Overview
This section discusses how living cells transfer and store energy, focusing on redox reactions, electron carriers, and the role of ATP as the primary energy currency in biological systems.
Redox Reactions in Living Systems
- Energy conversion in cells involves linked oxidation and reduction (redox) reactions.
- Oxidation is the loss of electrons from a molecule, reducing its potential energy.
- Reduction is the gain of electrons, increasing a molecule’s potential energy.
- Redox reactions transfer energy in the form of high-energy electrons for cellular processes.
Electron Carriers
- Electron carriers transfer high-energy electrons between molecules in metabolic pathways.
- Main electron carriers are NAD+ (oxidized form) and NADH (reduced form), derived from vitamin B3.
- NAD+ accepts electrons to become NADH; NADH can donate electrons in later reactions.
- FAD+ (from vitamin B2) can be reduced to FADH2 and also shuttles electrons.
- NADP+ is another variant important in anabolic reactions and photosynthesis.
ATP: The Cell’s Energy Currency
- Cells cannot store large amounts of free energy because excess energy can cause harmful heat.
- ATP (adenosine triphosphate) stores and releases energy as needed, acting like a rechargeable battery.
- ATP consists of adenosine monophosphate (AMP) with two additional phosphate groups.
- The bonds between ATP’s phosphate groups are unstable and release energy when broken (dephosphorylation).
- ATP hydrolysis produces ADP (adenosine diphosphate), inorganic phosphate, and free energy.
ATP Generation and Use
- Most ATP used by cells is regenerated from ADP via metabolism of sugars like glucose.
- ATP powers cell work by phosphorylating (adding a phosphate to) other molecules (phosphorylation).
- In substrate-level phosphorylation, a phosphate group is directly transferred from a substrate to ADP.
- The majority of ATP is produced via oxidative phosphorylation, involving chemiosmosis in mitochondria (eukaryotes) or plasma membrane (prokaryotes).
Key Terms & Definitions
- Oxidation — loss of electrons from a molecule, resulting in lower energy.
- Reduction — gain of electrons by a molecule, increasing its energy.
- Redox Reaction — a pair of linked oxidation and reduction reactions.
- Electron Carrier — molecule that transports electrons during metabolic reactions (e.g., NAD+, FAD+).
- ATP (Adenosine Triphosphate) — the primary energy carrier in cells.
- Phosphorylation — addition of a phosphate group to a molecule.
- Substrate-level Phosphorylation — direct transfer of a phosphate to ADP from a substrate.
- Oxidative Phosphorylation — ATP generation using energy from electron transport and chemiosmosis.
Action Items / Next Steps
- Review glycolysis and how energy is harvested from sugars.
- Study the structure and function of mitochondria related to ATP production.
- Prepare to discuss glycolysis in detail in the next section.