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Reviewed: 2/2/25
Short Answer: It depends on the system. In galvanic (voltaic) cells, the anode is negative (−), and the cathode is positive (+). In electrolytic cells (or any setup that requires an external power source, like electrophoresis), the anode is positive (+), and the cathode is negative (−).
Our guide below breaks down all of the fundamental concepts you’ll need to understand the answer above: oxidation and reduction, electron flow, and the fundamental differences between galvanic and electrolytic cells.
Regardless of whether the anode is (+) or (−), oxidation always occurs at the anode, and reduction always happens at the cathode.
Use the mnemonic “An Ox, Red Cat” to remember: Anode = Oxidation, Reduction = Cathode.
In every electrochemical system, electrons flow from the anode to the cathode. This direction of electron flow does not change, even if the signs of the electrodes do (depending on whether we have a galvanic or electrolytic setup).
Galvanic (Voltaic) Cells: Spontaneous reaction, no external power source needed, ΔG < 0, E_cell > 0. Here, the anode is negative, and the cathode is positive.
Electrolytic Cells: Non-spontaneous reaction, need a power source, ΔG > 0, E_cell < 0. In these setups, the anode is positive, and the cathode is negative.
Whether the anode is labeled “positive” or “negative” changes with the type of cell, but remember that oxidation always happens at the anode, and electrons always move from the anode to the cathode.
A galvanic (voltaic) cell doesn’t rely on an external power source; the reaction naturally wants to occur, so it’s spontaneous.
Since the reaction is spontaneous, electrons naturally flow from the negative anode to the positive cathode. This flow creates a positive cell potential (E_cell > 0), which is another way of saying the reaction is energetically favorable.
ΔG < 0: Indicates a spontaneous process (no external push required).
E_cell > 0: A positive voltage means you can measure a potential difference—useful if you’re powering something or measuring electrical current.
Unlike galvanic cells, electrolytic cells require an external power source (like a battery) to drive the reaction. Because we have to force the reaction to occur, it’s non-spontaneous.
In an electrolytic setup, the anode is positive, and the cathode is negative—the opposite of a galvanic cell. Even though the signs are flipped, oxidation still happens at the anode, and reduction still happens at the cathode.
ΔG > 0: We’re putting in energy to push electrons where they “don’t want” to go spontaneously.
E_cell < 0: The reaction isn’t favorable under standard conditions; you need external voltage to make it happen.
If the molecule is negative, it will move toward the positive anode.
If the molecule is positive, it will move toward the negative cathode.
SDS-PAGE: Molecules (e.g., proteins) are coated with SDS, giving them a net negative charge. So, they travel toward the positive anode.
Isoelectric Focusing (IEF): Proteins start out in a pH gradient. If the local pH is higher than the protein’s pI, the protein gains a negative charge and moves toward the positive electrode. If the local pH is lower than the protein’s pI, the protein is more positively charged and moves toward the negative electrode. Eventually, each protein lands where pH = pI (net charge = 0) and stops moving.
Electrophoresis (e.g., SDS-PAGE) and Isoelectric Focusing (IEF) are critical lab techniques often tested in the MCAT’s Biology/Biochemistry section. They help separate molecules (like DNA or proteins) based on size, charge, or isoelectric point.
Because these methods apply an external voltage to drive the movement of molecules, they behave like electrolytic cells and so require a power source.
Anode = (+) and Cathode = (−) in these setups.
SDS-PAGE: Proteins are coated with negatively charged SDS, so they migrate toward the positive anode.
DNA Electrophoresis: DNA is inherently negative (due to its phosphate backbone), so it also travels toward the positive electrode.
Proteins are placed in a pH gradient while an electric field is applied.
The low pH end of the gel is near the positive anode, and the high pH end is near the negative cathode.
Each protein moves to the point where its net charge is zero—its pI (isoelectric point)—and stops migrating.
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