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Cell Membrane Structure

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In this lesson we will examine the structure of the cell membrane. Specifically, we will answer the following questions: what are some functions of the cell membrane, what is the structure of the cell membrane, and how are the cell membrane structure and function related? Our cells carry out the necessary functions to maintain not only their own lives, but ours as well. To do so, their needs must constantly be met. They need to bring in building materials and nutrients for energy production so they can do their work, and they also generate waste products that must be cleared away. Yet cells work best when the mini-world inside them stays relatively constant. How can they maintain this constancy, called homeostasis, while allowing substances to both come and go? The answer is found in the cell membrane. Think about a house. Whether the temperature outside is sub-freezing or scorching hot, you can maintain a comfortable temperature inside. But you couldn't do this if you did not have walls around you providing a physical separation between the inside and the outside worlds. Cell membranes provide the same service, acting as a physical separation between the inside of the cell, called the intracellular compartment, and the world around the cell, the extracellular compartment. For the majority of our cells, that outside world is fluid that fills the spaces between the cells, called the interstitial fluid. Now back to our house. Let's say you get hungry. You call the pizza guy, and he arrives. He can't just toss a pizza through the wall. You get it through the door. And if someone else comes to the door, you decide whether or not to open the door. Now let's say you are painting in there. As fumes begin to build, you could open a window or a door to let them out and to bring fresh air in. You control what comes and goes. And so does the cell membrane. The cell membrane is permeable to some substances, so they can pass directly through it. But the membrane is impermeable, or not permeable, to other substances, so they cannot pass through it. Because of this the cell membrane is said to be semipermeable, or selectively permeable. Cell membranes, however, unlike you in your home, have neither eyes nor brains to determine what should or should not come in or go out. Instead, for cells, that control is done chemically. The chemical composition of the cell membrane determines what can and cannot pass. And like you in your home, if something cannot pass directly through the cell membrane, it may still be able to pass through a door. The cell membrane is composed primarily of a phospholipid bilayer. This means it is made of two layers, or a bilayer, of phospholipids molecules. Phospholipids look kind of like brass brads, fasteners you have likely used to hold papers together or in a folder. The head is formed by a phosphate group. And two fatty-acid tails make up the lipid part of a phospholipid. Lipids are nonpolar, but the phosphate group is polar. The cell membrane separates the cytoplasm, which contains water, and the interstitial fluid, which also contains water. Water is polar, so the phosphate heads are attracted to water. They are hydrophilic. Hydro- refers to water and philo- means loving, so hydrophilic molecules love water, so to speak. But the lipid tails are nonpolar and try to avoid the polar water. They are hydrophobic, with phobic meaning fear. The hydrophobic fatty acids try to avoid water, leading to the bilayer structure. The phospholipid molecules arrange in two layers, each with their polar phosphate heads facing the water and their nonpolar fatty-acid tails tucked inside. Here is another view with the details added. The membrane is largely nonpolar, which means smaller nonpolar molecules are welcomed through it, but hydrophilic substances or larger molecules cannot pass directly through the phospholipids. The phospholipid molecules can move around, so the cell membrane is often referred to as a sea of phospholipids. They make up the bulk of the membrane and give it a very fluid and pliable nature. Embedded within it are other molecules that also move around. Cholesterol molecules provide some rigidity, or some structure, to the fluid membrane. And there are also many proteins. Some proteins pass completely through the membrane and are called integral proteins. Peripheral proteins are attached to one side or the other of the membrane. Some proteins anchor the cell membrane to adjacent structures inside and out, while other serve as identifiers, or markers, so the cell can be recognized. Some proteins act as receptors, for hormones, for example, that the affect the cells functions. Some proteins serve as carriers that bind to and transport substances across the cell membrane. Still other proteins form channels, or doorways, if you will, that open to allow molecules to pass through that otherwise could not. Cell membranes also have some carbohydrates in a superficial layer called the glycocalyx, which plays a big role in cell recognition. Cell membranes are not merely a sack or a wrapper, nor are they fixed like the walls in your home. Instead, they are ever changing. We saw how the membrane is a pliable, double layer of phospholipid molecules, with other molecules embedded within it and that all these molecules can move around. This organization is referred to as the fluid mosaic model. The cell membrane forms a very responsive separator that changes frequently to meet the cell's needs. It regulates what enters and leaves the cell, maintains the cell's positions, and communicates with other cells, all while ensuring the cell's needs are met, so it can go about its usual work.