Biology ACST 112 - Chapter 6: ATP Production

Biology ACST 112 - Chapter 6: How Cells Make ATP: Energy-Releasing Pathways

• Learning Objectives: Students will be able to:
  • Write a summary reaction for aerobic respiration, showing oxidized and reduced reactants.
  • List and briefly describe the four stages of aerobic respiration.
  • Indicate the location of each stage of aerobic respiration in a eukaryotic cell.
  • Define chemiosmosis and describe how a proton gradient is established across the inner mitochondrial membrane.
  • Compare and contrast anaerobic respiration and fermentation, including ATP formation mechanisms, final electron acceptors, and end products.

Overview

• Living cells require energy from external sources.
• Animals obtain energy from eating plants or other animals.
• Energy enters ecosystems as sunlight and leaves as heat.
• Photosynthesis makes organic molecules.
• Cells use energy from organic molecules to regenerate ATP, powering cellular work.

Catabolism and ATP Production

• Breakdown of organic molecules is exergonic (releases energy).
• This occurs through cellular respiration, which can be aerobic or anaerobic.
• Aerobic respiration requires oxygen.
• Anaerobic respiration does not require oxygen, and includes fermentation.
• Aerobic respiration consumes organic molecules and oxygen, producing ATP.
• Anaerobic respiration is similar to aerobic respiration, but uses compounds other than oxygen.
• Fermentation is a partial sugar breakdown process that occurs without oxygen.

Redox Reactions

• Redox reactions involve electron transfer between molecules.
• Oxidation is loss of electrons; reduction is gain of electrons.
• Cellular respiration is a redox process.
• Glucose is oxidized, and oxygen ultimately is reduced.

The Four Stages of Aerobic Respiration

• Cellular respiration involves the oxidation of glucose.
• The stages are: glycolysis, formation of acetyl coenzyme A, citric acid cycle, and electron transport and chemiosmosis.
• Glycolysis occurs in the cytosol.
• Formation of acetyl coenzyme A, citric acid cycle, and electron transport and chemiosmosis occur in the mitochondria.
• Prokaryotes lack mitochondria; these processes occur in the cytoplasm and plasma membrane.

Glycolysis (Glucose Splitting)

• Glycolysis is the first step in cellular respiration.
• It’s a series of 10 biochemical reactions that extract energy from glucose by splitting it into two 3-carbon pyruvate molecules.
• Glycolysis occurs in the cytosol of all living cells.
• Glycolysis produces 2 ATP, 2 NADH, and 2 pyruvate molecules.

Formation of Acetyl Coenzyme A

• Each pyruvate is oxidized to a 2-carbon acetyl group (acetyl coenzyme A).
• NADH is produced. 
• Carbon dioxide is released.

The Citric Acid Cycle

• The acetyl group combines with oxaloacetate to form citrate.
• Citrate is recycled to oxaloacetate.
• Carbon dioxide is released.
• Energy is captured as ATP, NADH, and FADH₂.

Electron Transport and Chemiosmosis

• Electrons from NADH and FADH₂ are passed to an electron transport chain.
• Electrons are passed to the final electron acceptor, oxygen, forming water.
• Energy is used to pump protons (H+) across the inner mitochondrial membrane, creating a gradient.
• Chemiosmosis: H+ gradient used to generate ATP via ATP synthase.

Summary of Glucose Aerobic Respiration

• Total ATP production from glucose oxidation is 32-34 ATP.

Fermentation and Anaerobic Respiration

• Fermentation and anaerobic respiration enable ATP production without oxygen, often in the absence of oxygen.
• Glycolysis can occur in the absence of oxygen, coupled with fermentation or anaerobic respiration for ATP production.
• Anaerobic respiration uses an electron transport chain with an electron acceptor other than oxygen (e.g., sulfate).
• Fermentation has no electron transport chain and uses an organic molecule (e.g., pyruvate) to re-oxidize NADH.

Types of Fermentation

• Common types: alcoholic and lactic acid fermentations.
• Alcohol fermentation: pyruvate is converted to ethanol, releasing CO2.
• Lactic acid fermentation: pyruvate is reduced directly to lactate, with no CO₂ release.

Anaerobic Respiration

• Anaerobic respiration uses electron acceptors other than oxygen in electron transport chains.
• End products are carbon dioxide, reduced inorganic substances, and ATP.

Fermentation vs. Aerobic Respiration

• Both use glycolysis to oxidize glucose to pyruvate.
• Different final electron acceptors: organic molecules in fermentation; oxygen in aerobic respiration.
• Aerobic respiration produces 30-32 ATP; fermentation produces 2 ATP per glucose molecule.

Energy Yield of Nutrients Other Than Glucose

• Many organisms use nutrients other than glucose for energy.
• Fatty acids and amino acids are digested and used in glycolysis and the citric acid cycle to generate ATP.