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The plasma membrane, also known as the cell membrane, is an essential part of the cell that encloses the cell's interior components while only allowing certain parts of the outside environment to enter. This membrane is one of the few parts that prokaryotic, eukaryotic, plant, and animal cells all have in common. The plasma membrane is far more than a simple barrier; it controls what moves into and out from the cell, and it governs many of the interactions that occur between a cell and its environment. The membrane is composed of many different molecules and proteins that move somewhat fluidly, resulting in the "fluid mosaic" description of the plasma membrane.
The most abundant molecules in the plasma membrane are phospholipids, which are made up of a hydrophobic, "water-fearing" tail and a hydrophilic, "water loving" head. Two layers of phospholipids arranged with the hydrophobic tails on the inside form a phospholipid bilayer that provides the primary structure of the membrane. This bilayer prevents large substances or particularly polar substances from passively diffusing across the cellular membrane.
Many proteins that allow for the transportation of large or polar substances across the membrane are embedded in the phospholipid bilayer. Some allow for the passive diffusion of substances in and out of the cell; this requires no energy. Others actively carry substances from one side of the membrane to the other. This process, generally referred to as active transportation, does require a small expenditure of energy. Not all substances can move in and out of the plasma membrane at all times, so it is said to be "selectively-permeable."
The plasma membrane also plays an important role in positioning, anchoring, and shaping the cell while connecting neighboring cells. Extracellular structural components, which compose the extracellular matrix, connect to a cell at its cellular membrane. Cell walls, which provide rigidity to plant cells and to some bacteria and other small organisms, also tend to connect to a cell's plasma membrane.
Cellular communication is another important function of the plasma membrane. Proteins and protein receptors embedded in the membrane can send and receive chemical signals. Some of these signals prompt cells to some form of action, such as absorbing or expelling particular substances. Other chemical signals serve as identification mechanisms and allow cells to recognize each other. This is particularly important in the immune system so the body's immune response only targets harmful cells and does not harm the body's normal cells.
@indemnifyme - The plasma membrane was one of my favorites in biology too. Once you learn about the plasma membrane, a lot of other stuff about the cell is easy to understand.
For example, once you've got the idea of membrane proteins down, passive and active transport are easy to understand. Also, as you pointed out, there are membranes inside the cell too. Once you understand the plasma membrane around the cell, the interior membranes make sense too!
When I took biology I was totally fascinated by the plasma membrane. It really is integral to the cells functioning because of the way it allows some things, but not others, to pass through the membrane.
There are also membranes inside the cell. In fact, the compartmentalization inside the cell makes it possible for all kind of different chemical reactions to happen in the cell all at once.
I could keep going on and on-cells are really amazing!