A. Cells need energy so that they can manufacture new substances, perform mechanical work of motion, transport substances and DG enerate heat. Cells have evolved to use energy very efficiently.
Photosynthetic organisms convert light into all other forms of energy.
D G is the actual free energy change of a chemical reaction. It is a function of the actual concentration of components and temperature of the reaction. D G will change. D G , actual free energy change, of reacting system is always decreasing until equilibrium is reached, then it is zero. At that point, no more useful work can be done.