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, identifying oxidized and reduced reactants.
  • Outline and summarize the four stages of aerobic respiration.
  • Determine the location of each stage in a eukaryotic cell.
  • Define chemiosmosis and explain proton gradient formation across the inner mitochondrial membrane.
  • Contrast and compare anaerobic respiration and fermentation, including ATP formation mechanisms, final electron acceptors, and end products.

Overview

• Living organisms gain energy from external sources.
• Some animals consume plants or other animals.
• Energy enters the ecosystem as sunlight and exits as heat.
• Photosynthesis creates organic molecules from CO₂ and H₂O, using energy from light.
• Cells use this energy stored in organic molecules to regenerate ATP, powering cellular work.

Catabolism and ATP Production

• Breakdown of organic molecules is exergonic (releases energy stored in ATP).
• This occurs through cellular respiration (aerobic or anaerobic).
• Aerobic respiration needs O₂.
• Anaerobic respiration does not require O₂ and can be fermentation.
• Aerobic respiration consumes organic molecules and O₂ to produce ATP.
• Anaerobic respiration resembles aerobic respiration but uses a different electron acceptor than O₂.
• Fermentation is a partial sugar degradation that takes place without O₂

Redox Reactions

• Redox reactions involve electron transfer between molecules.
• Oxidation is loss of electrons.
• Reduction is gain of electrons.
• C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (in the chemical bonds of ATP) - Summary of Glucose Oxidation

The Four Stages of Aerobic Respiration

• Glucose oxidation has four main stages: glycolysis, formation of acetyl CoA, citric acid cycle, and electron transport chain and chemiosmosis
• Glycolysis in the cytosol: breaks down glucose to pyruvate, forming 2 ATPs and 2 NADH
• Formation of acetyl CoA in the mitochondria: oxidizes pyruvate to Acetyl CoA, forming 2 NADH and releasing CO₂
• Citric acid cycle in the mitochondria: oxidizes acetyl CoA to CO₂, producing 2 ATP, 6 NADH, and 2 FADH₂.
• Electron transport chain and chemiosmosis in the mitochondria: uses electron carriers (NADH and FADH₂) to create a proton gradient across the inner mitochondrial membrane, generates 32-34 ATPs through chemiosmosis.
• Prokaryotes lack mitochondria, so these processes happen in the cytosol and associated with the cell membrane.

Glycolysis (Glucose Splitting)

• Glycolysis is the first step in cellular respiration.
• It's a series of 10 biochemical reactions.
• Glycolysis breaks down glucose into two three-carbon pyruvate molecules, extracting energy.
• This process 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 enters the mitochondrion and is oxidized to a 2-carbon group (acetyl CoA).
• NADH is produced, and CO₂ is released as a byproduct.

The Citric Acid Cycle

• Acetyl CoA combines with oxaloacetate (a four-carbon molecule) to form citrate (a six-carbon molecule).
• The cycle regenerates oxaloacetate, releasing CO₂ as a waste product.
• Energy is captured as ATP and the reduced high-energy electron carriers NADH and FADH₂.

Electron Transport and Chemiosmosis

• Electrons from glucose are transferred to a chain of electron acceptors, culminating in oxygen reduction to water.
• Energy released from electron transfer pumps H⁺ ions across the inner mitochondrial membrane, creating a proton gradient.
• ATP synthase uses the energy stored in this gradient to produce ATP through chemiosmosis.

Summary of Glucose Aerobic Respiration

• Glycolysis produces 2 ATP, 2 NADH, and 2 pyruvate molecules.
• Formation of acetyl CoA produces 2 NADH and releases 2 CO₂.
• The citric acid cycle produces 2 ATP, 6 NADH, and 2 FADH₂ and releases 4 CO₂
• The electron transport chain and chemiosmosis produce 32-34 ATP.
• The overall total ATP yield is ~ 32-34 ATP.

Fermentation and Anaerobic Respiration

• Fermentation and anaerobic respiration enable ATP production without oxygen.
• Glycolysis occurs with or without O₂
• Anaerobic respiration uses an electron transport chain with an electron acceptor other than oxygen (e.g., sulfate).
• Fermentation is glycolysis coupled with reactions that regenerate NAD⁺, allowing glycolysis to continue.

Types of Fermentation

• Alcohol fermentation: Pyruvate is converted to ethanol in two steps (releasing CO₂).
• Yeast is used in brewing, winemaking, and baking.
• Lactic acid fermentation: Pyruvate is reduced to lactate by NADH (no CO₂ release).
• Used by some fungi, bacteria, human muscle cells, and human red blood cells.

Anaerobic Respiration

• Anaerobic respiration does not use oxygen but other inorganic molecules as the final electron acceptor.
• The process is similar to aerobic respiration, using an electron transport chain and chemiosmosis to produce ATP.

Fermentation vs. Aerobic Respiration

• Both use glycolysis to oxidize glucose to pyruvate.
• They differ in electron acceptors (organic molecule for fermentation, O₂ for aerobic respiration).
• Aerobic respiration produces significantly more ATP (30-32) than fermentation (2).

Energy Yield of Nutrients Other Than Glucose

• Many organisms use various nutrients (proteins, carbohydrates, fats) instead of glucose, which are converted to intermediates feeding into glycolysis or the citric acid cycle to generate ATP.