Is it always possible to convert a k-tape Turing machine to a single-tape one without increasing its running time complexity exponentially? I am studying the past exam paper of course Theory of Computation. I know it is always possible to convert a k-tape Turing machine to a single-tape one, but how will the running time complexity be changed?
It actually depends on the way you implement a k-tape Turing Machine as a single-tape one. 

Using the standard implementation, you can prove that the single-tape machine will take quadratically more computation time.