Cellular Respiration: Pathways, Mechanisms, and Regulation

Cellular Respiration

Overview of Cellular Respiration

Cellular respiration is a series of metabolic processes that convert biochemical energy from nutrients into adenosine triphosphate (ATP), releasing waste products. It involves multiple pathways and is essential for energy production in both prokaryotic and eukaryotic cells.

• Exergonic process: The breakdown of organic molecules releases energy.

• Four main metabolic pathways:

  1. Glycolysis (cytoplasm)

  2. Pyruvate Oxidation (cytoplasm to mitochondria)

  3. Citric Acid Cycle (mitochondrial matrix)

  4. Electron Transport Chain (inner mitochondrial membrane)

Redox Reactions

Redox (reduction-oxidation) reactions are fundamental to cellular respiration, involving the transfer of electrons between molecules.

• Oxidation: Loss of electrons by a molecule.

• Reduction: Gain of electrons by a molecule.

• Mnemonic: "LEO the lion says GER" (Lose Electrons = Oxidation, Gain Electrons = Reduction).

Key Reactants and Products

• Glucose (C6H12O6): Main fuel molecule, oxidized to release energy.

• Oxygen (O2): Final electron acceptor, reduced to form water.

• Water (H2O): Formed from hydrogen (from glucose) and oxygen (from air).

Reaction Terms

• Reactants: Starting substances (e.g., A + B).

• Intermediate products: Substances formed during the reaction (e.g., C, D, E, F).

• End product: Final substance produced (e.g., G).

Major Pathways of Cellular Respiration

Glycolysis

Glycolysis is the first step in cellular respiration, occurring in the cytoplasm. It breaks down glucose into pyruvate and generates a small amount of ATP and NADH.

• Location: Cytoplasm (cytosol).

• Oxygen requirement: Can occur with or without oxygen (anaerobic or aerobic).

• Phases of Glycolysis:

  1. Energy Investment (steps 1–3): ATP is consumed to phosphorylate glucose.

  2. Cleavage (steps 4–5): Six-carbon sugar is split into two three-carbon molecules.

  3. Energy Liberation (steps 6–10): ATP and NADH are produced.

• Reactant: Glucose

• End product: 2 Pyruvate

• ATP yield: Uses 2 ATP, produces 4 ATP (net gain = 2 ATP)

• Regulation: Controlled by feedback inhibition (high ATP slows glycolysis via noncompetitive enzyme inhibition).

• Occurs in: Both prokaryotes and eukaryotes.

• Significance: Can be the final step for some organisms; used for quick energy bursts in humans.

Pyruvate Pathways

After glycolysis, pyruvate can follow different pathways depending on the presence of oxygen and the type of organism.

• Transport: Pyruvate enters mitochondria via an outer membrane channel and an H+/pyruvate symporter in the inner membrane.

• Aerobic pathway: Continues to pyruvate oxidation and the citric acid cycle (requires oxygen).

• Anaerobic pathway: Undergoes fermentation (no oxygen required).

• Organism differences:

  • Prokaryotes: May stop after glycolysis (2 ATP is sufficient).

  • Eukaryotes: Continue to next stages for more ATP production.

Citric Acid Cycle (Krebs Cycle)

The citric acid cycle completes the breakdown of glucose derivatives, generating electron carriers and a small amount of ATP.

• Location:

  • Eukaryotes: Mitochondrial matrix

  • Prokaryotes: Cytoplasm

• Cycle runs: 2 times per glucose molecule (since 2 pyruvate are produced per glucose).

• Products per cycle: CO2, NADH, FADH2, small amount of ATP.

• ATP yield: Least amount of ATP among the four pathways.

• Basic steps:

  1. Oxaloacetate + Acetyl-CoA → Citrate

  2. Citrate → Isocitrate

  3. Isocitrate → α-ketoglutarate

  4. Series of reactions regenerate oxaloacetate

• Regulation: High levels of oxaloacetate slow the cycle (feedback inhibition).

Electron Transport Chain (ETC) & Oxidative Phosphorylation

The electron transport chain uses electrons from NADH and FADH2 to create a proton gradient, which drives ATP synthesis via chemiosmosis.

• Location: Inner mitochondrial membrane

• Process:

  • Electrons are transferred through a series of protein complexes.

  • Energy released pumps H+ ions across the membrane, creating a gradient.

  • ATP synthase uses this gradient to convert ADP + Pi into ATP.

• Equation for ATP synthesis:

$\mathrm{ADP} + \mathrm{P_i} \rightarrow \mathrm{ATP}$

• Total ATP yield: Approximately 30–32 ATP per glucose molecule (all stages combined).

Fermentation

Fermentation is an anaerobic process that allows glycolysis to continue by regenerating NAD+ when oxygen is not available.

• Purpose: Regenerates NAD+ so glycolysis can continue.

• Types of fermentation:

  1. Alcoholic fermentation: Occurs in yeast, some bacteria, and plant cells; produces ethanol and CO2.

  2. Lactic acid fermentation: Occurs in animal cells (e.g., muscle cells); produces lactate.

• ATP yield: Much less than aerobic respiration.

Types of Respiration

• External respiration: Breathing (exchange of gases with the environment).

• Internal respiration: Cellular respiration (metabolic processes inside cells).

Summary Table: Pathways of Cellular Respiration

Pathway	Location (Eukaryotes)	Main Reactant	Main Product(s)	ATP Yield
Glycolysis	Cytoplasm	Glucose	2 Pyruvate, 2 NADH, 2 ATP (net)	2
Pyruvate Oxidation	Mitochondrial Matrix	2 Pyruvate	2 Acetyl-CoA, 2 NADH, 2 CO2	0
Citric Acid Cycle	Mitochondrial Matrix	2 Acetyl-CoA	4 CO2, 6 NADH, 2 FADH2, 2 ATP	2
Electron Transport Chain & Oxidative Phosphorylation	Inner Mitochondrial Membrane	NADH, FADH2, O2	H2O, ~26-28 ATP	26-28
Fermentation	Cytoplasm	Pyruvate	Lactate or Ethanol + CO2	2 (from glycolysis only)

Additional info: Table values for NADH, FADH2, and CO2 are inferred from standard textbook sources for clarity.