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 that occur in different cellular compartments 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/cytosol)

  2. Pyruvate Oxidation (cytoplasm to mitochondria)

  3. Citric Acid Cycle (mitochondrial matrix in eukaryotes, cytoplasm in prokaryotes)

  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.

• Reduction: Gain of electrons.

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

Reaction Terms

Chemical reactions in metabolism can be described by the roles of their components:

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

• Intermediate products: Compounds formed along the pathway (e.g., C, D, E, F).

• End product: Final output of the pathway (e.g., G).

Key Reactants and Products

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

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

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

Major Pathways of Cellular Respiration

Glycolysis

Glycolysis is the first step in cellular respiration, breaking down glucose into pyruvate. It occurs in the cytoplasm and does not require oxygen.

• Location: Cytoplasm (cytosol).

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

• 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).

• Organisms: Occurs in both prokaryotes and eukaryotes; can be the final step for some organisms or used for quick energy in humans.

Pyruvate Pathways

Pyruvate, the end product of glycolysis, is transported into the mitochondria in eukaryotes for further oxidation or undergoes fermentation in the absence of oxygen.

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

• Pathways:

  • Aerobic: Continues to citric acid cycle and electron transport chain (requires oxygen).

  • Anaerobic: Undergoes fermentation (does not require oxygen).

• 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 oxidation of glucose derivatives, generating electron carriers and a small amount of ATP.

• Location:

  • Eukaryotes: Mitochondrial matrix

  • Prokaryotes: Cytoplasm

• Cycle turns: 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 allows glycolysis to continue in the absence of oxygen by regenerating NAD+. It produces much less ATP than aerobic respiration.

• 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

Summary Table: Pathways of Cellular Respiration

Pathway	Location (Eukaryotes)	Oxygen Required?	Main Reactant	Main Product(s)	ATP Yield
Glycolysis	Cytoplasm	No	Glucose	2 Pyruvate, 2 NADH, 2 ATP (net)	2
Pyruvate Oxidation	Mitochondrial Matrix	Yes	2 Pyruvate	2 Acetyl-CoA, 2 NADH, 2 CO2	0
Citric Acid Cycle	Mitochondrial Matrix	Yes	2 Acetyl-CoA	4 CO2, 6 NADH, 2 FADH2, 2 ATP	2
Electron Transport Chain & Oxidative Phosphorylation	Inner Mitochondrial Membrane	Yes	NADH, FADH2, O2	H2O, ATP	~26–28
Fermentation	Cytoplasm	No	Pyruvate	Lactate or Ethanol + CO2	2 (from glycolysis only)

Types of Respiration

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

• Internal respiration: Cellular respiration (metabolic processes within cells)

Additional info: The citric acid cycle is also known as the Krebs cycle or tricarboxylic acid (TCA) cycle. The electron transport chain is coupled to ATP synthesis via chemiosmosis, a process described by Peter Mitchell's chemiosmotic hypothesis.