Question

How does each nonspontaneous positive free energy step in glycolysis go to completion? Why does ATP help make the step/reaction spontaneous? Why does ADP to ATP rxn in glycolysis also help make free energy change negative, wouldn’t it do the opposite of ATP?

Answer 1

The completion of non spontnaeous positive energy step in glycolysis is supplemented by coupled reaction. Here the non spontaneous positive energy step is coupled with a spontaneous negative energy step which leads of overall negative energy for the coupled reaction leading to spontaneity of reaction.

Example:

Glucose + fructore ---> sucrose Delta G = +27KJ

ATP ---> ADP + iP   Delta G = -30 KJ

So overall energy Delta G = - 3KJ

Making the overall reaction spontaneous.

In such case, a phosphate group is transferred from ATP to glucose This leads to formation of phosphorylated glucose which is an endergonic reaction being the unstability of ATP.

The other reaction is in between phosphorylated glucose and fructose to form sucrose. Here the phosphorylated glucose is unstable making reaction energetically favourable.

When ADP is converted to ATP in glycolysis step

For example

PEP + ADP ---> Pyruvate + ATP the Delta G -31.6 KJ / mole

Here the PEP molecule has large Delta G negative energy of hydrolysis (-61 KJ / mole) leading to favourability of reaction.