Study Concepts Energy is the capacity to do work, or heat or anything transformed into heat. All cellular activity is based on energy supplied by the sun or from chemical bonds. Changes in energy are governed by first and second laws of thermodynamics. Differences in free energy between reactants and products govern whether reactions are spontaneous or non-spontaneous. Activation energy is required for reactions to proceed.
Enzymes lower the activation energy for biological reactions.
Enzymes interact with their substrates. Oxidation-Reduction Reactions Oxidation is the loss of an electron. Reduction is the gain of an electron. Key role in flow of energy in biological reactions. Electrons carry energy with them from one molecule to another.