Cellular Respiration

Cellular Respiration - Overview

Cellular respiration is a series of metabolic pathways that convert carbohydrates, primarily glucose, into adenosine triphosphate (ATP), which is the energy currency of the cell. The overall formula for cellular respiration is:

C6H12O6 + 6O2 → 6CO2 + 6H2O + ATP

This process allows cells to gradually harvest energy from glucose, ensuring that heat generation is minimized and maximizing energy retention for cellular activities. If cells were to utilize all potential energy from glucose at once, it could lead to excessive heat production, and not all energy would be effectively captured. The reactions in respiration occur in a stepwise fashion, where the products of one reaction serve as the reactants for the next.

Three Steps of Cellular Respiration

Cellular respiration comprises three main stages, which cumulatively yield a maximum of 36 molecules of ATP from one glucose molecule, representing approximately 39% of the total energy contained in glucose:

1. Glycolysis:

   • Glycolysis represents the first step and occurs in the cytoplasm. It involves the conversion of one glucose molecule (C6) into two molecules of pyruvate (C3).

   • Net Yield: 2 ATP molecules are produced from this process via substrate-level phosphorylation.

   • The pathway transforms glucose → fructose → PGAL → PGA → PEP → pyruvate.

   • Key Reaction: 2 ATPs are used, yielding a total of 4 ATPs, resulting in a net gain of 2 ATPs.

   • Glycolysis occurs in the absence of oxygen (anaerobic) and leads to further breakdown through the Krebs cycle when oxygen is available, or fermentation processes when oxygen is absent.

2. Krebs Cycle (Citric Acid Cycle):

   • The Krebs Cycle is the second stage, taking place in the mitochondrial matrix. Each turn of the cycle processes an acetyl group derived from pyruvate, releasing CO2 and capturing high-energy electrons.

   • Net Yield: 2 ATP are produced along with NADH and FADH2, which carry electrons to the next stage.

   • The cycle operates twice for each glucose molecule processed, making it a crucial step in energy extraction.

3. Electron Transport Chain (ETC):

   • The third stage occurs across the inner mitochondrial membrane (cristae) and utilizes electrons from NADH and FADH2.

   • Electrons are transferred through a series of proteins, causing proton (H+) pumping across the membrane, creating a proton gradient.

   • Net Yield: Approximately 32 ATPs are generated through oxidative phosphorylation, which utilizes chemiosmosis to convert the energy from the proton gradient into ATP.

   • This process ultimately produces water as a byproduct when electrons combine with O2 and protons.

Fermentation

In the absence of oxygen, cells can undergo fermentation to regenerate NAD+ for glycolysis. This less efficient process yields only 2 additional ATPs per glucose molecule:

• Types of Fermentation:

  • Alcoholic Fermentation: Typically occurs in yeast and some bacteria, converting pyruvate into ethanol and carbon dioxide.

  • Lactic Acid Fermentation: In animals, particularly in muscle cells during intense exercise, pyruvate is converted to lactic acid, which can lead to muscle soreness.

Aerobic vs Anaerobic Respiration

• Aerobic Respiration: Occurs in the presence of oxygen, yielding a total of 36 ATP.

• Anaerobic Respiration: Occurs without oxygen and yields significantly less ATP (only 2 ATP).

Importance of Mitochondria and ATP Synthase

• The mitochondria play a critical role in ATP production through the electron transport chain and Krebs cycle. The ATP synthase enzyme utilizes the proton motive force established by the ETC to synthesize ATP as protons flow back into the mitochondrial matrix.

• More mitochondria in muscle cells enhance ATP production capacity, improving endurance and reducing reliance on fermentation, thus minimizing lactic acid build-up and soreness in athletes.