The bus is exactly going from Tegel airport to Berlin Zoo station. I can not say to the account of Alexanderplatz exactly (I have never traveled), but this is east Berlin and the distance from Tegel airport is quite large, the bus takes about 30 minutes, taking into account the stops.

If it's a Berlin Tegel TXL airport in the north-west of Berlin, then you can get to the city center - Alexanderplatz by bus TXL. Travel time is about 20 - 30 minutes, depending on the traffic situation and time of day.

It is most convenient to travel around the city with a map or print out the required route. You can buy a ticket for a TXL bus in one of the machines or directly from the driver, and it will be valid for all modes of transport for two hours and for any number of trips.

TXL buses run every 10 to 20 minutes. Timetable: Mon - Fri from 05:07 to 00:27. On Saturdays from 05:47 to 00:27. On Sundays and holidays from 05:38 to 00:27.