Cellular Respiration Overview

Overview

This lecture covers the second half of cellular respiration: the Krebs (citric acid) cycle, electron transport chain, ATP production, and comparisons with anaerobic respiration and fermentation.

Krebs Cycle (Citric Acid Cycle)

• Each glucose molecule produces two pyruvate molecules, creating two acetyl coenzyme A for two Krebs cycle turns.
• Acetyl CoA combines with oxaloacetic acid to form citric acid, starting the cycle.
• The Krebs cycle regenerates oxaloacetic acid to repeat the process.
• For each cycle turn: 3 CO₂, 4 NADH, 1 FADH₂, and 1 ATP are produced (doubled per glucose).
• Per glucose: 6 CO₂, 8 NADH, 2 FADH₂, and 2 ATP are generated (net 4 ATP so far with glycolysis).

Electron Transport Chain & Oxidative Phosphorylation

• NADH and FADH₂ deliver hydrogen protons/electrons to the electron transport chain in mitochondria.
• Hydrogen protons are stripped and pumped across the mitochondrial inner membrane, creating a concentration gradient.
• Protons flow back through ATP synthase, powering ATP production (“chemiosmosis”).
• ATP synthase uses a rotor, rod, and knob to combine ADP + phosphate into ATP.
• The electron transport chain yields about 34 ATP per glucose.

Evolutionary Perspective on Mitochondria

• Mitochondria evolved from bacteria and retain independent, circular DNA.
• They use hydrogen gradients as ancient bacteria do, making ATP production highly efficient.
• Skeletal muscle cells have many mitochondria but yield slightly less ATP (~36 per glucose).

Location and Types of Respiration

• Glycolysis occurs in the cytosol; Krebs cycle and electron transport chain occur in mitochondria.
• Aerobic respiration uses oxygen as the final electron acceptor; most efficient (~36-38 ATP).
• Anaerobic respiration uses inorganic molecules (nitrate, sulfate, etc.) as final electron acceptors; less ATP (2-36).

Fermentation

• Fermentation occurs without oxygen, skips Krebs cycle and electron transport chain.
• Net yield: 2 ATP via glycolysis only.
• Bacteria produce lactic acid (used in dairy, cheese, yogurt); yeast produce ethanol (found in beer, wine, spirits).
• Fermentation is inefficient for ATP but important for food and beverage production.

Key Terms & Definitions

• Krebs (Citric Acid) Cycle — Series of reactions that generate CO₂, NADH, FADH₂, and ATP from acetyl CoA.
• Electron Transport Chain — Mitochondrial process using NADH/FADH₂ to create a proton gradient for ATP synthesis.
• ATP Synthase — Enzyme complex that makes ATP as protons move down their gradient.
• Oxidative Phosphorylation — ATP production driven by a proton gradient from electron transport.
• Fermentation — Anaerobic process producing ATP via glycolysis and converting pyruvate to lactic acid or ethanol.

Action Items / Next Steps

• Review provided summary tables of passive/active transport, endocytosis, and exocytosis.
• Review posted materials on mitosis and the cell cycle (lab content, no new video).
• Prepare for application-based topics in upcoming chapters.