Excel Formula - formula based on if cell contains

I'm not sure how to explain this situation. 

Cell   A2 has the following contents   D20   

D20 will be used in multiple formulas throughout the spreadsheet.  It is one of the variables.   

Cell C1  has a formula =SUM(D1: whatever the contents of A2, in this case it is D20)  

So when I change the contents in A2 to something else the second variable in C1 will be the contents of A2.