Cell Membrane Structure, Function,

 Cell membrane, specifically the fluid mosaic model, and what makes it semi-permeable. Okay, here we go! The cell membrane has several major roles. It protects the cell from its surroundings, maintains an internal environment that is different from its external environment, and determines which particles may enter or exit the cell. It has many components that allow it to perform all of these functions. These include phospholipids, cholesterol, channel proteins, carrier proteins, glycoproteins, and peripheral proteins. The membrane they create is both fluid and selectively permeable. Here's a picture of the cell membrane. Notice how it has two layers. These layers are comprised of what we call phospholipids. The phosphate head is both polar and hydrophilic. Attached to it are two nonpolar hydrophobic fatty acid tails. These tails can be either saturated or unsaturated. The phosphate groups have a charge, and the tails do not. The nonpolar tails face inwards towards one another, and the polar phosphate groups are on the outside. This arrangement keeps molecules from passing through too easily. In between every few phospholipids is a molecule you've probably heard of; Cholesterol. Despite what you might see in all those pharmaceutical commercials, cholesterol is actually really important. It helps the cell membrane maintain an appropriate level of fluidity by managing the space between phospholipids. At room temperature, the cell membrane has about the same consistency as vegetable oil, which is ideal. However, higher temperatures would cause the cell membrane components to move farther away from each other and the membrane would disintegrate. Conversely, colder temperatures would cause the phospholipids to clump together and make the cell membrane too rigid to work properly. Cholesterol has a very stable structure, and it helps the phospholipids stay at the optimal distance from one another... Almost like an overzealous chaperone at a middle school dance. So, how exactly does it control what molecules can pass in and out? Certain molecules like oxygen and carbon dioxide can easily squeeze through them without any special accommodations. This is lucky because we exchange a lot of oxygen-carbon dioxide during cellular respiration. These molecules are small and uncharged, so they can sneak through using just diffusion. However, other molecules can't get through without help, and this is where many of our proteins are going to come into play. Molecules that are large or charged can't get through the fatty inner layer. Specialized channel proteins allow large

or charged molecules to pass through the membrane. These molecules flow down the 

concentration gradient from high concentration to low concentration. 

Structures called carrier proteins to charge a toll to admit a molecule. In return for a small amount of ATP, these proteins allow certain molecules to pass through. They are especially useful because they can often pump items against their concentration gradients. A great example of this is the sodium-potassium pump, which pumps sodium out of cells, and potassium into cells, allowing neurons to work. The cell membrane also often includes glycoproteins, which have a protein component embedded in the cell wall and then a carbohydrate protrusion that dangles out away from it. Antibodies involve glycoproteins, but they also play a major role in conception. Human egg cells are surrounded by layers including the Corona Radiata and the Zona Pellucida. Sperm cells contain special enzymes in a compartment called the Acrosome. These enzymes break through the Corona Radiata, exposing the glycoproteins in the inner Zona Pellucida. The sperm attaches itself to these glycoproteins, which begins the process of fusing the two cell membranes so the genetic material can be combined. Finally, cell membranes are also studded with peripheral proteins on one side of the membrane. They can attach and remove themselves from either side of the cell membrane over and over again. You've probably come across a number of peripheral proteins if you've studied cellular respiration. The compartments of mitochondria are bound by a phospholipid membrane, which is studded with proteins, just like the ones earlier in this video. Cytochrome C is an important protein in the electron transport chain. The final steps of cellular respiration. Interestingly, enzymes associated with the Cytochrome C protein are one of the targets of the poison Cyanide. To wrap up, let's consider this idea called the Fluid Mosaic Model. We've learned that the cell membrane is fluid thanks to cholesterol,