Cell Membrane

Jina Joseph
AP Bio
10.3.14   

The Cell Membrane 

     The cell membrane is very important to the body. Yet, some people fail to see and appreciate the cell membrane and it's ability to keep the body functioning. Did you know that one of the earliest moments of evolution may have been the formation of a membrane that enclosed a solution? Every organism is made up of cells. All cells are contained by a cell membrane. The cell membranes are made up by phospholipids and proteins, and are sometimes known as "phospholipid bi-layer".

Above is a model of a cell membrane. The spheres are the phosphate end, which means they are soluble and hydrophilic. The extensions, also known as the tails, are hydrophobic.

      A membrane is held together by hydrophobic interactions, which are weaker than covalent bonds. The movement of phospholipids in the membrane are quick. Phospholipids, which are next to each other, switch positions about 10^7 times per second. A membrane is constantly fluid as a temperature decreases until the phospholipids create a closely packed arrangement. The membrane brings it's fluid to a lower temperature when there are rich phospholipids with unsaturated hydrocarbon tails. Due to the kinks located in the tails, unsaturated hydrocarbon tails cannot be placed closely together. 

      The cell membrane is selectively permeable. This means that the cell only allows certain things to go in and out of the cell. In fact, this is one reason the membrane is structured this way. Due to the way it is structured, there is a greater ratio of surface area to volume allowing the flow of molecules in and out of the cells at an easier way. Particles with no charge can pass through the membrane. Some molecules have charges so it is more difficult to get across the membrane. Proteins such as a channel protein and an aquaporin helps molecules to get in and out of the cell membrane. A membrane is also known a a mosaic fluid because its consists of various proteins.  Integral proteins and Peripheral proteins are two out of the hundreds of proteins in a person's body. Integeral proteins go through the hydrophobic center of the lipid bilayer, while peripheral proteins are not in the lipid bilayer, but they are appendages connected to the surface of the membrane. On the cytoplasmic side of the membrane, membrane proteins are retained in place by the cytoskeleton. On the extracellular side, some membrane proteins are connected to fibers of the extracellular matrix. The membrane alone could not provide support to the cell. 

     Diffusion is the result of thermal motion. Diffusion is the movement of molecules of any substance so that they spread out evenly into available space. Processes such as diffusion and facilitated diffusion use passive transport because there is no energy being used. The diffusion of water across the cell membrane is known as something specific, osmosis. Now this relates to the tonicity of a solution. The tonicity of a solution depends the concentration of the solution that cannot cross the membrane. Cells without walls are seen in three enviroments: isontonic, hypertonic, and hypotonic. In an isotonic enviroment, there will be no net movement. The solutes concentration is the same on the inside and outside of a cell meaning the state of equilibrium has been reached. A hypertonic solution is when there is more concentration on the outside of the cell. On the other hand, the hypotonic solution is when there is more concentration in the inside of the inside of the cell.

In a hypotonic solution, the cell can burst or result in lysed. This means that their is too much water in the cell. On the contrary, hypertonic solutions can result in cells shrinking because the solution continues to decrease.