Mixing 8/10speed crackset with 11speed components

Alex-x4 Posts: 15
edited January 2015 in Workshop
Dearest cycling Illuminati,

I have a 2014 Giant Defy 1 which has a 2x10 groupset (Mostly 105)

My question is, can I upgrade to a 2x11 without changing the Shimano R565 crankset?

Similarly a pal of mine has a Cannondale Synapse Claris 2014. Could he upgrade his bike to an 11speed groupset (latest 105) without swapping the 8 speed crankset?

Or is it that the narrower 11speed chain widths would not work on 10/8speed cranksets???

Thank you in advance!
Kind regards,