Overview
This lecture explains how energy is transferred and utilized within living systems, focusing on redox reactions, electron carriers, and ATP as the main energy currency of the cell.
Redox Reactions in Energy Transfer
- Oxidation is the loss of electrons from a molecule, leading to decreased potential energy.
- Reduction is the gain of electrons by a molecule, resulting in increased potential energy.
- Redox reactions always occur as paired oxidation and reduction events.
- Electrons transferred during redox reactions allow energy transfer in small, manageable amounts throughout metabolic pathways.
Electron Carriers
- Electron carriers like NAD+, FAD, and NADP function as shuttles for high-energy electrons during metabolism.
- NAD+ is reduced to NADH by accepting two electrons and a proton; NADH then stores and transfers energy.
- FAD is reduced to FADH2; both NAD+ and FAD are mainly involved in energy extraction from sugars.
- NADP (with an extra phosphate) is primarily used in photosynthesis and anabolic reactions.
ATP: Structure, Function, and Energy Release
- ATP (adenosine triphosphate) is the main energy currency in cells.
- ATP consists of adenine, ribose, and three phosphate groups; repulsion between phosphate groups makes it unstable.
- Breaking ATP’s terminal phosphate bond (dephosphorylation) releases usable energy for cell work.
- ATP is regenerated from ADP by adding a phosphate, using energy derived from glucose catabolism.
Phosphorylation
- Phosphorylation transfers a phosphate group from ATP to a substrate, forming an intermediate complex.
- Substrate-level phosphorylation directly transfers a phosphate from a substrate to ADP, forming ATP.
- Most ATP is formed via oxidative phosphorylation, which uses chemiosmosis and occurs in mitochondria or the plasma membrane.
Oxidative Phosphorylation and Chemiosmosis
- Chemiosmosis is the process by which most ATP is generated, driven by the movement of electrons and protons.
- Oxidative phosphorylation involves electron transport chains and the enzyme ATP synthase, using oxygen as the final electron acceptor.
Key Terms & Definitions
- Oxidation — loss of electrons from a molecule.
- Reduction — gain of electrons by a molecule.
- Redox Reaction — coupled oxidation and reduction process.
- Electron Carrier — molecule that transports high-energy electrons (e.g., NAD+, FAD).
- ATP (Adenosine Triphosphate) — main energy-carrying molecule in the cell.
- Phosphorylation — addition of a phosphate group to a molecule.
- Substrate-level Phosphorylation — direct transfer of a phosphate to ADP to form ATP.
- Oxidative Phosphorylation — ATP production via electron transport and chemiosmosis.
Action Items / Next Steps
- Review metabolic pathways involving NAD+, FAD, and ATP.
- Practice drawing and labeling ATP and electron carrier structures.
- Read the next section on glucose catabolism and its link to ATP regeneration.