First of all, /dev/sda is a disk not a partition. /dev/sda1 is a partition.
This types of operations are risky and you have to know what you are doing. Methods are changing according to the OS and the filesystem.
You can read my blog posts about these operations.
http://ermanarslan.blogspot.com.tr/2013/10/linux-extend-logical-volume-size-lvm.htmlhttp://ermanarslan.blogspot.com.tr/2013/10/linux-how-to-extend-partition-and.html