All of the chemical reactions in the body. Breakdown of complex molecules to simple ones = catabolism (cellular respiration). Build-up of complex molecules from simple ones = anabolism, energy is required. Complex molecules -> catabolism -> energy is produced (exergonic) -> energy captured -> anabolism -> energy is used (endergonic). Atp + h20 -> adp + pi (7. 3 kcal/mole energy) exergonic. Muscle cells -> require 10 million atp/sec when working. Energy from foods is converted to energy through cellular respiration. Glucose + o2 -> co2 + h2o (686 kcal/mole) Catalytic proteins that speed up the rate of a reaction, without altering the end products or change the reaction. Chemical reaction to start requires initial energy (activation energy), usually heat energy. Sucrose binds to an active site on the enzyme. Reaction occurs at 1000 x per second: affected by environment (sensitive) Temperatures can affect enzymes ability to function. Each enzyme has its own optimal temperature.