Re: Can you please help me with a bio-chemistry question?

This is a puzzling problem!  Let's review the science behind it.

What makes the light bulb light up is not voltage, but current.
If all of your connections are tight between the bulb and the battery,
then there is apparently not enough current reaching the bulb.

Voltage = current x resistance.  The "electrodes" in the potato and
the wires leading from them may have a higher resistance than the 
wires leading from the battery terminals.  If that were the case,
the voltage would be the same, but the current would be greatly
reduced, perhaps so low that the bulb does not light.

Here's a few things you can do to check:

-- Remove the potato and replace it with the battery, so that you are
using the same wires to light the bulb, and thus using the same
resistance.  If the resistance is too great, try using smaller 
electrodes and wire with a lower resistance rating.

-- Since you have a multimeter, switch it to "ammeter" and measure the
amperage (current) flowing out of the potato.  If the amperage is low,
try the same thing to fix it.

If you find that you are getting the same current flow out of the 
potato than out of the battery, and all of the connections are nice 
and tight so you have good contact, the bulb *should* light in both
cases.  If it doesn't...then I am stumped!