grant problem

 Hello all,

 i have a function that updates a table. I gave execute grant on it to
 a particular user but no grant on the table in question.

 when logging in as that user and executing the function, i get an error
 ERROR: permission denied for relation <table_name>

 How can i ensure that any update on this table is only done through
 that function, and that the function is oublicly executable?

 Please provide suggestions.

 Thanks & Regards,
Raj.