EXECUTE IMMEDIATE help

I'm trying to run an EXECUTE IMMEDIATE within a PL/SQL if loop but it doesn't work for me. I'm trying to create a column conditionally but it doesn't work. It fails because there are apostrophes within the statement. How do I override the apostrophe?

DECLARE cCount NUMBER;
BEGIN SELECT count(*)
INTO cCount
FROM all_tab_columns
WHERE owner = 'Owner'
AND table_name = 'table_name'
AND column_name = 'column_name' ;

IF cCount = 0 THEN

        EXECUTE IMMEDIATE 'ALTER  TABLE owner.table_name 
        ADD (column_name CHAR(1) DEFAULT ' ' NOT NULL)';
ELSE
	EXECUTE IMMEDIATE 'ALTER TABLE owner.table_name
	MODIFY column_name DEFAULT ' '';