Im pretty newbie in cryptocurrency. I heard that we need to keep secure our Private key. Do we get a Private Key each time we open an account on an Exchange ie: Bittrex, Poloniex, Binance, Coinbase, etc? or do we get Private Key on each currency? I have account on Bittrex and Binance and Coinbase, and as I can remember they never show me Private key. do I suppose to get one?

I also need to know; Do we need to save the generated address each time we send or receive coins between exchange? the address keep changing each time we send.

And where do I get a Paper Private key?