Cellular Respiration: Mechanisms and Pathways

Chapter 9: Cellular Respiration

Introduction to Cellular Respiration

Cellular respiration is a series of metabolic processes that convert biochemical energy from nutrients into adenosine triphosphate (ATP), releasing waste products. It is essential for the survival of most organisms, providing the energy required for cellular activities.

• Catabolic reactions: These reactions break down complex molecules into simpler ones, releasing energy. Cellular respiration is a catabolic process.

• Redox reactions: Oxidation-reduction reactions involve the transfer of electrons between molecules. Oxidation is the loss of electrons, while reduction is the gain of electrons.

• Activation energy: Energy input is required to initiate cellular respiration. Enzymes lower this energy barrier, allowing glucose and other fuels to be metabolized efficiently.

• Stepwise electron transfer: The transfer of electrons during cellular respiration occurs in a controlled, stepwise manner, maximizing energy capture in the form of ATP.

Catabolic Pathways and Types of Respiration

• Fermentation: An anaerobic process (no oxygen required) that partially degrades sugars to release energy.

• Cellular respiration: An aerobic process (requires oxygen) that completely breaks down organic molecules to carbon dioxide and water, releasing energy as ATP.

General equation for aerobic respiration:

$\text{Organic compounds} + O_2 \rightarrow CO_2 + H_2O + \text{Energy (ATP + heat)}$

For glucose:

$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy}$

• Exergonic reaction: Releases free energy ($\Delta G \approx -686$ kcal/mol for glucose).

• Carbohydrates, fats, and proteins can all serve as fuel for cellular respiration.

Redox Reactions in Cellular Respiration

Redox reactions are central to cellular respiration, involving the transfer of electrons from fuel molecules to oxygen.

• Oxidation: Loss of electrons (e.g., glucose is oxidized to CO2).

• Reduction: Gain of electrons (e.g., O2 is reduced to H2O).

• Oxidizing agent: The electron acceptor (e.g., O2).

• Reducing agent: The electron donor (e.g., glucose).

Example: Formation of table salt:

$\text{Na} + \text{Cl} \rightarrow \text{Na}^+ + \text{Cl}^-$

• Na is oxidized (loses an electron), Cl is reduced (gains an electron).

Example: Cellular redox reaction:

$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$

• O2 is the oxidizing agent, C6H12O6 is the reducing agent.

Electron Carriers and Energy Transfer

Electrons from glucose are transferred to electron carriers, such as NAD+ (nicotinamide adenine dinucleotide), which become reduced to NADH. These carriers store energy and transfer electrons to the electron transport chain.

• Dehydrogenase enzymes: Remove hydrogen atoms (2 electrons and 2 protons) from substrates, transferring electrons to NAD+.

• NADH: Stores energy and donates electrons to the electron transport chain.

Stages of Cellular Respiration

Cellular respiration occurs in three main stages:

1. Glycolysis: Occurs in the cytoplasm; splits glucose into two molecules of pyruvate.

2. Krebs Cycle (Citric Acid Cycle): Occurs in the mitochondrial matrix; completes the breakdown of glucose derivatives, generating electron carriers.

3. Electron Transport Chain and Oxidative Phosphorylation: Occurs in the inner mitochondrial membrane; uses electrons from NADH and FADH2 to generate ATP.

Glycolysis

Overview

Glycolysis is the process of splitting one molecule of glucose (6 carbons) into two molecules of pyruvate (3 carbons each). It occurs in two phases: the energy investment phase and the energy payoff phase.

• Energy investment phase: 2 ATP are used to phosphorylate glucose and its intermediates.

• Energy payoff phase: 4 ATP and 2 NADH are produced, along with 2 pyruvate and 2 H2O.

Net products per glucose: 2 ATP, 2 NADH, 2 pyruvate, 2 H2O

Substrate-level phosphorylation: ATP is produced directly by transferring a phosphate group from a substrate to ADP.

Krebs Cycle (Citric Acid Cycle)

Overview

The Krebs cycle completes the oxidation of glucose derivatives. Each turn of the cycle processes one acetyl CoA, generating electron carriers and ATP.

• Location: Mitochondrial matrix

• Entry: Pyruvate is converted to acetyl CoA (producing NADH and CO2).

• Key steps:

  • Acetyl CoA combines with oxaloacetate to form citrate.

  • Citrate is rearranged and oxidized, releasing CO2 and generating NADH, FADH2, and ATP (or GTP).

  • Oxaloacetate is regenerated to continue the cycle.

Per Acetyl CoA	Per Glucose (2 turns)
3 NADH	6 NADH
1 FADH2	2 FADH2
1 ATP (or GTP)	2 ATP (or GTP)
2 CO2	4 CO2

Electron Transport Chain and Oxidative Phosphorylation

Electron Transport Chain (ETC)

The ETC is a series of protein complexes embedded in the inner mitochondrial membrane. Electrons from NADH and FADH2 are transferred through these complexes to oxygen, the final electron acceptor, forming water.

• As electrons move through the chain, energy is released and used to pump protons (H+) from the matrix to the intermembrane space, creating a proton gradient (proton-motive force).

• Each carrier in the chain is more electronegative than the previous, ensuring a directional flow of electrons.

ATP Synthesis: Chemiosmosis

ATP synthase, an enzyme in the inner mitochondrial membrane, uses the energy stored in the proton gradient to synthesize ATP from ADP and inorganic phosphate (Pi) as protons flow back into the matrix.

• Oxidative phosphorylation: The production of ATP using energy derived from the redox reactions of the ETC.

• Each NADH yields about 2.5 ATP; each FADH2 yields about 1.5 ATP.

• Total ATP yield per glucose: About 30–32 ATP (including glycolysis, Krebs cycle, and oxidative phosphorylation).

Metabolic Pathways Related to Cellular Respiration

Fermentation

Fermentation allows glycolysis to continue in the absence of oxygen by regenerating NAD+ from NADH.

• Alcohol fermentation: Pyruvate is converted to ethanol, releasing CO2 and regenerating NAD+.

• Lactic acid fermentation: Pyruvate is converted to lactate, regenerating NAD+. Occurs in muscle cells and some bacteria (e.g., in cheese and yogurt production).

Fermentation produces much less ATP than aerobic respiration (only 2 ATP per glucose, from glycolysis).

Facultative Anaerobes

Facultative anaerobes can survive using either fermentation or cellular respiration, depending on the presence of oxygen.

Evolutionary Significance and Integration with Other Pathways

• Glycolysis is a universal pathway, present in nearly all organisms and predates the presence of oxygen in Earth's atmosphere.

• Intermediates from glycolysis and the Krebs cycle can be used in anabolic pathways to synthesize macromolecules.

• Catabolic and anabolic pathways are interconnected, allowing cells to adapt to varying energy and biosynthetic needs.

Regulation of Cellular Respiration

Cellular respiration is tightly regulated by feedback mechanisms, primarily through the levels of ATP. When ATP is abundant, catabolic pathways slow down; when ATP is scarce, catabolism accelerates to produce more ATP. Many allosteric enzymes are involved in this regulation.

Summary Table: ATP Yield from Cellular Respiration

Stage	ATP Produced (per glucose)	NADH Produced	FADH2 Produced
Glycolysis	2	2	0
Pyruvate Oxidation	0	2	0
Krebs Cycle	2	6	2
Oxidative Phosphorylation	~26–28	-	-
Total	~30–32	10	2

Note: Actual ATP yield may vary depending on cell type and shuttle mechanisms for transporting electrons into mitochondria.

Key Terms and Concepts

• ATP (Adenosine Triphosphate): The main energy currency of the cell.

• NAD+/NADH: Electron carrier involved in redox reactions.

• FAD/FADH2: Another electron carrier used in the Krebs cycle and ETC.

• Substrate-level phosphorylation: Direct formation of ATP by transferring a phosphate group to ADP from a substrate.

• Oxidative phosphorylation: ATP formation driven by the transfer of electrons through the ETC and chemiosmosis.

• Proton-motive force: The electrochemical gradient of protons across the inner mitochondrial membrane, used to drive ATP synthesis.

Example Application: During vigorous exercise, muscle cells may switch to lactic acid fermentation when oxygen is limited, allowing ATP production to continue, though less efficiently than aerobic respiration.

Additional info: The integration of catabolic and anabolic pathways allows cells to efficiently manage resources and energy, adapting to environmental and physiological changes.