Cell membrane Proteins

In the cell membrane, there are a wide composition of proteins present. The most common protein you'll find in the membranes are receptors and channels. They are the messenger of cells. Receptor senses whats going on outside the cells and sends signal down to adjust accordingly. On the other hand, channels are like doors for your cells. They allow passage of nutrients and biochemicals. The most common type of channels you might find in a cell are aquaporins. The clever amongst you may have already realised what this channel/transporter do. It allows movement of water molecule through the membrane. Whilst water is a polar molecule and can move through the membrane bi-layer, it predominately uses this transporter for efficient and fast transport of water molecules. Down below is an actual X-ray crystallography picture of aquaporin channel.

This is a top down view of the channel. Its made up of four sub-units.

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.