Phospholipid Bilayer

• All cell membranes consist of a phospholipid bilayer. A phospholipid has a hydrophilic (attracts water) glycerol head and two hydrophobic (deters water) fatty acid tails. The phospholipid bilayer gives the cell fluidity, meaning the cell can move. This bilayer is also semi-permeable, meaning that some molecules like oxygen, carbon dioxide and small hydrocarbons can get into the cell, while other molecules like water, charged ions and glucose cannot get in.
  
  

Proteins

• Since the phospholipid bilayer is only semi-permeable, proteins are present in and around the cell membranes in order to help cells communicate with each other as well as to exchange nutrients. Integral, or transmembrane, proteins are embedded within the phospholipid bilayer. At normal physiological temperatures, these proteins can allow different ions or nutrients into a cell as well as send signals to and from neighboring cells. Peripheral proteins are attached to either the inside or outside of the cell. These proteins help to provide structure to the cells.
  
  

Function

• The semi-permeable nature of the cell membrane provides it with its most vital function which is to protect the cell and the organelles within the cell from the outside environment. The fluidity of the cell membrane allows the cell to grow and respond to various environmental changes including temperature changes.
  
  

Increased Temperature

• At physiological, or body temperature, the cells function their best. As temperatures increase, both the cell membrane and the proteins can be affected. The fatty acid tails of the phospholipid bilayer can "melt" at high temperatures meaning that they become more fluid and allow more movement. This affects the permeability of the cell which may allow molecules into the cells that should not get in, thereby damaging the cell.
  The transmembrane or peripheral proteins can also be damaged by high temperatures. High temperatures cause proteins to denature, or break down. Increased temperatures also increase the reactions that happen within the cells, which may be acceptable to a point, until the temperature becomes too high which will destroy the protein, the reactions and the cells.
  
  

Decreased Temperatures

• A decrease in temperature also has an effect on cell membranes and cells. The fatty acid tails of the phospholipids become more rigid at cold temperatures. This affects the fluidity, the permeability and the cells ability to live. When the cells are less fluid, they cannot move or grow. The decrease in permeability means that vital molecules cannot get into the cell. In addition, cold temperatures can cause cellular reactions to slow down or even stop.