Activation Energy in Chemistry

Activation Energy

• Activation energy is the energy required for a chemical reaction to proceed by increasing the energy of the substrate.
• Catalysts, such as enzymes, reduce activation energy, allowing reactions to occur faster and at lower temperatures.
• The energy released during reactions results in the formation of products, and reactions can be reversible but often require external energy input for the reverse process.
• Biological processes are predominantly cycles of reversible reactions needing energy from the Sun.

Role of ATP

• Adenosine triphosphate (ATP) serves as the universal energy currency for cells, utilized in nearly all energy-requiring cellular processes.
• ATP is water-soluble and easily transported, ensuring efficient energy delivery within cells.
• It provides immediate energy through a one-step reaction and is hydrolyzed easily to release energy.
• ATP can be recycled quickly from adenosine diphosphate (ADP) through phosphorylation, allowing constant ATP availability.
• Each of the three phosphate groups in ATP is negatively charged, leading to unstable covalent bonds that release substantial energy (30.5 kJ/mol for the first two phosphates and 14.2 kJ/mol for the last).

ATP Synthesis

• ATP synthesis from ADP involves three mechanisms:
  • Photophosphorylation in chloroplasts during photosynthesis.
  • Oxidative phosphorylation across mitochondrial membranes during electron transport.
  • Substrate-level phosphorylation during metabolic reactions, like glycolysis.

Chemiosmotic Theory

• In mitochondria, hydrogen ions are pumped across membranes, creating a proton gradient that drives ATP synthesis via ATP synthase.
• Protons flow back through ATP synthase, converting ADP and inorganic phosphate into ATP, with electrons combining with protons and oxygen to form water.

ATP Usage

• ATP is not suitable for long-term energy storage; fats and carbohydrates serve that purpose more effectively.
• ATP is used in diverse cellular functions, including:
  • Anabolic processes: building macromolecules (e.g., polysaccharides, proteins).
  • Movement: muscle contraction and intracellular transport.
  • Active transport: moving molecules against concentration gradients.
  • Secretion: formation of vesicles.
  • Activation of chemicals: enhancing reactivity via phosphorylation.

Glycolysis

• Glycolysis occurs in the cytoplasm, converting one glucose molecule into two pyruvate molecules through ten enzyme-controlled reactions, grouped into four stages:
  • Activation of glucose: phosphorylation using ATP increases reactivity.
  • Splitting of sugar: fructose 1,6-bisphosphate is divided into two triose phosphates.
  • Oxidation: NAD+ is reduced to form NADH as hydrogen is removed from triose phosphates.
  • Production of ATP: converting triose phosphates into pyruvate yields a net gain of two ATP and two NADH.

Link Reaction

• Pyruvate from glycolysis enters mitochondria, undergoing decarboxylation and oxidation.
• Each pyruvate loses a carbon (as CO2), is oxidized to reduce NAD+ to NADH, and forms acetyl coenzyme A (acetyl CoA) for entry into the Krebs cycle.

Coenzymes

• Coenzymes like nicotinamide adenine dinucleotide (NAD) and flavine adenine dinucleotide (FAD) aid in carrying hydrogen atoms and electrons in respiration.
• NAD is crucial for the function of dehydrogenase enzymes that facilitate oxidation reactions in the metabolic pathway.