This should leave you wondering - the phosphorylation of glucose (as shown above) has a positive delta G and thus cannot occur, but every cell phosphorylates glucose! How can this be? In order for the reaction to occur, an source of energy is needed ( like the pen to the ceiling). A common source of energy that cells use is a compound callATP (adenosine triphosphate). ATP can undergo the hydrolysis (breaking off) of one of the phosphate groups to produce adenosine diphosphate, ADP and a free phosphate. This is written as:
ATP ----------> ADP + PO42-
This reaction has a negative delta G. Thus if the cell breaks up ATP at the same time that it is adding the phosphate to glucose, the cell can use the energy released from ATP to fuel the addition of the phosphate to glucose. This requires that the amount of energy released from the hydrolysis of ATP is greater than the amount of energy needed to add the phosphate. If this occurs, then these reactions are said to be coupled. If so, then the reaction would be written as:
ATP + PO42- + glucose ----------> glucose-6-phosphate +ADP + PO42-
This reaction can be simplified (just like in Algebra) by noticing that there is PO42- on each side of the reaction and can be cancelled.
ATP + glucose ----------> glucose-6-phosphate +ADP
Now, where do cells get the ATP? Obviously, if the breakdown of ATP has a negative delta G (gives off energy), the formation of ATP must have a positivedelta G.
ADP + PO42- ----------> ATP
Where do cells get the energy to make ATP (remember, most of the cellular reactions that require energy use the hydrolysis of ATP as the source - ATP is like money to the cell). The ultimate source of this energy is from the nutrients eaten by organisms. The breakdown of nutrients such as sugars and fats into carbon dioxide and water has a negative delta G. The cells are capable of taking these nutrients, breaking them down and using the energy in them to make ATP. You will know more about these processes later.