I am a little fuzzy on Withdrawl of Products. Could you explain how the drain of products pushes the unfavorable reactions in glycolysis? And what products are actually draining?
The direction of a chemical reaction will be determined by delta G. Delta G has 2 components, delta Go and RTlnQ. Q is made up of the concentrations of the reactants and products under the conditions you are considering to calculate delta G. Say the reactants are A and B and the products are C and D. And further suppose that the delta Go for A + B =--> C + D is large and positive (unfavorable). Now Q = A*B/C*D (concentrations). Now suppose D is also a reactant for a second chemical trasnsformation (a downstream reaction), say C --> X + Y. If the delta Go for c --> X + Y is very favorable, then the delta G for that reaction will be favorable (i.e., large and negative) even in the presence of a low concentration of C. So the concentration of C will be greatly reduced by the running of the second reaction. This reduction in C concentration will greatly affect the delta G of the first reaction, by making Q very very small (e.g., 10-7) and thus making delta G for A + B --> C + D negative even though delta Go for a+B --> C + D is large and positive (unfavorable). The products tha are draining are the products of the reaction under consideration for the determination of delta G. Note that, importantly, delta G and delta Go are different.