Cell Membranes

The cell or plasma membrane surrounds the cell and protects the cell contents from the external environment.  The main component of the membrane is the phospholipid.  The phosphate group (orange circles) sticks itself into the watery world of the environment or cytoplasm, while the water hating oily lipids  (black lines attached to orange circles) organise themselves so that they face each other away from the water. In doing so they form a 2 molecule thick bilayer.  This acts as a pretty effective barrier for the cells to keep their contents in and the environment out.

However, there are things that the cell needs to get in (like nutrients) and out (like wastes).  To help this the membrane incorporates proteins into this bilayer. These let materials cross the cell membrane according to their shape. If the materials are the right shape, they can pass through the channel and enter or exit the cell. If not, they stay where they are.

Proteins also act as way of cells recognising each other (important for the immune response not killing your own cells).

The other structures on the cells include carbohydrates and cholesterol.  Carbohydrates also help in cell recognition (blood typing being a good example) while cholesterol helps keep the membrane fluid.

When we draw the membrane in a 3 D manner, things get complex.



Basically the structure of the membrane is made up of many small pieces arranged together to create a functional unit.  This is like a mosaic picture, where the picture is made up of many fragments.  Because membranes have to be fluid to work, the model of the membrane is called the fluid mosaic model.



Because the membrane now only lets certain things though now, it is called a specifically permeable or semi-permeable membrane.

Shown below are examples of an impermeable (nothing gets through, on theleft) and fully permeable membrane (everything gets through on the right).

              



In the example below here, note that only 1 of the 2 substances gets through.  The membrane selected the red circles to pass through, but not the purple ones.  This is an example of selectively permeable.