Cellular Respiration: Key Stages

Cellular respiration is the process of converting chemical energy (usually from glucose) into a usable form of energy for the cell, primarily ATP (adenosine triphosphate). It can be aerobic (requiring oxygen) or anaerobic (not requiring oxygen).

1. Glycolysis

• Location: Cytoplasm
• Description: The breakdown of glucose (a 6-carbon molecule) into two molecules of pyruvate (a 3-carbon molecule).
• Inputs:
  • Glucose
  • 2 ATP (to initiate the process)
  • 2 NAD+
  • 4 ADP + Pi (Pi represents inorganic phosphate)
• Outputs:
  • 2 Pyruvate
  • 4 ATP (net gain of 2 ATP, as 2 were initially used)
  • 2 NADH (reduced form of NAD+)
  • 2 H+
• ATP Yield: Net gain of 2 ATP molecules per glucose molecule.

KEY TAKEAWAY: Glycolysis occurs in the cytoplasm and does not require oxygen.

2. Krebs Cycle (Citric Acid Cycle)

• Location: Mitochondrial matrix
• Description: A series of reactions that oxidize pyruvate (from glycolysis) to carbon dioxide. Pyruvate is first converted to Acetyl-CoA, which then enters the cycle.
• Inputs:
  • 2 Acetyl-CoA (derived from 2 pyruvate molecules)
  • 6 NAD+
  • 2 FAD
  • 2 ADP + Pi
• Outputs:
  • 4 CO2
  • 6 NADH
  • 2 FADH2 (reduced form of FAD)
  • 2 ATP
• ATP Yield: 2 ATP molecules per glucose molecule (1 ATP per turn of the cycle, and the cycle turns twice per glucose molecule).

EXAM TIP: Remember that the Krebs cycle occurs twice for each glucose molecule because glycolysis produces two pyruvate molecules.

3. Electron Transport Chain (ETC)

• Location: Inner mitochondrial membrane (cristae)
• Description: A series of protein complexes that transfer electrons from NADH and FADH2 to oxygen, releasing energy to pump protons (H+) across the inner mitochondrial membrane, creating an electrochemical gradient. This gradient is then used by ATP synthase to produce ATP (oxidative phosphorylation).
• Inputs:
  • 10 NADH (from glycolysis and Krebs cycle)
  • 2 FADH2 (from Krebs cycle)
  • O2 (final electron acceptor)
  • ADP + Pi
• Outputs:
  • H2O (produced when oxygen accepts electrons)
  • ATP
  • NAD+
  • FAD
• ATP Yield: Approximately 32-34 ATP molecules per glucose molecule. (Note: The exact yield can vary depending on the efficiency of the proton gradient and other factors).

COMMON MISTAKE: Students often forget that the majority of ATP is produced during the electron transport chain.

Summary Table

STUDY HINT: Create a flowchart to visualize the flow of molecules through each stage of cellular respiration.

Overall ATP Yield

• The total ATP yield from aerobic cellular respiration is approximately 36-38 ATP molecules per glucose molecule.
• This is a theoretical maximum; actual yield can vary.

REMEMBER: Glycolysis (cytoplasm), Krebs Cycle (mitochondrial matrix), ETC (inner mitochondrial membrane).

Anaerobic Fermentation

• Occurs in the absence of oxygen.
• Regenerates NAD+ so glycolysis can continue.
• Two main types:
  • Lactic acid fermentation (animals): Pyruvate is reduced to lactate.
  • Alcohol fermentation (yeast): Pyruvate is converted to ethanol and carbon dioxide.
• ATP yield is only 2 ATP per glucose (from glycolysis).

APPLICATION: Fermentation is used in the production of various foods and beverages (e.g., yogurt, bread, beer).

VCAA FOCUS: VCAA exams often test your understanding of the inputs, outputs, and locations of each stage of cellular respiration, as well as the overall ATP yield. Make sure you can recall these details accurately.