Cellular ATP Production Overview

Overview

This lecture covers how cells produce ATP, focusing on the processes of glycolysis (anaerobic) in the cytoplasm and further aerobic metabolism in the mitochondria.

Cellular ATP Production Pathways

• ATP can be produced in the cytoplasm without oxygen (anaerobic) or in mitochondria with oxygen (aerobic).
• Glycolysis occurs in the cytoplasm and does not require oxygen.
• Aerobic processes start when metabolites enter the mitochondria.

Glycolysis

• Glycolysis begins with a 6-carbon glucose molecule, which is phosphorylated by ATP to form glucose 6-phosphate.
• Glucose 6-phosphate is rearranged to fructose 6-phosphate, then phosphorylated again to fructose 1,6-bisphosphate.
• The 6-carbon fructose 1,6-bisphosphate is split into two 3-carbon glyceraldehyde-3-phosphate (G3P) molecules.
• G3P is phosphorylated and oxidized, transferring electrons to NAD⁺ to form NADH + H⁺.
• ATP is produced by substrate-level phosphorylation, resulting in a net gain of 2 ATP per glucose.
• Glycolysis produces two pyruvate molecules per glucose.

Fate of Pyruvate: Intermediate Step

• If oxygen is present, pyruvate enters mitochondria and is converted to acetyl coenzyme A (acetyl-CoA).
• This conversion produces CO₂ (waste) and more NADH + H⁺.
• If oxygen is absent, pyruvate is converted to lactic acid, leading to muscle fatigue.

Citric Acid Cycle (Krebs Cycle)

• Acetyl-CoA joins oxaloacetate to form citric acid, beginning the cycle.
• The cycle generates CO₂, NADH + H⁺, FADH₂, and a small amount of ATP (via GTP).
• Each turn of the cycle regenerates oxaloacetate.
• NADH and FADH₂ transport high-energy electrons to the electron transport chain for further ATP production.

Key Terms & Definitions

• ATP (Adenosine Triphosphate) — Energy-carrying molecule used by cells.
• Anaerobic — Processes that occur without oxygen.
• Aerobic — Processes that require oxygen.
• Glycolysis — Anaerobic breakdown of glucose into pyruvate, yielding ATP.
• Pyruvate — 3-carbon product of glycolysis.
• Acetyl-CoA — 2-carbon molecule entering the citric acid cycle.
• NAD⁺ (Nicotinamide Adenine Dinucleotide) — Electron carrier, becomes NADH after accepting electrons.
• FAD (Flavin Adenine Dinucleotide) — Electron carrier, becomes FADH₂ after accepting electrons.
• Citric Acid Cycle/Krebs Cycle — Aerobic process generating electron carriers and small ATP amounts.

Action Items / Next Steps

• Memorize the steps of glycolysis and their sequence.
• Practice spelling key biochemical terms for exams.
• Understand differences between aerobic and anaerobic ATP production.
• Complete any related handout exercises and review diagrams as needed.