The Delta G degree for the reaction catalyzed by glycogen phosphorylase (glycogen_(n)) + P_i doubleheadarrow glycogen_(n – 1) glucose 1-phosphate) is +3.1 kJ/mol but under physiological conditions (Delta G’) it is estimated to be -6 kJ/mol. What is the most likely explanation for this difference in the values of Delta G^0′ and Delta G’.