Using SELECT ... FOR UPDATE as a way to lock a whole table

I know I can lock a whole table using LOCK TABLE, but I am just curious if I can get the same result by using the FOR UPDATE statement, assuming that all transactions that update that table select a specific row from that table using FOR UPDATE at the beginning of their transaction (or maybe having a separate table called locks with one unique column named lock_name and using SELECT lock_name FROM locks WHERE lock_name = 'foo' FOR UPDATE to acquire a lock before each transaction)? I am asking this question because I have to save a tree data structure in a database table using either an adjacency list or a closure table. When multiple users try to add a new node or relocate a sub-tree, I have to select some rows and do some calculations and then update the table based on those calculations and this operation is not possible unless I lock the whole table.