 2 ATPs.

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

Maximum ATP Yield per Glucose

Anaerobic and Aerobic Conditions

Anaerobic Conditions

When oxygen is not present, cells can still produce ATP through fermentation. Fermentation regenerates NAD+ from NADH, allowing glycolysis to continue.

• Alcohol fermentation: Produces ethanol and CO2 (e.g., yeast).

• Lactic acid fermentation: Produces lactate (e.g., muscle cells, some bacteria).

• Commercially important products: cheese, yogurt, alcohol.

Aerobic Conditions

In the presence of oxygen, oxidative phosphorylation occurs, producing 4-6 more ATPs as NADH passes electrons to the electron transport chain.

• The oxidizing agent for glycolysis is NAD+, not oxygen.

• Fermentation can generate ATP by substrate-level phosphorylation if there is sufficient NAD+.

• Substrate-level phosphorylation: Direct enzymatic transfer of phosphate to ADP to form ATP.

Comparison of Fermentation and Respiration

Organism Classification by Oxygen Use

• Strict (obligate) aerobes: Require oxygen for growth and use oxygen as the final electron acceptor.

• Strict (obligate) anaerobes: Grow only in the absence of oxygen; oxygen is toxic to them.

• Facultative anaerobes: Can grow in either aerobic or anaerobic environments (e.g., yeast, many bacteria, mammalian muscle cells).

Fats, proteins, and carbohydrates can all be consumed by cellular respiration to form ATP.

Summary Table: Types of Organisms by Oxygen Requirement

Additional info: NAD+ stands for nicotinamide adenine dinucleotide, a key electron carrier in cellular respiration. Substrate-level phosphorylation and oxidative phosphorylation are two distinct mechanisms for ATP production.