COUPLED REACTIONS

 

This is very important!

 

Consider the phosphorylation of glucose to glucose 6-phosphate:

Glucose + phosphate®  Glucose 6-phosphate                                  DG = +3.3 kcal/mol (unfavourable)

 

Consider the hydrolysis of ATP:

ATP®ADP + phosphate                                                             DG = - 7.3 kcal/mol (favourable)

 

Summing these reactions together:

ATP + glucose ®ADP + glucose 6-phosphate                              DG = +3.3 + (-7.3) = - 4kcal/mol (still favourable)

 

We will see LOTS of unfavourable reactions that are driven by being linked to favourable ones. 

 

Click here for another example.

 

 

Energy barriers and DG

 

Oxidizing wood by burning it in air is clearly very favourable (large negative DG)

But wood is very stable in air - you need to put a match to it to make it burn.

There is an energy barrier to overcome even for reactions with a negative DG.

 

This energy barrier must be overcome in cells if reactions are to proceed at a rate suitable for life.

This is achieved by reducing the energy barrier by providing an alternative reaction route - enzyme catalysis.