Cell Membrane

Cell membrane plays the most important rule in cellular biology. It controls the input and output of any substances in cells. However, as it is an important component of cell it is at the same time has a very complex structure. The most current model on how a cell membrane works is that of fluid mosaic model - that proteins are embedded into the cytoplasmic membrane. Nonetheless, let us look at a diagram of the plasma membrane as depicted below:

I will discuss in more details about this figure later. The membrane is made out of amphipathic phospholipids. That in other  word means that it has a hydrophobic component and a hydrophilic component. As you can see on this figure: 

 The hydrophobic component is the tail part of the phospholipids, as it is a water fearing molecule meaning that it does not interact with water or any polar substances. On the other hand the hydrophilic component of phospholipid is exposed to the both outer and inner cellular fluid. The hydrophilic component interacts with water and other substances that are in both extracellular and intercellular fluid. This amphipathic characteristics is very important for cell membrane as it allows only  selective substances to pass through, making it semi-permeable. So if you wondering why the cell membrane is semi-permeable, than the simple answer is because of this hydrophilic and hydrophobic component that blocks access to important nutrient (large macro-molecule and ions) passing through it.