Most of the environment around us contains water: from lakes, oceans, and puddles to the juicy insides of fruits or the humidity in the air. It turns out that most of your body is also made of water. About 60% of it! Your body has a way of separating the water inside you from all of the water in your environment (think of yourself in a swimming pool!).
Oils and fats—known as lipids—do not mix with water. This separation occurs if you try to mix several tablespoons of oil and water in a glass (try it at home!)…
…but it also occurs in very small ways inside our bodies. The oily part surrounds drops of water to make little compartments called cells. The oil layer surrounding each cell is only two molecules thick and we call it the cell membrane or bilayer (bi- meaning two, so two layers of molecules).
In the drawing to the right, the top “blob” is a cell, with the inside of the cell colored yellow. The green insert is zooming in to the cell membrane—you can see the environment outside the cell colored blue and the environment inside the cell still yellow. The membrane of the cell is made of the two shiny gold layers facing in opposite directions. These two layers are the bilayer.
Our bodies are made up of many, many such cells…something around 50 trillion cells. That’s 50,000,000,000,000 little water-filled packets that are stuck close together to make up YOU!
Not all of these cells are the same—some are blood cells that make your blood red, some are muscle cells that make your body move, some are fat cells that store energy and keep you warm, and the ones we are particularly interested in are called neural cells, nerve cells, or neurons. Neurons allow for different parts of your body to communicate with each other. There are about 86 billion neurons…that’s 86,000,000,000 or 0.2% of all the cells in your body.
The membranes around each cell are oily and thus will not allow water to go in and out on their own. Cells develop more sophisticated ways to move water (and other things that don’t mix with oil) back and forth when they want to. Because each cell can control what things are inside or outside, cells are the smallest living things!
The “machines” responsible for moving water-like things in and out of cells are called proteins. And they work like machines would: each type of protein has a specific shape and each shape needs specific parts that fit together in order to work.
Some proteins work inside of cells to turn the food that you eat into energy so your body can do work. Other proteins sit in the membrane—at the barrier between “inside” and “outside”— and decide what things can come or go and when. Both of these types of functions are important to neuroscience, but some of the machines at the cell membrane give neurons particularly unique functions. Let’s take a closer look…