The primary source for energy on earth is the sun. The most common and important form of energy available for living organisms is heat. The products of a reaction can have less energy than the reactants. The products of a reaction can have more energy than the reactants. Reversible reactions tend to approach an equilibrium.
Most reactions are reversible.
They release energy. Glucose metabolism is an example. Their products have more energy than the reactants. There is an energy loss. Bonds are broken.