Overview
This lecture covers cellular respiration—a process all living organisms use to convert food energy (mainly glucose) into ATP, the energy currency for cellular functions. It details the major steps, key molecules, and the relationship with photosynthesis.
Energy Processes in Living Organisms
- Cellular respiration is performed by all living organisms (prokaryotes and eukaryotes) to make ATP from food.
- The main food molecule for cell respiration is glucose; ATP is the usable energy form for cells.
- Photosynthesis is performed by plants, some protists, and some bacteria, not by all organisms.
Connection Between Photosynthesis and Cellular Respiration
- Photosynthesis converts CO₂ and H₂O into glucose and O₂ using sunlight.
- Cellular respiration uses glucose and O₂ to produce ATP, releasing CO₂ and H₂O.
- These two processes are essentially the reverse of each other.
ATP Formation and Its Importance
- Making ATP means adding a phosphate to ADP (phosphorylation), storing energy.
- Breaking ATP’s high-energy terminal phosphate bond releases energy for cell work.
- Energy to re-form ATP comes from glucose bonds and from a concentration gradient set up during respiration.
Methods and Stages of Cellular Respiration
- Two methods to make ATP: substrate-level phosphorylation and oxidative phosphorylation.
- Four main stages: glycolysis, pyruvate oxidation, citric acid cycle (Krebs cycle), and oxidative phosphorylation (electron transport chain + chemiosmosis).
Glycolysis
- Occurs in the cytoplasm and can happen with or without oxygen.
- Glucose split into two pyruvate molecules (3-carbons each).
- Net gain of 2 ATP (4 made, 2 used) by substrate-level phosphorylation.
- 2 NAD⁺ reduced to 2 NADH as electron carriers pick up H.
Pyruvate Oxidation
- Occurs in mitochondrial matrix if oxygen is present.
- Each pyruvate loses a carbon as CO₂, producing acetyl-CoA (2 per glucose).
- 2 NADH produced (no ATP made).
Citric Acid Cycle (Krebs Cycle)
- Also occurs in mitochondrial matrix.
- Each acetyl-CoA enters cycle; all glucose-derived carbons released as CO₂.
- 2 ATP made by substrate-level phosphorylation.
- 6 NADH and 2 FADH₂ produced as electron carriers.
Oxidative Phosphorylation
- Takes place along the inner mitochondrial membrane.
- Electron carriers (NADH & FADH₂) donate electrons to the electron transport chain.
- H⁺ ions pumped to intermembrane space, creating a gradient.
- H⁺ flow back via ATP synthase (chemiosmosis), generating ≈32 ATP per glucose.
- O₂ is the final electron acceptor, forming H₂O.
Anaerobic Respiration (Fermentation)
- Occurs when oxygen is insufficient.
- Only glycolysis occurs, yielding 2 ATP per glucose.
- NAD⁺ is regenerated; products include lactic acid (in animals) or alcohol (in yeast).
Use of Other Organic Molecules
- Fats and proteins can be used for ATP production but are less efficient and preferred less than carbohydrates.
Key Terms & Definitions
- Cellular respiration — process by which organisms convert food energy into ATP.
- ATP (Adenosine Triphosphate) — primary energy carrier in cells.
- Glycolysis — splitting glucose into two pyruvate molecules; first stage of respiration.
- Phosphorylation — addition of a phosphate group to a molecule.
- Substrate-level phosphorylation — direct transfer of phosphate to ADP to make ATP.
- Oxidative phosphorylation — ATP production using electron transport chain & chemiosmosis.
- NAD⁺/NADH, FAD/FADH₂ — electron carriers that transfer high-energy electrons.
- Electron Transport Chain (ETC) — series of protein complexes transferring electrons, pumping H⁺.
- Chemiosmosis — ATP synthesis driven by H⁺ movement through ATP synthase.
- Fermentation — anaerobic process regenerating NAD⁺ and producing 2 ATP.
Action Items / Next Steps
- Complete the cellular respiration worksheet or study guide questions.
- Review diagrams for each stage and ATP production summary.
- Ask for clarification if unsure which worksheet or study guide to use.