Overview
This section explains how energy is transferred and used in living systems, focusing on electron transfer, ATP as the cell’s energy currency, and the chemical pathways involved.
Importance of Electrons in Energy Transfer
- Energy conversion in cells involves oxidation (electron loss) and reduction (electron gain) reactions, known as redox reactions.
- The removal of an electron (oxidation) lowers potential energy, while its addition (reduction) raises potential energy.
- Electron transfer allows incremental energy release, preventing cell damage from sudden energy bursts.
Electron Carriers in Cells
- Electron carriers shuttle high-energy electrons in metabolic pathways, mostly derived from B vitamins.
- NAD+ (from niacin/vitamin B3) accepts electrons and is reduced to NADH; NADH carries energy.
- FAD+ (from riboflavin/vitamin B2) and its reduced form FADH2 also transport electrons.
- NADP (variation of NAD) is involved in anabolic reactions and photosynthesis.
ATP: The Energy Currency of the Cell
- Cells use ATP (adenosine triphosphate) to safely store and release energy when needed.
- ATP consists of adenosine (adenine + ribose) and three phosphate groups; high energy is stored in phosphate bonds.
- Removal of a phosphate group (dephosphorylation) releases energy for cellular work.
ATP Structure and Hydrolysis
- Addition of phosphate groups to AMP forms ADP and ATP, with increasing instability due to charge repulsion.
- Hydrolysis of ATP (breaking with water) produces ADP, inorganic phosphate, and free energy.
- ATP is regenerated from ADP using energy from metabolizing sugars (e.g., glucose).
Phosphorylation and ATP Use
- ATP transfers its phosphate group to other molecules (phosphorylation), activating them or enabling reactions.
- In substrate-level phosphorylation, ATP is formed directly by transferring a phosphate from a substrate to ADP.
Oxidative Phosphorylation and Chemiosmosis
- Most ATP is produced through chemiosmosis in mitochondria (eukaryotes) or the plasma membrane (prokaryotes).
- Oxidative phosphorylation uses an electron transport chain and oxygen to drive ATP synthesis.
Key Terms & Definitions
- Oxidation — loss of electrons from a molecule, decreasing its potential energy.
- Reduction — gain of electrons by a molecule, increasing its potential energy.
- Redox Reaction — paired oxidation and reduction reactions.
- NAD+/NADH — electron carrier, NAD+ is oxidized, NADH is reduced.
- FAD+/FADH2 — electron carrier, FAD+ is oxidized, FADH2 is reduced.
- ATP — adenosine triphosphate, primary energy currency of the cell.
- Hydrolysis — process of breaking molecules with water, e.g., ATP to ADP.
- Phosphorylation — addition of a phosphate group to a molecule.
- Substrate-level phosphorylation — direct transfer of a phosphate group to ADP to form ATP.
- Oxidative phosphorylation — ATP production via electron transport chain and chemiosmosis.
Action Items / Next Steps
- Review how redox reactions function in metabolic pathways.
- Study ATP structure and how energy is released and used in cells.
- Prepare for upcoming sections on glycolysis and cellular respiration.