Cellular Respiration: Pathways, Mechanisms, and Regulation

Cellular Respiration

Overview

Cellular respiration is the process by which cells extract energy from organic molecules, primarily glucose, to produce ATP. This process involves a series of metabolic pathways that occur in different cellular compartments and is essential for both prokaryotic and eukaryotic organisms.

• Exergonic reactions: Breakdown of organic molecules releases energy.

• Four main metabolic pathways: Glycolysis, Pyruvate Oxidation, Citric Acid Cycle, Electron Transport Chain.

• Key reactants: Glucose (C6H12O6), Oxygen (O2).

• Key products: ATP, Water (H2O), Carbon dioxide (CO2).

Redox Reactions

Oxidation and Reduction

Redox 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).

Metabolic Pathways of Cellular Respiration

Pathway Overview

Cellular respiration consists of four sequential metabolic pathways:

1. Glycolysis: Occurs in the cytoplasm.

2. Pyruvate Oxidation: Cytoplasm to mitochondria.

3. Citric Acid Cycle (Krebs Cycle): Mitochondrial matrix (eukaryotes), cytoplasm (prokaryotes).

4. Electron Transport Chain (ETC): Inner mitochondrial membrane.

Key Reactants and Products

Glucose and Oxygen

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

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

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

Glycolysis

Phases and Regulation

Glycolysis is the first step in cellular respiration, breaking down glucose into pyruvate and generating ATP.

• Location: Cytoplasm (cytosol).

• Oxygen requirement: Can occur with or without oxygen.

• Phases:

  1. Energy Investment (steps 1–3): ATP is used.

  2. Cleavage (steps 4–5): Glucose split into two 3-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: Feedback inhibition by ATP; noncompetitive enzyme inhibition.

• Occurs in: Both prokaryotes and eukaryotes.

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

Pyruvate Pathways

Aerobic vs Anaerobic

Pyruvate produced in glycolysis can follow different pathways depending on oxygen availability.

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

• Aerobic pathway: Continues to citric acid cycle and ETC (requires oxygen).

• Anaerobic pathway: Fermentation (does not require oxygen).

• Organism differences:

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

  • Eukaryotes: Continue to next stages (2 ATP not sufficient).

Citric Acid Cycle (Krebs Cycle)

Cycle Steps and Regulation

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

• Location: Mitochondrial matrix (eukaryotes), cytoplasm (prokaryotes).

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

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

• Steps:

  1. Oxaloacetate + Acetyl-CoA → Citrate

  2. Citrate → Isocitrate

  3. Isocitrate → α-ketoglutarate

  4. Series of reactions regenerates oxaloacetate

• Regulation: High oxaloacetate slows cycle (feedback inhibition).

• ATP yield: Least ATP produced in this cycle.

Electron Transport Chain & Oxidative Phosphorylation

Mechanism and ATP Production

The electron transport chain (ETC) uses electron carriers to generate a proton gradient, which powers ATP synthesis.

• Location: Inner mitochondrial membrane.

• Electron carriers: NADH and FADH2 donate electrons.

• Oxidative phosphorylation: ATP synthase uses energy from proton gradient.

• Chemiosmosis: H+ gradient drives ATP production.

• ATP synthesis equation:

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

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

Fermentation

Types and Purpose

Fermentation occurs when oxygen is unavailable, allowing glycolysis to continue by regenerating NAD+.

• Purpose: Regenerates NAD+ for glycolysis.

• Types:

  1. Alcoholic fermentation: Yeast, some bacteria, plants; produces ethanol and CO2.

  2. Lactic acid fermentation: Animal cells; produces lactate (occurs in muscle cells).

• ATP yield: Much less than aerobic respiration.

Respiration Types

External vs Internal Respiration

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

• Internal respiration: Cellular respiration (energy production within cells).

Summary Table: Cellular Respiration Pathways

Pathway	Location	Key Reactants	Key Products	ATP Yield
Glycolysis	Cytoplasm	Glucose	2 Pyruvate, 2 NADH	2 (net)
Pyruvate Oxidation	Mitochondrial matrix (eukaryotes), cytoplasm (prokaryotes)	2 Pyruvate	2 Acetyl-CoA, 2 NADH, 2 CO2	0
Citric Acid Cycle	Mitochondrial matrix (eukaryotes), cytoplasm (prokaryotes)	2 Acetyl-CoA	4 CO2, 6 NADH, 2 FADH2, 2 ATP	2
Electron Transport Chain	Inner mitochondrial membrane	NADH, FADH2, O2	H2O, ATP	~26-28
Fermentation	Cytoplasm	Pyruvate, NADH	Lactate or Ethanol + CO2	2 (from glycolysis only)

Example: During intense exercise, human muscle cells switch to lactic acid fermentation, producing lactate and allowing glycolysis to continue for rapid ATP production.

Additional info: The citric acid cycle is also known as the Krebs cycle or tricarboxylic acid (TCA) cycle. Feedback inhibition is a common regulatory mechanism in metabolic pathways, ensuring energy production matches cellular demand.