Cell membranes are one of the tiny wonders of the world. More than just a barrier, they selectively keep things out or let things in to the cell. They have components which identify the cells they protect, receive signals that change what the cell is doing, and send out signals as well.
But what is more amazing is that they are self assembling. If you have a test tube with the molecules that make up a cell membrane mixed evenly in water, you'll find that they quickly form up into spheres of cell membrane. It's one of those things that is both simple and complex. I still have to admit to being a bit awed by them.
The main ingredients of a cell membrane are phosphoglycerides (more commonly known as phosphlipids). There are several versions, but their main construction is a single phosphate head and two fatty acid tails. The phosphate head likes to be around water and mixes with it regularly. The fatty acids are basically oil, and try to avoid water. When you mix these molecules up randomly in water, they come together in a double layered sheet of molecules with the water loving phosphate heads spread out on the outside and the water avoiding tails bunched up on the inside. Beautiful!
Only small molecules such as water, carbon dioxide, and oxygen can pass through membranes made of just phospholipids. These substances enter or exit the cell by diffusion, which basically means things bumping around each other enough that they get spread out evenly. Laws of probability and all that. But cells may need to ingest large molecules such as sugars and proteins. And they may want to keep a concentration of some substances higher or lower in the cell than its surrounding environment.
For this, there are proteins that imbed themselves all the way through the membrane, called integral proteins. Some of these proteins are just pores that certain molecules can fit through. This is how glucose enters the cell. A cell is constantly breaking down glucose to create energy, so there is usually less glucose in the cell than outside of it (the cell is always breaking down glucose to create energy), so it naturally diffuses into the cell. But because a glucose molecule is so big, it must move through the transport proteins in the membrane. No energy is needed to let these kinds of molecules pass.
Other integral proteins receive signals which don't have to enter the cell to affect it. These special signal molecules will change the shape of the protein when they attach to it. On the inside of the cell, this shape change starts a chain reaction which causes the cell to either start producing something, stop producing something, or in the case of muscle cells, to change shape.
Some proteins might be stuck to just the surface or partway through a membrane. These peripheral can act as identifiers. In blood cells, surface proteins determine the blood type.
There is a lot more to cell membranes, far more than can be covered in one post or even a series of posts. They are an essential component of all biological life, as important as DNA. Not only do they protect the cell from the outer environment, they protect the cells organelles and store molecules that they don't want to use yet. They can store things for later use, which would otherwise be quickly metabolized in the cell's cytoplasm.
We are learning from these natural technologies. Scientists have now created artificial membranes, an important step to creating artificial life forms. This could have applications in medicine, building, convenience, and many other things (some of them admittedly scary but that's another post). Other technologies which take their cue from the cell membranes are the liposomes found in our face lotions. Another possible technology is protecting buildings. While we'll still face pitfalls and dangers, I firmly believe that learning from nature and protecting it, rather than trying to circumnavigate it will propel us into a better world.
