Cellular Respiration Overview

Overview

This lecture explains the key processes and molecules involved in cellular respiration, including glycolysis, link reaction, Krebs cycle, electron transport chain, and differences between aerobic and anaerobic pathways.

Electron Carriers and Redox Reactions

• NAD (nicotinamide adenine dinucleotide) is an electron carrier, easily reduced (gains electrons) and oxidized (loses electrons).
• Oxidation is the loss of electrons; reduction is the gain of electrons; these occur simultaneously (redox reactions).
• Electron carriers transport both electrons and energy between molecules.

Steps of Cellular Respiration

• Cellular respiration includes four major steps: glycolysis, link reaction, Krebs cycle, and electron transport chain (ETC) with chemiosmosis.

Glycolysis

• Glycolysis breaks down glucose (6 carbons) into two pyruvate (3 carbons each) in the cytoplasm without oxygen (anaerobic).
• Begins with phosphorylation (adding phosphates from 2 ATP) to destabilize glucose.
• Glucose splits into two intermediates, which are oxidized, reducing NAD to NADH (reduced NAD).
• Net products: 2 ATP (4 produced, 2 used), 2 NADH, 2 pyruvate.
• In absence of oxygen, pyruvate is converted to lactate to regenerate NAD, allowing glycolysis to continue.

Anaerobic and Aerobic Respiration Pathways

• Without oxygen, humans/bacteria convert pyruvate to lactate; yeast perform alcoholic fermentation (produce ethanol and CO₂).
• Anaerobic respiration yields 2 ATP per glucose; aerobic respiration produces much more ATP.
• Yeast switch from aerobic to anaerobic when oxygen runs out; both processes generate CO₂ (important for bread and alcohol).

Mitochondrial Structure and Reaction Locations

• Glycolysis occurs in cytoplasm; link reaction and Krebs cycle in mitochondrial matrix; ETC and chemiosmosis on the inner mitochondrial membrane (cristae).

Link Reaction

• Pyruvate enters mitochondrial matrix, loses a carbon (decarboxylation, produces CO₂) and becomes acetyl-CoA (2 carbons).
• NAD is reduced to NADH during this step; this reaction occurs twice per glucose.

Krebs Cycle (Citric Acid Cycle)

• Acetyl-CoA combines with oxaloacetate (4C) to form citrate (6C), then undergoes decarboxylation and oxidation.
• Products per cycle turn: 2 CO₂, 3 NADH, 1 FADH₂, 1 ATP; happens twice per glucose.

Electron Transport Chain and Chemiosmosis

• NADH and FADH₂ donate electrons to ETC; electrons pass through carriers in the inner membrane, powering proton pumps.
• Protons are pumped into the intermembrane space, creating a gradient.
• Protons flow back into the matrix through ATP synthase (chemiosmosis), driving ATP formation from ADP and inorganic phosphate.
• NADH enables pumping of more protons (10) than FADH₂ (6), resulting in more ATP per NADH.
• Oxygen is the final electron acceptor in the ETC, forming water with electrons and protons.

Interdependence and Energy Substrates

• All steps depend on each other; ETC needs NADH/FADH₂ from previous steps.
• Carbohydrates provide ~4 kcal/g, lipids ~9 kcal/g due to their different structures.
• Carbohydrates must be converted to pyruvate via glycolysis; lipids can become acetyl-CoA directly.
• Only carbohydrates can fuel anaerobic respiration.

Key Terms & Definitions

• NAD/NADH — Electron carrier molecule (oxidized/reduced forms).
• Glycolysis — Metabolic pathway breaking down glucose to pyruvate.
• Phosphorylation — Addition of a phosphate group to a molecule.
• Decarboxylation — Removal of a carbon atom (as CO₂).
• Acetyl-CoA — Two-carbon molecule entering the Krebs cycle.
• Krebs Cycle — Series of reactions generating NADH, FADH₂, ATP, and CO₂.
• Electron Transport Chain (ETC) — Series of proteins transferring electrons and pumping protons, driving ATP synthesis.
• Chemiosmosis — Movement of protons through ATP synthase to produce ATP.
• ATP Synthase — Enzyme producing ATP as protons flow through it.
• Final Electron Acceptor — Oxygen in aerobic respiration, receives electrons at ETC end.

Action Items / Next Steps

• Review glycolysis, link reaction, Krebs cycle, and electron transport chain steps.
• Study key terms and their functions in respiration.
• Practice drawing/labeling mitochondrion and mapping where each process occurs.