Cellular Respiration Overview

Overview

This lecture covers the process of cellular respiration, detailing the stages, key molecules involved, enzyme functions, ATP production, and the differences between aerobic and anaerobic pathways.

Introduction to Cellular Respiration

• Cellular respiration is the process cells use to extract energy from glucose (C6H12O6) using oxygen, producing CO2, H2O, and ATP.
• The net reaction: Glucose + 6 O2 → 6 CO2 + 6 H2O + energy (ATP).
• ATP (adenosine triphosphate) is the cell’s energy currency, storing and supplying energy for endergonic (energy-requiring) reactions.

ATP Structure and Function

• ATP consists of a ribose sugar, adenine (a nitrogenous base), and three phosphate groups.
• Energy is released when ATP loses a phosphate group (exergonic reaction).
• Cells use ATP (not directly glucose) to improve efficiency and minimize energy loss as heat.

Stages of Cellular Respiration

• Four main stages: Glycolysis, Pyruvate Oxidation, Krebs Cycle (Citric Acid Cycle), and Electron Transport Chain (ETC).
• Glycolysis occurs in the cytosol; pyruvate oxidation and Krebs cycle in mitochondrial matrix; ETC in the inner mitochondrial membrane.

Glycolysis

• Glycolysis splits glucose (6C) into two pyruvate molecules (3C each) through 10 steps.
• Net result: 2 ATP (substrate-level phosphorylation) and 2 NADH produced per glucose.
• Has two phases: investment (uses 2 ATP) and payoff (produces 4 ATP).
• Involves key enzymes: kinase (transfers phosphate), isomerase (rearranges molecules), dehydrogenase (removes hydrogen).

Pyruvate Oxidation

• Each pyruvate is oxidized to acetyl coenzyme A (Acetyl CoA), releasing CO2 and reducing NAD+ to NADH.
• Catalyzed by pyruvate dehydrogenase enzyme (transfers hydrogen).

Krebs Cycle (Citric Acid Cycle)

• Acetyl CoA combines with oxaloacetate to form citrate; cycle regenerates oxaloacetate.
• Per glucose: yields 6 NADH, 2 FADH2, 2 ATP (by substrate-level phosphorylation), and 4 CO2.
• Dehydrogenase enzymes repeatedly remove hydrogen and transfer to NAD+ or FAD.

Electron Transport Chain & Oxidative Phosphorylation

• NADH and FADH2 donate electrons to ETC complexes; electrons travel to oxygen, forming water.
• Electron flow pumps protons, creating a gradient used by ATP synthase (chemiosmosis) to produce ATP.
• Oxidative phosphorylation: electron transport (oxidation) + ATP synthesis (phosphorylation).
• Oxygen is the final electron acceptor in aerobic respiration.

ATP Yield Summary

• Maximum theoretical ATP per glucose: 38; practical net yield is often 36 due to transport costs and inefficiencies.
• ATP production: 2 from glycolysis, 2 from Krebs cycle, up to 34 from ETC/chemiosmosis.

Anaerobic Respiration & Fermentation

• Without oxygen, glycolysis continues via fermentation (lactic acid in muscles, ethanol in yeast).
• Both types produce 2 ATP per glucose and regenerate NAD+ for glycolysis.

Enzyme Functions

• Kinase: transfers phosphate groups.
• Dehydrogenase: removes hydrogen.
• Isomerase: rearranges molecule structure.
• ATP synthase: synthesizes ATP during chemiosmosis.

Quiz & Exam Concepts

• Know products and locations of each stage.
• Oxygen is the final electron acceptor; glucose is oxidized, oxygen is reduced.
• Substrate-level phosphorylation occurs in glycolysis and Krebs; oxidative in ETC.
• Most ATP is produced by chemiosmosis, not directly by the ETC.
• Only glycolysis yields ATP in anaerobic conditions (2 ATP per glucose).

Key Terms & Definitions

• ATP (Adenosine Triphosphate) — main cellular energy carrier.
• Glycolysis — process converting glucose to pyruvate, generates ATP and NADH.
• Pyruvate Oxidation — conversion of pyruvate to Acetyl CoA, CO2, and NADH.
• Krebs Cycle — cycle that oxidizes Acetyl CoA, producing NADH, FADH2, ATP, CO2.
• Electron Transport Chain (ETC) — mitochondrial process transferring electrons to oxygen, generating ATP.
• Chemiosmosis — ATP production using a proton gradient through ATP synthase.
• Substrate-level Phosphorylation — direct transfer of phosphate to ADP to form ATP.
• Oxidative Phosphorylation — ATP synthesis using electron transport and chemiosmosis.
• Fermentation — anaerobic process to regenerate NAD+, producing lactic acid or ethanol.
• Redox Reaction — oxidation and reduction occurring simultaneously.

Action Items / Next Steps

• Review stage locations and products of cellular respiration.
• Practice quiz questions on ATP yield, enzyme functions, and stage outcomes.
• Memorize key enzymes and their roles for each step.