Why can't high current substitute low voltage
Consider using the water in pluming metaphor to explain voltage and current. Using this metaphor, the voltage is the pressure in the pluming and the current is the amount of water making it through the pluming. Batteries become water pumps.  Resistors become constrictions in the pipe.  Diodes become check-valves which need a little pressure to open:


  A diode is equivalent to a one-way check valve with a slightly leaky
  valve seat. As with a diode, a small pressure difference is needed
  before the valve opens. And like a diode, too much reverse bias can
  damage or destroy the valve assembly.