Can a transistor's emitter and collector be used interchangeably?
As I'm sure you've learned, a BJT is composed of two back-to-back P-N junctions. Knowing this, allow me to redraw Q1 for you, using the diode model of a BJT:







simulate this circuit &ndash; Schematic created using CircuitLab

When the voltage on either of the emitters is below the threshold set by Q2 and Q4, the appropriate emitter diode is forward biased and current flows through it. When both emitters are above the threshold, current flows out of the collector instead, activating Q2 and in turn deactivating Q3 and activating Q4, pulling the output low.