Re: How does cells make electricity?

Yan-
Good question! Cells use their inherent electricity for lots of different things; understanding how they develop that electricity is very important. To answer your question, first I'll have to give you some background Chemistry, and then we can delve into how cells become electrically charged.

First, we need to define electricity! Electricity could be several different things. For my explanation, I'll first have to talk about charge, which is the presence of “electricity”, then what are electric particles, and finally how electric particles interact to make something "electric."

Charge is how “electric” something is. It is measured in units called Coulombs. This charge is carried on particles, such that we would say a “particle is charged.” (Kind of like you would say a ball is blue – it’s a property of the particle). In a wire, these particles are electrons, and they carry negative charge. So when you plug a lamp into the wall, electrons carry the charge in the cord from the wall to the lamp, and then this charge is used to light the light bulb. It’s as though the electrons are charged marbles moving in a tube (the wire) to the lamp (though the specifics are more complicated).

In a cell, the charge carriers are atoms. Atoms are made up protons and electrons (and neutrons, but I’ll ignore those for now). Protons carry positive charge, and as I said before, electrons carry negative charge. When there are the same number of protons and electrons, the atom is said to be neutral. But if the balance is tipped, so that there are more protons than electrons, then the atom becomes positively charged. And if there are more electrons, then the atom is negatively charged. An atom that has a positive or negative charge is called an ion. These ions can carry charge around a cell, similar to the way that electrons traveled in the wire. It’s like there’s a bigger particle (that’s made up of some smaller ones, namely, protons and electrons – sort of like a popcorn ball) that carries the charge around the cell.

So we’ve established that there are electrical particles (ions) in a cell, and that they can carry charge. But that’s not the end of the story! Just because something has charged particles doesn’t mean it is electric or charged. A glass of water from the tap has many electric particles in it, but you don’t get shocked when you drink it. (But an important note is that if something that is plugged in touches water, that water CAN shock you, and quite badly. Those charged particles can carry the electricity from whatever is plugged in into you! That’s why they say to keep electrical appliances away from water).

So how does cells become electric?
Similar to the way that there are positive and negative particles in an atom, there are both positive and negative ions inside and outside the cell. The cell literally pushes the positive ions out across its cell membrane. Specifically, it pushes Potassium and Calcium ions out (among other ions). What this pushing does is tip the balance of positive and negative ions inside and outside the cell. Since the cell pushes out positive ions, the outside becomes more positive than the inside. Since we often think of the cell as being surrounded by a big ocean of stuff, we look at the cell's perspective: the cell is negative relative to its environment (the same as the outside being positive relative to the cell). This imbalance of electric charge is called Electrical potential, and is quantified in Volts (like in a 9 Volt battery). Since this electrical potential is formed by different ions on either side of the cell membrane, we refer to it as the membrane potential. This potential is the “electricity” we speak of when we say that cells are electric. It is a form of energy that the cell can use – I’ll talk about that in a bit.

As you may know, cell membranes don’t let many things flow through them, so ions and water don’t just cross the cell membrane. The cells push out the ions by special molecular pumps. These molecules are proteins which are able to grab some ions from the inside of the cell and let them pass to the outside of the cell. This process often requires a lot of energy. The more positive particles you push out of the cell, the harder it becomes to push more out. A good portion of the cell’s energy is spent on keeping these pumps working.

Cells also have specialized, strategically placed holes in their cell membranes that allow ions to come back into the cell, known as ion channels. Why? you might be asking. What a waste of energy, to pump out all these ions just to let them back in! But this re-entry of ions is highly regulated. This close regulation of ion re-entry allows the cell to control specific aspects of its function. It’s as if the cell uses ions as logs to light a fire in the fireplace. You keep all the logs outside until you want them, and then you go grab a few; just enough for the fire that you’ll be lighting that night. If you had all these logs around your house, not only would things get really cluttered, but you might accidentally set one of these extra logs on fire and burn your house down! Cells are really finicky about what ions they let in and when they let them in. If too many ions come in, it can kill the cell!

I know this was kind of long – you may want to print it and re-read it! I hope it helps you understand what electricity is and how it comes about in cells. In summary, the uneven balance of ions inside and outside the cell make it electric; movement of these ions across the cell membrane allows the cell to perform important functions. I’ll include some links below!

Some more about the membrane potential:

http://faculty.washing ton.edu/chudler/ap.html
http: //www.madsci.org/posts/archives/dec97/880395992.Ns.r.html

How the cell can utilize this potential can be found at: http: //www.madsci.org/posts/archives/aug98/901728002.Ns.r.html

Searching the madsci archives will give you even more about membrane potentials (I just listed a few above for your perusal!)