When the zinc-plated screw comes into contact with the citric acid in the lemon, it starts two chemical reactions. In one reaction, called oxidation, the acid begins to remove the zinc atoms from the plating on the screw. Two electrons are then removed from each zinc atom, giving the zinc atom a positive charge of two.
These charged zinc atoms, called zinc ions, remain in the lemon and darken the area near the screw after some time has passed.
The other reaction, called reduction, focuses on the positively charged hydrogen atoms, or hydrogen ions, in the citric acid near the screw.
These ions accept electrons released by the oxidation reaction and form hydrogen gas, which can sometimes be seen bubbling out around the screw.
The hydrogen ions are called oxidation agents, because of their tendency to remove electrons from the zinc.
These two reactions continue as long as the zinc-plated screw is in the lemon and there is zinc remaining on the screw. They do not depend on the presence of copper or any other material. The important thing to realize is that electrons are being released from the zinc and being accepted by the hydrogen ions in the acid.
The copper in the coin is also an oxidation agent. In fact, it is a slightly stronger oxidation agent than the hydrogen ions in the citric acid. That is, it can attract many of the available electrons that are released from the zinc. But it cannot do so until there is a connection between the copper and the zinc-plated screw. When a conducting pathway or circuit is established between the zinc-plated screw and the copper penny, the copper draws electrons out of the screw through the circuit and back into the lemon through the coin.
This movement of electrons through the circuit is an electric current. By convention, scientists have agreed that electrons move away from the negative terminal of a battery or electric cell and through the circuit toward the positive terminal. Thus, the zinc (source of electrons) is the negative terminal in a lemon electric cell, and the copper is the positive terminal.
When the circuit includes an LED, the electric current can make it light up.
The voltage of the lemon electric cell comes from the relative difference in the ability of zinc and copper to give up electrons. The electric current provided by the cell depends on the quantity of electrons released by the chemical reactions, among other things.
Reference: Snyder, Carl H. (1998). The Extraordinary Chemistry of Ordinary Things (3rd ed.). (pp 258-271). New York: John Wiles & Sons, Inc.
