Cell Respiration

Balanced Aerobic Cellular Respiration Equation

  • C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP
    This equation represents glucose (C₆H₁₂O₆) reacting with oxygen (O₂) to produce carbon dioxide (CO₂), water (H₂O), and ATP.

Parts of the Mitochondrion

  • Outer membrane – Protects and defines the shape.

  • Inner membrane – Contains enzymes for the electron transport chain (ETC) and ATP synthase.

  • Matrix – The innermost part, where the Krebs cycle takes place.

  • Cristae – Folds of the inner membrane that increase surface area for the ETC.

Aerobic vs. Anaerobic Respiration

  • Aerobic: Requires oxygen. Occurs in three main steps: Glycolysis, Krebs cycle, and Electron Transport Chain (ETC). It produces a large amount of ATP (36–38 ATP per glucose molecule).

  • Anaerobic: Does not require oxygen. Includes glycolysis followed by fermentation (lactic acid or alcoholic). Produces less ATP (2 ATP per glucose molecule).

Three Parts of Aerobic Respiration

  1. Glycolysis: Breaks down glucose into pyruvate (happens in the cytoplasm).

  2. Krebs Cycle (Citric Acid Cycle): Further breaks down pyruvate and produces electron carriers (NADH, FADH2) and ATP (occurs in the mitochondria's matrix).

  3. Electron Transport Chain: Uses NADH and FADH2 to generate ATP (occurs in the inner mitochondrial membrane).

Why Glycolysis is Considered Anaerobic

  • It doesn’t require oxygen to occur. This makes it the oldest step in cellular respiration, evolutionarily, because it can function in environments lacking oxygen.

ATP Synthesis in the ETC

  • The ETC uses NADH and FADH2 to create a proton gradient across the inner mitochondrial membrane. The protons flow back through ATP synthase, which converts ADP and Pi into ATP. Oxygen acts as the final electron acceptor, combining with electrons and protons to form water.

Organisms Capable of Krebs Cycle & ETC

  • Eukaryotes (animals, plants, fungi) and some prokaryotes (e.g., some bacteria) can perform these processes.

Inputs and Outputs of Each Part of Aerobic Respiration

  • Glycolysis

    • Inputs: Glucose, 2 ATP, NAD⁺

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

    • Occurs in the cytoplasm.

  • Krebs Cycle

    • Inputs: 2 Pyruvate (from glycolysis), NAD⁺, FAD, ADP

    • Outputs: 6 CO₂, 8 NADH, 2 FADH₂, 2 ATP

    • Occurs in the mitochondrial matrix.

  • Electron Transport Chain

    • Inputs: NADH, FADH₂, O₂, ADP + Pi

    • Outputs: Water, ATP

    • Occurs in the inner mitochondrial membrane.

ATP Yield per Glucose

  • Glycolysis: 2 ATP (net) and 2 NADH (which can generate 3-5 ATP later).

  • Krebs Cycle: 2 ATP (per glucose, since 2 pyruvate are produced from one glucose).

  • ETC: Up to 34 ATP (from the NADH and FADH₂ generated in glycolysis and the Krebs cycle).

Electron Transport Chain Function

  • Electron carriers (NADH and FADH₂) donate electrons, which travel down the chain.

  • As electrons move, they pump protons across the membrane, creating a proton gradient.

  • The gradient drives protons back through ATP synthase, creating ATP.

  • Oxygen acts as the final electron acceptor, combining with electrons and protons to form water.

Fermentation

  • Purpose: To regenerate NAD⁺, which is required for glycolysis to continue in the absence of oxygen.

  • Alcoholic Fermentation: Produces ethanol and CO₂ (used by yeast and some bacteria).

  • Lactic Acid Fermentation: Produces lactic acid (used by muscle cells in humans and some bacteria).

  • Reversibility: Lactic acid fermentation can be reversed because lactic acid can be converted back into pyruvate. Alcoholic fermentation is irreversible because ethanol cannot easily be converted back to pyruvate.

Examples of Organisms

  • Alcoholic fermentation: Yeast, some bacteria.

  • Lactic acid fermentation: Muscle cells in humans, some bacteria.

Switching Between Aerobic and Anaerobic Respiration During Exercise

  • Aerobic respiration is the preferred method when oxygen is available. As exercise intensity increases and oxygen demand exceeds supply, fermentation (lactic acid fermentation) kicks in temporarily to help produce energy without oxygen, leading to lactic acid buildup. Once oxygen becomes available again, aerobic respiration resumes.

Vocabulary Definitions

  • Aerobic: Requires oxygen.

  • Anaerobic: Does not require oxygen.

  • Breathing: The process of taking in oxygen and releasing carbon dioxide.

  • Respiration: The process by which cells produce energy (ATP) from glucose and oxygen.

  • Electronegative: A measure of an atom’s ability to attract and hold onto electrons.

  • Fermentation: The metabolic process that regenerates NAD⁺ under anaerobic conditions, allowing glycolysis to continue.

This should give you a solid foundation for studying cellular respiration! Let me know if you'd like to dive deeper into any specific section or if you have questions on any part of the guide.