The motor in your 3.5" hdd draws more power then your laptop power supply has available. Remember if you were JUST using the 3.5" hdd on the laptop it might work but the laptop power supply has to run the processor, light the screen, power the sound and graphics systems. Even though these draw little power they still consume a fair amount. If you read your laptop specification sheet you will see the max. PSU output. Then look at your 3.5" hdd and you might get a better understanding. We don't want to see you burn up your motherboard.
If you still want to try using your 3.5" hdd I would suggest you invest in the adapter whs suggested.
If you go the other way and purchase a PSU to powerup your 3.5" hdd you can buy an enclosure and you won't have your SATA cables draped across your lap or desk.