Cellular Respiration: Pathways for ATP Production

Chapter 9: Cellular Respiration

Introduction to Cellular Respiration

Cellular respiration is a fundamental metabolic process by which cells convert biochemical energy from nutrients into adenosine triphosphate (ATP), and release waste products. This process is essential for the survival of most organisms and involves a series of controlled redox reactions.

• Definition: Cellular respiration is the process by which the oxidation of glucose is linked to the production of ATP.

• Aerobic Respiration: Most species use oxygen as the final electron acceptor.

• Phosphorylation Methods: There are two main methods for producing ATP:

  • Oxidative phosphorylation

  • Substrate-level phosphorylation

Overview of Cellular Respiration Pathways

Cellular respiration consists of several interconnected metabolic pathways that extract energy from glucose and other high-energy compounds.

• Main Steps:

  1. Glycolysis

  2. Pyruvate Oxidation

  3. Citric Acid Cycle (Krebs Cycle)

  4. Oxidative Phosphorylation (Electron Transport Chain and Chemiosmosis)

• Alternative Pathway: Fermentation occurs when oxygen is absent.

Glycolysis

Overview and Steps

Glycolysis is the first stage of cellular respiration, occurring in the cytoplasm and does not require oxygen. It is an ancient pathway found in all living organisms.

• Definition: Glycolysis means "sugar splitting" and involves the breakdown of one glucose molecule (6C) into two pyruvate molecules (3C each).

• Phases:

  • Energy Investment Phase: 2 ATP are consumed to activate glucose.

  • Cleavage Phase: The 6C molecule is split into two 3C molecules.

  • Energy Payoff Phase: 4 ATP and 2 NADH are produced (net gain: 2 ATP).

• Substrate-Level Phosphorylation: Direct transfer of a phosphate group from a substrate to ADP, forming ATP.

Net Yield per Glucose:

• 2 ATP (net gain)

• 2 NADH

• 2 Pyruvate

Regulation: Glycolysis is regulated by allosteric enzymes such as phosphofructokinase, which is inhibited by high levels of ATP (feedback inhibition).

Fermentation

Emergency ATP Production Without Oxygen

Fermentation is an anaerobic process that allows glycolysis to continue producing ATP when the electron transport chain (ETC) is unavailable due to lack of oxygen.

• Purpose: Regenerates NAD+ so glycolysis can continue.

• ATP Yield: Only ATP produced is from glycolysis (2 ATP per glucose).

• Types of Fermentation:

  • Lactic Acid Fermentation: Pyruvate is reduced to lactate. Occurs in muscle cells and some bacteria.

  • Alcohol Fermentation: Pyruvate is converted to acetaldehyde (by loss of CO2), then reduced to ethanol. Occurs in yeast.

Pyruvate Oxidation

Preparation for the Citric Acid Cycle

Pyruvate oxidation is the "prep step" that converts pyruvate into acetyl CoA, which enters the citric acid cycle.

• Location: Mitochondrial matrix (in eukaryotes).

• Process:

  • Pyruvate undergoes decarboxylation (loss of CO2).

  • NAD+ is reduced to NADH.

  • Acetyl group is transferred to coenzyme A, forming acetyl CoA.

• Regulation: Controlled by feedback inhibition (high ATP, acetyl CoA) and activation by high NAD+.

Citric Acid Cycle (Krebs Cycle)

Oxidation of Acetyl CoA

The citric acid cycle is a cyclic pathway that completes the oxidation of glucose derivatives, producing electron carriers for the ETC.

• Location: Mitochondrial matrix.

• Steps:

  • Acetyl CoA (2C) combines with oxaloacetate (4C) to form citrate (6C).

  • Cycle regenerates oxaloacetate.

  • Each turn releases 2 CO2, produces 3 NADH, 1 FADH2, and 1 ATP (via substrate-level phosphorylation).

• Per Glucose (2 cycles):

  • 6 NADH (3 per cycle)

  • 2 FADH2 (1 per cycle)

  • 2 ATP (1 per cycle)

  • 4 CO2 (2 per cycle)

• Regulation: Allosteric and competitive inhibition by ATP and NADH.

Oxidative Phosphorylation

Electron Transport Chain and Chemiosmosis

Oxidative phosphorylation is the final stage of cellular respiration, where most ATP is produced using the energy from electrons carried by NADH and FADH2.

• Electron Transport Chain (ETC):

  • Located in the inner mitochondrial membrane.

  • Electrons from NADH and FADH2 are transferred through a series of protein complexes.

  • Oxygen is the final electron acceptor, forming water.

• Chemiosmosis:

  • Protons (H+) are pumped into the intermembrane space, creating an electrochemical gradient.

  • ATP synthase uses the flow of protons back into the matrix to synthesize ATP from ADP and Pi.

• ATP Yield: About 26-28 ATP per glucose (majority of ATP from cellular respiration).

Key Equations:

• Substrate-level phosphorylation: $\text{ADP} + \text{P}_{\text{sub}} \xrightarrow{\text{kinase}} \text{ATP} + \text{substrate}$

• Overall cellular respiration: $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{energy (ATP)}$

Summary Table: Cellular Respiration Pathways

Stage	Main Products	Location	ATP Yield
Glycolysis	2 Pyruvate, 2 NADH, 2 ATP	Cytoplasm	2 (net)
Pyruvate Oxidation	2 Acetyl CoA, 2 NADH, 2 CO2	Mitochondrial Matrix	0
Citric Acid Cycle	4 CO2, 6 NADH, 2 FADH2, 2 ATP	Mitochondrial Matrix	2
Oxidative Phosphorylation	H2O, ~26-28 ATP	Inner Mitochondrial Membrane	26-28
Fermentation	Lactate or Ethanol, 2 ATP	Cytoplasm	2

Types of Cellular Respiration

Aerobic vs. Anaerobic Respiration

• Aerobic Respiration: Oxygen is the final electron acceptor; produces the most ATP.

• Anaerobic Respiration: Final electron acceptor is an inorganic molecule other than oxygen (e.g., sulfate, nitrate); used by some bacteria; produces less ATP.

Key Terms and Concepts

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

• NAD+ / NADH: Electron carrier; NAD+ is reduced to NADH during glycolysis and the citric acid cycle.

• FAD / FADH2: Another electron carrier involved in the citric acid cycle and ETC.

• Substrate-Level Phosphorylation: Direct formation of ATP by transfer of a phosphate group to ADP from a substrate.

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

• Fermentation: Anaerobic process to regenerate NAD+ and allow glycolysis to continue.

Additional info: The notes have been expanded to include definitions, regulatory mechanisms, and a summary table for clarity and completeness.