AP Biology 9/14/01 Plasma Membrane The fluid mosaic of a plasma membrane is made up of a semi permeable double barrier of phospholipids called a bi layer. A phospholipid is a phosphorus element with two fatty acids acting as tails. Due to the hydrophilic property of phosphorous, and the hydrophobic property of the fatty acids, this causes the phospholipids to line up in two rows with the tails in together because there are aqueous solutions on either side. This causes a semi permeable barrier that is called the plasma membrane. Semi permeable means that some things are able to pass through while others are not. When molecules are transported though a membrane using either method, it is called diffusion.
Passive transport is called passive because it requires absolutely no energy transfer to move things through the membrane. It occurs when the membrane either separates and allows the molecules through, or a piece of the membrane breaks off and surrounds the molecule and brings it through. The reason that no energy is used, is that the concentration gradient is going the same way as the molecules entering. This is one form of diffusion. When water is diffused through a membrane, it is passive transport, also known as osmosis.
Another example of passive transport is glucose entering red blood cells. The other form of transport through a membrane is active, this is when proteins called permeates imbedded in the membrane are sticking out of either side of the membrane and surrounds the molecule and moves it through to the other side. This form of diffusion is called active transport, because energy is used to move the molecule. An example of passive transport within the cell would be RNA exiting.
On the opposite end, active transport requires energy because the molecules entering are going against the concentration gradient.