minor scale systems

[renfroejames]

Hi,
I was wondering if someone could explain to me why the harmonic minor scale is played the same way whether it is descending or ascending, while the melodic minor scale ( with raised 6 and 7) is played ascending but the natural minor is played descending. 
Also, let’s say I was composing an ascending melody using the melodic minor scale – do I need to use specific notes going up but vary them going down? My confused logic would say yes because I need to play the natural minor while descending.