Cellular respiration is the powerhouse of energy production in cells. It's a complex process that breaks down glucose to create ATP, the energy currency of life. This process involves three main stages: glycolysis, the Krebs cycle, and the electron transport chain.

These stages work together to maximize energy extraction from glucose. Glycolysis happens in the cytoplasm, while the Krebs cycle and electron transport chain occur in mitochondria. Understanding this process is key to grasping how cells power life.

Glycolysis and Fermentation

Glycolysis Overview

• Glycolysis first step in cellular respiration where glucose is broken down into two pyruvate molecules
• Occurs in the cytoplasm of the cell
• Does not require oxygen (anaerobic process)
• Produces a net gain of 2 ATP and 2 NADH molecules per glucose molecule
• Consists of two phases:
  1. Energy investment phase: Uses 2 ATP to phosphorylate glucose
  2. Energy payoff phase: Produces 4 ATP and 2 NADH, resulting in a net gain of 2 ATP and 2 NADH

Pyruvate and Fermentation

• Pyruvate end product of glycolysis
  • Can enter the Krebs cycle for further oxidation if oxygen is present
  • Converted to lactate or ethanol through fermentation in the absence of oxygen
• Anaerobic respiration cellular respiration that occurs without oxygen, such as fermentation
• Fermentation process that regenerates NAD+ from NADH in the absence of oxygen
  • Allows glycolysis to continue by providing NAD+ for the energy payoff phase
  • Two main types of fermentation:
    1. Lactic acid fermentation: Pyruvate is reduced to lactate (occurs in animal muscle cells during intense exercise)
    2. Alcoholic fermentation: Pyruvate is decarboxylated to acetaldehyde and then reduced to ethanol (occurs in yeast and some bacteria)

Krebs Cycle and Electron Transport Chain

Krebs Cycle (Citric Acid Cycle)

• Krebs cycle second stage of cellular respiration, occurs in the matrix of the mitochondria
• Acetyl-CoA (produced from pyruvate) enters the Krebs cycle by combining with oxaloacetate to form citrate
• Series of redox reactions that generate high-energy molecules (3 NADH, 1 FADH2, and 1 GTP/ATP) per acetyl-CoA
• Carbon dioxide is released as a byproduct
• Regenerates oxaloacetate to continue the cycle

Electron Transport Chain and Oxidative Phosphorylation

• Electron transport chain (ETC) final stage of cellular respiration, occurs in the inner mitochondrial membrane
• NADH and FADH2 (produced in glycolysis and Krebs cycle) donate electrons to the ETC
  • Electrons are passed through a series of protein complexes, releasing energy used to pump protons (H+) into the intermembrane space
  • Creates an electrochemical gradient (proton gradient) across the inner mitochondrial membrane
• Oxidative phosphorylation production of ATP using the energy from the proton gradient
  • Protons flow back into the matrix through ATP synthase, driving the synthesis of ATP
  • Process is called chemiosmosis: Movement of ions across a semipermeable membrane down their electrochemical gradient
• ETC and oxidative phosphorylation are highly efficient, producing around 34 ATP per glucose molecule

Cellular Respiration Overview

Aerobic Respiration

• Aerobic respiration cellular respiration that requires oxygen, includes glycolysis, Krebs cycle, and electron transport chain
• Occurs primarily in the mitochondria, the powerhouses of the cell
  • Mitochondria have a double membrane structure, with the inner membrane folded into cristae to increase surface area for the ETC
• Most efficient form of cellular respiration, producing around 38 ATP per glucose molecule
  • Glycolysis: 2 ATP
  • Krebs cycle: 2 ATP (directly) and high-energy molecules (NADH and FADH2)
  • Electron transport chain: Around 34 ATP (from oxidative phosphorylation)

Anaerobic Respiration and Efficiency

• Anaerobic respiration cellular respiration that occurs without oxygen, such as fermentation
  • Includes glycolysis followed by either lactic acid or alcoholic fermentation
  • Produces only 2 ATP per glucose molecule (from glycolysis)
• Comparison of aerobic and anaerobic respiration:
  • Aerobic respiration is much more efficient (38 ATP vs. 2 ATP per glucose)
  • Anaerobic respiration is faster and can provide energy in the absence of oxygen (such as during intense exercise)
  • Aerobic respiration produces CO2 and H2O as byproducts, while anaerobic respiration produces lactate or ethanol