The Cell: Plasma Membrane

 Ugh! The dreaded cell! The most basic unit to life, and yet, there is a lot that goes on in it! If the cell doesn't work right, disease and illness arise. So much for basic, right? Well, let's take a step back even further. If we know how each PART of the cell works, we can understand everything from fluid and electrolyte balance, the production of ATP (fancy acronym for cell energy), to function of entire systems of the body! Not basic at all...but we are going to do just that. Make it basic, so an understanding can be formed about how important each part is to sustaining life.

So let's hold onto our hats and dive into the Plasma Membrane. When I was brainstorming how I wanted to present the cell, I kept thinking about eggs and then I was thinking about fairy tales. Now, you may be asking yourself, "Liz, how the hell are these two related?" Well...there not. But bare with me, because I am going to use both to demonstrate the importance of the plasma membrane.

Before I talk about the egg, I want you to grab an egg and soak it overnight in vinegar. Should look something like this when it is done:

I know a lot of you have probably already done this before, but that thin layer that is holding the egg together is the plasma membrane. It is semi-permeable and keeps the egg from leaking contents all over the place. 

Now, what the hell does "semi-permeable" mean? Not impermeable like your rain boots, but not permeable like a cheese-cloth either...hmm. To be honest, I only heard the term "semi-permeable" when I started anatomy classes. I always thought that it either was or it wasn't. No grey. You'll learn real quick in healthcare, there is A LOT of grey. 

So this brings me to the bigger picture of the fairy tale. 

Imagine a city with high-walls, draw bridges, armed guards, and a moat. Okay, now this is getting interesting! This city is our cell. The moat and the fortress walls are the plasma membrane. It allows certain "people" to pass and others do not. 

The plasma membrane is a phospholipid bilayer:

The plasma membrane has hydrophilic ("water loving") heads and Hypdophobic ("water fearing") tails. It is picky who can come in an out without tickets or passwords to get through. 

Three very important VIP members can go right through, in and out of the cell at free will. No doors need to be opened, no turn-styles, no armed guards or passwords. They walk straight through the walls, like Harry Potter walking through the pillar to get to platform 9 3/4. These VIPs are water, oxygen and carbon dioxide. CO2 is "booed" out of the cell and O2 has a ticker-tape parade when it enters. This form of movement in and out of the wall is referred to as passive diffusion. Every molecule that moves through passive diffusion goes from an area of high molecule concentration to an area of lower molecule concentration. If you have 10 oxygens inside the cell and  and 2 oxygens on the outside of the cell, the oxygens will move from the higher area of inside the cell, to outside the cell ("peace out dudes, I need more room"), until it is equal. 6 oxygens on each side. 

Water moves in a similar fashion through osmosis. People often get this confused because it is not the particles moving, but the solvent (good old H2O) that is moving. The water is moving from LOWER particle concentration to HIGHER particle concentration. Just think the side that has more particles are saying, "I'm parched, can I get some water over here!" This can be seen with that egg that you left out over night. Put it in a hypertonic solution (something with a lot of particles in it) like corn syrup, and watch the egg shrivel. The water inside the egg is moving to the parched particles of the corn syrup. Once it has shrunk, you can put it in distilled water (low particle solution) and watch it expand. Water will fill the egg space because there are more particles in the egg than the water. 

Okay...we talked about passive diffusion and osmosis, how the hell does everything else make it through this membrane?  

Protein channels are used as another form to let particles into and out of the cell. The protein channels are the draw bridge. The draw bridge allows as many that can fit through the channel. Often water and ions are what can fit through these channels. Damn, skinny bitches can sneak right in. 

Other protein channels, called gated proteins, only allow one type of molecule in and it is a slower process. Make sure you scan your badge before going through this turnstile and only people who have 5 freckles on each cheek can pass. 

Even carrier proteins get molecules across. These molecules have to say the secret password the protein then shape shifts to carry the molecule through. Cool, right? I wish I could shape shift. 

All of these proteins mentioned above are all in the cell membrane and all are hard at work to moving molecules, particles from areas of higher concentration to an area of lower concentration. They are kicking some people out and letting other in. 

Well, this is a great system. Seems to work, except there is one problem. There are certain ions that we do not want to be equal on both sides of the cell membrane. Sometimes we want higher concentrations inside the cell than outside the cell. This is where active transport comes into play. People need to pay to get in. Armed guards are at these active transport site and there are strict rules. The most common type of active transport is the Na+/K+ pump. Sodium is saying to the guard, "hey, I NEED to get out of here! I will do anything". The guard,  "I can get you out, but you need to pay. I also need you to find two of the finest potassiums from the outside to take your place." ATP is used as currency. 3 sodiums move outside the cell and 2 potassiums move in. The cell is none-the-wiser. 

So this is the work of the plasma membrane. Instead of looking like the above picture of the phospholipid bilayer, it actually looks more like this: 

If you have any questions, please leave them down below. If anyone has any suggestion on the improvement of this post, please leave a comment. I hope this was helpful.