How does a two input transistor AND gate work?

Question

I'm trying to self study electronics. I started reading a book on digital electronics and I got stuck on the first chapter. It's about the logic gate circuit. There is a two input AND gate circuit using three transistors:

In my physics book, the circuit for an AND gate shown is pretty clearly. It used two transistors and I get that theory, but I don't get this.

In this image, why is it that when either A or B or both are at 0 V, X=0 V? I know that in these cases, at least one of the transistors is off, so current from 5V passes through R and to the base of transistor T3, making it on. The current should also flow through the resistor across R, so why does X have an output 0?

Answer 1

1. T3 goes conducts and goes to saturation when either A and B or both are A and B are O. Since path from 5v via T1 , T2 is not complete and in one way open circuit. Since in saturation has Vce less that .3V, you get X=0

2. When A and B are 1 , path from 5v via T1 and T2 is complete and both the transistor will be in saturation. Hence T3 will not turned on, as Vbe of T3 will be less than 0.7. hence you get X as high