Getting demoted from tier 5 to (tier-1) 1 is quite difficult, in that your mmr needs to be (tier-1) 5 level rather than simply that of the division below. If you got to plat by your own merits I wouldn't worry to much about being demoted due to a loss streak, as odds are you'll have a win streak later to counter balance it.
I currently get 25-26ish LP for a win and lose 16ish for a loss. As another post mentioned, it depends on your mmr relative to the average of your division, so if your mmr is higher than expected your gains will exceed your losses, and vice versa (with gains=losses if you're roughly right where your mmr indicates you belong).