Respiration Home Central Concepts module Respiration Biochemical reactions in living organisms are essentially energy transfers.
Reduction is the gain of an electron.
Such coupled reactions are referred to as redox reactions. Glycolysis and the Krebs Cycle Schematic representation of the controlled stepwise oxidation of sugar in a cell, compared with ordinary burning. At each step, an enzyme controls the reaction by reducing the activation energy barrier that has to be surmounted before the specific reaction can occur. Energy is the ability to bring about change or to do work. Thermodynamics is the study of energy.