Backpowering via input pins

I had the Arduino Uno connected and running with an external interrupt timer (555) connected to pin 2 of Arduino and set for input interrupt. I removed the USB cable and thus power. I noted that the green LED (ON) was flashing in time with the one second interrupt. My assumption was that voltage from pin 3 of the 555 (output) was providing power in some way. On viewing the schematic of Arduino I note that the LED is connected to 5v and the Arduino pin 2 only goes to the Atmega chip.

Is it the case that the Atmega chip is after all just a bunch of resistors inside, cleverly connected? :)

What is going on?