The iMac has no Thunderbolt port. Only minidisplay, USB, FireWire, Ethernet and audio in/out.
the MacBook has Thunderbolt, hdmi and usb.
I Really look forward to hearing a suggestion of how I can use the 27" screen as external screen for my MacBook Pro.
iMac, OS X El Capitan (10.11.2), iMac 27" ultimo 2009