Cellular Respiration: Pathways and Energy Yield

Cellular Respiration

Overview of Cellular Respiration

Cellular respiration is a series of metabolic processes by which cells convert biochemical energy from nutrients into adenosine triphosphate (ATP), releasing waste products. It involves the breakdown of glucose and other organic molecules in the presence or absence of oxygen.

• Aerobic respiration: Complete breakdown of glucose with oxygen, producing CO2, H2O, and ATP.

• Anaerobic respiration: Partial breakdown of glucose without oxygen, resulting in less ATP and different end products (e.g., lactic acid or ethanol).

• Fermentation: Metabolic process that converts sugar to acids, gases, or alcohol in the absence of oxygen.

General equation for aerobic respiration:

$\mathrm{C_6H_{12}O_6} + 6\mathrm{O_2} \rightarrow 6\mathrm{CO_2} + 6\mathrm{H_2O} + \text{energy (ATP)}$

Redox Reactions in Cellular Respiration

Oxidation and Reduction

Redox reactions involve the transfer of electrons between molecules, playing a central role in energy extraction from food molecules.

• Oxidation: Loss of electrons from a molecule (the molecule is oxidized).

• Reduction: Gain of electrons by a molecule (the molecule is reduced).

• Oxidizing agent: Accepts electrons and is reduced.

• Reducing agent: Donates electrons and is oxidized.

Example: In the reaction Na + Cl → Na+ + Cl-, sodium is oxidized and chlorine is reduced.

Stages of Cellular Respiration

1. Glycolysis

Glycolysis is the first step in the breakdown of glucose, occurring in the cytoplasm. It converts one molecule of glucose into two molecules of pyruvate, generating ATP and NADH.

• Location: Cytoplasm

• Phases:

  • Energy Investment Phase: 2 ATP are used to phosphorylate glucose and its intermediates.

  • Energy Payoff Phase: 4 ATP and 2 NADH are produced.

• Net yield per glucose:

  • 2 ATP (4 produced - 2 used)

  • 2 NADH

  • 2 Pyruvate

Key reactions:

1. Phosphorylation of glucose (using ATP)

2. Cleavage into two 3-carbon sugars

3. Oxidation and substrate-level phosphorylation to produce ATP and NADH

Summary equation:

$\mathrm{Glucose} + 2\mathrm{NAD}^+ + 2\mathrm{ADP} + 2\mathrm{P_i} \rightarrow 2\mathrm{Pyruvate} + 2\mathrm{NADH} + 2\mathrm{ATP} + 2\mathrm{H_2O} + 2\mathrm{H}^+$

2. Pyruvate Oxidation

Pyruvate produced in glycolysis is transported into the mitochondrion, where it is converted into acetyl-CoA, releasing CO2 and generating NADH.

• Location: Mitochondrial matrix

• Products per glucose:

  • 2 Acetyl-CoA

  • 2 CO2

  • 2 NADH

Summary equation:

$2\mathrm{Pyruvate} + 2\mathrm{NAD}^+ + 2\mathrm{CoA} \rightarrow 2\mathrm{Acetyl{-}CoA} + 2\mathrm{CO_2} + 2\mathrm{NADH}$

3. Citric Acid Cycle (Krebs Cycle)

The citric acid cycle completes the breakdown of acetyl-CoA to CO2, generating NADH, FADH2, and ATP (or GTP).

• Location: Mitochondrial matrix

• Per turn (per acetyl-CoA):

  • 3 NADH

  • 1 FADH2

  • 1 ATP (or GTP)

  • 2 CO2

• Per glucose (2 turns):

  • 6 NADH

  • 2 FADH2

  • 2 ATP

  • 4 CO2

Summary equation (per glucose):

$2\mathrm{Acetyl{-}CoA} + 6\mathrm{NAD}^+ + 2\mathrm{FAD} + 2\mathrm{ADP} + 2\mathrm{P_i} \rightarrow 4\mathrm{CO_2} + 6\mathrm{NADH} + 2\mathrm{FADH_2} + 2\mathrm{ATP} + 2\mathrm{CoA}$

4. Electron Transport Chain and Oxidative Phosphorylation

The electron transport chain (ETC) is a series of protein complexes in the inner mitochondrial membrane that transfer electrons from NADH and FADH2 to oxygen, generating a proton gradient used to produce ATP via chemiosmosis.

• Location: Inner mitochondrial membrane

• Key steps:

  • Electrons from NADH and FADH2 are transferred through complexes I-IV.

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

  • ATP synthase uses the proton gradient to synthesize ATP from ADP and Pi.

  • Oxygen acts as the final electron acceptor, forming water.

• ATP yield:

  • Each NADH yields approximately 2.5 ATP.

  • Each FADH2 yields approximately 1.5 ATP.

Summary equation:

$10\mathrm{NADH} + 2\mathrm{FADH_2} + 6\mathrm{O_2} + 34\mathrm{ADP} + 34\mathrm{P_i} \rightarrow 10\mathrm{NAD}^+ + 2\mathrm{FAD} + 12\mathrm{H_2O} + 34\mathrm{ATP}$

Fermentation

Types of Fermentation

When oxygen is not available, cells can generate ATP through fermentation, which regenerates NAD+ for glycolysis.

• Lactic acid fermentation: Pyruvate is reduced to lactate (e.g., in muscle cells).

• Alcoholic fermentation: Pyruvate is converted to ethanol and CO2 (e.g., in yeast).

Net ATP yield: 2 ATP per glucose (from glycolysis only).

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
Citric Acid Cycle	2	6	2
Oxidative Phosphorylation	~28	-	-
Total	~32	10	2

Additional info: Actual ATP yield may vary depending on cell type and shuttle mechanisms.

Integration of Metabolic Pathways

Catabolism of Other Molecules

Proteins, fats, and carbohydrates can all enter cellular respiration at various points:

• Proteins: Broken down into amino acids, deaminated, and enter as pyruvate, acetyl-CoA, or citric acid cycle intermediates.

• Fats: Broken down into glycerol (enters glycolysis) and fatty acids (converted to acetyl-CoA via beta-oxidation).

• Carbohydrates: Enter as glucose or other sugars feeding into glycolysis.